South Foehn Studies and a New Foehn Classification Scheme in the Wipp

South foehn studies and a new foehn classiﬁcation scheme in the Wipp and Inn valley

A dissertation submitted to the Department of Meteorology and Geophysics, University of Innsbruck

for the degree of Doctor of Natural Science

presented by Johannes Vergeiner

October 2004

Proposal for the composition of the examining committee presented to the dean of the Faculty of Natural Sciences at the University of Innsbruck as required for the submission of the dissertation under § 62 (7) UniStG:

Reviewing committee:

1. Ao. Prof. Dr. Georg Mayr (advisor), University of Innsbruck

2. Prof. Stephen Mobbs, University of Leeds, England

3. Prof. Dr. Josef Egger, University of Munich

Committee for oral examination:

Chairperson: Prof. Dr. Michael Kuhn, University of Innsbruck

Examiner:

1. Ao. Prof. Dr. Georg Mayr (advisor), University of Innsbruck Subject: Synoptic Meteorology

2. Ao. Prof. Dr. Ekkehard Dreiseitl, University of Innsbruck Subject: Climatology

3. Prof. Dr. Reinhold Steinacker, University of Vienna Subject: Climatology

Date: October 2004

For all those who believe in me

”How many years can a mountain exist, before it is washed to the sea? How many years can some people exist, before they’re allowed to be free? And how many times can a man turn his head, and pretend that he just doesn’t see?

The answer my friend is blowing in the wind, the answer is blowing in the wind.”

(Bob Dylan, 1962)

Abstract

The participation in the Mesoscale Alpine Programme (MAP) in fall 1999 and the resulting rich data set was a motivation to study foehn ﬂow in the Brenner target area. Chapter 1 introduces the reader to the goals of MAP, the measurement strategy in the Brenner target area and describes the tedious homogenisation of data measured by diﬀerent types of weather stations. The subsequent main part of this thesis consists of two case studies – one in the framework of MAP and one which resulted as a consequence of MAP – and a new method to classify foehn based on a general approach.

The first article in chapter 2 presents a case study of a complex and short-lived three-layer flow situation (”Sandwich foehn”) measured during MAP. Observations from an aerosol Doppler lidar, two radiosounding systems, and several weather stations are supplemented by model simulations. A ∼500m deep upvalley flow (northerly in the Wipp valley) transports cold air from the northern Alpine foreland to the region of interest. It almost reaches the Brenner pass at the border to Italy around 09 UTC, before it is pushed back by the stengthening foehn. The upstream radiosonde (in terms of foehn) shows flow blocking south of the main Alpine crest up to 1.5km MSL, above which southerly flow of 3 − 5 m s−1 was measured. A strong inversion at 3.8km MSL decouples the south flow from the northwesterly above. Downstream in the Wipp valley the neutrally stratified foehn layer has descended by 500m and the foehn jet topping the upvalley flow reaches 12 m s−1. Lidar measurements reveal two gravity waves exited by west-east oriented ridges and the decreasing depth of the cold surface layer particularly between 12 and 14 UTC. The numerical simulations with the MM5 capture the main flow features but underestimate the depth and southward extend of the northerly surface flow. Consequently the foehn layer is simulated too close to the surface. Additionally wind speeds in the foehn jet and the northwesterly flow aloft are overestimated. The horizontal flow speed in the foehn layer is stronger along the eastern slopes of the Wipp valley. While the westward bending of the valley axis has an obvious impact on this flow asymmetry, the influence of the northwesterly flow aloft is unclear.

v vi

In-situ measurements with a Falcon 20 E5/D–CMET run by the German Aerospace Center (DLR) are the backbone of the second article in chapter 3. The access to the aircraft was part of the STAAARTE (Scientific Training and Access to Aircraft Throughout Europe) programme, which aims at training young scientists. During two flights in February and November 2000 standard meteorological parameters were sampled at a 10- 100 Hz rate. Flight data were combined with measurements of radiosoundings, a mobile platform and surface stations. Both days were characterized by southwesterly winds at crest height and moderate pressure-driven foehn flow below. On 28 February a marked low level jet was found within the southern Wipp Valley at 1500 – 2000 m MSL below the strong inversion. Near the exit of the Wipp Valley at the same height a well-mixed zone with nearly no horizontal flow existed as a consequence of a hydraulic-jump-like feature. The inversion along the Brenner cross-section descended from south to north and split up into two weaker parts. On 7 November the oncoming southerly flow over the Inn Valley changed from southwesterly flow at crest height to highly orography- dependent flow below. Horizontal flow splitting was observed at the conjunction of major north-south orientated valleys with the perpendicular Inn Valley. Ver- tical motions on the order of 5 m s−1 showed strong wave activity east of Innsbruck.

Finally chapter 4 presents a new method of classifying foehn periods at a downstream location. It evolved from the identification of foehn during the Special Ob- serving Period (SOP) of MAP and an unsatisfactory and tedious manual classification up to then. The Objective Foehn Classification (OFC) is based on the lapse rate between a suitable mountain reference and the valley station as well as the wind direction at both stations. This new approach utilises the physical foehn mechanism on the lee side and is therefore thought to be applicable to foehn winds around the world with local adaptations. The foehn direction at the valley station has to be determined independently and the shape of the topography upstream has to be taken into account. The ability of the scheme to exclude valley winds with similar flow characteristics and to capture an interrupted foehn sequence is demonstrated exemplary. Statistics for all stations within the 70 day SOP of MAP are given as well. Contents

Abstract v

Contents vii

1 General introduction 1 1.1 Overview of the MAP experiment ...... 2 1.2 Homogenisation of MAP-SOP station data ...... 5 1.2.1 Processing logistics ...... 5 1.2.2 Calibration of sensors ...... 6 1.2.3 Quality control using sensor intercomparisons ...... 8 1.3 Goals and outline of the thesis ...... 9 APPENDIX A: Quality ﬂags ...... 11 APPENDIX B: Formulae and coeﬃcients for the Leeds sensor calibration . 12

2 Sandwich Foehn on 18 October 1999 15 2.1 Introduction ...... 17 2.2 Synoptic overview ...... 17 2.3 Description of model set-up ...... 21 2.4 Mesoscale ﬂow analysis ...... 23 2.4.1 Flow description with lidar data and model ﬁelds ...... 23 2.4.2 Vertical structure using rawinsondes and model data . . . . . 26 2.4.3 Surface wind and pressure analysis in the Wipp valley . . . . . 27 2.4.4 Horizontal wind as simulated by the MM5 model ...... 29 2.5 Summary and conclusion ...... 31

3 Case studies of foehn ﬂow in a T-shaped valley 35 3.1 Introduction ...... 37 3.2 Setup of measurements and location ...... 39 3.3 The 28 February 2000 foehn ...... 42 3.3.1 Flight pattern ...... 42 3.3.2 Foehn characteristics ...... 43 vii viii CONTENTS

3.3.3 Wipp Valley cross section ...... 44 3.3.4 The Inn Valley ...... 50 3.4 The 7 November 2000 foehn ...... 52 3.4.1 Flight pattern ...... 52 3.4.2 Foehn characteristics ...... 52 3.4.3 Flow in and above the Inn Valley ...... 55 3.5 Discussion and Concluding Remarks ...... 58

4 An objective foehn classification scheme 63 4.1 Introduction ...... 65 4.2 Previously proposed foehn criteria ...... 66 4.3 Definition of foehn and gap flow ...... 68 4.3.1 Foehn definition ...... 68 4.3.2 Gap flow definition ...... 69 4.3.3 Connection of foehn and gap flow ...... 69 4.4 General concept of the objective foehn classification ...... 70 4.4.1 Conserved quantities and their application as a tracer . . . . . 70 4.4.2 Wind over a ridge or a single gap ...... 70 4.4.3 Wind through and over complex topography ...... 71 4.5 Application of the OFC to gap flow in the Brenner target area . . . . 74 4.5.1 Weather stations and sensor accuracies ...... 74 4.5.2 The wind criterion for foehn in the Wipp valley ...... 75 4.5.3 The Θ criterion for foehn in the Wipp valley ...... 77 4.6 Illustration of method ...... 78 4.6.1 Gap flow period at 04(Zagl) with foehn break ...... 78 4.6.2 Valley wind regime at 06(Tienzens) ...... 80 4.7 Statistical evaluation for the MAP-SOP ...... 82 4.7.1 South wind characteristics during the MAP-SOP ...... 82 4.7.2 Comparison of OFC to the traditional classification scheme . . 83 4.8 Concluding Remarks ...... 85 4.8.1 Potential and limits of the new scheme ...... 85 4.8.2 Application to other mountain areas ...... 86 4.8.3 Outlook ...... 86 Error estimates ...... 87 4.8.4 Condensation/ Evaporation ...... 87 4.8.5 Radiation ...... 88 4.8.6 Turbulent mixing ...... 88 Gap flow classification at 16 Wipp valley stations ...... 90

5 Conclusions and outlook 99 CONTENTS ix

Bibliography 101

Acknowledgments 107

Curriculum Vitae 109

Epilogue 111 x CONTENTS Chapter 1

General introduction

The Mesoscale Alpine Programme (MAP) with its Special Observing Period (SOP) in fall 1999 was a welcome opportunity for the Mountain Meteorology Group of the Department of Meteorology to revive the foehn research in the Austrian Alps. It promised insight into the planning of an international project and a unique data set to work on. This thesis is a direct result of the so-called GAP experiment, a part of MAP studying the south foehn and gap flow in the Brenner target area near the Austrian/Italian border. The two south foehn studies in 2000 also evolved as a side product of MAP. All in all it took more than two years to choose the instrumentation and measurement locations, set up the equipment, take the measurements and maintain the instruments, calibrate and format the weather station data, and write proper analysis tools. A description of some of the relevant preparation work is given in this chapter. Section 1.1 gives an overview over MAP with its eight projects studied in three dedicated target areas. In the subsequent section 1.2, the processing of the data collected by weather stations is highlighted. It is meant to give an insight into the necessary steps to obtain highly accurate, homogenised and quality controlled data from raw data taken by different types of weather stations. Finally section 1.3 addresses the goals and outline of this thesis. The author is aware of the fact, that a general introduction usually contains a history of the research topic. Nevertheless the reader interested in the history of Alpine foehn and gap flow studies is referred to the Gen- eral introduction in Gohm (2003), which is up to date, gives an excel- lent overview over the foehn research and can be downloaded from the web (http://meteo9.uibk.ac.at/dissertations/Gohm Alexander 2003 Diss.pdf).

1 2 General introduction

1.1 Overview of the MAP experiment

In mountainous terrain such as the Alps intense weather situations in the form of floods and windstorms bring a high cost to the society. As a response from the scientific community, the Mesoscale Alpine Programme (MAP, Bougeault et al. 2001) was designed as a major field program in order to better understand and predict these events. The following scientific objectives were published in the MAP Design Proposal (Binder and Schär1996).

1a. To improve the understanding of orographically influenced precipitation events and related flooding episodes involving deep convection, frontal precipitation and runoff.

1b. To improve the numerical prediction of moist processes over and in the vicinity of complex topography, including interactions with land-surface processes.

2a. To improve the understanding and forecasting of the life-cycle of Foehn-related phenomena, including their three-dimensional structure and associated boundary layer processes.

2b. To improve the understanding of three-dimensional gravity wave breaking and associated wave drag in order to improve the parameterization of gravity wave drag eﬀects in numerical weather prediction and climate models.

3. To provide data sets for the validation and improvement of high-resolution numerical weather prediction, hydrological and coupled models in mountainous terrain.

The Special Observing Period (SOP) took place from 7 September to 15 November 1999. Experimental facilities included a number of aircrafts, ground- based Doppler lidars, sodars, radiosounding systems, Doppler radars, wind proﬁlers and surface stations. Special products and model forecasts for MAP operations were provided by an international team at the MAP Operation Centre in Innsbruck. These included 5-min rapid scans from EUMETSAT over the Alpine region, a special Alpine radar composite, high resolution forecasts from the Canadian MC2 model and several mesoscale models. The SOP was organised into eight projects, referred to as P1 to P8 (cf. table 1.1).

The ﬁeld activities were concentrated in three target areas. The Lago Maggiore Target Area was the main focus of the so-called Wet-MAP (Projects P1 and P3, parts of P8). Nonstationary aspects of foehn in a large valley (P5) and parts of P8 were treated in the Rhine valley Target Area. The Brenner target area was 1.1 Overview of the MAP experiment 3

Name Short description Target area P1 Orographic precipitation mechanisms Lago Maggiore TA P2 Incident upper-tropospheric PV anomalies - P3 Hydrological measurements and ﬂood forecasting Lago Maggiore TA P4 Dynamics of gap ﬂow Brenner TA P5 Nonstationary aspects of foehn in a large valley Rhine valley TA P6 Three-dimensional gravity wave breaking - P7 Potential vorticity banners - P8 Structure of the PBL over steep terrain Lago Maggiore Rhine valley TA

Table 1.1: The eight scientific projects of MAP. P2, P6 and P7 were not dedicated to a specific Target Area (TA). dedicated to the dynamics of gap flow (P4, acronym: GAP). The specific scientific questions for GAP are stated in Bougeault et al. 1998:

• To determine the relative importance of gap width versus terrain elevation changes along the floor axis on deep, continuously stratified flow through realistic topography

• To determine the relationship between the gap flow and the flow above mountain-top level; in particular, whether the gap flow is reinforced by flow aloft along the axis of the gap or by a mean-state critical level which caps the low-level cross-mountain flow

• To study the vertical and cross-gap distribution of wind speed and thermodynamic properties. These are controlled by inviscid stratified dynamics together with surface friction along the valley floor and side walls. The frictional effects as well as dissipation and mixing of the low-level high speed flow need to be included in realistic models

An overview of GAP is given in Mayr et al. (2004). The synoptic conditions leading to gap flow and foehn are explained and a case study of 30 October 1999 demonstrates the usage of the rich data set for a strong foehn case. The ground- based instrumentation in the Brenner target area is shown in figure 1.1. A NOAA scanning aerosol Doppler lidar was located at Gedeir (GED) in the middle of the Wipp valley. Three radiosounding systems gave detailed information on the vertical structure. One at Sterzing (STZ) 15km south of the Brenner pass, one at Gedeir and the system of the Austro Control at the Innsbruck airport. Launches were focused on Intensive Observing Periods (IOP) and altogether 258 balloons were re- leased. A phased array sodar at Brennerbad (BRB) measured the wind distribution in the vertical to determine the mass flux through the gap up to ∼2000m MSL. The 4 General introduction mini-sodar at the roof of the University building in Innsbruck (IBK) was able to measure turbulence up to 140m a.g.l. which is especially valuable to study processes just before foehn break through. A car-based mobile platform was used to measure relevant parameters (pressure, temperature, humidity) at a 1Hz rate between Innsbruck and Sterzing during interesting periods (Mayr et al. 2002). A backbone of the experiment were the remote and in-situ measurements by research aircrafts. GAP missions were performed by the NOAA-P3, the NCAR-Electra with a SABL backscatter lidar on board, the French Fokker ARAT with a Leandre2 differential absorption lidar and the DLR Dornier 228. A summary on the aircraft missions is given in Mayr et al. (2004).

18'

IBK

12'

GED

BRE 47oN BRB

54' STZ

12' 18' 24' 11oE 36' 42' 48' 30.00'

Figure 1.1: Topographic map of the instrumentation of the Brenner pass Target Area during MAP. Lidar and rawinsonde measurements where taken at Gedeir (GED, 1068m MSL). The rawinsonde stations Sterzing (STZ, 942m MSL) and Innsbruck airport (581m MSL) are marked with a big blue circle. Two sodars at Brennerbad (BRB, 1310m MSL) and Innsbruck university (IBK, 610m MSL) are represented by magenta asterisks. Black cross markers denote weather stations measuring temperature, humidity, pressure and wind; the Brenner pass (BRE, 1370m MSL) is labelled. Temperature sensors are marked with small light blue circles. Topography contours every 400m starting from 600m MSL from a 1 km GLOBE topography set. 1.2 Homogenisation of MAP-SOP station data 5

Nature was quite kind and provided an above-normal foehn frequency during the SOP. All in all gap ﬂow in the Wipp valley was observed about one third of the time. For more details on the gap ﬂow frequency see Appendix B in chapter 4.

1.2 Homogenisation of MAP-SOP station data

1.2.1 Processing logistics

To study foehn it is important to have an accurate quality controlled data set. Pressure differences on the order of 0.1hPa and temperature differences of a few tenths of a degree should be recognised. For this purpose a thorough data calibration was performed. Additionally a special data processing procedure from raw data to quality controlled data was developed. It guarantees that each processing step is repeatable, if changes should occur. This procedure is shortly described in the following section, since it may give readers confronted with large and inhomogeneous data sets some useful hints. First the raw data from the four different station types were formatted. Two columns with uniform date format were introduced (YYYYMMDDhhmm and julian day), and data columns were sorted. Data values were not changed in this first step, but easier data handling was assured. In the second step the calibration polynoms were applied. The calibration of the sensors is described in more detail in 1.2.2. Thirdly a semi-automatic quality control was applied to the calibrated data (prequality control stage). It checks each parameter for values outside of physical limits, outliers, inconsistencies and icing and adds automatic quality flags to the files (cf. table A.1).

Since it turned out that it is impossible to find all erroneous data with an automatic program, the data were also checked graphically. A second set of manual quality flags was generated by the above mentioned program with an initial value of -1. Whenever the graphical check revealed erroneous interpretation of the automatic control, a manual flag was set. It is important to realize that the data themselves were not changed during the prequality control. In the final quality control the data can be averaged to a chosen time step and at the same time the interpretation of the flags and the appropriate change in the data is performed. A manual setting different to -1 also overwrites the automatic flag. In the given 2 averaging interval at least 3 of the data have to be ”good”, i.e. flagged with a value between 0 and 3, otherwise they will be set to missing value. New quality flags are produced as listed in table A.2. 6 General introduction

Finally a file header of fixed length was added to each station file with all available meta information like station name, position, sensor information, applied calibrations, parameter and unit information, quality flag description and special remarks.

1.2.2 Calibration of sensors

An important part in the data processing was the calibration of the sensors from different types of weather stations. Three station types were available prior to the MAP-SOP, namely the Campbell, Davis Weather Monitor II (short: Davis) and Friedrich stations, the latter ones provided by the University of Mu- nich. The forth type of instruments was provided by the University of Leeds and most sensors were only available during the field phase due to a dense experiment schedule. Therefore a different calibration procedure had to be applied. The calibration of the pressure, temperature and humidity sensors of these stations are shortly described in this section. For a more detailed information on the processing of the first three station types the reader is referred to Pippan (2000) (a pdf-file of the Diploma thesis can be downloaded from http://meteo9.uibk.ac.at/diploma theses/pippan christoph 2000 dipl.pdf). The pressure sensors of type Vaisala PTB100B and PTB101B (used for the Campbell weather stations) and of the Davis stations were calibrated against a Vaisala PA11A reference, which was in turn compared to a highly accurate Vaisala PTB220B. The calibration measurements were carried out at the roof of the University building (Bruno Sander Haus) from 19 to 30 May 1999. The calibrated pressure is a function of the measured pressure and the temperature (first or second order depending on sensor characteristics). Two comparison measurements from 24 June to 26 July 1999 at the University roof and from 26 June to 17 August 1999 in the garden of the old Meteorology building were used to test the calibration independently. A drift of one Vaisala sensor was detected and corrected for. Constant offsets after the calibration of the less accurate Davis sensors to the reference detected during the second test period (which was closest to the SOP) were additionally applied. One Davis sensor mounted at a mountain peak (Blaser) failed during the SOP on 30 September and had to be replaced by a new sensor. This new sensor was calibrated after the measurement period between 30 November and 1 December at the University roof again comparing it to the reference PA11A. The pressure sensors of type Vaisala DPA21 used for the Friedrich stations, which were not available yet in May, were calibrated using the 26 June to 17 August 1999 period. The calibrated Vaisala sensor of type PTB101B with the lowest standard deviation was taken as reference. 1.2 Homogenisation of MAP-SOP station data 7

The temperature sensors of type Vaisala HMP35AC, HMP45D and Rotronic MP100A (Campbell) and the Davis sensors were calibrated in a Haake K20 Eichwanne between -10 and 30 ◦C at the Department of Meteorology, University of Innsbruck. The reference sensor was a Testoterm Testo 781 silicon thermometer. A calibration for the humidity sensors was tried with a calibration facility at the German Aerospace Center (DLR) in Oberpfaﬀenhofen. Unfortunately the humidity correction did not work well, possibly because the housing was not hermetically sealed. Therefore a comparison to the average of all humidity readings of the Friedrich stations during the above mentioned two comparison periods was chosen to calibrate the Campbell and Davis humidity sensors. The reason is that the humidity of the Friedrich stations is calculated from psychrometers (dry and wet bulb temperature), which is a more accurate principle compared to capacity measurements as long as it is assured that the wet bulb sensor does not run dry. First the wet bulb temperature readings of the Friedrich stations were slightly corrected using periods of saturation, setting the wet bulb temperature equal to the calibrated dry bulb temperature. The temperature of the Friedrich stations was calibrated against a Vaisala HMP45D in the same period as the pressure between 26 June and 17 August 1999.

Since the Leeds stations were not available prior to the SOP and most sensors were used for another field phase on the isle of Arran, Great Britain, immediately afterwards, a direct calibration as described above was not possible. Therefore other ways of homogenising the Leeds data had to be found. The methods used are described below. The applied formulae and the coefficients for each sensor are listed in Appendix B. To calibrate temperature, the Campbell sensor of the Steinach station was sent to Leeds and used as a reference. This sensor was run together with five temperature sensors from Leeds in a box and a 2-point measurement at 17 and 29 ◦C was performed. A linear relation between the true and measured temperature value was assumed. From the box measurements a mean slope (1.01157) was calculated and assumed to be valid for all Leeds sensors, which were all build in the same way and have similar sensor characteristics. Then the Arran field data were analysed to determine the offset of each sensor. Since the hills on Arran are not very high, the surface coincides with an isentropic surface under windy conditions. Only periods with wind speed greater than 7.5 m s−1 were used to ensure the validity of this assumption. Therefore the stations (with the same sensors as in MAP) could be compared even though they were in different locations and at different heights. The difference in height was accounted for by using adiabatic lapse rate. 14 out of 20 8 General introduction sensors could be calibrated that way. Of the six remaining sensors two had failed during the MAP field phase and were not available anymore on Arran, for three others the determined offset proofed to be inaccurate in an intercomparison of all SOP temperature data (the method is outlined in section 1.2.3). Four of these five sensors were calibrated by comparing to SOP night time measurements between 18 and 06 UTC – excluding radiational effects – of suitable nearby reference stations. The southernmost station was too far off to apply that method. But since it was very close to the launching point of the Sterzing rawinsonde, the offset could be determined from comparisons to readings of a hand-held Vaisala thermometer taken prior to each ascent. The last sensor at the mountain station Sattelberg was calibrated against a Campbell sensor during the MAP-SOP in a horizontal distance of ∼ 30m. The relative humidity calibration was done from three weeks of intercomparison measurements in a garden near Leeds, England in June 2000. All sensors still working were referenced to the Campbell sensor of the Steinach station. The average slope and offset of these Leeds sensors was applied to the remaining devices, again relying on similar sensor characteristics. The self-built Leeds pressure sensors measure pressure bits directly. The conversion to pressure in hPa (which is simultaneously a calibration) was done by comparison to a high-precision Paroscientific barometer. In order to get the necessary range of 850 to 1000hPa, the comparison measurements were carried out twice in a car on the way from Leeds to Innsbruck, using routes leading to 1500m MSL. The pressure intercomparison during MAP periods with neglectable horizontal pressure gradients showed very good accuracy and consistency of the Leeds pressure data (see section 1.2.3).

1.2.3 Quality control using sensor intercomparisons

Temperature intercomparison Leeds vs. Campbell sensors

Two Leeds sensors M12 and M15 were still available after the end of the SOP. Between 24 November and 3 December 1999 an intercomparison at the roof of the University building was performed. It involved the two Leeds T-sensors and the Campbell Steinach sensor. Comparison of the resulting linear calibration polynom to the one calculated from Arran data shows very good agreement, namely an oﬀset of 0.24◦C (−0.03◦C) for M12 (M15) at 0◦C.

