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Thermally Driven Flows at an Asymmetric Valley Exit: Observations and Model Studies at the Valley Exit

THOMAS SPENGLER Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland

JAN H. SCHWEEN Institute for Geophysics and Meteorology, University of Cologne, Cologne,

MARKUS ABLINGER,GU¨ NTHER ZA¨ NGL,* AND JOSEPH EGGER Meteorological Institute Munich, University of Munich, Munich, Germany

(Manuscript received 2 September 2008, in final form 31 March 2009)

ABSTRACT

The summertime thermal circulation in the region of an asymmetric valley exit is investigated by means of observations and high-resolution model simulations. The northeastward-oriented Alpine Lech Valley opening into the Bavarian Alpine foreland has an eastern slope exceeding the western slope by about 15 km. Northerly winds along the eastern slope are frequently observed, reaching substantial strength during fair weather conditions. A field experiment has been conducted to explore this phenomenon and to pinpoint the connection of the northeasterly flow to the Lech Valley wind circulation. Numerical simulations have also been carried out to support the interpretation of the observations. It is found that the northerlies owe their existence to the dominantly easterly flow along the foothills of the Alps, which is partly induced by the Alpine heat low but may be strengthened by favorable synoptic conditions. Examples for both situations will be discussed. The diurnal flow in the Lech Valley has little obvious impact on these northeasterlies. On days with moderate synoptic easterly flow, a wake is present on the lee of the eastern slope of the exit region, ac- companied by a shear zone along the edge of the wake. This shear zone is forced southward during the daytime because of thermally initiated pressure gradients between the Alpine foreland and the Alps, leading to sudden wind changes in the exit area at the time of its passage.

1. Introduction the Inn Valley exit region, strong nocturnal low-level jets (10–14 m s21) extending some tens of kilometers into the Thermally driven wind circulations can have a signifi- foreland were observed during clear nights. Similar di- cant impact on the diurnal flow evolution in the vicinity of urnal flow evolutions were also found at the Loisach valley exit regions. At the Inn Valley exit, a day–night (Sla´dkovicˇ and Kanter 1977) and Salzach (Ekhart 1944) asymmetry of the flow was identified during the Meso- Valley exits. The nocturnal low-level jet can generally be scale Experiment in the Region of Kufstein-Rosenheim linked to hydraulic theory applied at a constriction at the (MERKUR; Freytag and Hennemuth 1983; Mu¨ller et al. valley exit (Za¨ngl 2004). 1984; Freytag 1985; Pamperin and Stilke 1985). Although Despite the often very complex and different nature the daytime up-valley flow was found to be rather weak in of these valleys, they have one feature in common: a symmetric opening at the valley exit (i.e., the bounding * Current affiliation: Deutscher Wetterdienst, Offenbach, Germany. slopes of the valley terminate at the same location; see Figs. 1a,b). Henceforth we will refer to valley exits as being asymmetric when one bounding slope is longer Corresponding author address: Thomas Spengler, Institute for Atmospheric and Climate Science, ETH Zurich, Universita¨tsstrasse than the other at the valley exit, as shown in Figs. 1c,d. 16, ETH Zentrum, CH-8092 Zu¨rich, Switzerland. The daytime and nocturnal flow conditions for a sym- E-mail: [email protected] metric setup are shown in Figs. 1a,b. In the exit region

DOI: 10.1175/2009MWR2779.1

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FIG. 1. Schematic illustration of different valley exits with possible flow features. (a) Daytime up-valley flow and (b) nocturnal jetlike down-valley outflow for a symmetric valley exit, re- spectively. (c) Possible up- and down-valley flow for an asymmetric valley exit. (d) Impact of a background flow on the thermally driven circulation in an asymmetric valley exit area. weak winds are expected during the daytime (Fig. 1a), an example of flow over and around a promontory, a whereas a strong jetlike outflow during the night reaches situation which might be of relevance with respect to a significant distance downstream from the valley exit Fig. 1d. Crook et al. (1990) showed that a low Froude into the foreland (Fig. 1b). number flow setup yields flow around the topography, Although many examples of symmetric exits have been which generates a vortex in the lee of the Palmer Divide. discussed in the literature, there has not been much focus The vortex subsequently moves downstream, leaving on the effects of asymmetry of valley exits on the diurnal behind a wake of stagnant air. The effects of diabatic flow evolution. Figures 1c and 1d illustrate two scenarios heating were included in Crook et al. (1991), showing for an asymmetric valley exit region. Figure 1c illustrates strong interactions of shear and thermal instabilities the possible daytime (nighttime) flow setup with up-val- along the convergence vorticity zone. ley (down-valley) flow conditions. The question mark In the European Alps there are several large valleys highlights the issue of how the asymmetry might affect the with asymmetric openings into the Alpine foreland (e.g., daytime (nighttime) inflow (outflow) in the foreland re- the Rhine Valley at Lake Constance and the Rhone gion—that is, whether the mass inflow (outflow) for the Valley at Lake Geneva). Another asymmetric Alpine up-valley (down-valley) winds reaches farther into the valley exit is that from the Lech River into the Ostallga¨u, foreland compared to a symmetric valley exit. Finally, in southwestern (Fig. 2). The Allga¨u Field Ex- Fig. 1d depicts the open question of how an ambient flow periment (AllgEx) was conducted in the latter area to in the foreland might change the inflow (outflow) in the explore the impact of the geographic asymmetry on the vicinity of an asymmetric valley exit. There have been flow evolution near the exit region. some studies on the impact of large-scale flow over valleys a. Geographic setting on the valley flow but none on the flow near valley exits. Wippermann and Gross (1981) showed that the The rather flat Ostallga¨u area is bounded by the pressure gradient related to the ambient flow can lead to Ammer Mountains to the southeast and by the Allga¨uer a channeling of the flow in wide valleys such as the Rhine Alps to the southwest (Fig. 2). The Lech River Valley Valley. Whiteman and Doran (1993) presented similar is bound by the Allga¨uer Alps to the northwest and channeled flow behavior for the Tennessee Valley in the the Lechtaler Alps to the south. The highest adjacent United States. The Denver Cyclone (Szoke 1991), or mountains are around 3000 m. The valley slopes from Denver convergence vorticity zone (Szoke et al. 1984), is approximately the southwest to the northeast with a

