Propagation Characteristics of Thunderstorms in Southern Germany

InstitutfurPhysik der Atmosphare Report No. 97

Propagation characteristics of thunderstorms in southern Germany

by

Martin Hagen, Blasius Bartenschlager, Ullrich Finke

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Oberpfaffenhofen Mai 1998

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Portions of this document may be illegible electronic image products. Images are produced from the best available original document. Institut fur Physik der Atmosphare Report No. 97

Propagation characteristics of thunderstorms in southern Germany

by

Martin Hagen, Blasius Bartenschlager, Ullrich Finke

* DLR Propagation characteristics of thunderstorms in southern Germany

M. Hagen, B. Bartenschlager, U. Finke, Deutsches Zentrumjur Luft- und Raumfahrt, Institutfur Physikder Atmosphare, Oberpfaffenhofen, 82234 Wefiling, Germany

Paper submitted to Meteorological Applications

The propagation of thunderstorms in southern Germany was investigated. The thunderstorms were observed by a lightning position system duringthe summer months of the years 1992 to 1996. On average every second day thunderstorms were observed anywhere in southern Germany. In general thunderstormsapproach from westerly and south-westerly directions. The average speed is 13 m/s. No significant relation between the occurrence of thunderstormsand the large scale synoptic pattern described by the Grosswetterlagen (large scale weather pattern) was found. Thunderstorms were observed during almost all Grosswetterlagen. The reduction to 8 weather pattern based on the low- level flow in southern Germany showed that thunderstorms are likely when the flowhas westerly di­ rections (43%) or easterly directions (20%). Three distinct groups of differentlightning patterns could be identified; stationary, moving thunderstorms and thunderstorm lines. The convective avail­ able potential energy (CAPE) and the wind shear were retrieved from the radio soundings from Munchen and Stuttgart. On average CAPE was 583 J/kgfor stationary, 701 J/kgfor moving thunder­ storms, and 876 J/kgfor thunderstorm lines. The average bulk Richardson numbers are 152, 80 and 52 for stationary, moving thunderstorms and thunderstorm lines, respectively. The steering level was found to be at about 3 and 6 km m.s.l. However, it should be noted, thatin most cases the soundings do not completely describe the local environmentof thunderstorms, since radio soundingsare only available twice a day.

1 Introduction for thunderstorms in the state of maturity. By means of a phenomenological classification based on different patterns and structures of the spatial lightning distri­ During summer thunderstorms are a frequent weather bution the thunderstorms have been divided into three phenomena in southern Europe. They have an essential types. These phenomenological types of thunderstorms contribution to the total precipitation in southern Ger­ then have been related to environmental and synoptic many. Thunderstorms are connected with lightning conditions. strikes, heavy precipitation, hail and strong turbulence, thus having a considerable potential of endangering Only a few related studies for Central Europe are and damage. For that reason more knowledge of the known. The analysis by Pelz (1984) is based on syn­ development and motion of thunderstorms is of great optic observations (1893 until 1907) and gives the interest and represents a central problem in nowcasting spatial distribution of thunderstorm days in Germany. and short range forecast. More recent studies have been performed with radar data by Holler (1994) and by Finke and Hauf The object of this paper is to show how the propaga­ (1996a+b) using lightning observations. Houze et al. tion characteristics and the organisation of thunder­ (1993) and Schiesser et al. (1995) investigated the storms in southern Germany is related to environ­ mesoscale structure of severe precipitation systems in mental and synoptic conditions. For this purpose a Switzerland. great number of thunderstorm days (1992-96) in southern Germany have been investigated. A lightning Holler (1994) showed how the mesoscale organisation location system operated by the Badenwerk AG and of thunderstorms in southern Germany is related to the the Bayern werk AG in southern Germany has been occurrence of hail. Based on radar observations thun­ used for the determination and classification of thun­ derstorms where classified as single cells, multicell derstorm days. Lightning is an unambiguous indicator storms and supercell storms. Their mesoscale organi ­