Temperature intercomparison from SOP data

To check the consistency of the different temperature calibration methods, all temperature sensors were compared simultaneously to the Campbell station Ellbögen, located in the middle of the Wipp valley during five selected periods of the SOP. The 1.3 Goals and outline of the thesis 9 chosen periods were three stably stratified situations after passage of a cold front, two weeks of high pressure (valley wind system dominant), and the whole SOP. Temperature was reduced with a standard gradient of −0.65K/100m to the height of 1100m MSL. Based on statistics for each period an temperature offset to Ellbögen was estimated for each station. Offsets lower than ±0.5K are hardly detectable with this method. The knowledge, that the temperature gradient is stronger during stably stratified situations and weaker in situations with mixed boundary layer, was taken into account. As a result of this intercomparison the offset of three Leeds sensors as determined from the Arran field data were changed as described in section 1.2.2.

Pressure intercomparison from SOP data

All pressure sensors were compared to the reference station Meteodat Innsbruck (701, calibrated with the high precision barometer Parascientific of Leeds) in two selected high pressure periods with –on average– neglectable horizontal north-south pressure gradients and for the whole SOP. If the horizontal or vertical distances between a station and the reference was too high, the comparison was performed with a reliable nearby station, which was then linked back to the reference. Therefore this method works best for stations in the Inn valley or the northern Wipp valley. A pressure offset was estimated comparing reduced pressures. Pressure is reduced to an appropriate level using the temperature of the station, a dry adiabatic gradient and the hydrostatic assumption. In case of doubt, more cross checks between two stations were performed. For mountain stations above 1500m MSL, the Campbell station Sattelberg at 2210m MSL was chosen as a reference. Based on statistics an offset was estimated for each station. Offsets lower than ±0.2hP a (for stations near to Innsbruck ±0.1hP a) are hardly detectable with this method. Two obviously wrong calibration offsets of two mountain stations and a sensor drift of one station were detected and corrected for. Additionally it turned out that the Leeds stations using static pressure ports with flexible tubes are the best basis for analysing small horizontal pressure differences. Therefore the surface pressure analysis in the Wipp valley in chapter 2 relies on the Leeds pressure devices.

1.3 Goals and outline of the thesis

The aims of this thesis are closely related to the questions posed by the Mesoscale Alpine Programme. The overall scientiﬁc objectives, which are published in the MAP Design Proposal (Binder and Sch¨ar1996), are listed in chapter 1.1. Two of these objectives are relevant for this work: 10 General introduction

2a. To improve the understanding and forecasting of the life-cycle of Foehn-related phenomena, including their three-dimensional structure and associated boundary layer processes.

3. To provide data sets for the validation and improvement of high-resolution numerical weather prediction, hydrological and coupled models in mountainous terrain.

The understanding of the life-cycle of foehn is improved by detailed case studies, where the synoptic scale as well as the small-scale features triggered by local topography in the Wipp and Inn valley are considered. Analysis of lidar scans in chapter 2 and airborne in-situ measurements in chapter 3 describe essential three dimensional flow features. The article on Sandwich foehn (chapter 2) uses up-to-date high resolution numerical model with realistic topography to test the model performance in mountainous terrain for a complex weather situation. The above mentioned analyses are also related to the specific scientific questions for GAP (Bougeault et al. 1998). Especially the third objective ’To study the vertical and cross-gap distribution of wind speed and thermodynamic properties’ is addressed with the case studies.

The development of a new foehn classification scheme (chapter 4) is a consequence of the needs which surfaced in the data processing stage. At first the practical question ’How exactly do we analyse foehn periods?’ was discussed, and soon it became apparent that this is not a trivial question and the methods used so far do not seem sufficient. The starting point was found in the principle of the foehn descent downstream of the obstacle. Three important objectives are combined in the article: The general description of this classification scheme, which is applicable to foehn winds around the world –whatever their local may be. The discussion of the application to the downstream stations in the Brenner pass Target Area with respect to the shape of the gap or ridgeline and orientation of the downstream valley axis. Finally the evaluation of the foehn periods including a comparison to manual foehn analyses using the traditional 3-criteria definition. Appendix A 11

APPENDIX A: Quality ﬂags used for MAP weather stations

In the pre-quality control stage, flags between 0 and 6 are applied to each parameter for each time step. Automatic flags are generated by a matlab programme, hand-set flags give the option to overwrite the automatic flag after manual inspection. The description of the flags and the applied action is given in table A.1. The final quality flags (0 – 2) after data averaging reveal how many values within an averaging interval were marked as ”good”. The description of the final quality flags is found in table A.2.

Flag Description Applied action during quality control 0 Value okay None 1 Value slightly out of physical range None (e.g. relative humidity of 100.2%) 2 Value suspect, use with caution None 3 Value slightly out of physical range Correct into physical range (e.g. wind speed of -0.1m s−1 changed to 0.0m s−1) 4 Value out of physical range or an outlier Set to missing value 5 Sensor icing or transmission error Set to missing value 6 Error value already from data logger None. Leave at missing value

Table A.1: Description of quality ﬂags applied in the prequality control stage.

Flag Description 0 All values within averaging interval were good 2 1 ≥ 3 of values within averaging interval were good 2 2 < 3 of values within averaging interval were good, set to missing value

Table A.2: Description of ﬁnal quality ﬂags. 12 General Introduction

APPENDIX B

The applied formulae and coeﬃcients for the calibration of the Leeds sensors are given below. An overview of the calibration process can be found in section 1.2.2.

Temperature calibration

The temperature calibration formula is:

TCal = −0.23828 + 1.00928 ∗ (Offset + Slope ∗ TRaw) where TCal is the calibrated temperature, TRaw the raw (measured) temperature, both in degrees Celsius. The Slope was determined from comparison measurements of ﬁve Leeds sensors to the Steinach Campbell sensor and the average of 1.01157 is used for all sensors except for sensor 161, which was already calibrated. The two sensors M01 and M02 failed during the SOP and had to be replaced by new ones. The date of replacement is given in the remarks.

Name Location(MAP) Offset Std. dev. Method Remark M01 Brenner -1.65412 0.72815 MAP field data night, M03 as ref Before 2 Nov 1999 M01 Brenner -0.64889 0.41541 Arran field data, M11 as ref After 2 Nov 1999 M02 Tienzens -2.46336 0.37030 MAP field data night, M07 as ref Before 2 Oct 1999 M02 Tienzens -2.33313 0.28457 Arran field data, M11 as ref After 2 Oct 1999 M03 Pontigl -3.28621 0.31930 Arran field data, M11 as ref M04 Schönberg -1.46867 0.33975 Arran field data, M11 as ref M05 Luegg -2.63691 0.11251 Arran field data, M11 as ref M06 Stafflach -3.77096 0.18685 Arran field data, M11 as ref M07 Matreiwald -2.97952 0.40736 Arran field data, M11 as ref M08 Sterzing -0.64284 0.31120 Comparison to hand-held Vaisala M09 Sattelberg middle -3.27662 0.36361 Arran field data, M11 as ref M10 Sattelberg top -0.63300 n.a. Comparison with IMGI station M11 Hall -1.37573 0.46493 MAP field data night, M15 as ref M12 Innsbruck -0.80675 0.33727 Arran field data, M11 as ref M13 Sattelberg bottom -1.06581 0.23092 Arran field data, M11 as ref M14 Sterzing -0.64284 0.31120 Comparison to hand-held Vaisala Same sensor as M08 M15 Volders -0.38611 0.16407 Arran field data, M11 as ref M16 Patsch -1.01020 0.27930 MAP field data night, M04 as ref 161 Pad.berg top 0.00000 n.a. Hobo temperature sensor Slope = 1.0 163 Pad.berg middle -0.10559 0.76835 Arran field data, M11 as ref 261 Vennspitze -6.19424 0.42608 Arran field data, M11 as ref 263 Pad.berg bottom -1.50019 0.40335 Arran field data, M11 as ref

Table B.1: Temperature calibration information Appendix B 13

Humidity calibration

The humidity calibration was performed during three weeks in June 2000 in the garden of Martin Hill, a technician at the Department of Prof. Stephen Mobbs. The Steinach Campbell sensor was used as a reference. The humidity calibration formula is:

RHCal = Offset + Slope ∗ RHRaw where RHCal is the calibrated relative humidity, RHRaw the raw (measured) humidity, both in %.

Name Location(MAP) Offset Slope Method Remark M01 Brenner -1.813821 0.980699 Average of other sensors Before 2 Nov 1999 M01 Brenner 2.334630 0.946226 Comparison at Martin’s house After 2 Nov 1999 M02 Tienzens -1.813821 0.980699 Average of other sensors Before 2 Oct 1999 M02 Tienzens -2.445516 0.970836 Comparison at Martin’s house After 2 Oct 1999 M03 Pontigl -3.790640 0.995877 Comparison at Martin’s house M04 Schönberg -2.570900 0.984044 Comparison at Martin’s house M05 Luegg -1.813821 0.980699 Average of other sensors M06 Stafflach -1.813821 0.980699 Average of other sensors M07 Matreiwald -0.373000 0.969266 Comparison at Martin’s house M08 Sterzing -4.566693 1.027746 Comparison at Martin’s house M09 Sattelberg middle -1.813821 0.980699 Average of other sensors M10 Sattelberg top -1.221780 0.972234 Comparison at Martin’s house M11 Hall -1.813821 0.980699 Average of other sensors M12 Innsbruck -3.642939 0.990030 Comparison at Martin’s house M13 Sattelberg bottom 0.447857 0.966868 Comparison at Martin’s house M14 Sterzing -4.566693 1.027746 Comparison at Martin’s house Same sensor as M08 M15 Volders -1.813821 0.980699 Average of other sensors M16 Patsch -1.413695 0.984354 Comparison at Martin’s house 161 Pad.berg top -1.813821 0.980699 Average of other sensors 163 Pad.berg middle -1.813821 0.980699 Average of other sensors 261 Vennspitze -1.813821 0.980699 Average of other sensors 263 Pad.berg bottom -1.813821 0.980699 Average of other sensors

Table B.2: Relative humidity calibration information 14 General Introduction

Pressure calibration

The calibration of the self-built Leeds pressure devices is done by referencing the measured pressure bits (PBits) to a pressure p in hPa using comparison to a known standard. The last term of the empirical formula accounts for hysteresis by taking the internal temperature half an hour ago.

The conversion formula is:

2 p = C1 + C2 ∗ P Bits + C3 ∗ P Bits ∗ Tin + C4 ∗ P Bits ∗ Tin + C5 ∗ Tin(t − 30min) where Tin denotes the internal temperature inside the box where the pressure device is mounted.

Name Location(MAP) C1 C2 C3 C4 C5 M01 Brenner 276.7962 200.1477 0.64947780 0.11448594E-02 -0.15754698 M02 Tienzens 242.1113 153.0234 0.48249898 0.54653688E-03 -0.08921028 M03 Pontigl 286.6352 162.2379 0.50284551 0.55780847E-03 -0.06527352 M04 Sch¨onberg 210.1755 145.2084 0.44437885 0.21439316E-03 -0.11525505 M05 Luegg 196.8512 134.5463 0.40824928 0.17422824E-03 -0.09544870 M06 Staﬄach 250.1494 177.4880 0.57265317 0.62476002E-03 -0.09914532 M07 Matreiwald 211.6627 142.6241 0.43285549 0.36649568E-03 -0.09985716 M08 Sterzing 185.9521 120.7915 0.35521115 0.21734050E-03 -0.08387409 M09 Sattelberg middle 218.2129 154.9924 0.49862079 0.30564980E-03 -0.99285490 M10 Sattelberg top 177.3898 240.7580 0.69534107 0.10920575E-03 -0.08645155 M11 Hall 123.2179 133.2805 0.36585634 0.16901570E-03 -0.08923380 M12 Innsbruck 182.8043 141.9234 0.40788092 0.21496028E-03 -0.04932761 M13 Sattelberg bottom 212.8148 143.1206 0.41095829 0.45755969E-03 -0.08111220 M14 Sterzing 216.3866 279.1313 0.84044231 0.20177480E-03 -0.11257181 M15 Volders 170.0346 122.1419 0.35447414 0.23915063E-03 -0.10079038 M16 Patsch 182.6480 151.4316 0.43622217 0.56238337E-03 -0.11092003 161 Pad.berg top -138.4684 300.8513 0.70005260 0.21891660E-05 0.0 163 Pad.berg middle -67.0695 302.4968 0.75951470 0.68558007E-03 0.0 261 Vennspitze -144.3853 301.2055 0.66653694 0.41604807E-03 0.0 263 Pad.berg bottom -151.0396 303.3103 0.69934408 0.31485774E-03 0.0

Table B.3: Pressure calibration coeﬃcients

Note that the new sensor type named 161 – 263 does not show a hysteresis anymore. Chapter 2

Sandwich Foehn on 18 October 1999

Sandwich Foehn on 18 October 1999 1 Johannes Vergeiner2 , Alexander Gohm, Georg J. Mayr Department of Meteorology and Geophysics, University of Innsbruck, Austria Gunther¨ Zangl¨ , Meteorologisches Institut, University of Munich, Germany

Summary

A case study of a complex and short-lived three-layer flow situation (”Sandwich foehn”) measured during the Mesoscale Alpine Programme on 18 October 1999 in the Wipp valley, Austria is presented. Observations were collected with a ground-based scanning aerosol lidar, two radiosounding systems and several weather stations. Additional simulations with the fifth-generation Pennsylvania State University-NCAR Mesoscale Model (MM5) were performed. The causes for the north-south-north flow structure are examined on a synoptic scale. The study focuses on detailed descriptions of the flow including its time evolution and comparisons of the observations to the simulation in the Wipp valley and its northern exit region. The Alpine region at the southwestern flank of a pressure low experiences northwesterly flow above crest height (∼3km MSL) throughout the day. Shallow south foehn with increasing strength is caused by weak southwesterly winds at the eastern flank of another pressure low and a south-north oriented pressure gradient due to cold air advection south of the main Alpine crest up to 2km MSL. A ∼500m deep upvalley flow (northerly in the Wipp valley) transports cold air

1To be submitted to Quarterly Journal of the Royal Meteorological Society 2Corresponding author: Department of Meteorology and Geophysics, University of Innsbruck, Innrain 52, A-6020 Innsbruck, Austria; E-mail: [email protected] 15 16 CHAPTER 2. SANDWICH FOEHN ON 18 OCTOBER 1999 from the northern Alpine foreland to the region of interest. It almost reaches the Brenner pass at the border to Italy around 09 UTC, before it is pushed back by the stengthening foehn. Rawinsondes around noon show flow blocking south of the main Alpine crest up to 1.5km MSL, above which southerly flow of 3 − 5 m s−1 was measured. A strong inversion at 3.8km MSL decouples the south flow from the northwesterly above. Downstream in the Wipp valley the neutrally stratified foehn layer has descended by 500m and the foehn jet topping the upvalley flow reaches 12 m s−1. Lidar measurements reveal two gravity waves exited by west-east oriented ridges and the decreasing depth of the cold surface layer particularly between 12 and 14 UTC. The numerical simulations with the MM5 capture the main flow features but underestimate the depth and southward extend of the northerly surface flow. Consequently the foehn layer is simulated too close to the surface. Additionally wind speeds in the foehn jet and the northwesterly flow aloft are overestimated. The across-valley asymmetry of the foehn strength in the MM5 simulation was found in previous case studies as well. While the impact of the westward bending of the valley axis is obvious, the influence of the northwesterly flow aloft is unclear.

KEYWORDS: lidar, MAP, MM5, sandwich foehn, Wipp valley 2.1. INTRODUCTION 17

2.1 Introduction

During the Special Observing Period (SOP) of the Mesoscale Alpine Programme (MAP) in autumn 1999 one of the aims was to better understand the dynamics and flow structure during foehn events Mayr et al. (2004). One of the two regions of investigation for this purpose was the target area Brenner (cf. figure 2.1). On 18 October 1999 an interesting event occurred, which is the topic of this article. A shallow layer of northerly upvalley flow was topped by shallow south foehn up to crest height changing to northwesterly flow above ∼3000m MSL. The name Sandwich foehn 3 was given to this phenomenon of layers of similar flow direction at the surface and the top with the ”real” thing – the south foehn layer with opposite flow direction – in between.

While no aircraft operations were carried out on that day, a NOAA aerosol lidar located in the middle of the Wipp valley supplemented by two rawinsondes in the target area (and additional ones in the greater vicinity), a sodar and a dense network of surface stations establish a rich data set. Therefore it is possible to gain insight in small-scale flow structures and explain the opposing flows on a small vertical scale from the viewpoint of fluid dynamics. Furthermore this case study reveals, how up- to-date 4-D model simulations with realistic topography and full physics can cope with this complex situation. Section 2.2 points out the synoptic influence which makes the rare and short-lived Sandwich foehn possible. Analysis data from the European Centre for Medium Range Weather Forecast (ECMWF) are used as well as a pressure analysis by the Vienna Enhanced Resolution Analysis (VERA, Steinacker et al. 1997). The three layer wind structure is shown using valley and mountain stations and rawinsonde ascents. The set-up of the MM5 model is discussed in section 2.3. The analysis in section 2.4 using a lidar, model data, rawinsondes and the surface network highlights the detailed flow structure and temperature profile in the Wipp valley. The influence of local topographic features is discussed as well. A conclusion is offered in section 2.5.

2.2 Synoptic overview

This section gives an overview over the synoptic-scale conditions on 18 Oct 1999 and their influence on the flow in the Wipp valley. From top to bottom the causes for the north-south-north flow structure are described using analysis of models and

3The expression Sandwich foehn was ﬁrst used by A. Lanzinger in a weather brieﬁng at the Department of Meteorology and Geophysics, University of Innsbruck. 18 CHAPTER 2. SANDWICH FOEHN ON 18 OCTOBER 1999

30'

ZUG

IV 20'

IBK

10' GED

TIE

NOE

SAT 47oN BRE

STZ

50'

11oE 15' 30' 45' 12oE

Figure 2.1: Topographic map of the Brenner pass area. Lidar and rawinsonde measurements where taken at Gedeir (GED, 1068m MSL), the along valley scan direction of the lidar and equidistant circles every five km are plotted as well. The rawinsonde station Sterzing (STZ, 942m MSL) is marked with a big circle. Cross markers denote other mentioned locations with: Brenner (BRE, 1370m MSL), Sattelberg (SAT, 2110m MSL), Noesslachjoch (2230m MSL), Tienzens (TIE, 1120m MSL), Innsbruck (IBK, 580m MSL) and Zugspitze (ZUG, 2960m MSL). The dashed rectangle represents the subdomain of the MM5 model shown in figure 2.7. Italic letters indicate locations of the Wipp valley (WV) and Inn valley (IV), respectively. Topography contours every 500m from a 1 km GLOBE topography set, the 1500m contour line is plotted as a solid black line for comparison with figure 2.6. 2.2. SYNOPTIC OVERVIEW 19 measurements. Above 3km MSL northwesterly flow is dominant throughout the day. This is revealed by the geopotential height analysis at 500hPa of the European Centre for Medium Range Weather Forecast (ECMWF). As can be seen in the top panels of figure 2.2, Austria is at the southwestern flank of a pressure low with centre over Slovakia. Even though the west-east gradient weakens between 06 and 18 UTC, the flow direction is maintained. Note a second pressure low over the southwest of France, which is important for the southerly flow component at crest height.

54oN 540

546 542 552 544 544 544 550

548 548 o 546 51 N 552 550 546

550 548

540 542 550 48oN 548 548 542 542 546 544 546 544 546 o 548 546 45 N 546 550 550 548 548 546 550 552 552 552 554 42oN 554 548 554 556 550 556 550 550 558 552 552 552 556 558 o o o o o558 o o o o o 560 o o o 554 o o o 560o o 0 4 E 8 E 12 E 16 E 20 E 0 4 E 8 E 12 E 16 E 20 E 0 4 E 8 E 12 E 16 E 20 E

49oN 1019 1020 1022 1025 1021 1023 1021 10191020 1022 1024 1017 1021 1018 1018 48oN

10191020

1019 1020 1023 10171016 1014 1022 1016 1022 1017 1018 1017 o 1022 47 N 1023 1021 1013 1014 1015 1020 1024 1015 1019 1017 1022 1018 1021

1021 1025 1018 1017 1018 1021 1017

1020 o 1022 1019 46 N 1023 1022 1016 1020 1016 1020 1019 1019 1019 1020 1018 10171018 1020 1021 1018 1016 10 m/s 10 m/s 10 m/s 45oN 10oE 11oE 12oE 13oE 14oE 15oE 10oE 11oE 12oE 13oE 14oE 15oE 10oE 11oE 12oE 13oE 14oE 15oE

Figure 2.2: Synoptic analyses on 18 Oct 1999 at 06, 12 and 18 UTC (from left to right). Top panels show geopotential height (gpdm) at 500hPa from ECMWF analysis on a 0.4degree grid. The dashed area marks the region depicted in the lower panel, where solid lines show reduced surface pressure (hPa) as analysed by VERA. The wind vectors give the corresponding surface wind information. Dark (light) grey shading denotes a negative oﬀset of the potential temperature at 850hPa as analysed by the ECMWF by more than 6 (3-6) K. The mean value is computed in the domain shown in the top panels. The dot marks the location of Innsbruck, Brenner pass is additionally marked in the lower panels.

Up to approximately 3km MSL, a southerly wind component develops in the morning of the day described. It is due to two different processes happening at the same time. One is the weak southwest flow at 700 hPa at the eastern flank of 20 CHAPTER 2. SANDWICH FOEHN ON 18 OCTOBER 1999 the low over France (not shown). The other important cause is a density driven flow through the Brenner gap. The cold front associated with the pressure low over Slovakia advanced the Alps at the previous day. Relatively cold air was flowing along the eastern flank of the Alps (below crest height). Therefore a cold air pool over Bavaria forms as well as one south of the main Alpine crest associated with higher pressure at both sides relative to the inneralpine region, as can be seen from the ECMWF analysis of the potential temperature deviation at 850hPa in the lower panels of figure 2.2. In terms of flow in the atmospheric boundary layer (ABL) the pressure pattern and surface wind vectors are of great importance. The 6 hourly analysis of the Vienna Enhanced Resolution Analysis (VERA) is presented in the lower panels of figure 2.2. VERA is capable of reproducing mesoscale patterns below the station density, by using topographically controlled thermal and dynamic fingerprints (Steinacker et al. 1997). In the depicted 12 hour period the lowest pressure is inneralpine (cf. the reduced pressure pattern in figure 2.2). This causes the relatively cold air south of the Alps to approach the main Alpine crest. At the same time a very shallow layer of ground near air in the Alpine foreland flowed along the Inn valley and halfway up the Wipp valley. This upvalley flow was capped by an inversion and a stratocumulus deck at ∼1500m MSL. Around 10 UTC the stratocumulus deck started to break up. First the clouds dissolved in the southern Wipp valley – presumably due to entrainment with drier foehn air from aloft. Innsbruck airport at the northern exit of the Wipp valley reported 6/8 of Sc at 11 UTC, but already one hour later only 1/8 of Sc along the hill slopes was left. During the morning hours these clouds played a significant role in preventing the warming of the lower ABL. This delayed foehn breakthrough and helped to prolong the cold upvalley flow. The time series of the vertical wind profile was obtained from the Sterzing rawinsonde and hourly wind data selected from weather stations at three different heights (figure 2.3). The rawinsonde at 4km MSL measured northwesterly flow of ∼ 10 m s−1 until noon turning to slightly weaker north flow later during the day. The transition zone between the foehn flow and the northwest wind in the mid troposphere is represented by the mountain station Zugspitze (2960m MSL) located 50km north of the Alpine crest. It starts of with northwesterly flow as well, followed by four hours of west wind. At 11 UTC the foehn flow sets in as the foehn layer deepens. In the evening the wind direction turns to westerly flow again. Sattelberg (2110m MSL) in a broad gap at the Alpine crest is in the foehn flow all day, strongest between 6 and 14 UTC with ∼ 10 m s−1. The wind at Tienzens (1120m MSL) in the southern part of the Wipp valley changes between down- and upvalley directions during the day. The Sandwich foehn period as valid for the Tienzens location is highlighted with a grey box in figure 2.3. Note that the northerly upvalley flow lasted longer in 2.3. DESCRIPTION OF MODEL SET-UP 21 the northern part of the valley.