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FIG. 2. Topographic map of the Ostallga¨u and the Lech Valley. Letters indicate AWS positions [V1 (, 1093 m), V2 (, 940 m), V3 (, 841 m), P1 (, 792 m), P2 (Buching, 796 m), P3 (Steingaden, 786), and P4 (, 811 m)]. The pertinent mountain ranges are also indicated, as well as the Vilser Mountains (VM, highlighted by the dashed circle) and the Valley (HV). The line indicates the position of the cross section in Fig. 12. varying bottom width between about 300 m near Holzgau near the ground is regularly observed in the Ostallga¨u. (V1; 1093 m) and 4–5 km in the basin of Reutte (V3; The onset of the northeasterly flow is usually charac- 838 m). The valley has several tributaries mostly directed terized by a continuous change from almost calm con- toward the south into the Lechtaler Alps. Between ditions in the morning to northeasterlies peaking around Reutte (V3) and Schwangau (P1) the Lech River Valley 5–8 m s21 in the afternoon. This feature is well known by is bound by the Vilser Mountains (VM), which rise about the local residents and is often used by paragliders and 400 m above the valley floor, creating an obstacle to at- hang gliders for dynamic soaring in the area. These mospheric flow between the valley and the foreland. The winds are most likely related to the larger-scale cyclonic asymmetry of the exit region is manifested by the dis- flow around the Alps in summer due to the Alpine heat tinct protrusions of the ranges embedding the Lech Valley low (Burger and Ekhart 1937; Hafner et al. 1987). exit area, where the northeastern flank of the valley exit Weissmann et al. (2005) pinpointed that a significant (Ammer Mountains) extends a significant distance into the transport of air mass from the Alpine foreland into the Alpine foreland, producing a barrier in ambient easterly Alps is driven by this thermal circulation with maximum flow. The major measuring site was located at Buching northeasterly flow in the late afternoon and evening. (P2), which is located northwest of the nearby Halblech During the nighttime, rather calm conditions domi- Valley (HV). There is also a small ridge of about 100 m nate the Ostallga¨u. Strong nocturnal jetlike outflow, as height between Buching (P2) and Steingaden (P3). in the aforementioned Inn Valley, is usually not ob- served near the surface away from the Lech Valley exit. b. Summer flow conditions in the Ostallga¨u A remarkable feature of the daytime northeasterly During the summer months between April and Sep- flow in the area around Buching is its aforementioned tember, a thermally driven daytime northeasterly flow strength since it is still located in the Alpine foreland.

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The nearby Lech Valley circulation might be one driver TABLE 1. Dates of intensive observation periods (IOPs) during of the flow, but theory would suggest that the strongest AllgEx 2005 with no. of pilot balloons (PB) released and Kali as- winds during the daytime occur in the valley itself. cents (KA). In addition, and in contrast to the daytime evolution Date No. PB No. KA described above, abrupt onset of northeasterly flow is 15 Jul 2005 9 8 also observed in Buching on certain days during sum- 16 Jul 2005 8 8 mer. This can be a hazard to the local pilots because 18 Jul 2005 9 7 wind speeds sometimes exceed the maximum flight ve- 22 Jul 2005 8 8 locity of paragliders, implying that they have to land 23 Jul 2005 8 8 24 Jul 2005 4 3 while flying backward. A preliminary analysis carried 27 Jul 2005 12 8 out together with the local paragliding school (DAeC 28 Jul 2005 12 6 Gleitschirmschule) showed that events of strong north- 29 Jul 2005 9 7 easterly flow in the area are often accompanied by the 1 Aug 2005 12 8 approach of a low with its frontal system from the 4 Aug 2005 6 6 5 Aug 2005 10 10 northwest. This synoptic setup usually has a tendency 9 Aug 2005 9 7 toward foehn conditions in southern Bavaria, which are 10 Aug 2005 8 4 often accompanied by northeasterly flow in the Bavar- 11 Aug 2005 7 2 ian Alpine foreland (Hoinka 1980; Heimann 1997). This 12 Aug 2005 10 0 northeasterly flow, however, is not observed in the area 13 Aug 2005 6 0 around Buching until about noon. The primary goals of AllgEx were to identify the role of the Lech Valley in driving the afternoon northeast- Buching (P2), Schwangau (P1), Reutte (V3), Stanzach erly flow in the Ostallga¨u area and to explore the dy- (V2), and Holzgau (V1) are approximately aligned namics of the sudden onset of northeasterly flow on along a line from northeast to southwest, stretching from certain days during the summer months. These two the Bavarian Alpine foreland into the upper Lech Val- questions strongly relate to the question marks high- ley in . An additional station was created in lighted in Figs. 1c and 1d. Lengenwang (P4), which is located about 20 km west of This paper is organized as follows: The instrumenta- P3, and is primarily utilized as a reference station for the tion of the field campaign is described in section 2. Se- ambient flow to the north of the area of interest. Each of lected surface observations and soundings from the area these AWSs is equipped with sensors for temperature, of investigation are presented in section 3, followed by humidity, wind speed and direction, and pressure (res- section 4 with numerical simulations. Discussion of the olution 0.1 hPa, accuracy 60.3 hPa). Two-minute mean results and concluding remarks are presented in section 5. values were sampled at a height of 2 m, whereas pressure was measured near the ground. At the beginning of the campaign we determined the 2. Instrumentation difference of each station barometer to a reference ba-