5000

4000

3000

Height (m MSL) 2000

1000

10 m/s

0 00:00 03:00 06:00 09:00 12:00 15:00 18:00 21:00 00:00 18−Oct−1999

Figure 2.3: Time series of wind on 18 Oct 1999 at four diﬀerent heights as measured by a rawinsonde (top) and three weather stations. Up-pointing arrows denote south ﬂow. The Sandwich foehn period at Tienzens is highlighted by the grey box. A westerly reference wind vector is displayed in the lower right corner.

2.3 Description of model set-up

The numerical simulations were conducted with the ﬁfth-generation Pennsylvania State University NCAR Mesoscale Model (MM5) version 3.3 (Grell et al. 1995). MM5 is a nonhydrostatic model based on a terrain-following sigma coordinate system. Five interactively nested grids were used with horizontal mesh sizes of 64.8, 21.6, 7.2, 2.4 and 0.8km, respectively. The coarsest domain with an area of 4000 x 4000km covers most of the European continent. The results presented in this case study are taken from the innermost domain, which is centered around 47.3◦N, 11.5◦E. The model orography of domains 1 – 2 (3 – 5) was interpolated from terrain data with 5’ (30”) resolution. Elevations in domain 5 along the Inn 22 CHAPTER 2. SANDWICH FOEHN ON 18 OCTOBER 1999

Valley, Wipp Valley, and their tributaries were manually corrected based on readings from a terrain map with a scale of 1:100 000. Information on land use was obtained from United States Geological Survey (USGS) data with the same horizontal resolution as for orography. In the vertical, 43 unevenly spaced sigma levels were used. The lowest half-sigma level is about 14m above the ground and is referred to as surface level. The vertical distance between the model levels is about 50 m close to the ground and increases up to 800 m near the upper boundary, which is located at 100 hPa. For cloud-microphysical processes the so-called Reisner1 scheme was used (Reisner et al. 1998). In the three coarser domains 1 –3 a cumulus parameterization scheme was used (Grell 1993), in the two finer domains 4 and 5 convection is explicitly resolved. Further, a turbulent kinetic energy (TKE) based parameterization of the planetary boundary layer was used (the Gayno – Seaman scheme; Shafran et al. 2000). The radiation scheme accounts for the interactions with moisture and clouds (Grell et al. 1995; Mlawer et al. 1997). It was modified by the last author to include the effects of sloping orography on the flux of direct solar radiation (Garnier and Ohmura 1968). An improved version of the radiative boundary condition by Klemp and Durran (1983) was applied as an upper boundary condition (Zängl2002b). The horizontal diffusion was modified to account for numerical errors in narrow valleys (Zängl2002a). Furthermore, a generalized sigma-coordinate system similar to that proposed by Schäret al. (2002) was used (see Zängl2003 for the MM5 implementation). It allows for rapid vertical decay of small-scale topographic structures and substantially improves the accuracy of horizontal advection above steep terrain. The MM5 simulations are driven with ECMWF-analysis on a 0.5x0.5 degree grid on standard pressure levels initialized on 18 Oct 1999 at 00 UTC. The three standard pressure levels 850, 925 and 1000hPa are below the ECMWF model topography in the Alpine region and the data are extrapolated. Due to this ECMWF-analysis problem in the Alpine ABL the stratocumulus deck in the Inn valley was not simulated correctly by the MM5 simulation in a first attempt. Consequently initial values were corrected to fit the Innsbruck rawinsonde as follows: The temperature at the grid points 47.5◦N, 10.5 − 13◦E was reduced by 1.5K (2K, 3K) at pressure levels of 850hPa (925hPa, 1000hPa). Relative humidity values were enhanced by 5% at the same three levels between 47.5 − 48◦N, 12 − 13◦E (representing the Inn Valley and the southern rim of the Alpine foreland). At 850hPa (925hPa) the relative humidity was additionally restricted to ≥ 96%(≥ 88%) in the same area. These modifications improved the simulations by prolonging the endurance of the stratocumulus deck in the Inn valley until approximately 10 UTC. 2.4. MESOSCALE FLOW ANALYSIS 23

2.4 Mesoscale ﬂow analysis

To get a detailed picture of the atmospheric layering and the flow in the Wipp valley on 18 October 1999, measurements from different platforms are combined with the numerical simulation. The results of a NOAA aerosol lidar and of the MM5 model are compared in section 2.4.1. Section 2.4.2 concentrates on the vertical structure using rawinsonde data, again comparing it to model profiles. Time series of pressure and wind at selected stations in the target area are shown in section 2.4.3, which give a detailed insight in the evolution of the surface flow during the day. Section 2.4.4 presents the horizontal flow field as simulated by the MM5 model in the foehn layer and at the surface level. The model output is used in two ways: The ability of the model to simulate the observed Sandwich foehn structure is discussed and discrepancies to the observations are pointed out. Additionally the skill of the model to qualitatively describe the horizontal wind variations in the foehn layer during the day as a function of the onflow conditions (from north and south!) and topographic control is utilised.

2.4.1 Flow description with lidar data and model ﬁelds

The NOAA/ETL Transverse Excitation Atmospheric Pressure (TEA) CO2 Scanning Doppler lidar was located at Gedeir (GED) about halfway between the Brenner pass and Innsbruck. The Brenner pass is 17.4km south of GED, Innsbruck 12.8km NNW of it. The lidar emits pulses at 10.59µm, which are backscattered from aerosols that move with the flow. The radial velocity component along the beam direction is gained from the Doppler-shifted frequency of the backscattered signal in 300m range gates. Detailed descriptions of the lidar are given, among others, in Post and Cupp (1990) and Gohm et al. (2004). Durran et al. (2003) compared the accuracy of the lidar with airborne in-situ measurements during three MAP-SOP events. They found an unexplained bias, in which the wind speeds from the National Oceanic and Atmospheric Administrations (NOAA) WP-3D aircraft exceeded those from the lidar. This bias was small down valley (0.4 m s−1) and much larger up valley (2.4 m s−1). In our definition winds towards the lidar have positive radial velocities. For an easier representation of the measured wind field in a vertical, roughly south-north transect along the Wipp Valley, i.e. in an x–z plane, we computed the valley-relative radial velocity vrr based on combined up- and downvalley scans of vr. The definition is

vrr = vr for x ≤ x0 (2.1)

vrr = −vr for x > x0 24 CHAPTER 2. SANDWICH FOEHN ON 18 OCTOBER 1999

with x0 defining the horizontal location of the lidar site and with x increasing from south to north. This definition implies that vrr has always positive values for down- valley (southerly) flow and negative values for upvalley (northerly) flow regardless of the scanning direction. In contrast, vr changes sign whenever the flow passes the lidar site. Figure 2.4 shows the wind field in the Wipp Valley as observed with the lidar and as simulated by the MM5. The south-north transect is oriented at an azimuth angle of 178◦ (320◦) south (north) of the Doppler lidar site Gedeir, which is located at x = 20 km. The observations are averaged over a series of azimuthal and elevation scans conducted within a period of approximately 40 minutes near 1200 UTC and 1400 UTC 18 October 1999, respectively. The model fields are snapshots valid for the corresponding times of 1200 and 1400 UTC. Note that for purely horizontal flow low scan elevations result in a higher radial velocity than higher ones. Directly above the lidar only vertical motions are captured. The most striking flow feature represented in both the observations and the simulation, i.e. the essential characteristic of the sandwich foehn, is a three-layer flow structure: A surface layer with nearly stagnant or light northerly winds, especially obvious in the northern part of the Wipp Valley and in the Inn Valley, followed by a foehn layer with southerly winds between approximately 1 and 3 km MSL, and topped by a layer with northerly winds above ∼3 km MSL. A major discrepancy between the observations and the simulation is the underestimation of the southward extent of the shallow surface layer. In other words, the simulated warm foehn flow has become fully established at the surface, except in the northern third of the valley, whereas the observations indicate that the foehn flow is kept aloft by the influence of the cool surface layer which extends about 10 km further upvalley than in the simulation. Similar to the simulation of the 24 October 1999 foehn case discussed by Gohm et al. 2004, the model underestimates the depth of the foehn layer by approximately one third equivalent to 500 m. Maximum radial wind speeds in the foehn layer are approximately 50% (4 m s−1) higher in the simulation than in the observations. The distribution of potential temperature and of wind velocity in the model both indicate downslope flow underneath a gravity wave to the lee of the southernmost obstacle represented in the shown transect (Sattelberg). Some evidence for this feature is also found in the lidar observations at 1400 UTC, however, the significance is limited due to sparse data coverage at large distances from the lidar. Nevertheless, the lidar data indicate a second gravity wave, represented as an isolated region with nearly zero radial velocities especially at 1400 UTC, to the lee of a second obstacle (Nösslachjoch) north of Sattelberg. In the simulation, this feature is not represented at 12 UTC and only weakly apparent at 14 UTC. A closer inspection of the temporal evolution of the model fields reveals that most of these discrepancies might be a result of a temporal phasing error. The 2.4. MESOSCALE FLOW ANALYSIS 25

0 −4−2 4 4 −2 −2 −2 −2 −2 −4

−2

−2 0 −4 −4 0 3.5 3.5 0 −2 −2 0 −2 0 4 0 0 3 4 0 0 3 4 4 4 4 0 4 4 4 2.5 4 2.5 4 8 4 8 2 2 4 4 4 4 0 8 8 8 4 4 4 0 8 z (km MSL) 1.5 4 0 z (km MSL) 1.5 4 4 −20 −4 4 0 0 1 0 1 0.5 (a) 0.5 (b) 0 0 0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 40 Distance (km) Distance (km)

−10 4 −8 −8 4 −8 −6 −8 −6 −6 −4 −6 3.5 −4 −2 −6 −6 3.5 −2 0 −4 −4 0 0 −4 −2 −4 −4 −2 −2 3 −2 3 0 0 0 0 −2 2.5 2.5 0

0 4 4 4 4 4 12 4 4 2 8 2 8 12 8 12 8 8 8

4 z (km MSL) 1.5 8 z (km MSL) 1.5 12 12 12 8 0 8 4 0 1 8 0 −2 1 8 12 4 0 44 0.5 (c) 0.5 (d) 0 0 0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 40 Distance (km) Distance (km)

4 4 300 300 300 300 300 300 3.5 3.5 295 295 295 295 3 3 295 295

2.5 2.5 290 2 2 290

z (km MSL) z (km MSL) 290 1.5 1.5 290 290 290 290 1 285 1 285 0.5 (e) 0.5 (f) 0 0 0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 40 Distance (km) Distance (km)

Figure 2.4: Vertical cross-section along the Wipp Valley as indicated in Fig. 2.1. The south-north transect is oriented at an azimuth angle of 178◦ (320◦) south (north) of the Doppler lidar site Gedeir, which is located at x = 20 km. Orography is displayed as ﬁlled black areas. Valley-relative radial velocity (a)–(b) observed with the Doppler lidar and (c)–(d) simulated by the MM5: contour lines with 2 m s−1 (1 m s−1) for positive (negative) values; negative values are dashed; shaded contours for positive velocities, i.e. downvalley foehn winds, above 4, 8, and 12 m s−1. (e)–(f) Potential temperature (contour lines, 1-K increment) and vectors for the wind components parallel to the cross-section simulated by the MM5. Figure columns from left to right represent times of approximately 1200 and 1400 UTC 18 October 1999, respectively.

earlier model field at 09 UTC seems to describe the observed flow at noon and in the early afternoon much better than the later ones. In detail, the agreement is better concerning the extent of the surface layer, the strength of the foehn flow, and the 26 CHAPTER 2. SANDWICH FOEHN ON 18 OCTOBER 1999 gravity-wave structure over Nösslachjoch.

2.4.2 Vertical structure using rawinsondes and model data

Rawinsonde data from Sterzing (Gedeir) show the foehn related differences of the vertical wind and temperature structure south (north) of the Alpine crest (see figure 2.1 for location). Figure 2.5 compares data at both sites to MM5 model profiles, which are calculated along the balloon trajectory.

5 5 5 (a) (b) (c) 4.5 4.5 4.5

4 4 4

3.5 3.5 3.5

3 3 3

2.5 2.5 2.5

Height (km MSL) 2 Height (km MSL) 2 Height (km MSL) 2

1.5 Obs 1.5 1.5 MM5 1 1 1 280 290 300 310 0 5 10 15 20 0 90 180 270 360 Potential Temperature (K) Wind Speed (m/s) Wind Direction (deg)

5 5 5 (d) (e) (f) 4.5 4.5 4.5

4 4 4

3.5 3.5 3.5

3 3 3

2.5 2.5 2.5

Height (km MSL) 2 Height (km MSL) 2 Height (km MSL) 2

1.5 Obs 1.5 1.5 MM5 1 1 1 280 290 300 310 0 5 10 15 20 0 90 180 270 360 Potential Temperature (K) Wind Speed (m/s) Wind Direction (deg)

Figure 2.5: Comparison of radiosoundings at (a) – (c) Sterzing (12 UTC) and (d) – (f) Gedeir (14 UTC) with the corresponding model proﬁles. All model proﬁles were derived from grid point data interpolated onto balloon trajectories.

The observed upstream profile (in terms of foehn) at 12 UTC shows easterlies of 4 ms−1 topped by nearly stagnant (blocked) flow up to approximately the Brenner pass height (1.5km MSL). A marked inversion at 3.2km MSL decouples southerly flow of 3 − 5 ms−1 from the stably stratified dry northwesterlies aloft. Wind speeds pick up above another stable layer at ∼3.8km MSL. The MM5 model atmosphere is too cold, especially near the surface by about 4K. As could be expected the foehn 2.4. MESOSCALE FLOW ANALYSIS 27 inversion at ∼3.8km MSL is smoothed out, but the height is well captured. The flow in the lowest 500 m does not stagnate. Up to 5km MSL the wind is well simulated, above wind speeds are overestimated by 3 − 5 ms−1 (not shown). Since the Gedeir sonde at 12 UTC was lost at 3km MSL, the 14 UTC sounding is displayed. The lowest 300m a.g.l. in the middle of the Wipp valley are marked by the cold northerly flow peaking at 4 ms−1. The foehn layer between 1.3 and 2.8km MSL shows wind speeds of 8 − 12 ms−1 and is topped by an isothermal layer. Comparison to the upstream sounding reveals, that the foehn inversion has weakened and descended by 500m. The major discrepancy between observations and the model is that no upvalley flow at Gedeir is simulated. Consequently the foehn layer touches the ground in the simulations. It has to be noted, that the maximum of the simulated foehn jet is 100 m a.g.l. The wind speed at the lowest half-sigma level is ∼ 10m s−1, which is not evident from figure 2.5, because the data are interpolated to the corresponding height of the rawinsonde. The top of the southerly flow and the stable layer (foehn inversion) is ∼500 m too low compared to the measurements, which was already seen in the comparison to the lidar measurements in section 2.4.1. Since the upstream profile (wind speeds above pass height and height of inversion) was simulated well, the downstream discrepancy most likely stems from the absence of the shallow cold air pool connected to the northerly flow. Closer inspection reveals other small discrepancies: For one – expect for the lowest 300m a.g.l. – the model profile is a bit too cold and generally smoothed out. Hence the second stable layer is much thinner and stronger in the sounding (3.7 - 3.9km MSL) than in the model (3.5 and 4km MSL). Furthermore the wind speed in the northwesterly flow above 4km MSL is overestimated by the MM5 run, confirming a similar observation for the upstream profile.

2.4.3 Surface wind and pressure analysis in the Wipp valley

For the analysis described in this subsection only stations developed and mounted by the University of Leeds were used, because they have a consistent pressure measurement minimizing the dynamic pressure eﬀect and are well spread within the target area. Calibration against a high precision reference ascertains high relative accuracies. The pressure is reduced to 900m MSL height instead of sea level to keep the temperature related reduction error small. Pressure values are displayed as pred − 910hP a for simplicity. Note that the valley bottom rises from north to south starting at 750m MSL to the pass height of 1370m MSL (second location from south) and drops 100m to the southernmost station location. Wind speeds are given in knots for better accuracy regarding the partly weak surface ﬂow. 28 CHAPTER 2. SANDWICH FOEHN ON 18 OCTOBER 1999

4.3 4.2 2.1 a) b) c)

3.9 3.6 1.1 3.6 3.5 1.1 3.7 3.5 1.0

4.7 4.0 0.7

4.7 3.5 0.1 4.0 2.9 1.0 4.5 3.9 1.7

4.7 4.5 2.4

0.3 0.5 0.6 d) e) f)

−0.7 0.2 0.5 −0.2 0.0 0.3 −0.3 0.0 0.3

−0.1 0.2 0.7

0.3 0.9 1.1 0.8 0.8 0.9 1.6 1.9 2.0

2.3 2.4 2.4

Figure 2.6: 3 hourly wind and pressure anlyses in the Inn and Wipp valley on 18 Oct 1999 between 06 UTC and 21 UTC. Short wind barbs represent 5kn, long ones 10kn. Numbers denote station pressure reduced to 900m MSL relative to a reference value of 910hPa. The dashed line is the borderline between up- and downvalley ﬂow at the surface. The 1500m height contour line (solid) is depicted from a 1km GLOBE topography set.

Figure 2.6a at 06 UTC shows a rather typical morning situation. Weak drainage flow with wind speeds below 3 kt dominates throughout most of the Wipp valley. Accordingly a slight pressure drop from south to north was observed. The Inn valley however already depicts easterly (upvalley) flow with 5kt due to the higher pressure in the Alpine foreland as already described in section 2.2. As a consequence the pressure is higher there than at the Wipp valley exit. Three hours later at 09 UTC (cf. figure 2.6b) the upvalley flow has flooded most of the Wipp valley with moderate wind speeds of 5kn. On the other hand the foehn flow at the surface has strengthened and reaches further downvalley, before it is 2.4. MESOSCALE FLOW ANALYSIS 29 lifted off the ground to establish the middle south flow layer of the Sandwich foehn. At 12 UTC (figure 2.6c) the upvalley flow in the northern Wipp valley has strengthened reaching almost 10kn, which corresponds well with a stronger pressure difference between the Inn valley and the pressure minimum in the southern Wipp valley. The opposing foehn flow of 10kt has displaced the upvalley flow a few kilometres to the north. Consequently the location of the pressure minimum is also shifted northward. The overall pressure drop compared to the morning hours is caused by warming due to solar radiation and advection of warm air by the south foehn. For example Stafflach, the fourth station from south, experienced a pressure drop of 4.6hPa in six hours. The afternoon and evening hours (figure 2.6d-f) are marked by a balance between the two opposing flows. The upvalley flow was pushed back and is restricted to the northern third of the valley. At 15 UTC the inflow in the Inn valley reaches its maximum strength. This is enabled by the dissolving stratocumulus deck, which leads to the establishment of a thermally induced valley circulation reinforcing the synoptic flow component. Later on towards the evening the solar radiation input vanishes, the pressure differences decrease and the upvalley flow slows down. At 21 UTC it has almost ceased, three hours later thermally driven drainage flow dominated (not shown). The foehn flow at the surface experiences little change throughout the afternoon, it covers two thirds of the Wipp valley with a magnitude of 5 - 10kn. The general pressure increase between 15 and 21 UTC can be attributed to the daily pressure cycle.

2.4.4 Horizontal wind as simulated by the MM5 model

As pointed out in sections 2.4.1 and 2.4.2 comparison to the aerosol lidar and rawinsondes reveal a few weaknesses of the model simulation. The underestimation of the northerly inflow (southward extent, layer depth) leads to a foehn layer that is too close to the ground. A temporal phasing error seems to develop as a consequence. The foehn jet magnitude is slightly overestimated, as well as wind speeds above 4km MSL. An interpretation of the model fields has to keep these discrepancies in mind. Nevertheless there are a few things we can learn from the model. Since the flow below crest height is highly topographically controlled, some flow features concerning preferred locations of wave excitation and horizontal inhomogenuities of the flow speed have been observed during other events and seem very plausible. They are described in subsection (1). The simulated upvalley flow on the other hand is easier to check against surface observations. A critical discussion of the model results is found in subsection (2). 30 CHAPTER 2. SANDWICH FOEHN ON 18 OCTOBER 1999

South foehn ﬂow

Figure 2.7 a – c shows the horizontal wind field simulated by the MM5 at the height of 2km MSL. The location of the subdomain is shown in figure 2.1. The point of origin (0,0) represents the Brenner pass. Note that the simulated foehn jet is below 2km MSL, while the lidar observations show the maximum near that height as can be seen in figure 2.4. Consequently the model underestimates the foehn flow at the selected height. The across-valley asymmetry with stronger flow on the eastern valley side is evident throughout the Wipp valley. For the valley part north of the lidar site this flow asymmetry has been described for other MAP events (cf. Flamant et al. 2002; Durran et al. 2003; Gohm et al. 2004; Mayr et al. 2004. Gohm et al. (2004) suggest the cause in the valley bending westward north of the lidar site in combination with the asymmetry of the ridges protruding into the valley from the western and eastern side. Flamant et al. (2002) on the other hand proposed that the southwest orientation of the ridges to the west in southwesterly flow aloft lead to this asymmetry. The shallow nature of the Sandwich foehn and a stronger flow on the eastern side not only north of the lidar site points towards the influence of the (north)westerly flow component above 3km. On the other hand the flow deflection caused by the westward bending Wipp valley is evident, so that for this case study both explanations might be part of the truth. During the course of the day the strongest foehn branch reaching the Inn valley moves from east of Innsbruck to the western side. Quantitatively the simulations on the western side are supported by the rawinsonde ascents at Innsbruck, with wind at 2km turning from 215 to 140 deg between 05UTC and 11 UTC and wind speed increasing from 7 to 10 ms−1.

Up-valley surface ﬂow

Figure 2.7 d – f displays wind arrows at the lowest half-sigma level of the MM5 model at approximately 14m MSL. The focus is on the flow in the valleys (which have no topographic shading corresponding to heights below 1000m MSL). At 09 UTC the foehn flow has established at the pass and reaches 10km downstream of it. The middle of the Wipp valley is characterized by flow stagnation. Weak upvalley flow in the Inn valley (∼ 2 ms−1) is simulated. The northerly flow into the Wipp valley reaches approximately 10km south. The position of the confluence zone corresponds well with the surface observations as shown in figure 2.6b. Three hours later the southerly model wind at the surface has reached the intersection with the major side valley (”Stubai”). Comparison with aerosol lidar and surface observations reveal that the depth of the simulated cold up-valley flow was too 2.5. SUMMARY AND CONCLUSION 31

MM5, wind, z=2000 m, 0900 UTC MM5, wind, z=2000 m, 1200 UTC MM5, wind, z=2000 m, 1500 UTC 35 35 35 (a) (b) (c) 30 30 30

25 25 25

20 20 20

15 15 15 y (km) y (km) y (km)

10 10 10

5 5 5

0 0 0

−5 10 m/s −5 10 m/s −5 10 m/s −20 −10 0 10 −20 −10 0 10 −20 −10 0 10 x (km) x (km) x (km) MM5, wind, surface, 0900 UTC MM5, wind, surface, 1200 UTC MM5, wind, surface, 1500 UTC 35 35 35 (d) (e) (f) 30 30 30

25 25 25

20 20 20

15 15 15 y (km) y (km) y (km)

10 10 10

5 5 5

0 0 0

−5 10 m/s −5 10 m/s −5 10 m/s −20 −10 0 10 −20 −10 0 10 −20 −10 0 10 x (km) x (km) x (km)

Figure 2.7: MM5 wind ﬁelds of the 00 UTC run on 18 Oct 1999 at (a),(d) 9UTC, (b),(e) 12UTC and (c),(f) 15 UTC. Horizontal wind arrows at 2km MSL are shown in the top row, surface winds at the lowest half-sigma level 14m above ground in the bottom row.

shallow. Additionally the lidar data suggest that the strength of the foehn flow is a bit overestimated. As a consequence the northerly flow is eroded too quickly, as an inspection of figure 2.6c clearly proves. This discrepancy is maintained at 15 UTC. While the model fields show foehn throughout the Wipp valley, in reality the foehn front at the surface was near y=17km (17km north of Brenner). Nevertheless the up-valley flow in the Inn valley is still present in the simulations and wind speeds correlate well with the observations.

2.5 Summary and conclusion

A case study of a complex three-layer flow situation measured during the Mesoscale Alpine Programme on 18 October 1999 is presented. During a twelve hour period 32 CHAPTER 2. SANDWICH FOEHN ON 18 OCTOBER 1999 the northern Wipp valley experienced shallow upvalley (northerly) flow topped by south foehn. Above crest height winds turned to northwest again. The upvalley flow is shown to be caused by advection of cold air from the Alpine foreland and a stratocumulus deck which is removed by the developing foehn flow from south to north. The foehn flow is a consequence of a dynamic and hydrostatic influence: the dynamic part is a weak synoptic south flow on the eastern flank of a pressure low over France. The hydrostatic part is caused by cold air (from the same air mass as the one in the Alpine foreland) flowing around the Alps building up a higher pressure than inneralpine. The pressure low over Slovakia causes the northwesterly flow aloft. Analysis of the surface stations in the Wipp valley showed the evolution during the day. Weak drainage flow in the night is followed by the interaction of the cold shallow upvalley flow from north with the foehn. In the morning the northerly flow floods most of the valley, before it is gradually pushed back by the south wind. A 2 quasi-stationary balance is reached in the afternoon with the foehn covering 3 of the valley surface. Aerosol lidar measurements in the middle of the Wipp valley reveal details of the flow structure and evolution: two gravity waves excited by west-east oriented ridges, break through of the foehn in the southern Wipp valley, and the decreasing depth of the cold surface layer between 12 and 14 UTC due to the foehn influence. The averaged maximum radial wind velocity is close to 10 m s −1. Numerical simulations with the MM5 model were able to capture the main flow features. The south wind layer upstream is well represented, while the shallow northerly flow is underestimated (southward extend and depth of layer). As a consequence the top of the foehn layer is too low, although the depth of the foehn layer compares well to the lidar measurements. Simulated foehn break through is earlier as was observed and therefore the model fields at 09 UTC represent the observed flow situation at noon better. Simulated wind speeds in the foehn jet and of the northwesterly flow aloft are overestimated. The model simulations show stronger foehn flow on the eastern Wipp valley side. While a connection with the northwesterly flow aloft is hypothesised but unclear, the westward bending of the northern valley part evidently leads to flow deflection towards the eastern valley side. 4-D numerical simulations with realistic topography are certainly able to capture complex flow structures in mountaineous terrain. Therefore they need initial model analysis fields that capture the main mesoscale air mass characteristics. In the presented case study the upstream conditions (wind speed and direction, temperature profile including foehn inversion) for the foehn flow were well represented, while the strength and duration of the inneralpine stratocumulus deck proved crucial for 2.5. SUMMARY AND CONCLUSION 33 the shallow upvalley (northerly) flow but difficult to simulate inspite of corrected ECMWF-analysis profiles using rawinsonde data of Innsbruck. Deviations of the modelled foehn flow in the Wipp valley from the observed one (timing, breakthrough at the surface, layer height) can be partly explained by interaction with a different upvalley flow acting as a lower boundary for the foehn flow.

Acknowledgements

Bob Banta, Lisa Dary and Mike Hardesty from NOAA/ETL are acknowledged for the Doppler lidar measurements and the scientiﬁc support. Stephen Mobbs and Samantha Arnold provided and maintained the weather stations used in this study. This research was supported by the Austrian Science Fund FWF under the Grants 13655 and 15077.

Chapter 3

Case studies of foehn ﬂow in a T-shaped valley system

Case studies of foehn ﬂow in a T-shaped valley system 1 Johannes Vergeiner2 , Alexander Gohm, Georg J. Mayr Department of Meteorology and Geophysics, University of Innsbruck, Austria Robert Baumann, DLR- Oberpfaﬀenhofen, Germany

Summary

During two flights with the DLR Falcon in February and November 2000 in-situ measurements of standard meteorological parameters sampled at a 10- 100 Hz rate were taken to determine foehn flow characteristics in the Wipp and the Inn Valley in the Eastern European Alps. Both days were characterized by southwesterly winds at crest height and moderate pressure driven foehn flow below. Flight data were combined with measurements of radiosoundings, car-based and surface stations especially placed for foehn situations. On 28 February a marked low level jet was found within the southern Wipp Valley at 1500 – 2000 m MSL below the strong inversion. Near the exit of the Wipp Valley at the same height a well-mixed zone with nearly no horizontal flow existed as a consequence of a hydraulic-jump-like feature. The inversion along the Brenner cross-section descended from south to north and split up into two weaker parts. On 7 November the oncoming southerly flow over the Inn Valley changed from southwesterly flow at crest height to highly orography- dependent flow below. Horizontal flow splitting was observed at the conjunction

1To be submitted to Monthly Weather Review 2Corresponding author: Department of Meteorology and Geophysics, University of Innsbruck, Innrain 52, A-6020 Innsbruck, Austria; E-mail: [email protected] 35 CHAPTER 3. CASE STUDIES OF FOEHN FLOW IN A T-SHAPED VALLEY 36 SYSTEM of major north-south oriented valleys with the perpendicular Inn Valley. Ver- tical motions on the order of 5 m s−1 showed strong wave activity east of Innsbruck.

KEYWORDS: foehn, MAP, Wipp valley 3.1. INTRODUCTION 37

3.1 Introduction

Foehn flow in the European Alps has always had a strong influence on the people living in this area. As a consequence we can look back to a more than 100 year long meteorological tradition to monitor, describe and explain this warm and dry mountain wind. In order to capture the complex flow features connected to foehn, information on the vertical structure of the atmosphere is essential. This was already realised by some early foehn pioneers. For example Ficker (1931) flew in dangerous foehnwinds with a gas balloon from Innsbruck to wherever the wind carried him around 1912. He gave an exciting report on the smooth ascent upstream of an elongated ridgeline and the dangerous downdrafts downstream of it. But up to the 80s scientists were lacking proper means to perform mesoscale analysis of orographic flow modifications. A need for field experiments to supplement ground-based measurements with airborne ones became apparent. Knowledge concerning the orographic influence on flows had been gained in a number of field experiments outside of the Alpine area (Colorado Lee Wave Programme 1970; Lilly et al. 1971; Measurements above the Colorado Rocky Mountains 1971; Lilly 1978; WAMFLEX 1973; Lilly et al. 1982; Measurements above the British Isles 1976 – 81, Brown 1983). The experimental gap in the Alps was supposed to be filled by ALPEX in 1982 (Küttner1992), but unluckily no deep foehn events occurred over the Alps, only one over the Pyrenees (Hoinka 1984; Cox 1986). Therefore the DFVLR-foehn experiments between 1982 and 1987 were set up, where Hoinka and Clark (1991) were able to calculate the momentum flux across the Alps using two aircraft and three motor gliders. Seibert (1990) reviewed the knowledge on foehn after ALPEX. Still little was known about the mesoscale structure of foehn flows.

The Mesoscale Alpine Programme (MAP) (Bougeault et al. 2001) should fill this gap. During the Special Observing Period (SOP) in autumn 1999 measurements of gap flow in the Brenner target area (cf. Fig. 3.1) were one of the main objectives (Mayr et al. 2004; Mayr et al. 2003). A number of aircraft missions (NOAA- P3, NCAR-Electra, DLR-Dornier, the French Arat) took in situ measurements and used remote techniques to measure the properties of the foehn layer. Nance (2000) analysed the P3-flight as well as the data from a lidar in the Wipp Valley during a shallow foehn event with channelled south flow in the Wipp Valley showing a wavy structure downslope of Brenner and westerlies above. Flamant et al. (2002) analysed the structure of the upper boundary of the foehn layer on 30 October 1999 as observed by an airborne differential absorption lidar. A hydraulic-jump- like feature was found 10 km north of Brenner. Towards the exit into the Inn Valley a CHAPTER 3. CASE STUDIES OF FOEHN FLOW IN A T-SHAPED VALLEY 38 SYSTEM

Figure 3.1: Topography of the Alps including sounding stations Milano (MIL), Verona (VER) and Munich (MUN) and the stations Bozen (BOZ) and Innsbruck (IBK) with close-up of the Brenner target area. Sounding stations Sterzing (STZ) and Innsbruck (IBK) in the zoomed plot are marked with an asterisk, additionally used meteorological stations with a black circle. BRE denotes Brenner, SAT Sattelberg and PAK Patscherkofel. Mountain ranges are written in capital letters. Isolines are 600, 1300, 2000 and 2700 m MSL from a 1 km GLOBE topography set. 3.2. SETUP OF MEASUREMENTS AND LOCATION 39 transition to supercritical flow was suspected. Gohm et al. (2000) studied the inter- ruption of the foehn flow by a strong cold front. Gohm and Mayr (2004) examined the applicability of single-layer hydraulic theory to foehn in the Wipp Valley. From comparisons to the extensive data set measured during the MAP-SOP they found that a hydraulic model with realistic topography is able to reproduce the basic flow features including the location of steeply amplified or breaking waves resembled by a hydraulic jump. In autumn 1999 two foehn flights with the DLR Falcon 20 E5 / D-CMET were planned as part of the SOP, but for various reasons the flights with the Falcon could not be performed during MAP. Therefore after the SOP favorable foehn conditions were sought and found on 28 February and 7 November 2000. The main purpose was to gain further knowledge on the flow structure of south foehn crossing the lowest gap in the Alps, following the crest-perpendicular Wipp Valley and encountering the crest-parallel Inn Valley (cf. Fig. 3.1). The deflection of the flow at the ridgeline north of Innsbruck as a consequence of the T-shape alignment of those two valleys was investigated as well as the intrusion of foehn air west and east of the Wipp Val- ley into the west-east oriented Inn Valley. The high frequency in-situ measurements allowed for analysis of small-scale turbulent motion down to ≈ 1m. This paper presents the results of these measurements. Section 3.2 describes the setup of the measuring systems and points out the topographic features important to understand the flow. Section 3.3 deals with the first flight, where the main focus was on the flow in the crest-perpendicular Wipp Valley and the differences north and south of the main Alpine crest. Section 3.4 describes the second flight focussing on the flow into the mostly west-east oriented, crest-parallel Inn Valley through a number of gaps and valleys. Section 3.5 offers a discussion of the results and concluding remarks.

3.2 Setup of measurements and location

The DLR Falcon Research Aircraft measured all basic meteorological and naviga- tional data at a sensor-dependent rate of 10 – 100 Hz. Temperature was taken with a fast response open-wire Pt-100 and a slower but robust Pt-500 inside two de-iced Rosemount housings and was corrected for errors due to the adiabatic compression and the de-icing facility (Helten et al. 1999). Humidity was measured by three diﬀerent sensor types. A fast Lyman-Alpha Hygrometer (Busen and Buck 1995) based on the absorption of UV radiation at 121.56 nm, which is a measure of the absolute humidity, a Vaisala Humicap based on the measurement of the capacitance of a small ceramic capacitor with a response time of the order 1 – 10 seconds and a very stable, but slow Dewpoint Mirror. The Lyman-Alpha and CHAPTER 3. CASE STUDIES OF FOEHN FLOW IN A T-SHAPED VALLEY 40 SYSTEM

Vaisala were mounted inside the nose of the aircraft. The humidity data were transformed to environmental air conditions assuming constant mixing ratio of the air while passing the measurement channel. Wind was derived from the difference between the velocity components of the aircraft relative to the ground and of the aircraft relative to the air. The latter was measured by a Rosemount-5-Hole probe on the tip of the nose-boom. A ring of ports around the shaft of the probe allowed for the calculation of the static pressure. The altitude, position and velocity relative to the ground were measured with an Inertial Reference System (IRS; Honeywell YG 1779). Further information on algorithms, calibration procedures and definitions of parameters is given in Bögeland Baumann (1991), Baumann (1994) and Meischner (1985). Aircraft intercomparisons are described e.g. in Quante et al. (1996), Strömet al. (1994) and Helten et al. (1999). Abso- lute accuracies are 0.5 ◦C for the static temperature, 0.25 g m−3 for the absolute humidity of the Lyman-Alpha, 5% for the Vaisala-humidity, 0.5 hPa for the pressure, 2 m s−1 for the horizontal wind, 0.3 m s−1 for the vertical wind. Relative accuracies, i.e. random errors changing from point to point, are much better, typically by one to two orders of magnitude. In addition to the aircraft data, vertical soundings and near ground information are an essential supplement. A thermodynamic and wind profile upstream (in our cases: south) of the main Alpine crest is crucial for understanding cross Alpine flows. For the first south foehn case on 28 February three soundings at 09, 12 and 15 UTC were performed at Sterzing, 943 m MSL (cf. Fig. 3.1). Additional soundings were made in Innsbruck, 581 m MSL, at 11 and 15 UTC. During the flight car-based measurements of temperature, humidity and pressure were taken along the Autobahn between Innsbruck and Sterzing (Mayr et al. 2002). The mobile sounding system in Sterzing was not available for the second south foehn case on 7 November. Therefore a missed approach at Verona was flown with the Falcon to get the profile on the windward side. Also a sounding was made in Innsbruck at 12 UTC. The measuring car drove in the Wipp Valley between Innsbruck and Sterzing and in the Inn Valley. For the analysis of the Wipp Valley cross section, a number of weather stations were incorporated. They are partly operated by the University of Insbruck in places ideally suited to forecast and analyse foehn events. For location of the stations see Fig 3.2.

The most important topographic features encountered by south foehn in the Brenner target area and their influence on the flow are described below. It is aimed at helping readers not familiar with the location to visualize the highly topographically controlled flow. The main Alpine crest west and east of the Brenner pass is on 3.2. SETUP OF MEASUREMENTS AND LOCATION 41

47.3 47.8

47.2 47.6

47.1 47.4 B 47.0

47.2 46.9 11.3 11.4 11.5 11.6 47.3 47.0

A 47.2 46.8

47.1

46.6 47.0

46.4 46.9 10.8 11.0 11.2 11.4 11.6 11.8 12.0 11.3 11.4 11.5 11.6

Figure 3.2: (Left) Simplified flight track on 28 Feb 2000. Marked are the southern (A) and northern (B) end points of the Wipp Valley cross section. Isolines are 600, 1300, 2000 and 2700 m MSL from a 1 km GLOBE topography set. Crosses depict additional weather stations used for the analysis. The box marks the area of the close-ups on the right. (Right) Close-up of the two south-north flight paths above the Wipp Valley at a height of (top) 3100m MSL and (bottom) 2500m MSL. Isolines every 350m starting from 600m MSL with same grey scales as on the left. For further description see text. CHAPTER 3. CASE STUDIES OF FOEHN FLOW IN A T-SHAPED VALLEY 42 SYSTEM average 2900m MSL high. Embedded in it is a double gap structure. The larger amount of foehn air flows over the 15km broad saddle enclosed by the Stubaier and Zillertaler Alps at a height of about 2000 – 2200 m MSL. The Brenner pass itself is a deep and narrow incision within the broader gap down to 1370m MSL. After passing the Alpine crest the foehn flow is channelled horizontally and vertically into the NNW - SSE oriented Wipp Valley and speeds up. Ridges perpendicular to the valley axis have the potential to modify the flow. They can even cause wave overturning or breaking, thereby slowing the flow down. At the exit of the Wipp into the perpendicular Inn Valley, the topography drops by 200 m. If the Inn Valley is filled with cold air at the beginning of a foehn period, the warm foehn flow does not always manage to sweep it away. Outflow in the Inn Valley (”vorföhnigerWest”) helps to substitute the colder air by warmer foehn air from the south. North of Innsbruck the west-east oriented Nordkette with a height of approximately 2300 m MSL is a second barrier, especially for the foehn flow leaving the Wipp Valley. The role of the Nordkette will be examined in section 3.3.

3.3 The 28 February 2000 foehn

Throughout the day moderate foehn conditions with channelled flow into the Wipp Valley below a well defined foehn inversion at about crest height and southwesterlies above prevailed. Traverses along the Wipp Valley and in the Innsbruck area were flown and are analysed together with radiosoundings and additional ground-based observations.

3.3.1 Flight pattern

Starting at Oberpfaffenhofen at 1235 UTC five legs at 7400, 6800, 6200, 5600 and 5000 m MSL were flown from the Alpine foreland crossing Innsbruck and the Wipp Valley down to Bozen to measure the flow characteristics above the foehn inversion and get an overview of the cloudiness in the area of interest (long north-south leg in Fig. 3.2). Unexpectedly few low- and mid-level clouds in the basin of Sterzing (just west of point A) allowed flying with VFR (Visual Flight Rules) in the Wipp Valley at 4300, 3700 and twice at 3100 and 2500 m MSL between Sterzing and Innsbruck (shorter north-south leg in Fig. 3.2). Results from these Wipp Valley flight legs are presented in section 3.3.3. At the end 3 legs were flown inside the Inn Valley at 2200, 1900 and 1550 m MSL (west-east tracks in Fig. 3.2). They were expected to give an overview of the divergence of the foehn flow as it approached the Nordkette, the different air mass characteristics at the exit of the Wipp Valley, and the vertical velocity north of the Patscherkofel. For results see section 3.3.4. The landing was 3.3. THE 28 FEBRUARY 2000 FOEHN 43 again at Oberpfaffenhofen at 1544 UTC.

3.3.2 Foehn characteristics

Foehn characteristics can be expressed by the dynamic and hydrostatic driving forces for foehn as well as the moisture content of the advected flow. The following paragraphs describe the strength of the oncoming flow, the pressure difference across the Alps as a consequence of differing air mass properties and the upstream cloudiness. On 28 Feb 2000 the Alps experienced the beginning of a foehn period with diffluent SW flow at crest height which strengthened in the afternoon. Table B.1 shows the south wind component at 700hPa measured by six-hourly rawinsonde launches at Milano, 150km upstream of the Alpine crest. It is a good measure for the strength of the oncoming flow. While at 0000 UTC westerly winds prevail, the deepening trough strengthens the south wind component during the day to approximately 6 to 7 m s−1.

Time (UTC) v700 ( m s−1) ∆P (hPa) 00 0.2 1.9 06 6.4 3.8 12 6.9 8.4 18 5.9 5.1

Table B.1: Wind at 700hPa from rawinsonde data at Milano and reduced pressure diﬀerence between Bolzano and Innsbruck on 28 Feb 2000.

The higher pressure south of the main Alpine crest in comparison to the northern foehn regions is one of the driving mechanisms for the foehn flow. It is partly caused hydrostatically by relatively colder air masses in the upper Italian region, which in this case was brought about by the passage of a warm front north of the Alps. But partly the pressure gradient is also a consequence of the foehn flow itself, which brings warmer and therefore less dense air masses to the foehn regions. The early foehn stage on 28 February is marked by the build up of the pressure difference (Table B.1). At 0000 UTC the reduced pressure difference between Bozen 60 km south of the Brenner pass and Innsbruck was only 1.9 hPa, at 0600 UTC –still before foehn breakthrough in Innsbruck– 3.8 hPa. At 1200 UTC the foehn had reached the Inn Valley and the pressure difference accordingly strengthened to 8.4 hPa, and at 1800 UTC when the foehn had decoupled from the ground it had decreased to 5.1 hPa. But also the vertical structure of the pressure difference is of interest. It was CHAPTER 3. CASE STUDIES OF FOEHN FLOW IN A T-SHAPED VALLEY 44 SYSTEM calculated from two rawinsondes launched at 1200 UTC, one at Sterzing and one at Innsbruck (cf. Fig. 3.1). As shown in table B.2 the south-north pressure difference increased pronouncedly towards the ground. There was a slight negative difference at crest height and the largest impact (1.6 hPa) came from 2500 – 3000 m MSL.

Height (MSL) ∆P (hPa) 3000 −0.3 2500 1.3 2000 2.1 1500 2.6 1000 3.1

Table B.2: Pressure diﬀerence ∆P Sterzing – Innsbruck as a function of height taken from the 12 UTC 28 Feb 2000 rawinsondes.

In this layer, the height of the inversion bottom was lowered from 2700 m MSL on the windward side to 2300 m MSL above Innsbruck as shown in Fig. 3.3 causing a marked temperature contrast (see Fig. 3.4). Hence part of the pressure difference across the Alps is due to descending flow on the lee side lowering the foehn (temperature) inversion and the associated temperature difference in this layer.

The typical upstream foehn wall was only partially established during the first foehn flight. Scattered altocumulus was found in the basin of Sterzing, while the clouds west and east of it extended further to the north. Cloud cover in the western part and dissolving clouds due to the foehn descent just north of the Alpine crest are shown in Fig. 3.5. Note the downward moving cloud rolls in the middle of the picture as a consequence of the flow of about 15 m s−1 impinging the topography.

3.3.3 Wipp Valley cross section

In this section a closer look is taken at the foehn flow in the Wipp Valley, which is oriented perpendicular to the main Alpine crest. Since the lowest possible flight level with a twin jet aircraft in this area was 2400m MSL, the layer below that is not captured by in-situ aircraft measurements. This gap is filled by two soundings in Sterzing and Innsbruck (cf. Fig. 3.1), car-based measurements along the motor-highway between these two towns and seven weather stations. Nevertheless, the main focus will be on the upper part of the foehn flow. First we have to check, if the flow in the given time frame is quasistationary. Changes in the important parameters at the Station Sattelberg (2110 m MSL) 3.3. THE 28 FEBRUARY 2000 FOEHN 45

307 5000 306 305 305 4500

4000 304 303 302 3500 301

300 300 3000 295 292 2500 295 Height (m MSL) C 292 C 292 2000 291 290 C 290 1500 291 C W 291 1000 W C 291 292 292 500 46.8 46.9 47 47.1 47.2 47.3 47.4 Latitude (deg N)

Figure 3.3: Cross-section of potential temperature with an iso spacing of 1K on 28 Feb 2000 from Falcon ﬂight data (5 quasi-horizontal lines, 1358 - 1444 UTC), soundings Sterzing (1510 - 1530 UTC) and Innsbruck (1442 - 1454 UTC) depicted as quasi-vertical lines, weather stations (crosses) and car-based measurements (exemplary points marked by circles); × - x - maximum topography, —— average topography, - - - minimum topography, all three in the longitude range 11.3 − 11.7 ◦E. Dark grey areas indicate wind speed above 17.5 m s−1, light grey areas below 5 m s−1. CHAPTER 3. CASE STUDIES OF FOEHN FLOW IN A T-SHAPED VALLEY 46 SYSTEM located in the broad saddle between the Stubaier and the Zillertaler Alps (see Fig. 3.1 ) during the ﬂight are listed in Table B.3.

Parameter Flight in Wipp Valley Whole Flight 1358 – 1444 UTC 1235 UTC – 1544 UTC Temperature (◦C) −2.4 ± 0.1 −3.1 to −2.3 no trend ﬁrst increasing, then decreasing Wind direction (◦) 179 ± 8 178 ± 10 no trend no trend 1-min Wind speed ( m s−1) 20 ± 3 20 ± 4 no trend slightly increasing Mixing ratio ( g kg−1) 2.66 ± 0.03 2.66 ± 0.03 no trend no trend

Table B.3: Changes of meteorological parameters at the Station Sattelberg (2110 m MSL) during the Falcon ﬂight on 28 Feb 2000. The given range marks maximum deviation from the mean value.