A variety of observing systems were in use during rometer (dp0). However, this was not accurate enough to AllgEx, including a network of seven surface automatic adjust for ‘‘correct’’ absolute pressure differences between weather stations (AWSs) with continuous measure- different stations. To obtain pressure measurements of ments during the general observation period (GOP) maximum accuracy, we used the following procedure: from 1 July 2005 until 25 August 2005, as well as pilot Assuming a linear vertical temperature profile between balloons and remotely piloted vehicles equipped with the stations, the pressure of each station was reduced to sensor units, which were used during the 17 days of in- theheightofP1(pi) and averaged over the entire GOP tensive observation periods (IOPs; see Table 1). IOPs ( pi). It should be noted that other temperature depen- were carried out on subjectively selected days when the dencies (e.g., mean temperature) yield negligible differ- thermal circulation was expected to develop during the ences in pressure with the given height differences. Using daytime; in particular, we looked for no frontal activities the mean value ( pi), we determined the mean difference in the area of interest, rather weak upper level flow, and to P1 (dpi 5 pi pP1) for each station. We assume that low cloudiness (i.e., unimpeded insolation). the pressure gradients relative to P1 average out over the GOP and correct for the difference of the P1 barometer to a. Automatic weather stations the reference barometer (dp0), yielding the ‘‘exact’’ pres-

AWSs were deployed by the Meteorological Institute sure for every station: p9i (t) 5 pi(t) dpi dp0.One Munich (MIM) (Fig. 2). The stations Steingaden (P3), should be aware that the assumption that pressure

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FIG. 3. Scatterplot of 3-hourly mean wind for V2 vs P2, morning (0430–0730 CEST; square) and afternoon (1530–1830 CEST; circle). Filled symbols indicate that the daily mean global radiation exceeded 10 MJ m22. A threshold for wind speed .0.5 m s21 was applied. The horizontal and vertical lines indicate the upward (solid) and downward (dashed) direction of the valley axis. The diagonal is indicative for identical values at V2 and P2 and the dotted diagonals are the 458 deviation from it. gradients average out over the GOP is violated if a mean entation of the basis was chosen to be perpendicular to pressure gradient between different stations exists. How- the main wind direction in the target area. The mea- ever, we did not find systematic deviations in the mean suring system is identical to that described in Egger et al. values and thus argue that the proposed procedure is an (2000). During the IOPs, 149 ascents were carried out, appropriate approach to obtain a consistent dataset. aiming for an ascent every hour during the daytime. The For a dynamical interpretation of the wind evolution, vertical resolution is ;20 m and, depending on cloudi- wecalculated the 20-min running mean distance-weighted ness and visibility, profiles were usually taken up to pressure differences between different stations relative 2–4 km above ground level (AGL). to P1 [(pi9 2 pP91)/dL, where dL, is the distance between the respective stations]. We will refer to these weighted c. Remotely piloted vehicles (Kali) differences as pressure gradients. Further access to data from mountain AWSs in the vi- Battery-powered miniature airplanes (1.29 m in length cinity was provided by the local avalanche services in with a wingspan of 2.10 m) collected vertical profiles of Bavaria and (10-min-mean data of temperature, pressure, temperature, and humidity. The observing sys- humidity, wind speed and direction, and global radiation). tem is named Kali and is described in further detail in The station located at Tegelberg (TB in Fig. 2) on a Egger et al. (2002, 2005). During the IOP, 97 flights were mountaintop south of P1 was closest to the area of interest. launched, always aiming for parallel profiling with the pilot balloon. An average height of about 1000 m b. Pilot balloon soundings with theodolite (maximum ’2000 m) AGL was reached with a vertical Two theodolites tracking a helium-filled balloon were resolution of ;7 m. The use of an aircraft in a highly located 2 km southwest of P2 with a basis length and populated area such as Germany is subject to several orientation of 1561 m and 1178, respectively. The ori- rules. However, the AllgEx research area was far away