Especially during the time of the Wipp Valley ﬂights, temporal changes are negligible and show no remarkable trend. Therefore it is feasible to interpret the Wipp Valley cross section as a quasistationary picture of the foehn ﬂow along the

STZ IBK

5000

4000

3000 Height (m MSL) 2000

1000 STZ IBK

286 288 290 292 294 296 298 300 302 304 306 308 310 Potential Temperature (K) Wind (kn)

Figure 3.4: Vertical proﬁles of potential temperature in (K) and wind from the Sterzing sounding at 1146 UTC and the Innsbruck sounding at 1107 UTC on 28 Feb 2000. For position of sounding stations see Fig. 3.1. 3.3. THE 28 FEBRUARY 2000 FOEHN 47

Figure 3.5: View from the Falcon ﬂying in the middle of the Wipp Valley at a height of 7200 m MSL looking to the southwest. Beneath the strong inversion between 2700 and 3000 m MSL (cloud top) channeled south ﬂow is evident. Clearly visible are the downward moving cloud rolls along the main Alpine crest marked by the arrow. Above the inversion southwesterly winds reached 15 m s−1. Note the slight increase of cloud top heights towards the Alpine crest. CHAPTER 3. CASE STUDIES OF FOEHN FLOW IN A T-SHAPED VALLEY 48 SYSTEM

Wipp Valley transection. Furthermore, as moist processes play no significant role in this foehn event the isentropes closely resemble streamlines along the transection A – B in Fig. 3.2. The cross-sectional analysis in Fig. 3.3 highlights the most important characteristics of this foehn case on first sight. At a height of 2500 – 3000m MSL the strong potential temperature gradient - identifying the foehn inversion - decouples smooth flow above from mainly topographically controlled flow below. Above 3000m MSL moderate wave patterns with uniform winds from SW with 15 m s−1 are dominant. It is clearly the part below the (top of the) foehn inversion, where most of the action takes place. Let us take a look at the evolution of the foehn inversion in flow direction from south to north. The upper part - represented by the 300K isentrope - descends by about 400m into the middle of the Wipp Valley, superimposed by small amplitude waves. When approaching the chain north of Innsbruck (”Nordkette”), the air is lifted by about the same amount. The lower part on the other hand - look at the 291 and 292K isentrope - descends all the way to the valley ground. And even the air starting from 2700m MSL descends by 400m. Thus a thinning of the foehn inversion in flow direction is observed. The dry foehn air mass can be identified by a adiabatic temperature gradient or constant potential temperature with height. Thus the lower Wipp Valley and the Inn Valley clearly experience foehn. Note for example the constant Θ up to the foehn inversion in the mixed region behind the strongest wave at 47.14 ◦N. The radiosonde launched at Innsbruck also shows a dry adiabatic temperature gradient up to 2000m MSL.

In-situ measurements of the vertical motion and potential temperature along the two lowest flight levels as shown in Fig. 3.6 shed light on some details. Both levels were flown twice from south to north, so we can distinguish stationary features such as standing waves downstream of a ridge from features which are embedded in and advected by the flow. Flight tracks are depicted in Fig. 3.3. Generally a good correlation between downdrafts bringing warmer air from aloft and vice versa is observed. Since the aircraft flew closer to the eastern side of the valley, the sequence of up- and downdrafts is mainly controlled by the ridges perpendicular to the flow on the eastern side. These waves triggered by orography are found at the same locations during the first and second flight at 3000m MSL (cf. Fig. 3.6 top). The potential temperature sequence is marked by a few cold air spikes connected to updrafts. Note that this level is close to the inversion top with strong gradients below that flight level. Therefore air from 100m below is about 2K potentially colder. But all in all no significant temperature change from south to north was found at that level. The situation in 2400m MSL is more complex. North of the Brenner pass descending 3.3. THE 28 FEBRUARY 2000 FOEHN 49

Figure 3.6: Vertical motion ( m s−1), potential temperature (K) and corresponding height of airplane (km ASL) above the Wipp Valley on 28 Feb 2000 for two legs at flight level (FL) 100 (top) and two legs at FL80 (bottom). CHAPTER 3. CASE STUDIES OF FOEHN FLOW IN A T-SHAPED VALLEY 50 SYSTEM motions are dominant. Warmer air from aloft leads to a higher potential temperature level compared to the upwind side. All in all the temperature rise from Brenner to Innsbruck at 2400m MSL is 2 ◦C. In the middle of the Wipp Valley at 47.10 ◦N the strongest downdrafts reaching 5 m s−1 were recorded during the first flight (cf. Fig. 3.6 bottom left). Immediately downwind strong updrafts follow. Strong turbulence was observed while flying through this region, which is all in all inducive for wave breaking. Three kilometres downwind during the first flight the correlation w − Θ is positive. In contrast to the typical relation warm air is advected by ascending motion. The horizontal wind speed in this region is reduced by 10 m s−1 and the wind direction does not reflect channeled flow but is quite variable. It is hypothesized that this marks the mixed zone downwind of breaking waves dominated by small-scale turbulence. During the second flight at the same level 25 minutes later the same sequence of downward-upward motion was recorded, although with smaller amplitudes (cf. Fig. 3.6 bottom right). The region of maximum potential temperatures was seemingly advected northward and is now located near the Wipp Valley exit at 47.16 ◦N. If it is the same feature as before, it was moving downstream with a speed of 2.65 m s−1. Less is known about the atmospheric conditions below 2400m MSL. The upwind profile measured by the radiosonde launched at Sterzing reveals the blocking effect of the main Alpine crest below 1500m MSL. The wind speed is reduced to below 5 m s−1. The temperature profile is nearly dry adiabatic below the foehn inversion. The conditions at the entrance to the Inn Valley and the role of the ridge north of Innsbruck (Nordkette) are discussed in the following section.

3.3.4 The Inn Valley

The most prominent features of the foehn flow encountering the crest-parallel Inn Valley are described in this section. The emphasis is put on the flow out of the crest-perpendicular Wipp Valley encountering the Nordkette and the region downstream (north) of ridge A (cf. Fig. 3.1). As shown in Fig. 3.2, three 30 km legs at heights of 2200, 1900 and 1550 m MSL were flown in the Inn Valley around Innsbruck. The most remarkable feature is a pronounced warm foehn branch downstream of ridge A. The two lower flight legs showed potential temperatures being 2 K warmer than at the exit of the Wipp Valley (not shown). During strong foehn conditions a rotor cloud is sighted regularly in that position, since the foehn flow rushes down towards the Inn Valley and the wave may overturn. Even the moderate foehn conditions on 28 February produced strong turbulent motions after passing the ridge south of the Inn Valley at about crest level. Strong downdrafts characterised that region with peak amplitudes reaching 6 m s−1. Downstream 3.3. THE 28 FEBRUARY 2000 FOEHN 51 of the Patscherkofel (PAK) slope strong updrafts with more than 6 m s−1 were found in all three levels. This indicates the foehn air coming out of the Wipp Valley, which impinges the slope and is lifted. Towards the upper Inn Valley west of Innsbruck vertical motions were smoother (±1 m s−1).

(a) (b) 36' 36'

30' 30'

24' WETTERSTEIN 24' WETTERSTEIN

NORDKETTE NORDKETTE

18' 18' Zillertal Zillertal 12' 12' Oetztal Oetztal

6' Wipp Valley 6' Wipp Valley

STUBAIER STUBAIER ZILLERTALER ZILLERTALER

47oN 47oN 45' 11oE 15' 30' 45' 12oE 45' 11oE 15' 30' 45' 12oE 10 m/s 10 m/s

30' 30'

24' WETTERSTEIN 24' WETTERSTEIN

NORDKETTE NORDKETTE

18' 18' Zillertal Zillertal 12' 12' Oetztal Oetztal

6' Wipp Valley 6' Wipp Valley

STUBAIER STUBAIER ZILLERTALER ZILLERTALER

47oN 47oN 45' 11oE 15' 30' 45' 12oE 45' 11oE 15' 30' 45' 12oE 10 m/s 10 m/s

Figure 3.7: Air ﬂow as measured by the Falcon on 7 Nov 2000 between 1355 and 1531 UTC at (a) 3050 m MSL; (b) 2450 m MSL; (c) 1800 m MSL and (d) 1500 m MSL. Underlying topography isolines are 700, 1300, 2000 and 2700 m MSL; mountain chains in capital letters, important valleys in lower case.

Below 2000 m MSL diffluence due to the mountain chain north of Innsbruck (Nordkette) was present (not shown). While the stronger east branch with westerly flow rushed through the lower Inn Valley into the Alpine foreland, the wind turns to southeast west of Innsbruck towards the upper Inn Valley. This flow does generally not become very strong, since there is only one minor outlet of the upper Inn Valley towards the Alpine foreland (between Wetterstein and Nordkette, cf. Fig. 3.7). Most of the air coming out of the Wipp Valley up to 1200 m MSL, which is deflected into the upper Inn Valley, will not be able to flow through this outlet. It hits a more or less stagnant layer and is slowed down. The foehn flow in the Inn Valley is studied in more detail in section 3.4 using Falcon measurements on 7 November 2000. CHAPTER 3. CASE STUDIES OF FOEHN FLOW IN A T-SHAPED VALLEY 52 SYSTEM

3.4 The 7 November 2000 foehn

On 7 November 2000 the foehn ﬂow was moderate to weak without a marked inversion. A closer look was taken at the oncoming south ﬂow towards the Inn Valley.

3.4.1 Flight pattern

Take-off time in Oberpfaffenhofen was 1208 UTC. Since the main focus was on the variation of the flow along and across the Inn Valley, only three upper north-south legs between the Alpine foreland and Bozen at 5900, 5000 and 4500 m MSL were flown (see Fig. 3.8).

The latter leg was extended to Verona 120 km south of Bozen to make a missed approach to get the upstream thermodynamic and wind profile. After flying back to Innsbruck four Inn Valley legs were performed. Except for the highest leg at 3050 m MSL, which was flown straight from A to B all other three legs at 2450 m, 1800 m and 1500 m MSL had the following flight pattern: from A to D, back to C and on to B. The inner loop in the Innsbruck area was performed to get some information on the cross-valley variations flying a northern, midvalley and southern section. After the Inn Valley turns, the airplane returned to Oberpfaffenhofen landing at 1548 UTC.

3.4.2 Foehn characteristics

Similar to section 3.3.2 the strength of the oncoming flow, pressure gradient across the Alps and the cloud cover are inspected. A broad trough over southern Europe brought south-westerly flow for the European Alps on 6 and 7 November 2000. The six-hourly rawinsonde data at Milano at 700hPa in table B.4 show a strong south wind component of 12.8 m s−1 12 hours before the flight.

Time (UTC) v700 ( m s−1) ∆P (hPa) 00 12.8 3.2 06 7.9 2.1 12 7.5 4.3 18 4.9 3.2

Table B.4: Wind at 700hPa from rawinsonde data at Milano and reduced pressure diﬀerence between Bolzano and Innsbruck on 7 Nov 2000.

During the day, the ﬂow weakens. Wind direction and strength in the early 3.4. THE 7 NOVEMBER 2000 FOEHN 53

40'

20' C D A −

47oN − 40'

40' 11oE 20' 40' 12oE

Figure 3.8: Simplified flight track on 7 Nov 2000, marked are the most western (A) and eastern (B) points reached in the Inn Valley as well as the turning points for the Innsbruck legs (C,D). The north-south legs were flown at heights of 5900, 5000 and 4500 m MSL. Inn Valley turns at 3050, 2450, 1800 and 1500 m MSL. Isolines are 600, 1300, 2000 and 2700 m MSL from a 1 km GLOBE topography set. CHAPTER 3. CASE STUDIES OF FOEHN FLOW IN A T-SHAPED VALLEY 54 SYSTEM afternoon are very similar to our first case study. In the evening the oncoming flow is a bit more westerly and the south wind component decreases again.

VER IBK

5000

4000

3000 Height (m MSL) 2000

1000 VER IBK

286 288 290 292 294 296 298 300 302 304 306 308 310 Wind (kt) Potential Temperature (K)

Figure 3.9: Vertical proﬁles of potential temperature in (K) and wind from Falcon ﬂight data after missed approach towards Verona, 1334 - 1340 UTC, and the Innsbruck sounding at 1122 UTC on 7 Nov 2000. For position of stations see Fig. 3.1.

Figure 3.9 displays the upstream profile measured from a missed approach towards Verona 1334 – 1340 UTC and the 1122 UTC sounding from Innsbruck. In Verona above a 300 m thick cold pool with easterly winds –which is typical for the Po valley during foehn events– south west winds gradually increased from 4 to 8 m s−1 in the lower troposphere. Below the two isothermal layers at 2200 m and just above 4000 m the strongest winds reached 8 m s−1 from WSW with little directional shear. The lower moist layer at about 2000 m MSL (not shown) represented the cumuli fracti at the southern edge of the foehn wall. In the Inn Valley, the nocturnal cold air was flushed out towards the Alpine foreland by westerly winds. The radiosonde reveals a shallow cold air layer with westerly winds at 1122 UTC. Above that shallow layer the foehn was clearly visible in the southeast winds reaching 8 m s−1 already at that early foehn stage. At the time of the approach to Verona (1340 UTC) the cold air pool in Innsbruck had already been removed by the warmer foehn air. The radiosounding station is at the western edge of Innsbruck and therefore in the western branch of the deflection at the Nordkette giving the wind an easterly component. Above 3000m MSL the wind turns gradually to WSW and strengthens with height. Assuming dry adiabatic descent and that the Verona profile is representative for upstream conditions, then below 3000m MSL the 3.4. THE 7 NOVEMBER 2000 FOEHN 55 foehn air descended from Verona to Innsbruck by about 100 to 400 m. Above 3000 m MSL the Verona profile is potentially warmer than the Innsbruck one.

3.4.3 Flow in and above the Inn Valley

The analysis of the second flight concentrates on the modification of the foehn flow by the Inn Valley around Innsbruck and the surrounding mountain chains. After a discussion of the stationary assumption the horizontal flow field and its south-north variation in the Inn Valley are examined. This analysis is then combined with the vertical velocity field along the flight track. The time evolution of relevant meteorological parameters at the mountain station Sattelberg during the analysis time is shown in table B.5.

Parameter Flight in Inn Valley Whole Flight 1355 – 1531 UTC 1208 UTC – 1548 UTC Temperature (◦C) −3.3 ± 0.15 −3.4 to −2.9 slightly decreasing decreasing Wind direction (◦) 170 ± 15 170 ± 15 no trend no trend 1-min Wind speed ( m s−1) 14 ± 3 16 ± 5 no trend peaking between 1256 – 1324 UTC Mixing ratio ( g kg−1) 3.88 ± 0.05 4.03 to 3.83 slightly decreasing decreasing

Table B.5: Changes of meteorological parameters at the Station Sattelberg (2110 m MSL) during the Falcon ﬂight on 7 Nov 2000. The given range marks maximum deviation from the mean value.

Data are checked, if a quasistationary snap-shot interpretation of the flow is justified. While no significant change in the wind direction or humidity was recorded at Sattelberg during the Inn Valley flight period from 1355 to 1531 UTC, the station pressure increased by 1.1 hPa and the temperature dropped slightly from −3.2 to −3.4 ◦C. This means that the potential temperature altogether decreased by 0.4 K, which had to be considered when looking at the whole Inn Valley flight. The change of 0.1 K for an individual leg could be neglected. Now let us take a look at the topographic flow modification in the lower troposphere. It is characterized by low level jets through gaps and valleys impinging the Inn Valley and the following mountain barrier, which blocks and deflects the foehn flow. Above crest height the topographic influence vanishes and the flow field is much smoother. CHAPTER 3. CASE STUDIES OF FOEHN FLOW IN A T-SHAPED VALLEY 56 SYSTEM

Figure 3.7 shows arrows representing the horizontal flow field at four different heights from top to bottom. At 3050m MSL the flow along the Inn Valley (see Fig. 3.7a) was quite uniformly SW with wind speeds of 10 to 15 m s−1. Still the direction varied by about ± 30 degrees due to flow around individual mountain massifs north of the Inn Valley that protrude above that height. 600 m lower in Fig. 3.7b the influence of the orography was much more pronounced. The flow in the Innsbruck airport region just west of the Wipp Valley exit was deflected to the west and slowed down. This deflection was not dominant on the eastern side of the Wipp Valley exit, where southerly wind of more than 10 m s−1 prevailed. Note the outflows from two other major south-north oriented valleys, Otz¨ and Ziller Valley. The strong jet from the Wipp Valley at 1800m MSL shown in Fig. 3.7c was again deflected up-valley west of Innsbruck with even some northerly component. East of 11.5 ◦E the deflection was much weaker. Here the flow seemed to be influenced by several gaps south of the Inn Valley, because the south wind component with a lot of variety in direction and speed was dominant. In that area 1800 m was the lowest level, to which the southerly flow penetrated. The wind at the eastern exit region of the Ziller Valley turned from W to SW giving evidence of foehn air coming out of that valley. Weak divergence at the exit of the Otz¨ Valley showed that some air also flowed out of there, but wind speeds were only about 3 m s−1. In the lowest leg at about 1500 m MSL, the strongest wind was the south foehn coming out of the Wipp Valley with wind speeds of about 15 m s−1 (see Fig. 3.7d). The upvalley flow west of Innsbruck was much weaker, due to blocked air or even weak valley outflow in the upper Inn Valley. Part of the Inn Valley flow was heading towards the Alpine foreland through the gap (max. elevation 1200m MSL) between Wetterstein and Nordkette. The eastern branch of the divergence caused by the Nordkette was well established at that height. The turning of the wind direction to SW at the eastern exit region of the Ziller Valley was evident as in the level above. The vertical velocity above the Inn Valley completes the flow analysis. It reveals, where significant upward or downward motions are forced by topographic features or wave dynamics. A comprised picture is given in figure 3.10, which is best looked at from bottom to top. It contains the topography south of the Inn Valley, three panels of the vertical velocity at 1500, 1800 and 2450m MSL in the southern, middle and northern Inn Valley and the topography north of the Inn Valley. The dashed lines represent the three different levels and the vertical velocity is the deviation from this zero line. The scale of 5 m s−1 is given in panel SOUTH on the right hand side.

For the interpretation of the vertical motion above the Inn Valley we have to 3.4. THE 7 NOVEMBER 2000 FOEHN 57

km 3

1 3

2 NORTH

1 3

2 MIDDLE

1 3

2 5 m/s SOUTH

1 3 PAK

2 IBK

1 11.25 11.30 11.35 11.40 11.45 11.50 11.55 11.60 11.65 11.70 11.75 Lon (deg E)

Figure 3.10: Vertical velocity from 100 Hz data of Falcon flight on 7 Nov 2000 at three different heights above the northern, middle and southern Inn Valley region. Top and bottom: highest orography within 0.1 degree latitude bands north and south of the Inn Valley from a 100 m topography and in dashed lines the heights of the three flight legs (2450, 1800 and 1500 m MSL); Locations of Innsbruck (IBK) and the mountain station Patscherkofel (PAK) are depicted. CHAPTER 3. CASE STUDIES OF FOEHN FLOW IN A T-SHAPED VALLEY 58 SYSTEM keep the horizontal flow speed shown in Fig. 3.7a – d in mind. At the two lowest flight legs at 1500 m and 1800 m MSL channeled flow at the outlet of the Wipp Valley dominated. It produced strong updrafts in the lowest flight level when the flow impinged upon the Nordkette (panel NORTH in Fig. 3.10). The wave activity downstream of ridge C between 11.45 and 11.51 ◦E did not reach the lowest level, but at 1800 m MSL downdrafts on the order of a few m s−1 were measured in the southern Inn Valley followed by similarly strong updrafts in the middle of the valley. The wave pattern is still present at 2450 m MSL, although the amplitude is reduced. Clearly the region between 11.35 and 11.60 ◦E was a ”hot spot” for the foehn action. The reason is the location directly north of the main foehn gap at the Alpine crest between the Stubaier and Zillertaler Alps (cf. Fig. 3.1). A more pronounced west component in the oncoming foehn flow might shift this region to the east. West and east of the Innsbruck area the south component of the foehn flow was small due to mostly (redirected) along valley flow (cf. Fig. 3.7). Therefore the vertical velocity field was much smoother. At 3050 m MSL, where the flow did not encounter any obstacles, the vertical motion decayed (not shown).

3.5 Discussion and Concluding Remarks

The intention of this chapter is to compare the findings of this case study to typical foehn conditions. Since the flow below crest height is highly controlled by the topography, the same features are found time and again with variations depending on flow speed, direction and stability of the atmosphere downstream of the main Alpine crest. A lot of local foehn experience has been gained in Innsbruck over the last one and a half centuries, not much of which has entered the peer-reviewed literature. So some of the arguments will have a lack of reference, but the named phenomenon has non the less been frequently observed by local forecasters and researchers. The higher pressure south of the Alps as driving force for the foehn flow is primarily restricted to below crest height. The typical thermodynamic upstream profile with nearly stagnant cold air pool below was already pointed out by Pümpel (1978) and Steinacker (1983). Seibert (1985) analysed the period 1978 – 1982 and found the strongest barometric mean temperature difference between Munich and Mi- lano/Udine in the layer 850 to 700 hPa rather than below 850hPa. In comparison, on 28 February 2000 slightly potentially colder air at the ground was present (cf. Fig. 3.3). The flow reached 5 m s−1. The largest impact for the pressure difference across the main Alpine ridge came from a height of 2500 to 3000m MSL and not from the near-surface layer. Therefore the upstream cold air pool at the ground was not so important compared to the air mass difference due to the lowering of 3.5. DISCUSSION AND CONCLUDING REMARKS 59 the inversion height north of the Brenner pass on that day. In all the observations so far the wind speed maximum between Brenner and Innsbruck has been located in the lower Wipp Valley. This feature was also observed in the Lidar scans during the MAP-SOP for various foehn periods (Gohm and Mayr 2004; Gohm et al. 2003; Flamant et al. 2002). Potentially warmer air is accelerated when mixed downward, thereby enhancing the gustiness as well.

Figure 3.11: Sketch of the foehn ﬂow at the junction Wipp - Inn Valley.

The flow situation at the junction of the Wipp and Inn Valley is sketched in figure 3.11. It reflects the measurements made on 7 November 2000 as well as earlier observations, especially during the MAP-SOP. The dark grey arrows represent flow along the valley floor, the medium grey arrows the flow just below crest height and light grey arrows the flow originating higher than 2000m MSL. The flow out of the Stubai Valley, the major side valley of the Wipp Valley, was not measured by the Falcon and is therefore tagged by a question mark. The strong CHAPTER 3. CASE STUDIES OF FOEHN FLOW IN A T-SHAPED VALLEY 60 SYSTEM low-level jet out of the Wipp Valley impinges on the Nordkette, the air is slowed down and a local pressure high is produced (Seibert 1985). Below crest height a divergence in low levels and upward motion at the foothills of the Nordkette due to the forced overflow are present. The divergence and upward motion at the Nordkette were measured on 7 November as well. They are evident in Fig. 3.7d (divergence) and Fig. 3.10 (upward motion). It is hypothesised that the vertical splitting of theta surfaces on 28 February 2000 is also connected to the forced ascent of part of the air over the Nordkette. While the upper part is the remainder of the weakened inversion already present at the main Alpine crest, the lower part is formed by reduction of the flow thickness when passing the top of the Nordketten ridge. The foehn branch in the eastern part of the Wipp Valley passes some narrow and shallow west-east oriented side valleys with undular motion. It can sometimes be observed by watching small cumulus clouds at about 2000m MSL on their way towards the Patscherkofel. At a height of 1800m MSL southerly flow through gaps in the massif east of Innsbruck (ridge A) is evident. This was also true on 7 Novem- ber, as can be seen in Fig. 3.7c. Further downstream the first wide and deep valley (the Inn Valley) is encountered: the first opportunity for the eastern foehn branch to rush down to the valley floor. Therefore the vertical motions there are very strong (cf. Fig. 3.11). Airplanes starting at the Innsbruck airport towards the east are instructed to avoid that region and rather choose the smooth lifting along the foothills of the Nordkette. On 28 February as well as on 7 November 2000 the turbulent downward and following upward motion were of the order 10 m s−1. The further way of the foehn air from Innsbruck where Wipp and Inn Valley merge towards the Bavarian foreland is typically the following: The part of the flow, which was deflected westwards towards the upper Inn Valley hits a nearly stagnant layer or even westerly winds. Therefore it is slowed down. If the flow is still strong enough – which will be preferably occur at higher levels rather than near the ground– it can proceed through an outlet between the Wetterstein and Nordkette. This was already pointed out by Ficker (1910). On 7 November 2000 this flow was measured by the Falcon at 2400 and 1500 m MSL (cf. Fig. 3.7). It is confirmed by a weather station there. During the first foehn flight on 28 February 2000 the same weather station reported southerly winds towards the Alpine foreland as well. Another branch of the Wipp Valley flow is deflected eastwards into the lower Inn Valley. This westerly wind can be topped by south winds through narrow south- north oriented side valleys and gaps. During both flights in the Inn Valley none of these tiny foehn branches reached the lowest flight level (1500m MSL). A considerably stronger mass flux can be expected through the broad south-north oriented Otz¨ and Ziller Valleys. On 7 November 2000 the signal of the foehn flow 3.5. DISCUSSION AND CONCLUDING REMARKS 61 was found at the exit of both these major valleys. Outflow from the Otz¨ Valley was only observed down to 1800 m MSL. At the Ziller Valley exit the flow in 2400 m MSL was even stronger than from the Wipp Valley, but weak below 2000 m MSL. To conclude, let us try to explain the main flow features analysed in this paper in the hydraulic framework. In the last few years it was realized, that the main features of the low-troposphere foehn part can be described by shallow water theory (Flamant et al. 2002; Mayr et al. 2004; Mayr et al. 2003; Gohm and Mayr 2004). This simplified theory basically treats the lowest foehn flow controlled by topographic features similar to water in a river bed. The main benefit of the shallow water theory is that the flow is analyzed for virtual controls. These separate regions, where the flow speed is less than the wave speed (subcritical) from those, where it is higher (supercritical). In subcritical flow waves (and information) can propagate both up- and downstream. In contrast waves in supercritical mode can only propagate downstream and thus the flow does not respond to changes taking place downstream. The transition from super- to subcritical flow takes place through a hydraulic jump. In the atmosphere this jump corresponds to a wave breaking region associated with strong turbulence. Thinking in one-layer hydraulics, the upstream flow in the basin of Sterzing is subcritical. The top of the layer is approximately given by the height of the foehn inversion. The reservoir height south of the Alpine crest is higher than the one north of it (cf. the 292 K isentrope in Fig. 3.3). The resulting south flow compensates for the height difference. Part of the flow between the Stubaier and Zillertaler Alps descends into the Wipp Valley. Thereby it is channelled and accelerated. A transition from sub- to supercritical flow takes place, where and if the Froude-number – the ratio of the fluid velocity to the speed of propagation of linear shallow-water gravity waves– exceeds one. By rushing down into the Wipp Valley north of Brenner the top of the channelled flowing layer will be below the level at the Nordkette. There- fore the isentropic surfaces will have to rise eventually from south to north between say the middle of the Wipp Valley and the foothills of the Nordkette. Through a hydraulic jump kinetic energy of the flow is converted into turbulent motion and a well mixed layer with lower horizontal flow speeds is formed. This retransition from supercritical to subcritical flow takes place on 28 February 2000 as shown in the cross-sectional analysis in figure 3.3.