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3. Results During the 56 days of the GOP we identified three different thermal flow regimes in the area of interest. Days with a flat synoptic pressure distribution and daily total global radiation exceeding 10 MJ m22 (total daily insolation end of July at 488Nis;35 MJ m22 at the top of the atmosphere) are defined as days with a thermal circulation if the diurnal circulation is subjectively identified in the daily wind cycle. In the first regime, a thermally driven circulation is evident in the Lech Val- ley and the foreland (25 days). In the second, only the Lech Valley experiences a thermal circulation (7 days), and in the third, no thermal circulation developed dur- ing the day (24 days). During the observation period, the circulation in the foreland never occurred without the FIG. 4. MSLP with contour interval (CI) of 2 hPa (solid) and Lech Valley circulation being evident as well. Whereas geopotential (CI 5 20 gpm) at 500 hPa (dashed) for 1200 UTC the first regime is found on days with weak synoptic- 28 Jul. Topography shaded in increments of 500 m (white for sea level). The X marks the area of the field experiment. scale forcing, significant large-scale pressure gradients favor the third regime, where the wind at the experi- mental site was dominated by the large-scale synoptic penetrate down to the valley floor because of the south- flow. southwest orientation of the Lech Valley. To pinpoint some differences between the character- In the following we focus on the first regime where the istics of the flow behavior in the Lech Valley and the thermal circulation is also evident in the Alpine foreland. foreland, we briefly discuss the 3-hourly mean wind di- The selected behavior is depicted by two cases, 28 July and rection in the morning and afternoon at V2 and P2 17 August. Whereas the synoptic forcing is almost absent (Fig. 3). In the morning [0430–0730 central European for the first case, weak large-scale easterly flow is present summer time (CEST, which will be used throughout and in the second case. The rapid onset of northeasterly flow at which is UTC 1 2 h); squares], a channeling along the P2 mentioned in the introduction was observed once Lech Valley axis (dashed line) is clearly evident at V2, during the GOP (17 August) but fell outside an IOP. whereas for P2 the direction is much less constrained, a. 28 July indicating that the flow at P2 is not influenced by the Lech Valley outflow at this time. During the afternoon The flow evolution on 28 July appeared to be typical (1530–1830 CEST; circles) the flow is again channeled (according to the obtained dataset) for conditions with along the Lech Valley axis (solid line) at V2. At P2 two weak background flow. Rather weak mean sea level different regimes are evident during the afternoon, fa- pressure (MSLP) gradients are present in the area of voring either southerly or northerly flow. The latter di- interest with a low pressure system southwest of Great rection corresponds to the wind direction of the thermal Britain, and at 500 hPa the geopotential field implies a circulation at P2. mild southwesterly flow across the Alps (Fig. 4). The data points can be further distinguished by sepa- A down-valley wind is evident at V2 in the early rating them into dates on which the daily mean global morning with wind speeds of 2 m s21 (Fig. 5). Southerly radiation is above (filled symbols) or below (blank sym- wind (’4ms21) at P1 indicates some influence from the bols) 10 MJ m22. For values above this threshold, there Lech Valley outflow, whereas almost no wind was re- are more days with up-valley flow at V2 and concomitant corded at P2 and southeasterly winds around 1–2 m s21 northerly flow at P2 in the afternoon, indicative of a were evident at P3. Around 0830 CEST sudden changes daytime thermal circulation in the foreland. There are a at P1, P2, and P3 mark the breakup of the surface-based few outliers in the top left corner (solid circles), which can temperature inversion. Northerly wind speed at P2 in- mainly be related to south foehn events and days with creases with time after 1000 CEST, marking the onset of strong synoptic westerly flow with reduced cloudiness. the thermal circulation in the Alpine foreland, which Southerly (during foehn) and westerly flow are able to lasts until 1900 CEST (at P2). It should be noted that the

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21 FIG. 5. Time series of wind speed (solid lines), y (m s ), and direction (8, crosses) for selected AWS stations—(a) P3, (b) P2, (c) P1, and (d) V2—for 28 Jul.

flow at P4 is easterly, commencing at 1030 CEST and V2 shows the clear signature of a diurnal valley wind lasting until 2030 CEST (not shown). Maximum surface circulation with the aforementioned down-valley winds wind speeds at P2 (;4ms21) were sustained between decreasing from 1000 to 1200 CEST before the wind di- 1400 and 1800 CEST, with the evolution at P1 being rection changes to 08, indicating up-valley flow. The onset similar but with maximum wind speeds 1–2 m s21 lower. of the up-valley wind is rather abrupt around 1300 CEST The evolution at P3 is less clear because of strong vari- with wind speeds reaching 4 m s21 between 1400 and ations in wind speed and direction. 1800 CEST.

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FIG. 6. Potential temperature u and specific humidity q from descents of Kali and wind speed, y, and direction, dir, from pilot balloon ascents during 28 Jul near P2. Different times during the day are indicated by different line styles or symbols.

Inspecting the potential temperature profile, a shift 1.8 hPa, with a maximum in the morning hours and a toward higher values is evident over the course of the minimum in the evening. This diurnal cycle is typical for day indicating warm air advection, as also shown by a thermally driven circulations like sea breezes and valley clockwise turning (with height) of the wind direction winds. However, one should keep in mind the atmo- (Fig. 6). Around 1000 CEST a temperature inversion spheric pressure tide, which has a diurnal contribution as around 400 m AGL is evident, accompanied by weak well (Chapman and Lindzen 1970). According to the wind speed and varying wind directions in the lower empirical formula of Haurwitz and Cowley (1973), one 1000 m. At 1148 CEST, the mixed layer depth (defined would expect the diurnal maximum at P3 around 0930 by constant potential temperature with height) reaches CEST and the diurnal minimum 12 h later around 2130 600 m along with increased wind speed up to 4 m s21 CEST with an amplitude of about 0.38 hPa. Hence, the over the same vertical extent at a fixed wind direction of diurnal tide might explain up to a quarter of the signal 458. The low-level wind speed peaks around 1340 CEST evident in Fig. 7a. The pressure gradient P3–P1 (Fig. 7b) at 8 m s21 at a height of 200 m. It is interesting to note remains almost constant around zero, while the gradient that the depth of the layer with constant wind direction P2–P1 has a minimum of 20.05 hPa km21 in the after- (750 m) remains constant after 1208 CEST. This vertical noon. This implies a local pressure minimum at P2, extent of northeasterly flow appeared to be typical for which was observed on several days with a thermal cir- days with a thermal circulation near P2 and corresponds culation present in the Alpine foreland. The reversal in to the height of the nearby mountains to the south. The sign of the pressure gradient P3–P2 (which can be in- wind above 1000 m AGL is from southerly directions, ferred from the pressure gradients P3–P1 and P2–P1) at implying a counterflow setup in comparison to the lower 0900 CEST leads the onset of northeasterly flow at P2 by levels, and increases from 0943 to 1208 CEST but de- about one hour. The increase in wind speed until 1400 creases thereafter. It is interesting to note that the pilot CEST can also be explained by the increasing pressure balloons commonly recrossed the baseline of the the- difference between P2 and P3. Bearing in mind that the odolites some 20 min after release at about 2500 m AGL distance P3–P2 is about twice the distance P2–P1, the if the thermal circulation was present at P2. southward acceleration P3–P2 of the flow acts over a The surface pressure time series at P1 (Fig. 7a) has larger horizontal extent than its deceleration P2–P1. a diurnal cycle with a peak-to-peak amplitude of about Thus, the weak northeasterly flow at P1 is due to the

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FIG. 7. (top) Time series of surface pressure at P1 and (bottom) surface pressure gradients between selected AWSs (P3, P2, V2) and P1 for 28 Jul. inertia of the accelerated air parcels. The wind variation thereafter until the wind changes its direction to up- before 0930 CEST at P1 can be explained by the peaks in valley flow at 1200 CEST. The sign of the pressure gra- the pressure gradients P3–P1 and P2–P1 and is most dient changes again around 2030 CEST, coincident with likely linked to a local flow phenomenon rather than to a the onset of the down-valley flow. However, there might larger-scale thermal circulation. still be inflow at 100 m AGL. Similar time offsets in the The diurnal cycle for the pressure gradient V2–P1 flow response were also found for the Inn Valley (Za¨ngl shows forcing of down-valley flow in the morning until 2004). the pressure gradient changes its sign at 0930 CEST, The wind at TB (not shown) was from the south with marking the onset of up-valley flow forcing accompa- 5m s21 in the morning until 1200 CEST and then shifted nied by a continuous deceleration of down-valley flow toward the northeast, peaking around 3 m s21 in the late

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P2–P1, which explains about 80%–90% of the observed values (see Fig. 7b). The residual is most likely due to the pressure deficit at P2 connected to the thermal circula- tion of the adjacent Halblech Valley. It should be noted that the time series of the pressure gradient P3–P1, which levels around zero, is somewhat equivocal. In general one would expect a gradual pres- sure decrease toward the Alpine crest in the afternoon related to the larger-scale Alpine heat low. The rather flat curve implies a balance between the pressure de- crease in the area of P1 and the increase in pressure due to local effects such as the gravity wave response. How- ever, the distance between the AWS sites is too large to fully resolve this phenomenon. b. 17 August The flow evolution on 17 August features the sudden onset of northeasterly flow in the area of P2 mentioned FIG. 8. As in Fig. 4, but for 1200 UTC 17 Aug. in the introduction. The MSLP field implies easterly surface flow with rather weak gradients of geopotential at 500 hPa (Fig. 8). afternoon. At 1900 CEST the wind weakened and The wind speed (’6ms21) and direction (’1208)at turned southerly again. P3 are almost constant after 0900 CEST for the entire Analysis of the full dataset reveals that during fair day (Fig. 9), which is also the case for the AWS at P4 weather conditions the thermal circulation features (not shown). This continuance is in contrast to the flow northerlies along the foothills of the Alps (see Fig. 3), evolution at the other stations, where the wind speed at accompanied by a consistently identified local pressure P2 remains below 2 m s21 in the morning until 1130 minimum at P2. The most plausible cause for the after- CEST when a sudden onset of northeasterly flow occurs, noon pressure minimum at P2 is a pressure perturbation with surface wind speeds peaking around 7 m s21 in the pattern initiated by a gravity wave response to the ridge early afternoon. The time series at P1 shows very similar between P2 and P3 as well as to the ridge between P1 features with calm conditions before 1330 CEST fol- and V3 (Vilser Mountains). For two-dimensional (x–z), lowed by a sudden onset of northeasterly flow peaking hydrostatic, Boussinesq flow over orography, gravity around 5 m s21. The striking difference between the wave theory yields p^95 iruNh^ (e.g., Smith 1979), time evolution at P2 and P1 is the time shift in the onset where ^ denotes the Fourier transform of the variable of northeasterly flow by about 2 h. The wind at P2 and p9, r, u, N, and h denote the perturbation pres- weakens around 1500 CEST but northeasterly flow is sure, background density, basic state flow, Brunt–Vaisala evident until the late evening, whereas at P1 the wind frequency, and the heightpffiffiffiffiffiffiffi of the orography, respec- speed decreases after 1900 CEST and shifts to southerly tively. The factor i 5 1 indicates that the perturba- directions, attaining wind speeds around 2 m s21. tion pressure is 908 out of phase with the orography, At V2 the diurnal valley wind circulation is evident yielding positive (negative) pressure perturbations on with down-valley flow until 1100 CEST and increasing the windward (lee) side of the ridge. In prevailing up-valley flow thereafter, with maximum values around northeasterly flow, P2 is on the lee of the P3–P2 ridge 6ms21. The upslope flow starts to weaken after 1900 and P1 is on the windward side of the P1–V3 ridge CEST. The wind at TB (not shown) was from southerly (Vilser Mountains). Thus, we expect a negative gravity to easterly directions for the entire day with wind speeds wave pressure perturbation at P2 and a positive one at between 1 and 5 m s21. P1. We can estimate the relative contribution to the The surface pressure time series at P1 (Fig. 10) fea- observed pressure gradients by assuming N 5 0.01 s21, tures a maximum in the morning and a minimum in the r 5 1kgm23, and u 5 5ms21. The amplitude of the late afternoon similar to 28 July. The pressure gradient pressure perturbation corresponding to the ridge be- P3–P1 indicates southward forcing of flow from 1100 tween P3 and P2 (100 m) together with the pressure CEST onward with almost zero pressure gradients P2–P1, perturbation related to the Vilser Mountains (400 m) indicating that the flow is first accelerated between yields a pressure gradient of 20.038 hPa km21 for P3 and P2. It should be noted that the pressure gradient

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FIG. 9. As in Fig. 5, but for 17 Aug. increase leads the sudden onset of wind at P2 by about Thus, a zone of strong surface wind shear must exist half an hour. The marked pressure gradient increase for between P2 and P3. The presence of the Ammer P2–P1 at 1400 CEST is followed by a sudden onset of Mountains on the windward side suggests that P1 and P2 wind at P1 within 15 min. The V2–P1 time series behaves might be in the wake of this ridge, which possibly gen- very similarly to 28 July. erates the shear zone on its lee. Once the pressure gra- The analyses presented above indicate that P3 and P4 dients are initiated by heating, it appears that the shear tend to record the synoptic easterly flow, whereas P1 and zone is forced southward. It should also be noted that the P2 experience rather calm conditions in the morning. large-scale thermal circulation is stronger than on 28 July.