Acknowledgments

This STAAARTE (Scientiﬁc Training and Access to Aircraft Throughout Europe) project was funded by the European Commission through the TMR (Training and Mobility of Researchers) programme. The co-ordination work by Dr. A. Giez and CHAPTER 3. CASE STUDIES OF FOEHN FLOW IN A T-SHAPED VALLEY 62 SYSTEM

Dr. M. Krautstrunk from DLR Oberpfaffenhofen was appreciated a lot. We would also like to thank the Flight Facility Oberpfaffenhofen, who did a great job when flying within the Wipp and Inn Valley. The technical support of Mr. H. Wendt from the University of Munich concerning the installation of the mobile sounding system in Sterzing is greatly acknowledged. Johannes Vergeiner was supported by FWF 13655 and FWF15077, Alexander Gohm by FWF 13489 TEC. Chapter 4

An objective foehn classiﬁcation scheme

An objective foehn classiﬁcation scheme - General concept and application to the Wipp Valley 1 Johannes Vergeiner2 , Georg J. Mayr Department of Meteorology and Geophysics, University of Innsbruck, Austria

Summary

In the frame work of the Mesoscale Alpine Programme (MAP), foehn periods had to be identified objectively for the stations in the target area Brenner. Since a subjective classification using abrupt temperature and humidity changes is not satisfactory, the classification was based on the lapse rate between a suitable mountain reference and the valley station as well as the wind direction at both stations. This new approach utilises the physical foehn mechanism on the lee side and is therefore thought to be applicable to foehn winds around the world with local adaptations. The foehn direction at the valley station has to be determined independently and the shape of the topography upstream has to be taken into account. For the target area Brenner it is shown how to determine across which part of the topography the flow went in the presence of differently deep incisions at the crest. The ability of the scheme to exclude valley winds with similar flow characteristics and capture an interrupted foehn sequence is demonstrated exemplary. Statistics for all stations within the 70 day Special Observing Period of

MAP in 1999 are given as well.

KEYWORDS: foehn classiﬁcation, MAP, foehn, gap ﬂow 4.1. INTRODUCTION 65

4.1 Introduction

During the international Mesoscale Alpine Programme (MAP) (Bougeault et al. 2001) in 1999 a dense network of surface stations was installed in the Brenner target area (Mayr et al. 2004). As a basis for further examinations foehn periods during the Special Observing Period (SOP) had to be determined for all the stations north of the Alpine crest. The problem of finding a general acceptable foehn definition has been known for a long time. Frey (1957) emphasized the necessity of standardizing the foehn definition to allow for the comparison of results. Brinkmann (1971) discussed the most important approaches and concluded that the variations in location and season and possible transition forms make it difficult to find an acceptable definition. Consequently, a number of unsatisfactory definitions are present in the literature. A selection and short discussion of proposed foehn criteria is given in 4.2. The most common approach for the foehn identification is the 3-criteria definition by Conrad (1936), Osmond (1941) and Obenland (1956). According to this definition foehn is characterized by abrupt temperature and humidity changes in opposite direction and a surface wind from the direction of the mountain range. This method is based on a phenomenological rather than a physical description of foehn. Unfortunately, the analysis is not only tedious, but also dependent on the subjective rating of the analyst and open to misjudgment. As a consequence a generic foehn definition was needed. Schuetz and Steinhauser (1955) laid the foundation by including the criterion of a dry adiabatic lapse rate between a mountain and valley station due to the descent of foehn air. Unfortunately, they also included the not conclusive criterion of ascending temperature and descending humidity together with relative humidity being less than an arbitrary threshold of 70 %. For the Objective Foehn Classification (OFC) the two criteria which are a direct consequence of the foehn mechanism are used: first a dry adiabatic lapse rate between a suitable mountain reference and the valley station; and secondly a wind criterion, which is dependent on the location of the valley station but basically wind from the mountain range. The algorithm is easily applied to any data set, once the foehn wind direction and the location relative to the reference station are known. It considers the properties at the origin of the air mass and the mandatory physical process in the lee of the mountain range. The results are comprehensible, reproducible and consistent. In principal the general approach makes this scheme applicable to all sorts of foehn type winds around the world. This study describes the method as well as the practical application to the above mentioned weather stations during the MAP-SOP. 66 CHAPTER 4. AN OBJECTIVE FOEHN CLASSIFICATION SCHEME

The special stacked double gap profile at the Brenner cross-section suggests a further distinction in foehn flow through the lower gap versus one through the upper gap. Although the separation criterion is dependent on the downstream location of the station, the different air mass properties of these two flows make a distinction reasonable. Further away from the mountain chain, the air mass properties (as well as cloud cover etc.) may reflect the foehn situation without the proper wind. For the Alpine foreland Hoinka (1990) introduced the English term foehnic clearing for this phenomenon. It is not considered a foehn, after all foehn is a wind. Section 4.2 offers a brief discussion of previously used foehn criteria. The connection between foehn and gap flow and their connection are discussed in section 4.3. The general concept of the OFC is presented in section 4.4. Section 4.5 applies the scheme to the Brenner target area. Two examples in section 4.6 illustrate the capability of the OFC to classify foehn for different weather situations and locations. Section 4.7 presents statistical evaluations within the Special Observing Period of MAP at 16 stations. Potential and limits of the new scheme, the application to other mountain areas and an outlook are given in section 4.8.

4.2 Previously proposed foehn criteria

For the purpose of a foehn classification it is sufficient to consider the processes downstream of the crest. Furthermore it is important to note, that with increasing distance from the crest air from successively higher altitudes is brought to the ground during foehn conditions. The originating height of the foehn air can be higher than the crest and therefore potentially warmer (Mayr et al. 2004). A number of approaches have been proposed regarding the foehn classification. Their general applicability for foehn winds around the world are discussed below. As already mentioned, the 3-criteria definition – henceforth called traditional foehn classification – by Conrad (1936), Osmond (1941) and Obenland (1956) is the most common scheme. Foehn is analysed from a phenomenological point of view. The time series of measured parameters at a given location are interpreted. Wind is looked at for direction (including the variance), mean wind speed and gusts. This method is quite straightforward, but it is not sufficient to discriminate foehn flow non-ambiguously from strong thermally driven down-valley wind or outflow from cumulonimbus clouds. Additionally, the temperature and humidity properties at the station are examined. Temperature increase and relative humidity decrease at foehn onset and vice versa for cessation are expected. The theoretical background is the warming of the foehn air during the descent in the lee of a mountain. The relative humidity drop along the way is not due to a decrease of water vapor content, 4.2. PREVIOUSLY PROPOSED FOEHN CRITERIA 67 but is a consequence of the temperature increase of the foehn air. The specific humidity remains constant (unless there is precipitation on the downstream side). The temperature at a downstream station during foehn can be written as

Tstation = Torig + γ · ∆h (4.1)

where Torig is the original temperature at the mountain crest, ∆h the height difference crest to station and γ the effective temperature gradient for the descending air parcel. Only if the atmosphere between station and crest is stably stratified, the warming ∆h·γ during dry adiabatic descent will be sufficient to let the temperature of the foehn air exceed the one of the replaced air mass. Especially during summer days with a deep and well mixed boundary layer and superadiabatic gradients near the ground, foehn may well be cooler than the air mass it replaces. So basically warming and drying of the foehn air due to descent on the lee side of a mountain range are confused with foehn air being warmer and drier than the replaced air mass. Thus it can not be taken as a mandatory criterion. Experienced analysts know about that problem and may classify a foehn based on the wind measurements only, even without the appropriate temperature or humidity signal. But this is at best a tedious job, not reproducible and open for misjudgments. Two example periods discussed in section 4.6 highlight the problem and show the capability of the OFC to properly detect foehn. Seibert (1985) classified foehn in Innsbruck for the period October 1978 until September 1982. He summarized his experience in a flow chart diagram for south foehn in Innsbruck, which is based on the traditional foehn classification, but uses additional information such as wind gusts, wind direction at suitable mountain stations and weather observations like ”no thunderstorm in the surrounding”. Other studies used foehn criteria, which are only valid in the area of interest. Ekhart (1949) found characteristic fluctuations in the direction of mean southerly winds for foehn at Innsbruck. For valleys at right angle to the mountain massif, Ungeheuer (1952) proposed a mean wind direction and a threshold of 2 m s−1.A gustiness factor Vmax/Vmin > 2.0 together with characteristic temperature and humidity changes was used by Schlegel (1952) for the Alpine foreland. Lewis et al. (2004) found that the onset of foehn in the Wipp valley is connected to a sharp drop in the time series of the turbulent momentum flux u0w0. Foehn onset is characterized by a sudden increase in downward momentum flux values, as a faster moving air mass descends to the surface. He compared the flux signatures of foehn to the OFC and found very good agreement. The drawback is that turbulence measurements, e.g. by 3-axis sonic anemometers, are needed which are not (yet) part of the standard meteorological measurement procedure. On the other hand no reference station is needed to classify foehn. 68 CHAPTER 4. AN OBJECTIVE FOEHN CLASSIFICATION SCHEME

Another approach for the evaluation of foehn events is the consideration of surface pressure patterns and the upper air flow. It analyses mesoscale weather features, but is not ideal to decide between foehn or no foehn at a given location. Since in critical situations not only the pressure difference between two points but also the shape of the so called foehn nose is significant, the analysis has to be based on weather charts. Brinkmann (1970) selected chinook wind periods with this approach. He found that the percentage of misclassified cases was too large, primarily due to some non-chinook winds which had similar surface properties as chinook winds but were the result of descending air in an anticyclone. Especially climatological studies tend to use empirical and therefore quite arbitrary limits. For example Nkemdirim (1996) in a climatological study of chinook events in Canada used the foehn criterion, that the temperature must exceed the normal (climatological) daily maximum temperature. As a result there is a bias to strong winter-time events, while weak to moderate or summer events can easily be missed out. Longley (1967) estimated the chinook frequency in Alberta, Canada, with the arbitrary criterion of maximum temperature exceeding 4.4◦C during the winter months. Lockwood (1962) assumed that a foehn wind was blowing if unusually high temperatures for the season were reported in the immediate lee of the British Isles. All these criteria lack a physical basis and give a high percentage of misclassified cases.

4.3 Deﬁnition of foehn and gap ﬂow

To investigate a phenomenon like foehn scientifically, it is important to generalize the properties of individual cases to those of the whole class. For this purpose the class has to be defined accurately. Hence a clear and exact definition is the basis for a foehn wind classification. Besides the foehn terminology a second point of view is introduced where the origin of the foehn flow is attributed to topographic features at the crest (gaps). Both flow types are defined below and their connection in this context is shown in 4.3.3.

4.3.1 Foehn deﬁnition

Some meteorological glossaries define foehn as a warm dry wind on the lee side of a mountain range (American Meteorological Society 1959; Lewis 1991). Since foehn is a downslope wind, it will in general appear as a dry and warm wind due to adiabatic warming; but if the air mass before the descent is particularly cold, downslope winds can also appear as cold winds, because the warming is not sufficient to exceed the temperature of the replaced airmass. E. g. the Dinaric Bora is a relatively cold 4.3. DEFINITION OF FOEHN AND GAP FLOW 69 windstorm, because it is associated with advection of cold air. In contrast to the above mentioned definitions we will base ours on the physical foehn mechanism. Thus we define foehn following the formulation of the World Meteorological Organization (1992): Foehn is a wind warmed and dried by descent, in general on the lee side of a mountain. The driving force is either a cross-mountain synoptic flow or a cross-mountain pressure gradient, but not katabatic effects. In the context of this study, every wind meeting this definition after passing a mountain range is regarded as foehn, no matter what the local name is. This includes chinook in the Rocky Mountains, zonda in Argentine, puelche in the Andes, halny wiatr in Poland, aspre in the Massif Central, the Canterbury northwester of the Newzealand Alps, and many others.

4.3.2 Gap ﬂow deﬁnition

A further refinement occurs by considering the shape of the mountain range which the flow crosses. Incisions at the crest line lead to the concept of gap flows. In this framework gap winds are simply defined as flow through a gap with a vertical and/or lateral contraction.

4.3.3 Connection of foehn and gap ﬂow

For the purpose of this article it is important to understand the connection of foehn and gap flow. After passing a gap with a vertical constriction, the flow following the topography has to descend. Due to the compression warming will take place along the lee slope and the relative humidity will drop as a consequence of the warmer temperature. The airmass is accelerated on the descent and is faster than the adjacent air. Therefore every gap flow through a vertical constriction meets the foehn criteria above and is classified as foehn wind. Whenever the term gap flow is used in the following chapters, it is always assumed that it contains a vertical constriction. Gap flows are consequently a sub-category of foehn. The advantage of this alternative terminology is that an accurate specification of the air mass origin is possible in the presence of two or more stacked gaps. Lower gap flow is essentially foehn through a lower gap, upper gap flow refers to foehn flow through a higher gap in a mountain barrier. 70 CHAPTER 4. AN OBJECTIVE FOEHN CLASSIFICATION SCHEME

4.4 General concept of the objective foehn classi- ﬁcation

The general foehn classification method is described in this chapter. The use of conserved quantities as tracers is discussed in 4.4.1. Section 4.4.2 illustrates the concept for the simplest case of flow over a ridge line or through a single gap. The problems encountered for flow through and over more complex topography is dealt with in section 4.4.3.

4.4.1 Conserved quantities and their application as a tracer

In principal every quantity, which is conserved along a streamline under foehn conditions, can be used as a tracer. The two most suitable quantities are the mixing ratio, the mass of gaseous H2O per unit mass of dry air, and the potential temperature. The mixing ratio is unfortunately highly dependent on an accurate measurement of the relative humidity. A typical measurement error of the latter of 5% changes the mixing ratio by 0.3 to 0.4gkg−1. The relative error between two stations can therefore reach 0.7gkg−1 or 10% of the absolute water content! After an accurate calibration of the humidity the usage of mixing ratio as an (additional) tracer is conceivable and easy to introduce. For the OFC it was not included in order for the method to the usable for networks which do not measure humidity or not accurately enough. If the problems were overcome, possibly more information from this second independent variable could be gained for weather situations where non-adiabatic processes are important (see Appendix A). So let us take a look at the concept of potential temperature to explore its feasibility for the classiﬁcation of the air mass origin. Potential temperature is deﬁned as

Ã ! R 1000 cP Θ = T (4.2) p

where temperature T is in Kelvin, the pressure p in hPa and R and cP being the gas constant for dry air and the speciﬁc heat for constant pressure, respectively. Potential temperature is conserved along streamlines under the assumption of dry adiabatic (=isentropic) conditions, which exclude moist as well as diabatic processes. If the assumptions hold, potential temperature can be used as a tracer for the foehn air downstream of the mountain range.

4.4.2 Wind over a ridge or a single gap

First we want to deal with the more simple ridge ﬂow. The following two criteria have to be fulﬁlled to classify a foehn type wind. 4.4. GENERAL CONCEPT OF THE OBJECTIVE FOEHN CLASSIFICATION 71

Wind criteria

Purpose of the wind criteria is simply to exclude situations, where the air did not flow over the barrier. This requires the appropriate cross-mountain flow direction at a mountain reference station and at the classified station, where it is possibly channelled. In the case of an elongated ridge merging into a plateau, a wind direction at the station away from the mountain could be sufficient. If the flow downstream of the mountain is channelled into a valley the wind direction is usually easily determined by the direction of the valley axis. Of course local topographic features have to be taken into account.

Potential temperature criterion

The wind criterion alone still includes flows that do not cross the ridge. In the case of a downstream valley these are thermally driven nocturnal outflows from the main valley and tributaries (if they exist). Also frontal passages or convective cells can produce strong winds at the surface. Fortunately all these weather features have one thing in common: the potential temperature at the surface does not reach or exceed the values at a reference mountain station. This will only happen, if the pseudovertical profile between the reference station at the ridge and the surface is dry adiabatically mixed. In combination with the wind criterion, which makes sure that the reason is not a well mixed deep boundary layer due to solar heating, the only possibility for this is that the air has descended from the height of the mountain reference station! Consequently the second compulsory condition of the OFC is that the potential temperature at the classified station is equal or higher than the one at the mountain reference station.

4.4.3 Wind through and over complex topography

Number and location of reference stations needed for the classification depends on the shape and height of the crest. One representative station for each gap/ridgeline is needed. They should be positioned at the lower edge of the gap to represent the flow through it. The formulation for flow over a double gap is given in the following two subsections. Both the wind and the potential temperature criteria have to be fulfilled. The potential temperature criterion discriminates uniquely in no foehn, possible foehn over the lower gap (= lower gap flow) and possible foehn over the upper gap (=upper gap flow) for any given time. All possible foehn periods have to meet the wind criteria at the classified station and the ”proper” reference (i.e. the station in the lower/upper gap for lower/upper gap flow) as well. The application to the Brenner target area will be discussed in 4.5. An expansion to even more complex topography is straightforward and follows the same methodology. 72 CHAPTER 4. AN OBJECTIVE FOEHN CLASSIFICATION SCHEME

Wind criteria

A simple but effective formulation for the wind criteria is: For foehn flow the wind direction at both the reference and the downstream valley station has to be in a sector of ±45◦ from the estimated cross-barrier foehn wind direction. An additional minimum wind speed threshold at the reference station and/or the classified station may be introduced. The threshold serves to exclude light wind periods. After all, foehn is a wind. It has to be adapted to the specific location and should not exceed 2 m s−1. If, as is often the case, the gap merges with a downstream valley, the scheme has to be able to separate valley wind regimes from cross-mountain flows. The wind direction constraints serve to exclude thermally and dynamically driven up-valley flows as well as slope winds. If the wind at the station is caused by outflow from smaller side valleys or katabatic winds that have been redirected to along-valley flows, the wind criterion at the reference will not be fulfilled. The most difficult task is clearly to distinguish between thermally driven down-valley flow and foehn. Temporal evolution from one to the other is quite typical. Nocturnal outflow can evolve into gap flow at the beginning of a foehn period. This transition is typically gradual. The traditional foehn analyst is often forced to set the foehn starting date quite arbitrarily. The wind criteria above help to exclude down-valley flows without the proper wind at the reference in the gap. But additionally the potential temperature criterion (see section 4.4.3) ascertains that the origin of the air mass is at the gap. Only the combination of both constraints allows for the discrimination of foehn and thermally driven down-valley flow. An example that valley wind regimes in a high pressure regime are not classified as gap flow is given in section 4.5.2.

Potential temperature criterion

In contrast to the case of one ridgeline or gap only, not only a distinction foehn or no foehn is possible, but also which kind of foehn, i.e. upper or lower gap ﬂow. Again it is rather simple to exclude periods without foehn. The reference for this purpose is the station in the lower gap. No foehn is present, whenever

Θstation < Θlower gap. It is more difficult to distinguish from which gap the foehn air is coming. If Θstation > Θupper gap , the air must come from the upper gap. But inspection of the MAP data showed that this limit is too strict. Downstream of the crest air from the lower gap is partly mixed into the upper gap flow. This results in a lower potential temperature compared to the upper gap, even though wind speed and gustiness still show dominant upper gap flow characteristics. Therefore analysis of measured potential temperatures during foehn periods at each station is proposed. Let us define a normalized potential temperature difference, 4.4. GENERAL CONCEPT OF THE OBJECTIVE FOEHN CLASSIFICATION 73

Θ − Θ ∆Θ∗ = station lower gap (4.3) Θupper gap − Θlower gap ∆Θ∗ is zero, if the potential temperature of the station equals the one at the reference station in the lower gap. Whenever the potential temperature is as high as the one at the reference in the upper gap, ∆Θ∗ is one. The two gap flows should have different temperature characteristics representing the different height of origin at the crest. This difference should result in a dual mode in the frequency spectrum of ∆Θ∗ during foehn periods.

4 t i r 3 * c ∆Θ

Relative frequency (%) 1

0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 ∆Θ*

Figure 4.1: Sketch of dual mode of frequency distribution during foehn and choice of ∗ ∆Θcrit. The maximum to the left(right) represents the colder lower (warmer upper) gap ﬂow.

If such a dual mode exists, the minimum in the frequency spectrum in between separates the two ﬂows as sketched in ﬁgure 4.1. The critical value is then given by

Θcrit = Θlower gap + ∆Θcrit (Θupper gap − Θlower gap) (4.4)

Upper gap ﬂow is possible whenever Θstation ≥ Θcrit, lower gap ﬂow whenever

Θlower gap < Θstation < Θcrit. If additionally the wind criteria as described in 4.4.3 are met, a foehn period is recognised. The travel time of air from reference to clas- siﬁed station (O(1h)) is neglected. The application of the OFC to the stations in the Brenner target area is described in chapter 4.5. 74 CHAPTER 4. AN OBJECTIVE FOEHN CLASSIFICATION SCHEME

4.5 Application of the OFC to gap ﬂow in the Brenner target area

4.5.1 Weather stations and sensor accuracies

Most stations were only in use during the Special Observing Period of the Mesoscale Alpine Programme (MAP-SOP) between 7 September and 16 November 1999. Only the reference stations REFL(Sattelberg) and REFU(Brenner), 10(Ellboegen) and 16(Innsbruck) are installed permanently (see ﬁgure 4.2 and table B.1). Note that the valley station abbreviations are increasing downstream. Therefore the number is an indicator for the distance to the crest. Alternatively, for readers with local knowledge the name of the location is given in brackets.

Figure 4.2: Topography of the Alps with close-up of the Brenner target area. For details concerning the weather stations see table B.1. Isolines of topography from GLOBE data set every 400m between 600 and 2600m MSL.

The data from ﬁve diﬀerent station types were carefully postprocessed. To get a optimal data set the pressure, temperature and humidity sensors were calibrated against standard references before the MAP-SOP (Pippan 2000). Additionally, com- 4.5. APPLICATION OF THE OFC TO GAP FLOW IN THE BRENNER TARGET AREA 75

Short name Station name Latitude (degN) Longitude (degE) Height m (EGM96) REFL Brenner 47.0072 11.5081 1372 REFU Sattelberg 47.0111 11.4795 2108 01 Lueg 47.0278 11.4932 1224 02 Gries 47.0369 11.4736 1278 03 Staﬄach 47.0658 11.4872 1100 04 Zagl 47.0697 11.4812 1287 05 Steinach 47.0909 11.4729 1116 06 Tienzens 47.1132 11.4600 1119 07 Gedeir 47.1593 11.4531 1084 08 Matreiwald 47.1714 11.4311 936 09 Gleins 47.1723 11.4042 1443 10 Ellboegen 47.1875 11.4294 1080 11 Schoenberg 47.1901 11.4049 988 12 Patsch 47.2062 11.4060 910 13 Zenzenhof 47.2268 11.3896 715 14 Lans 47.2376 11.4383 891 15 Natters 47.2384 11.3714 811 16 Innsbruck 47.2643 11.3852 611

Table B.1: Weather stations used for applying the foehn detection algorithm. REFL denotes the reference station in the lower gap, REFU the reference in the upper gap. The station abbreviation is increasing downstream. parisons with all instruments at the same site and from the field data were carried out. For the use of potential temperature as a tracer, the relevant total accuracies according to manufacturer specifications are typically ±1hP a for the pressure in the temperature range 0 to +40◦C and ±0.2K for the temperature. However, the absolute accuracies are considerably improved due to the calibration against a standard reference. Note that potential temperature is more sensitive to temperature than to pressure changes. A temperature difference of 1K will change the potential temperature by about the same amount, but a change of 1hP a will only result in a Θ change of 0.1K.

4.5.2 The wind criterion for foehn in the Wipp valley

Let us take a look at the Alpine crestline in the Brenner target area (4.2 to make sure that the OFC can be applied to the MAP-SOP data.

Figure 4.3 shows the west-east cross-section and the location of the reference stations at the crest. Simplified, the crestline can be seen as a stacked double gap, the eastern crest line being a bit higher. The flow through the well defined deep insection 76 CHAPTER 4. AN OBJECTIVE FOEHN CLASSIFICATION SCHEME at x = 0 is represented by REFL (Brenner) in the lower gap. As soon as the air flows over the higher ridge between x = −9 and −2km it is represented by REFU (Sattelberg). Sometimes in southeasterly flow the air passes the approximately 2700m high ridge east of Brenner between x = 2 and 6km. For simplicity it is also counted as upper gap flow, although a further distinction with a third reference station is possible and follows the same method as described in section 4.4.3. To apply the wind criteria of 4.4.3, the foehn direction at each station including the two references have to be determined in advance. Channeling effects of the valley have to be taken into account in defining the local direction of the foehn flow. The orientation of the Wipp valley axis was estimated from geographic maps of the orientation at each valley location to get the foehn directions of the downstream stations. Additionally the flow directions for all stations - including the references at the crest- during known gap flow periods were determined. As expected the results match very closely. For most stations the foehn direction is well defined in a sector of ±20◦ standard deviation except for Natters and Lans near the exit to the Inn valley, where the standard deviation is close to 30◦. The minimum wind speed for foehn at the reference and at the classified downstream stations was set to 1 m s−1 to exclude light wind situations.

3500

3000

2500 upper gap

REFU 2000 lower gap

1500 REFL Height (MSL)

1000

500

0 −10 −8 −6 −4 −2 0 2 4 6 8 10 W Distance from Brenner (km) E

Figure 4.3: W-E cross-section of the Alpine crest in the vicinity of the Brenner pass and location of the two reference stations. 4.5. APPLICATION OF THE OFC TO GAP FLOW IN THE BRENNER TARGET AREA 77

4.5.3 The Θ criterion for foehn in the Wipp valley

As a consequence of the double gap structure at the main Alpine crest two air masses with diﬀerent properties exist in the Wipp valley during a south foehn period. Since no weather stations were in place close to the Alpine crest until 1998, this subtlety was not recognised. The discrimination strategy based on the potential temperature relative to the two references, which was described in section 4.4.3, requires the determination of relative frequencies of the normalized potential temperature diﬀerence.

10 Luegg Steinach 9 Ellboegen Matreiwald InnsbruckIMGI 8

Relative frequency (%) 3

0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 ∆Θ*

Figure 4.4: Relative frequency of the normalized potential temperature diﬀerence ∆Θ∗ at 5 selected stations during south wind periods in the MAP-SOP. For explanation see text. The vertical dashed line marks a 50% mixture between air from the lower and from the upper reference station.

Figure 4.4 shows the relative frequency of ∆Θ∗ for five stations (for locations see figure 4.2 and table B.1) during all south wind periods of the MAP-SOP. They are chosen to represent different distances from the crest as well as different heights 78 CHAPTER 4. AN OBJECTIVE FOEHN CLASSIFICATION SCHEME above the valley surface. Stations close to REFL (Brenner), e.g. 01(Luegg), are clearly in the lower gap flow regime. 30km downstream of the pass at 16(Inns- bruck), upper gap flow is highly dominant. The measures at the three selected stations in between represent a mixture of the two flows. The relative frequency at 05(Steinach), at the valley bottom 10km north of the pass, shows a bimodal structure. A peak at ∆Θ∗ = 0.28, the lower gap flow, and another one at 0.81. This can be interpreted as 81% of the air coming from REFU(Sattelberg), 19% from REFL(Brenner). 08(Matreiwald), another 9km further north, shows no significant lower gap flow maximum anymore, but two upper gap flow maxima. 10(Ellboe- gen) exhibits three different regimes. A clear lower gap flow regime, upper gap flow peaking at 0.65 and deep foehn (maximum at 1.12, which is from higher than 2100mMSL). The clear signal of the different air masses gives enough confidence to set the dis- ∗ tinction mark between lower and upper gap flow to a fixed value of ∆Θcrit = 0.5. 1 Using equ. (8), the critical value Θcrit simplifies to 2 (Θupper gap + Θlower gap). Upper gap flow is recognised at a station, whenever the wind criteria of 4.4.3 are met and

Θstation ≥ Θcrit, indicating that at least 50% of the air has has flown over the higher ridge. Lower gap flow requires accordingly Θlower gap < Θstation < Θcrit besides the ”proper” wind conditions. Values from the reference station and the valley station are compared for same times. The travel time of air from reference to classified station (O(1h)) is neglected.

4.6 Illustration of method

The OFC provides a consistent way of classifying foehn, which is physically based. This section demonstrates the ability of the method to classify foehn in the Wipp valley. Furthermore an example of a valley wind period is shown, where no foehn is classiﬁed.

4.6.1 Gap ﬂow period at 04(Zagl) with foehn break

In the night to 4 November 1999 weak southerly valley outﬂow had become established (cf. ﬁgure 4.5).

It is marked by air potentially colder than in the lower gap, showing that the flow is coming off the slopes and/or from side valleys. Therefore no data from the upper reference station are needed for a classification which is as well since these data are missing. The first gap flow period on 4 November starts and ends with air from the lower gap with more than seven hours of upper gap flow in between. Wind 4.6. ILLUSTRATION OF METHOD 79

Traditional

OFC

295

290 REFL(Brenner) Theta (K) REFU(Sattelberg) 285 04(Zagl) 360

270

180

90 direction (deg) 0 8

2 speed (m/s)

0 00 12 00 12 00 12 00 04 Nov 99 05 Nov 99 06 Nov 99

Figure 4.5: Comparison of traditional foehn classification and OFC at station 04(Zagl) from 4 November 1999 00 UTC to 7 November 1999 00 UTC. Lower gap flow in light grey, upper gap flow in black. Periods with missing data are marked by a thin line. Based on quality controlled 10 minute data. Locations of stations are shown in figure 4.2 with coordinates given in table B.1. 80 CHAPTER 4. AN OBJECTIVE FOEHN CLASSIFICATION SCHEME direction turns 15 degrees further south and wind speed is increasing. In the evening a foehn break sets in, clearly visible in the cold temperature at 04(Zagl), but also in the wind turning from S to SW, indicating flow from the Obernberg valley (to the SW from 04(Zagl) and 02(Gries) in figure 4.2). On 5 November the foehn sets in again in the late morning. Although the stratification between REFL(Brenner) and REFU(Sattelberg) is nearly dry adiabatic, which makes a distinction of the two flows more difficult, the evolution of higher into lower and then again higher gap flow before the break down is clearly visible. Note the concomitance of upper gap flow and high wind speed. A strong gust cold front ended at 6 November the foehn period around noon. It brought the first snow of the season which unfortunately also covered the temperature sensor at 04(Zagl). These data were flagged as ”bad” in the quality control procedure. Comparison with the traditional foehn classification reveals agreement, albeit not completely. On 4 November the OFC reveals lower gap flow periods before and after the upper gap flow, which was not recognised by the traditional foehn subjective classification.

4.6.2 Valley wind regime at 06(Tienzens)

The two day period between 9 and 11 October 12 UTC is marked by weak northwesterly flow in 500hPa in the beginning and turning to W towards the end of the period. The anticyclonic influence at the surface leads to the development of a thermally driven valley wind regime. It is important to check, whether the southerly flow associated with it can be distinguished from foehn by the OFC.

As is to be expected for the time of the year, the NNW diurnal up-valley wind at 06(Tienzens), 11 km north of REFL(Brenner), lasts for only eight hours (cf. figure 4.6). It starts around 8:30 UTC and ends at 16:30 UTC. For the remaining two thirds of the day SSE down-valley wind persists. During the first night, where the superimposed synoptic-scale forcing is opposed to the radiative forcing, the down-valley flow has a strength of about 2 m s−1. However, in the night from 10 to 11 October without any relevant cross-Alpine pressure gradient, the mean wind speed is close to 4 m s−1. But most important of all, the outflow of cold air results in lower potential temperature at the valley bottom than at REFL(Brenner). Therefore properly no foehn wind is classified! Similar results are achieved for the other stations in the Wipp valley and for different valley wind periods. During this anticyclonic period at REFL a southerly wind component was only recorded in the morning hours of 11 October (not shown). 4.6. ILLUSTRATION OF METHOD 81

Traditional

OFC

300 REFL(Brenner) REFU(Sattelberg) 295 06(Tienzens)

290 Theta (K)

285 360

270

180

90 direction (deg) 0 8

2 speed (m/s)

0 12 00 12 00 12 09 Oct 99 10 Oct 99 11 Oct 99

Figure 4.6: As in ﬁgure 4.5 but for station 06(Tienzens) from 9 October 1999 12 UTC to 11 October 1999 12 UTC. 82 CHAPTER 4. AN OBJECTIVE FOEHN CLASSIFICATION SCHEME

4.7 Statistical evaluation for the MAP-SOP

4.7.1 South wind characteristics during the MAP-SOP

The climatological maximum frequency of south foehn periods in the Alps is in the cold season when troughs over Europe may extend further south and lead to frequent southerly flow in the Alps. The gap flow duration during the MAP-SOP in autumn 1999 was even 15% above the ten year mean 1989-98 (Steinacker 2000). Every foehn in the Wipp valley is a mixture of lower gap flow topped by upper gap flow. The lower gap flow tends to flow along the valley bottom. Further downstream it is basically mixed up by the higher gap flow carrying considerable more mass. Therefore the upper gap flow frequency increases with station height and distance to the Alpine crest. The OFC was only applied to stations in the Wipp Valley down to 16(Innsbruck) at its exit region. The concept of comparing a streamline tracer between a valley station and a reference station at the crest is no longer feasible in the west-east oriented Inn Valley due to possibly different paths of the air than through the Wipp Valley. Station locations are shown in the close-up of figure 4.2. Table B.1 summarizes the station names and their location. The lower and upper gap flow frequencies in the Brenner target area during the 70 day period of the MAP-SOP shown in table B.2 are discussed in the following paragraph.

Due to the extremely complex topography of the target area, the variation in foehn frequencies is high and depends on the distance of the station to the baseline of the Alpine crest as well as on the height above the valley bottom. The highest foehn frequency was observed at stations well above the valley bottom. 04(Zagl) had a 45% foehn frequency during the MAP-SOP, 10(Ellboegen) 40%! 16(Innsbruck) on the other hand had less than 10%, which is significantly less than all the Wipp Valley stations. Especially during the night a cold pool may form at the bottom of the Inn Valley, which can not be removed by the warmer air exiting from the Wipp Valley. As a consequence, foehn periods at 16 are significantly less frequent and shorter than in the Wipp Valley. Only 01(Luegg) and 03(Stafflach) had more lower gap flow than upper gap hours in the MAP-SOP. They are at the valley bottom close to the pass and the flow over the upper gap does not easily reach there. All the other stations had more upper than lower gap flow hours. Evidently the further downstream the stations in the Wipp Valley are, the more upper gap flow they experience. 02(Gries) and 04(Zagl) are a bit higher up, therefore they experience more than 50% upper gap flow. 03(Stafflach) and 08(Matreiwald) are at the valley bottom, the lower gap flow percentage is high compared to surrounding stations and the total foehn frequency low. 05(Steinach), 06(Tienzens), 07(Gedeir) 4.7. STATISTICAL EVALUATION FOR THE MAP-SOP 83

Station Data avail. LGF duration LGF perc. UGF duration UGF perc. Lower/Total GF (%) (h) (%) (h) (%) (%) 01 65.5 340.2 30.9 5.3 0.5 98.5 02 69.4 98.8 8.5 259.5 22.3 27.6 03 76.4 170.2 13.3 110.5 8.6 60.6 04 83.1 276.0 19.8 349.7 25.1 44.1 05 76.3 162.3 12.7 290.2 22.6 35.9 06 69.8 115.2 9.8 173.2 14.8 39.9 07 82.1 102.2 7.4 202.7 14.7 33.5 08 79.2 104.7 7.9 125.8 9.5 45.4 09 15.4 9.3 3.6 98.3 37.9 8.6 10 75.7 96.5 7.6 407.2 32.0 19.2 11 83.0 117.2 8.4 238.7 17.1 32.9 12 70.2 40.8 3.5 260.8 22.1 13.5 13 83.1 17.1 1.3 203.8 14.6 7.7 14 80.6 13.3 1.0 226.2 16.7 5.6 15 64.4 7.5 0.7 149.5 13.8 4.8 16 74.2 10.7 0.9 105.5 8.5 9.2

Table B.2: Gap flow statistic for different stations during MAP-SOP. LGF denotes lower gap flow, UGF upper gap flow. and 11(Schoenberg) are characteristic for locations in the middle of the Wipp Valley which are well exposed to southerly foehn winds. At these four stations every third 2 to fourth hour in autumn 1999 showed foehn, 3 of which came over the upper gap. 09(Gleins) had a serious problem with the wind sensor due to very strong foehn winds. The valid period is too short to make proper conclusions. 14(Lans) and 15(Natters) are influenced by the flow in the Inn Valley. Consequently the foehn frequency – predominantly upper gap flow – is reduced.

4.7.2 Comparison of OFC to the traditional classiﬁcation scheme

A statistical approach serves to estimate not only the quality of the OFC, but also highlight the similarities and differences to the traditional foehn classification as described in section 4.2. Three stations were chosen for this purpose. 04(Zagl) in the southern Wipp valley 7km downstream of the crest, 10(Ellboegen) two thirds down the valley and 16(Innsbruck) just north of the exit region into the Inn valley. But first the main causes for differences in the two schemes have to be examined. The manual analysis error is caused by periods of southerly flow without a distinct temperature or humidity signal. In that case the decision foehn or thermally driven down-valley winds within the traditional classification framework is highly 84 CHAPTER 4. AN OBJECTIVE FOEHN CLASSIFICATION SCHEME subjective and the tendency to classify only stronger flows as foehn is hard to circumvent. The warming and drying foehn effect is least pronounced during lower gap flow at a position close to the Alpine crestline, where the height difference between the gap and the station is small. The manual analysis error therefore increases with decreasing distance to the Alpine crestline. The sector of the wind direction during foehn is generally well known and the minimum wind speed threshold of the OFC excludes only nearly stagnant wind periods at the gap or the station. Nevertheless a more restrictive minimum wind speed criterion can advance the accordance to the traditional classification, which is –as discussed above– due to analysis problems of the traditional scheme rather than the OFC. Consequently the main source of errors with the OFC is the potential temperature criterion. The potential temperature error may be caused by diabatic processes between the gap and the station as revealed in Appendix A or by temperature advection at the crest during the travel time.

Station Available Periods A B C D JCR OOR (%) (%) ZAG 8373 3248 506 189 4430 94.5 13.5 ELL 7630 2337 685 41 4567 98.3 22.7 IBK 7480 498 43 32 6907 94.0 7.9

Table B.3: Comparison of foehn classification by tracer method versus traditional scheme during the MAP-SOP at three selected stations. A denotes foehn in both schemes, B foehn classified by the tracer method only, C foehn classified manually only, D no foehn in both schemes, JCR is the Joint Classification rate in (%) defined as A/(A + C) and OOR is the OFC Only Rate in (%) defined as B/(A + B)

Based on the 10 minute data for the whole MAP-SOP the contingency table reveals the number of foehn periods analysed by both schemes (A), only by the OFC (B), only by the traditional classification (C), and rejected by both (D), respectively (cf. table B.3). The deduced joint classification rate (JCR) is defined as the total number of foehn events analysed by both schemes (A) divided by the total number of events in the traditional classification (A+C). Perfect accordance gives a JCR of 100%. The OFC only rate (OOR) is the number of events only analysed by the OFC (B) divided by the total number of events in the OFC (A+B). At all three stations the OFC detects more foehn cases than the traditional scheme, although this is not very pronounced at 16(Innsbruck). The main cause is the manual analysis error. An enhanced minimum wind speed threshold at the downstream station in the OFC at 04(Zagl) and 16(Innsbruck) increases the correspondence significantly. This proves the problem of detecting weak foehn periods with the 4.8. CONCLUDING REMARKS 85 traditional method! If e.g. the required wind speed at 10(Ellboegen) is set to 5 m s−1 instead of 1 m s−1, the agreement with the traditional classification is much better and the OOR decreases from 22.7% to 8.1% with nearly unchanged JCR. A relaxation of the potential temperature criterion allowing Θ at the station to be up to 1K colder than the reference enhances the JCR only slightly, while considerable enhancing the OOR at all three analysed stations and consequently reducing the scores. Requiring Θ at the station to be at least 1K warmer than REFL(Brenner) leads to much higher OOR and therefore worse results at 04(Zagl) and a nearly unchanged score at 10(Ellboegen) and 16(Innsbruck). This is a hint that further downstream, where the lower gap flow ceases to exist, a stricter criterion resembling the upper gap flow would do as well. Nevertheless the standard criterion produces no significant false alarms in that respect.

4.8 Concluding Remarks

4.8.1 Potential and limits of the new scheme

With some extra cost and effort in running one or more reference stations at the crest a proper distinction between different flows in a mountainous environment can be gained with the proposed OFC. Its great advantage is that it is physically based by considering the origin of the air and the downstream process of dry adiabatic descent. It includes foehn types regardless of their strength and of the temperature and humidity relative to the replaced air mass. It is objective, can be automatised and is especially suited for classifying long data periods. In rare cases it may be difficult to discriminate lower from upper gap flow, but then this is not possible at all using the traditional classification. Two potential sources for an incorrect analysis remain. One is connected to the fact that the descent of the foehn air might not be purely dry adiabatic, the second one is due to the neglection of the temperature advection during the travel time from reference to downstream station. The two limiting factors for the OFC are discussed below. Maximum estimations of errors due to diabatic and moist adiabatic processes are given in Appendix A. Evaporation downstream of the barrier in a foehn cloud decreases the temperature gain and leads to an underestimation of foehn. This problem could possibly be overcome, if a parameter that is conserved along streamlines in the presence of moist processes is used. Candidates are the mixing ratio and the equivalent potential temperature. Estimations of the accuracy of the humidity measurements discouraged the use for this article. The radiational influence is believed to be small, since the time scale of the air mass in the region of interest under wind 86 CHAPTER 4. AN OBJECTIVE FOEHN CLASSIFICATION SCHEME conditions typical for foehn is small. The maximum effect of turbulent mixing by friction in the boundary layer, wind shear or wave overturning is on the order of 1K. In a borderline case of analysing a weak, shallow foehn event this could lead to a wrong classification. Nevertheless the analyses so far proofed that the discrimina- tional power of the OFC is at least as good as the traditional method with a largely reduced effort. The temperature advection during the travel time of an air parcel from the reference to the downstream station is neglected for the sake of simplicity. It could be accounted for by calculating a travel time from a mean velocity between reference and downstream station at each time step. The values at the reference station are then taken the calculated interval before the specified time at the downstream station. For the application to the Brenner target area this correction did not improve the OFC. Close to the crest the travel time from the reference station is short, the expected error due to temperature advection small. Further downstream, where travel times are of the order of 1h, the potential temperature criterion is more easily fullfilled, because the air is coming from higher altitudes than the reference station.

4.8.2 Application to other mountain areas

Since the mechanism is similar for any foehn type wind, the proposed OFC is in principal applicable to every foehn wind around the world. However, the shape and height of the upstream topography will determine the exact formulation. In the absence of a deep incision it might be sufficient to classify foehn or no foehn. One or more gaps make it possible and necessary to make a more refined discrimination. Additionally the allowed foehn wind direction range at all classified stations has to be determined in advance either from prior observations or – in the absence of any data set at the location – from topographical considerations. Typically this is quite easy, if the flow location is not disturbed by nearby obstacles like houses, trees, hills etc. A serious restriction arises, if is is not clear, where the foehn air at a particular location comes from or if air from side valleys is mixed into the foehn flow.

4.8.3 Outlook

Besides the application to an extensive data set measured during the MAP-SOP, the OFC is currently applied to a multi-year time series for the Brenner target area and other Alpine locations. It is also used to derive objective foehn forecasts from numerical weather prediction models. APPENDIX A 87

APPENDIX A

Error estimates concerning the assumption that the potential temperature is conserved along a streamline are given below.

4.8.4 Condensation/ Evaporation

Moist processes downstream may rarely occur in case of a foehn wall extending to north of the mountain range. If the crest is in the foehn cloud, the temperature of the descending parcel on the lee side will increase less than in dry air. Figure A.1 shows a sketch of the moist adiabatic descent in clouds.

Figure A.1: Sketch of the moist adiabatic descent in clouds. The solid line is the streamline, the dashed line represents the apparent streamline under a dry adiabatic assumption.

Part of the energy is used for the evaporation. Hence the temperature ∆h below the ridge line will be lower than without clouds and therefore by approximately the same amount also the potential temperature. Since for a subsiding parcel the dry adiabatic temperature increase is 9.8 · 10−3 K m−1 , but under moist adiabatic conditions only 6.5 · 10−3 K m−1, the potential temperature decrease due to clouds is 0.33K per 100m. The underestimation of the original streamline height depends on the temperature proﬁle at the crest. The more stable the stratiﬁcation the less the error. A possible solution for this problem is the incorporation of the mixing ratio, if saturation occurs at the crest. The mixing ratio is conserved along a streamline, if the water content in the foehn air is constant, i.e. in absence of precipitation in or out of the cloud. 88 CHAPTER 4. AN OBJECTIVE FOEHN CLASSIFICATION SCHEME

4.8.5 Radiation

The amount of warming or cooling due to divergence of the radiation ﬂux is dependent on the time scale. The time scale for a distance of 20km with a speed of 5 m s−1 is 4 × 103 s or approximately 1 hour. Since the radiative heating is of the order of 2 K day−1, the total change in (potential) temperature is approximately 2 −1 24 K h × 1h ≈ 0.1 K, which is close to the measurement accuracy.