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FIG. 10. As in Fig. 7, but for 17 Aug.

To support the hypothesis of the moving shear zone Center for Atmospheric Research (NCAR) Mesoscale and to obtain a more complete three-dimensional pic- Model (MM5), version 3.6 (Grell et al. 1995). Five two- ture of the flow evolution, we conducted several model way nested model domains are used with a horizontal studies to pinpoint the inherent dynamical features. mesh size of 64.8 km, 21.6 km, 7.2 km, 2.4 km, and 800 m, respectively; 38 levels are used in the vertical. The lowermost is located at about 10 m AGL. The vertical 4. MM5 experiments distance between the model levels increases from about 40 m close to the ground to 800 m near the upper a. Setup boundary at 100 hPa. The initial and boundary condi- The numerical simulations have been conducted with the tions used for the real-case simulations are obtained fifth-generation Pennsylvania State University–National from operational European Centre for Medium-Range

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but also extending some 30 km to the north of the valley exit (Fig. 11a). Inspection of the vertical structure sug- gests that the wind maximum at the exit is caused by hydraulic effects with a coincident dipping of isentropic surfaces in the lee of the Vilser Mountains (not shown). This strong outflow into the foreland is absent in the surface observations. However, rather intense winds during nocturnal and early morning valley outflow are well known by the local wind surfers of Lake Weissensee just on the lee side of the Vilser Mountains. The jetlike flow disappears rapidly after sunrise because of vertical mixing of momentum initiated by surface heating. It should be noted that the early morning temperature has a positive bias in the model simulations, most likely due to an underestimation of the surface-based nocturnal temperature inversion. However, during the day dif- ferences in temperature are usually smaller than 1 K. Probably because of the underestimation of the low- level inversion, the surface winds are overestimated in the model compared to the observations. The wind during the daytime has small deviations compared to the observations. In the afternoon (Fig. 11b), up-valley flow is evident in the Lech Valley, accompanied by north to northeasterly flow in the Alpine foreland with wind speeds around 4ms21, which is in accordance with the observations. A vertical cross section through the line marked in Fig. 2 is shown in Fig. 12a, indicating that the flow crosses the Vilser Mountains southwest of P1 and subsequently contributes to the up-valley flow. Figure 13a shows pressure perturbation and wind at 1200 m above sea level at 1600 CEST, indicating a pressure gradient forcing flow from the foreland toward the Alps and up-valley. The wind in the vicinity of the valley follows the pressure gradient as is usual for highly ageostrophic thermally driven flow. In the Alpine fore- FIG. 11. Wind vectors at ;180 m AGL, with shading indicating total wind speed, for (a) 0800 and (b) 1600 CEST 28 Jul. Topog- land a geostrophic (easterly) component is evident. raphy of the model domain is contoured; CI 5 200 m. In the pressure gradient time series for selected sta- tions (Fig. 14a), a clear diurnal signal is evident for V2–P1 depicting the forcing of the valley wind circulation; Weather Forecasts (ECMWF) analysis data. In addi- P3–P1 and P2–P1 have a much less pronounced diurnal tion, two semi-idealized simulations have been con- cycle and indicate weak pressure gradients forcing flow ducted by combining realistic and slightly modified toward P1 during most of the day. Comparing the time topography with idealized large-scale flow conditions. series of pressure gradients with observations (Fig. 7), a Further details are given in the appendix. good qualitative agreement is evident but the model fails to predict the observed pressure minimum at P2 and tends b. Results to underestimate the amplitude of the diurnal pressure gradient V2–P1. However, the difference between model 1) CASE STUDY:28JULY and observations is smaller than 0.02 hPa km21,except The flow at 180 m AGL at 0800 CEST features a at P2. The absence of the P2 pressure minimum in the down-valley flow along the Lech Valley with a strong numerical simulations is most likely due to the coarser jetlike outflow at the Lech Valley exit reaching maxi- resolution of the topography, which does not feature the mum wind speeds around 10 m s21 in the area near P1 ridge to the north of P2 and the finer structure of the

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FIG. 12. Vertical cross section along the line marked in Fig. 2 showing contours u (K) and the projected wind vectors and wind speed (shaded) onto the cross section. Both panels are valid for 1600 CEST (a) 28 Jul and (b) 17 Aug.

Halblech Valley and which underestimates the height of shear zone reaches the Lech Valley exit, leaving the Vilser Mountains between P1 and V3. northeasterly flow behind it (not shown). At 1600 CEST the model produces rather uniform northeast- 2) CASE STUDY:17AUGUST erly flow in the Ostallga¨u (Fig. 15c). In the vertical cross At 0800 CEST (Fig. 15) valley outflow is only evident section at 1600 CEST (Fig. 12b), some reminiscence of in the region of the lower Lech Valley, corresponding the wind shear zone is evident near V3 (x ’ 20 km). The to very calm conditions in the P1 and P2 area. However, time evolution of the surface flow compares well with a strong easterly flow is present to the north of the the observations, especially the wake in the P2 area Ammer Mountains and at the crests of the mountains and the easterly flow farther away from the Alps. How- in the area. This easterly flow is related to the synoptic ever, the modeled passage of the wind shear zone at P2 conditions (see Fig. 8). By 1100 CEST (Fig. 15b) up- and P1 leads the observations by one and three hours, valley flow develops in the Lech Valley, whereas wind respectively. speeds near P1 and P2 are still insignificant. However, Stronger gradients of pressure perturbations, com- there is a zone of strong wind shear to the north of P2 pared to the 17 July case, are evident in Fig. 13b, forcing near P3. This shear zone subsequently progresses south- flow toward the south and up the Lech Valley. This fact ward, penetrating into the P2 area. By 1200 CEST the is also depicted in Fig. 14b, where the time series of the

FIG. 13. Pressure perturbation (shaded) and wind vectors at 1200 m above sea level at 1600 CEST for (a) 28 Jul and (b) 17 Aug. Topography of the model domain is contoured; CI 5 400 m.

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FIG. 14. Time series of surface pressure gradients from MM5 model simulations for (a) 28 Jul and (b) 17 Aug. All stations (P3, P2, V2) were reduced to the height of P1. pressure gradients is shown. Again, the forcing of the the generation of the aforementioned shear zone in valley wind circulation is clearly seen in the V2–P1 time prevailing synoptic easterly flow. To pinpoint the sen- series. Looking at the pressure gradients P3–P1 and sitivity of the flow evolution to the existence of the P2–P1, a sudden increase between 1100 and 1200 CEST Ammer mountains, two semi-idealized simulations with is evident, with a time lag of about 30 min between the easterly flow (see appendix for details) were carried out two curves indicating that a positive gradient is first to investigate the dynamical differences. established between P2 and P3. The amplitudes of the There are striking similarities in the wind field between diurnal cycles are about 2–3 times larger than the ones the case study for 17 August (Fig. 15b) and the idealized for the 28 July case. However, one should bear in mind simulation with real topography (Fig. 16a). Note the re- the contribution from the synoptic-scale pressure gra- semblance of the shear zone just north of P2 in the lee of dient related to the easterly flow. Comparing the modeled the Ammer Mountains. The shear zone is completely pressure gradients P3–P1 and P2–P1 to observations (Fig. absent in Fig. 16b where the mountain range was removed, 10), the model leads the changes in pressure gradients by featuring prevailing easterly flow in the Alpine foreland. about one and three hours, respectively, in accordance This test highlights the importance of the Ammer Moun- with the wind (see above). The amplitude of the diurnal tains in generating the wake, which is accompanied by the cycle of the pressure gradient V2–P1 is again under- shear zone. Another interesting feature to point out is the estimated by the model (maximum error ’0.02 hPa increased northeasterly flow component in the area of P2 km21), yet the wind speed in the Lech Valley compares evident in Fig. 16a relative to Fig. 16b, pinpointing a flow well to the observations. The model also produces higher channeling by the asymmetric valley exit. wind speeds in the Lech Valley compared to 28 July, in agreement with the observations. 5. Discussion and concluding remarks In general it should be noted that the model lacks the observed variations in wind, temperature, and pressure In this study, we tried to look for the relationship on time scales smaller than 1 h. However, this is not between the Lech Valley circulation and the north- unexpected because the MM5 model relies on parame- easterly flow near P2. We found that the northeasterly terizations of turbulent processes and is not able to ex- flow near P2 only occurs in tandem with the Lech Valley plicitly resolve eddies on these time scales. circulation. However, we also detected several days on which the Lech Valley circulation was evident, while no 3) IDEALIZED STUDY WITH MODIFIED northeasterly flow developed near P2. Analyzing the TOPOGRAPHY onset times of the thermal circulation, there is no evi- The case study for 17 August suggests that the wake dence that the Lech Valley wind circulation is leading effect of the Ammer Mountains plays a crucial role in the northeasterly flow at P2.

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Lugauer and Winkler (2005) suggested a local heat low situated to the west of the Ostallga¨u as the forcing mechanism for the northeasterly flow. This hypothesis cannot be ruled out using our observations, but the model simulations did not produce a local heat low in the proposed area in either of the case studies. The far- reaching (150–200 km outward from the main Alpine crest) convergent flow into the Alps on summer days forced by the Alpine heat low (Burger and Ekhart 1937; Weissmann et al. 2005) can be seen as a more general mechanism forcing the northeasterly flow near P2. Based on the arguments above, we argue that the Lech Valley circulation is not the sole driving mechanism. The northeasterly flow near P2 is rather a result of the easterly flow related to the Alpine heat low, which is partly deflected by the Ammer Mountains to the south- west into the Lech Valley and cannot be seen as a simple extension of the Lech valley winds. We identified a local pressure minimum on days with a thermal circulation at P2, which is most likely due to hydrostatic pressure perturbations initiated by a gravity wave response above the ridges between P1 and V3 as well as between P3 and P2. The higher frequency of the Lech Valley circulation compared to the occurrence of the thermal circulation in the Alpine foreland is most likely linked to the different spatial scales of the wind systems and a channeling of the flow in the valley. The difference in magnitude of ther- mally initiated pressure gradients can usually be attrib- uted to the so-called volume effect [also called the topographic amplification factor (TAF); see Wagner 1932; Steinacker 1984; Whiteman 1990]. The TAF de- scribes the ratio of the volume of air within a valley to a volume of air comprising the same horizontal area but over a flat surface. Assuming that the total solar energy input for a given surface area is the same, the TAF de- scribes the ratio of energy being available for heating the volume of air. Thus the TAF is closely linked to differ- ences in the diurnal temperature evolution between the valley and the plane and hence to the hydrostatic pres- sure gradients and the intensity of the diurnal valley winds. For an idealized V-shaped valley the TAF is ex- actly 2; if the valley slopes become more convex (con- cave), the TAF will become larger (smaller) than 2. It should be noted that if tributaries are included the TAF becomes even larger (Steinacker 1984). The Lech Valley TAF, excluding tributaries, is 2.02 and is thus comparable to the TAF of the Austrian Inn Valley in- cluding tributaries (TAF 5 2.1; Steinacker 1984). For comparison, the smaller and higher Alpine valleys like the Stubai, Wipp, and Dischma Valleys have TAFs of 3, FIG. 15. As in Fig. 11, but for (a) 0800, (b) 1100, and (c) 1600 CEST 3, and 2.7, respectively (Steinacker 1984; Mu¨ ller and 17 Aug. Whiteman 1988). Even though the TAF appears quite