4.8.6 Turbulent mixing

Turbulence may be caused by friction in the boundary layer, wind shear layers or hydraulic jump-like features. The concept of taking potential temperature as a tracer for the air mass is limited by turbulence along the way. Entrainment causes vertical exchange of air mass characteristics. In a stable atmosphere typically warmer air from aloft and colder air from below are mixed together. To estimate the possible influence of turbulent mixing, the law of conservation of heat is formulated in a horizontally homogeneous dry environment. The relevant prognostic equation for the change of the potential temperature with time following the air parcel is ³ ´ dΘ ∂ w0Θ = − (4.5) dt ∂z Bars denote the mean state, accents deviations of it. The vertical gradient of the turbulent heat flux on the right hand side can be parameterised with a simple local first-order small-eddy closure-technique

∂Θ w0Θ = −K (4.6) ∂z where K is the eddy viscosity with typical values in the literature on the order of 1 to 10m2 s−1 (cf. Stull, 1988). Therefore Ã ! dΘ ∂ ∂Θ ∂K ∂Θ ∂2Θ = K = + K 2 (4.7) dt ∂z ∂z |∂z{z∂z} | {z∂z } T erm A T erm B

Mixing length theory suggests, that Ã ! ∂U K ≈ k2z2 (4.8) ∂z ³ ´ ∂U where k is von Karman’s constant, z the depth of the layer and ∂z the shear of the mean horizontal wind. If foehn layer depth relevant for mixing is 100m and the vertical shear 10 m s−1 over a kilometer as an upper limit, the eddy viscosity APPENDIX A 89

2 2 10 2 −1 can reach values of K ≈ 0.4 100 103 = 16 m s . Term A is on the order of 5 m2s−1 0.5 K −5 −1 −1 500 m 100 m = 5·10 K s ≈ 0.18 K h . In the boundary layer typical variations of the eddy viscosity with height show a maximum some 100m above ground. Below the height of maximum eddy viscosity, term A is positive, above negative. Term B 2 −1 1 1 K −4 −1 −1 can reach values of 16 m s 103 m 100 m = 1.6·10 K s ≈ 0.58 K h . While the ∂Θ gradient of ∂z can be large near the top of the foehn layer, the accompanying stable layer on the other hand will reduce the turbulence. Therefore the total change of potential temperature due to turbulence could be as high as 0.76K per hour. Again the error in the estimate of the upstream streamline height depends on the temperature profile at the crest. In standard atmosphere 0.76K correspond to a vertical height difference of 230m, less for more stable stratification. The maximum effect of the mixing will be at the upper and lower boundaries of the foehn flow, where the temperature gradient and/or the wind shear are large. Typically it will have a cooling effect from below and warming from aloft. This mixing process is taken into account in the separation of lower and upper gap flow in section 4.4.3. Summarising the error estimates, strong turbulence over a considerable distance along the foehn flow and clouds extending well into the lee of the mountain range limit the applicability of the foehn classification. Radiative heating on the other hand has a minor impact. APPENDIX B

The following ﬁgures present the analysis of foehn periods in the Brenner target area during the 70 days of the MAP-SOP in 1999. The time series of lower and upper gap ﬂow of 16 stations between Lueg just north of the Brenner pass and Innsbruck were computed as described in this article. Also note the thin grey line, which marks missing data periods.

90 APPENDIX B 91

Gap flow periods 199909070000 − 199909120000

LUE

GRI

STA

ZAG

STE

TIE

GED

MAT

GLE

ELL

SBG

PAT

ZEN

NAT

LAN

IBK

250 250.5 251 251.5 252 252.5 253 253.5 254 254.5 255 Julian day 1999

Gap flow periods 199909120000 − 199909170000

LUE

GRI

STA

ZAG

STE

TIE

GED

MAT

GLE

ELL

SBG

PAT

ZEN

NAT

LAN

IBK

255 255.5 256 256.5 257 257.5 258 258.5 259 259.5 260 Julian day 1999

Figure B.1: (Top) Gap flow periods in 1999 between julian day 250 and 255. Lower gap flow in blue, upper gap flow in red. Missing data periods are indicated by a thin grey line. (Bottom) Same for julian days 255 – 260. 92 CHAPTER 4. AN OBJECTIVE FOEHN CLASSIFICATION SCHEME

Gap flow periods 199909170000 − 199909220000

LUE

GRI

STA

ZAG

STE

TIE

GED

MAT

GLE

ELL

SBG

PAT

ZEN

NAT

LAN

IBK

260 260.5 261 261.5 262 262.5 263 263.5 264 264.5 265 Julian day 1999

Gap flow periods 199909220000 − 199909270000

LUE

GRI

STA

ZAG

STE

TIE

GED

MAT

GLE

ELL

SBG

PAT

ZEN

NAT

LAN

IBK

265 265.5 266 266.5 267 267.5 268 268.5 269 269.5 270 Julian day 1999

Figure B.2: (Top) Same as above for julian days 260 – 265. (Bottom) Same for julian days 265 – 270. APPENDIX B 93

Gap flow periods 199909270000 − 199910020000

LUE

GRI

STA

ZAG

STE

TIE

GED

MAT

GLE

ELL

SBG

PAT

ZEN

NAT

LAN

IBK

270 270.5 271 271.5 272 272.5 273 273.5 274 274.5 275 Julian day 1999

Gap flow periods 199910020000 − 199910070000

LUE

GRI

STA

ZAG

STE

TIE

GED

MAT

GLE

ELL

SBG

PAT

ZEN

NAT

LAN

IBK

275 275.5 276 276.5 277 277.5 278 278.5 279 279.5 280 Julian day 1999

Figure B.3: (Top) Same as above for julian days 270 – 275. (Bottom) Same for julian days 275 – 280. 94 CHAPTER 4. AN OBJECTIVE FOEHN CLASSIFICATION SCHEME

Gap flow periods 199910070000 − 199910120000

LUE

GRI

STA

ZAG

STE

TIE

GED

MAT

GLE

ELL

SBG

PAT

ZEN

NAT

LAN

IBK

280 280.5 281 281.5 282 282.5 283 283.5 284 284.5 285 Julian day 1999

Gap flow periods 199910120000 − 199910170000

LUE

GRI

STA

ZAG

STE

TIE

GED

MAT

GLE

ELL

SBG

PAT

ZEN

NAT

LAN

IBK

285 285.5 286 286.5 287 287.5 288 288.5 289 289.5 290 Julian day 1999

Figure B.4: (Top) Same as above for julian days 280 – 285. (Bottom) Same for julian days 285 – 290. APPENDIX B 95

Gap flow periods 199910170000 − 199910220000

LUE

GRI

STA

ZAG

STE

TIE

GED

MAT

GLE

ELL

SBG

PAT

ZEN

NAT

LAN

IBK

290 290.5 291 291.5 292 292.5 293 293.5 294 294.5 295 Julian day 1999

Gap flow periods 199910220000 − 199910270000

LUE

GRI

STA

ZAG

STE

TIE

GED

MAT

GLE

ELL

SBG

PAT

ZEN

NAT

LAN

IBK

295 295.5 296 296.5 297 297.5 298 298.5 299 299.5 300 Julian day 1999

Figure B.5: (Top) Same as above for julian days 290 – 295. (Bottom) Same for julian days 295 – 300. 96 CHAPTER 4. AN OBJECTIVE FOEHN CLASSIFICATION SCHEME

Gap flow periods 199910270000 − 199911010000

LUE

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Figure B.6: (Top) Same as above for julian days 300 – 305. (Bottom) Same for julian days 305 – 310. APPENDIX B 97

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Figure B.7: (Top) Same as above for julian days 310 – 315. (Bottom) Same for julian days 315 – 320.

Chapter 5

Conclusions and outlook

The present thesis presents two detailed case studies of south foehn and a concept for a foehn classiﬁcation based on the involved physical processes rather than a phenomenological point of view. While detailed summaries are given in sections 2.5, 3.5 and 4.8, respectively, the present chapter oﬀers a brief summary of the main topics and results and points out ongoing activities and future perspectives concerning the foehn research.

The ”Sandwich foehn” period, which took place during the SOP of MAP and is presented in chapter 2, was chosen because of its interesting complex flow structure. The first part explains, how the vertical north-south-north flow layering is accomplished. On the synoptic scale, the influence of two pressure lows is important, especially the one northeast of the Alps enclosing the Alps with cold air resulting in an inneralpine pressure low. On the local scale the dissolving of the stratocumulus deck in the Wipp and Inn valley around midday plays an important role. It is partly caused by the just developing foehn flow. Numerical simulations with the MM5 model were able to capture the main flow features, even though the southward extend and depth of the shallow northerly flow in the Wipp valley is underestimated. It is concluded that the initial conditions to start the model are crucial. While the upstream model profiles – in terms of foehn – represent the main characteristics from rawinsonde measurements, the inneralpine initial conditions proved difficult to simulate in spite of corrected ECMWF-analysis profiles. As long as global models are not able to dissolve mountainous areas realistically, it seems like a modeller can at best correct important initial conditions retrospectively from available measurements. Ground based Doppler lidar measurements reveal details of the foehn structure and evolution in the Wipp valley, such as the gravity wave structure exited by west-east oriented ridges and the decreasing depth of the cold surface layer due to the foehn influence aloft.

99 100 CHAPTER 5. CONCLUSIONS AND OUTLOOK

Chapter 3 describes two moderate foehn cases measured in the framework of STAAARTE (Scientific Training and Access to Aircraft Throughout Europe) with the DLR-Falcon in February and November 2000. In situ measurements by the aircraft are complemented by soundings and ground based measurements. On 28 February a marked low level jet was found within the southern Wipp Valley at 1500 – 2000 m MSL below the strong inversion. Near the exit of the Wipp Valley at the same height a well-mixed zone with nearly no horizontal flow existed as a consequence of a hydraulic-jump-like feature. The inversion along the Brenner cross-section descended from south to north and split up into two weaker parts. On 7 November the oncoming southerly flow over the Inn Valley changed from southwesterly flow at crest height to highly orography- dependent flow below. Horizontal flow splitting was observed at the conjunction of major north-south oriented valleys with the perpendicular Inn Valley. Ver- tical motions on the order of 5 m s−1 showed strong wave activity east of Innsbruck.

The need to determine foehn periods for each weather station subsequent to MAP led to the development of an objective foehn classification scheme, which is presented in chapter 4. It is based on the lapse rate between a suitable mountain reference and the valley station as well as the wind direction at both stations. The concept is first discussed in general terms keeping the applicability to other foehn areas around the world in mind. The special topographic structure of the upstream Alpine crestline makes a distinction between a lower and an upper gap flow reasonable and possible. Exemplary periods demonstrate the ability of the scheme to distinguish foehn flow in the Wipp valley from other wind regimes. A statistical evaluation compares the results to subjective classifications at selected stations.

Ongoing research activities at the Department concentrate on the extension of the foehn classification. Three Diploma theses from the Department of Meteorology are coming up which build up on the objective foehn classification introduced in chapter 4. The first thesis by Werner Verant examines the applicability to another area, namely north foehn periods in two valleys in South Tyrol. The second thesis by Fritz Föstextends the classification to the Inn valley, with foehn winds being deflected by the mountain chain north of Innsbruck (Nordkette). The third thesis by Susanne Drechsel examines the foehn analysis and forecast skills of the ECMWF model using the pressure difference upstream to downstream, the potential temperature difference ∼ 400ma.g.l. between crest and the first downstream grid point and the downstream south wind component. The objective foehn classification is used to determine the foehn periods that have occurred. Bibliography

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Helten, M., H. Smit, D. Kley, J. Ovarlez, H. Schlager, R. Baumann, U. Schumann, P. Nedelec, and A. Marenco, 1999: In-flight intercomparison of MOZAIC and POLINAT water vapor measurements. J. Geophys. Res., 104, 26087 – 26096. Hoinka, K., 1984: Observation of a mountain-wave event over the Pyrenees. Tel- lus, 36, 369 – 384. Hoinka, K., and T. Clark, 1991: Pressure drag and momentum fluxes due to the Alps : Comparison between numerical simulation and observation. Quart. J. Roy. Meteor. Soc., 117, 495 – 525. Hoinka, K.P., 1990: Untersuchung der alpinen Gebirgsüberströmung bei Südföhn. Forschungsbericht DLR-FB 90-30, 186 pp., DLR Oberpfaffenhofen, Germany. Klemp, J.B., and D.R. Durran, 1983: An upper boundary condition permitting internal gravity wave radiation in numerical mesoscale models. Mon. Wea. Rev., 111, 430–444. Küttner,J., 1992: An overview of ALPEX. Ann. Meteor., 19, 3 – 12. Lewis, H.W., S.D. Mobbs, A. Gohm, M.K. Hill, S.J. Arnold, and G.J. Mayr, 2004: Turbulent fluxes measured in the Wipptal during MAP. Submitted to Quart. J. Roy. Meteor. Soc. Lewis, R.P.W., 1991: Meteorological Glossary. HMSO. Lilly, D., J.M. Nicholls, R.M. Chervin, P.J. Kennedy, and J.B. Klemp, 1982: Air- craft measurements of wave momentum flux over the Colorado Rocky Moun- tains. Quart. J. Roy. Meteor. Soc., 108, 625 – 642. Lilly, D., Y. Pann, P. Kennedy, and W. Toutenhoofd, 1971: Data catalog for the 1970 Colorado Lee Wave Observational Program. 190 p pp. Lilly, D.K., 1978: A severe downslope windstorm and aircraft turbulence event induced by a mountain wave. J. Atmos. Sci., 35, 59 – 77. Lockwood, J.G., 1962: Occurrence of foehn winds in the British Isles. Met. Mag., 91, 57 – 65. Longley, R., 1967: The frequency of winter chinooks in Alberta. Atmosphere, 5, 4 – 16. Mayr, G.J., L. Armi, S. Arnold, R.M. Banta, L.S. Darby, D.R. Durran, C. Fla- mant, S. Gaberˇsek,A. Gohm, R. Mayr, S. Mobbs, L.B. Nance, I. Vergeiner, J. Vergeiner, and C.D. Whiteman, 2004: Gap flow measurements during the Mesoscale Alpine Programme. Meteorol. Atmos. Phys., 86, 99 – 119. Mayr, G.J., I. Vergeiner, A. Gohm, J. Vergeiner, and R. Mayr, 2003: Gap flow – an overview and preliminary results. Osterr.¨ Beitr. Meteor., 29, 15 – 54. 104 BIBLIOGRAPHY

Mayr, G.J., J. Vergeiner, and A. Gohm, 2002: An automobile platform for the measurement of foehn and gap flows. J. Atmos. Oceanic Technol., 19, 1545– 1556. Meischner, P.E., 1985: Nutzerhandbuch fürdas FALCON-System. DFVLR Mit- teilung, 85 – 08, 131 pp. Mlawer, E.J., S.J. Taubman, P.D. Brown, M.J. Iacono, and S.A. Clough, 1997: Radiative transfer for inhomogeneous atmospheres: RRTM, a validated correlated-k model for the longwave. J. Geophys. Res., 102, 16663–16682. Nance, L., 2000: A look at the 30 October 1999 south foehn event in the Wipp Valley. In: AMS Mountain Meteorology Conference. Nkemdirim, L.C., 1996: Canada’s chinook belt. National Journal of Climatol- ogy, 16, 441 – 462. Obenland, E., 1956: Untersuchungen zur Föhnstatistikdes Oberallgäus. Berichte des Deutschen Wetterdienstes, 23. Osmond, H., 1941: The chinook wind east of the Canadian Rockies. Can. J. Res., 19, 57 – 66. Pippan, C., 2000: Umgang mit einem heterogenen, hochaufgelöstenMeßnetz währendder Feldphase des Mesoscale Alpine Programme. Diploma thesis, University of Innsbruck, 76 pp. Post, M.J., and R.E. Cupp, 1990: Optimizing a pulsed Doppler lidar. Appl. Opt., 29, 4145–4158. Pümpel, H., 1978: Analyse der atmosphärischen Struktur über dem Alpenraum auf isentropen Flächen. Quante, M., P. Brown, R. Baumann, B. Guillemet, and P. Hignett, 1996: Three aircraft intercomparison of dynamical and thermodynamical measurements during the Pre-EUCREX campaign. Contr. Atmos. Phys., 69, 129 – 146. Reisner, J., R.M. Rasmussen, and R.T. Bruintjes, 1998: Explicit forecasting of supercooled liquid water in winter storms using the MM5 mesoscale model. Quart. J. Roy. Met. Soc., 124, 1071–1107. Schär,C., D. Leuenberger, O. Fuhrer, D. Lüthi,and C. Girard, 2002: A New Terrain-Following Vertical Coordinate Formulation for Atmospheric Predic- tion Models. Mon. Wea. Rev., 130, 2459 – 2480. Schlegel, M., 1952: Die Böigkeit der Winde an einer Voralpenstation unter Berücksichtigung des Föhns. Bericht des Dt. Wetterdienstes in der US- Zone, 42, 47 – 50. BIBLIOGRAPHY 105

Schuetz, J., and F. Steinhauser, 1955: Neue Foehnuntersuchungen aus dem Sonnblick. Arch. fürMet., Geoph. und Bioklim., Series B, 6, 207 – 224. Seibert, P., 1985: Fallstudien und statistische Untersuchungen zum Südföhnim Raum Tirol. Ph.D. thesis, University of Innsbruck, 368 pp. Seibert, P., 1990: South foehn studies since the ALPEX experiment. Meteorol. Atmos. Phys., 43, 91–103. Shafran, P.C., N.L. Seaman, and G.A. Gayno, 2000: Evaluation of numerical predictions of boundary layer structure during the Lake Michigan Ozone Study (LMOS). Appl. Meteorol., 39, 412 – 426. Steinacker, R., 1983: Fallstudie eines Süd-und eines Nordföhnsüber den Alpen. Aereo-Revue, 8, 41–44. Steinacker, R., 2000: The ”Map Sop Man”. MAP Newsletter, 12, 2 – 3. Steinacker, R., W. Pöttschacher, and M. Dorninger, 1997: Enhanced Resolution Analysis of the Atmosphere over the Alps Using the Fingerprint Technique. Annalen der Meteorologie, 35, 235 – 237. Ström,J.R., R. Busen, M. Quante, B. Guillemet, P. Brown, and J. Heintzen- berg, 1994: Pre-EUCREX intercomparison of airborne humidity-measuring instruments. J. Atmos. Oceanic Technol., 11, 1392 – 1399. Ungeheuer, H., 1952: Zur Statistik des Foehns im Voralpengebiet. Bericht des Dt. Wetterdienstes in der US-Zone, 38, 117 – 120. World Meteorological Organization, 1992: International Meteorological Vocabu- lary. Zängl,G., 2002a: An improved method for computing horizontal diffusion in a sigma-coordinate model and its application to simulations over mountainous topography. Mon. Wea. Rev., 130, 1423–1432. Zängl,G., 2002b: Stratified flow over a mountain with a gap: Linear theory and numerical simulations. Quart. J. Roy. Meteor. Soc., 128, 927–949. Zängl,G., 2003: A Generalized Sigma-Coordinate System for the MM5. Mon. Wea. Rev., 131, 2875 – 2884. 106 ACKNOWLEDGMENTS Acknowledgments

Without any doubt the last ﬁve years have been an important period in my life. This thesis would not be like it is without the support of many people, on a professional and personal level.

First of all I want to thank Georg J. Mayr. He did not only advise me regarding my large- and mesoscale thesis problems, but also respected me as a person in the best sense. I am lacking words to express all the other aspects of his inﬂuence on my work, but I am sure he will know.

Secondly, my colleague Alexander Gohm accompanied my way for many years now. He was always there when I needed him, his dedication and responsibility for Mete- orology is quite unique and infectuous.

My father, Ignaz Vergeiner, introduced me to the beauty of studying motions in the atmosphere. He demonstrated a lot of his skills to me, and never objected to my sometimes diﬀerent approach.

Thanks also to my mother, Monika Vergeiner. It is a great pleasure to come back to you.

My two sisters, Magdalena and Lena, mean a lot to me. And Josef, you are the best brother-in-law I can imagine.

Of course I should mention all the other students, who contributed to the great atmosphere. The coﬀee breaks, student parties and card playing tournaments (”La- dinisch Watten”, of course) were and are an essential and integral part of Uni-life. Thanks Manni, Andi, Nori, Buchi, Carmen, G¨unther, Roli, Tom, Christoph, Vroni, Fritz, Susanne, Caro, Philipp, Holger, Georg, ...

Stephen Mobbs and Samantha Arnold from the University of Leeds had a huge impact on the success of MAP. They showed so much enthusiasm, working in the ﬁeld until late in the evening, walking and skiing in these strange mountains. Of course

107 108 ACKNOWLEDGMENTS

I also liked your good humour and the positive attitude towards almost everything.

Dr. A. Giez and Dr. M. Krautstrunk from DLR Oberpfaffenhofen did a great job in coordinating the STAAARTE programme. I would also like to thank the Flight Facility Oberpfaffenhofen, who did a great job when flying the Falcon within the Wipp and Inn Valley.

A big hand goes to Andy Sturman at the University of Canterbury for inviting me to New Zealand. This year was a real good experience for me, in many ways. Also the social aspect within the Department was really special. Thanks everyone at Canti!

Last but not least I want to mention my friends out there, who are present when I need them, to make fun and give support. Big hug to Pam, Robs, Heidi, Flo, Hiasl, Karin, B¨arbel, Anton, the people from Bits of Stone (Manni, Flo, Berndt, Rico), Elli, Sammy, Toi, Matsi, Astners, Georg T., Klex, Mike, Marie, Alex N., J¨org,Siggi, Jan, ... Curriculum Vitae

Johannes Vergeiner Reichenauerstrasse 84 / 19, A - 6020 Innsbruck, Austria Born on 16 November 1968 in Boulder, Colorado, USA

Education and Professional training: 1979–1987 Akademisches Gymnasium Innsbruck, Matura. 1987–1997 Diploma study at the University of Innsbruck. Master of Natural Science (Magister rerum naturalium) in Meteorology. 1994–1997 Diploma thesis under the guidance of Prof. H. Pichler, Department of Meteorology and Geophysics, University of Innsbruck: ”Uber¨ den Ein- ﬂuß eines isolierten Gebirges auf eine idealisierte Frontalzirkulation”. 1995 Civil service. 1997–2002 Research assistant and Ph.D. student in the group of Dr. G. J. Mayr at the Department of Meteorology and Geophysics, University of Innsbruck. 2002–2003 Working stay at the Department of Geography, University of Canter- bury in Christchurch, New Zealand. 2003–2004 Research assistant and Ph.D. student in the group of Dr. G. J. Mayr at the Department of Meteorology and Geophysics, University of Inns- bruck.

Participation in meteorological studies: Air pollution dispersion for a fac- tory in Hochfilzen, Tyrol, 1992; Meteorological conditions for dispersion of radioac- tive material after hazardous incident at power plant Three Mile Island, Pennsyl- vania, USA, 1995–1996; Assessment of landfill site Graslboden, Tyrol, 1996–1997; Car-based foehn measurements, Austria, 1998-1999; Gap flow study (MAP), field experiment, Austria, 1999; Winter-time PM10 profiles in an urban environment, Christchurch, New Zealand, 2003.

Meteorological training courses: ”Parametrization of Diabatic Processes”, ECMWF, 1999. 109

Epilogue

Ich schrieb schon eine ganze Weile an der Erinnerung an das Feuer, und je mehr ich schrieb, desto tiefer verstrickte ich mich in die Geschichten, die ich zu erzählenhatte. Es fiel mir bereits schwer, die Vergangenheit von der Gegenwart zu unterscheiden: Was damals geschehen war, geschieht immer noch, und es geschieht neben mir, und Schreiben ist meine Art, auf dieses Geschehen zu reagieren, schreibend schlage ich um mich und finde mich in Umarmungen. Trotzdem, es wird erwartet, daß Geschichtsbüchernicht subjektiv sind. Ich sprach mit JosèCoronel Urtecho über dieses Problem: Dem Buch, an dem ich gerade arbeite, sieht man auf den ersten Blick meine Wut und meine Liebe an. Am Ufer des R´ıoSan Juan sagte mir der alte Dichter, daß man sich einen Dreck um die Fanatiker der Objektivitätscheren soll: ”Hörnicht auf sie”, sagte er. ”So muß es sein. Die Priester der Objektivitätlügen. Sie wollen nicht objektiv sein, das ist eine faustdicke Lüge: Sie wollen Objekte sein, um sich vor menschlichem Leid zu retten.”

aus / from: Eduardo Galeano, ”Das Buch der Umarmungen”

111