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FIG. 16. Wind vectors at ;180 m AGL, shading indicating total wind speed, for idealized setup with easterly flow (a) including real topography and (b) without the Ammer Mountains, both at 1100 CEST. Topography of the model domain is contoured; CI 5 200 m. useful in comparing the strengths of different valley vortex in the lee, which subsequently moves downstream wind systems, it is not obvious how one can explain the leaving a wake behind. However, in our simulations nei- difference in frequency between the two wind systems ther a cyclone in the lee nor a subsequent downstream observed at the Lech Valley exit by only using TAF development is evident. There are two possible reasons arguments. for these differences: (i) different scales of the two phe- Regarding the mechanism yielding the sudden onset nomena, which are separated by an order of magnitude, of northeasterly flow near P2, the observations suggest or (ii) the wake in the lee of the Ammer Mountains is the existence of a shear zone between P2 and P3 moving affected by the Lech Valley wind circulation. Rerunning southward during the daytime. This hypothesis is sup- the idealized model simulation but stretching the real ported by the numerical simulations, where synoptic topography by a factor of 5 yields basically the same easterly flow yields a wake in the lee of the Ammer results presented above, indicating that the scale is most Mountains during low Froude number conditions in the likely not the key parameter determining the difference. night and early morning. At the northern edge of the We believe that the absence of the lee vortex in the wake a shear zone is present (Fig. 15a), which is subse- Ostallga¨u is primarily related to the wind field at the quently forced southward by the pressure gradients re- valley exit region. The nocturnal valley outflow coun- lated to differential heating between the foreland and teracts the lee vortex formation and the daytime low- the Alps. According to Wippermann and Gross (1981) level wind field acts to remove low-level air into the and Whiteman and Doran (1993), the pressure gradient Lech Valley hampering the wake formation. related to the synoptic easterly flow can also lead to a The results for an asymmetric valley exit presented in channeling of flow in the presence of mountainous ter- this study are inherent to the Lech Valley. However, we rain. This could be an additional mechanism moving the are confident that the discussed features are also evident shear zone southward. However, the pressure gradient at other asymmetric valley exits, especially the differ- related to a geostrophically balanced 5 m s21 easterly ence between the higher frequency of valley winds wind only explains 10%–20% of the observed pressure compared to the frequency of the larger-scale thermal gradients. Removing the Ammer Mountains in a semi- circulation in the foreland, as well as possible wake ef- idealized model experiment illustrates the effects of the fects due to the topographic setting and the synoptic asymmetry of the valley exit, since no wind shear zone is flow. It would be worthwhile to compare our findings to generated with a symmetric valley exit and easterly flow flow evolutions at other valley exits as well as utilizing prevails in the entire Alpine foreland (see Figs. 16a,b). idealized model studies to pinpoint the influence of the The Denver Cyclone (convergence vorticity zone; Szoke degree of asymmetry on the flow evolution. 1991) evolves in a similar flow setup with a wake behind a mountain range. Crook et al. (1990) showed that a low Acknowledgments. Theauthorsaregratefultothe Froude number flow around a mountain range produces a communities of Rieden and Buching for their support.

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Special thanks are given to the DAeC Gleitschirmschule The procedure to generate the idealized flow condi- for helpful input in the planning phase and the Forg- tions for the semi-idealized experiments is described in gensee Yachtschule, where we had our base during the Za¨ngl (2007). The flow field considered here has a wind entire campaign. We also thank all the farmers (A. and direction of 908 and positive vertical shear with the flow M. Falger, E. and I. Falger, T. Storf, T. Velle, J. and speed increasing from 7.5 m s21 at sea level to 20 m s21 R. Bellmund, T. Ha¨ußrer, J. Christa, J. Fritsch and J. and at 250 hPa, with constant values higher above. The H. Greißl) for providing some space for measuring ambient wind field is in geostrophic balance, assuming a equipment and last but not least the staff for AllgEx: constant Coriolis parameter of 1024 s21. The tempera- H. Lo¨sslein, H. Wendt, H. Aschenbrenner, P. Kolb, ture profile in the domain center starts from a sea level C. Schmidt, S. La¨mmlein, M. Garhammer, I. Nudelmann, temperature of 158C, followed by an isothermal layer up S. Raith, C. Schmidt, M. Zink, and M. Zwanzger. We also to a pressure of 900 hPa. Higher above, the temperature appreciate interesting discussions with C. Davis on the decreases at an average rate of 7 K km21 up to the material pointing out some similarities to the Denver tropopause, which is located at 250 hPa. The strato- Cyclone. We thank Tracy Ewen for a careful proofread- sphere is again assumed to be isothermal. To avoid the ing of the manuscript. The comments by the anonymous formation of clouds in the idealized simulation, the referees were constructive and helpful. relative humidity is set to 50% within the low-level isothermal layer and to 30% in the remainder of the troposphere. Radiation in the semi-idealized simula- APPENDIX tions is computed for an artificial date of 10 August.

Model Setup REFERENCES

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