1 sation was classified as isolated storms, clusters, line- 2 Types of thunderstorms and envi­ oriented storms and squall lines. Further details will be ronmental conditions given in Section 2. The analysis of polarimetric radar data showed that at 61% of the thunderstorm days hail was observed. The development of specific storm characteristics such as the organisation of the cells, life cycle, or propaga­ Finke and Hauf (1996 a) statistically evaluated the tion velocity depends mainly on environmental condi ­ tracks of moving thunderstorms in southern Germany tions. This environment is described by the synoptic by observations from a lightning network for a 3-year state of the atmosphere and, more detailed, by the time period (1992 - 1994). According to the lightning vertical profile of the wind, temperature and humidity. patterns the thunderstorms have been divided into three classes. More details can be found in Section 3. Single-cell storms were observed at 37%, storm tracks 2.1 Organisation of thunderstorms at 57% and fronts at 6% of the thunderstorm days, respectively. A frequency distribution of storm tracks Holler (1994) summarised the types of thunderstorms shows that for 56% of all cases the storms approach in southern Germany based on the storm scale and from south-west or west. The mean velocity is 47 mesoscale organisation. On the storm scale three basic km/h. The most frequently observed lifetime of storms types can be identified (after Foote, 1985): is 4 hours. Storm tracks longer than 250 km can be found on an average of 18 days a year. - Single cells, where the complete cycle of pre­ cipitation development and fallout takes place in Characteristic properties of thunderstorms like the life one cell within roughly half an hour. cycle and the type of organisation principally depend on environmental conditions. Numerical studies by - Multicell storms. All stages of development are Weisman and Klemp (1982 and 1984) show typical coexisting each one forming a part of the same values of environmental buoyancy and wind shear for storm system. Those systems can last for a few different types of thunderstorm organisation. The bulk hours. Richardson number was shown to be a suited indicator for the storm type: low values are observed in the - Supercell storms. The storm is formed by a large surrounding of supercells, higher values are observed quasi-stationary cell. All the different stages take with multicell storms. place in the same cell but at different locations. Supercell storms can last up to 12 hours. More recently thunderstorms in Switzerland have been investigated and environmental parameters have been This classification requires knowledge of the dynamic calculated to separate heavy hail thunderstorms from and microphysic structure of the storm. Alternatively, other thunderstorms. It was found that at thunderstorm phenomenological approaches classify the storms on days with stronger hail damage the mean value of their appearance as seen by an observation system. CAPE (convective available potential energy, see Based on the mesoscale shape of radar echoes the Section 2) is 656 J/kg and for the most heaviest hail ­ following mesoscale organisations are observed in storms the mean value is 876 J/kg (Schiesser et al., southern Germany (Holler, 1994; after Houze et al., 1997). 1990):

In the Central-European region almost no attempt has - Isolated storms. Thunderstorms exist isolated been undergone to investigate the great variety of and are not embedded in common reflectivity known indices which are used to describe the prob­ contours. ability of thunderstorm occurrence. However, Huntrie- ser et al. (1997) tested several indices for their useful­ - Clusters or complexes of storms. Thunderstorms ness in Switzerland. Their results show that CAPE is are closely grouped together. A cluster does not one of the successful indices to be used for forecasting have a preferred direction of alignment. thunderstorms. - Line-oriented storms. Isolated storms or storms The classification of thunderstorms, their initiation, within a cluster are aligned along lines. and the environmental conditions are outlined in Sec­ tion 2. Section 3 describes the data base and the proc­ - Squall lines. Special form of line-oriented sys­ essing of the lightning data, and Section 4 gives the tems. Squall lines are often characterised by a results of this study. leading line of heavy convection followed by a region of widespread stratiform precipitation.

2 In principle there is no direct relation between these observed in favourable Grosswetterlagen. The two kinds of classification. Except for the isolated Grosswetterlagen and a reduced classification (see storms the mesoscale organisations require long living Section 3.2) are listed in Table 1. multi- or supercell storms. For the characterisation of the storm environment we For this study we will use a scheme which focus on use the convective available potential energy {CAPE), properties like lifetime and motion of lightning clus­ wind shear energy and the bulk Richardson number. ters. This methodology is described in Section 3.3. These quantities are defined in the following sections.

During summer normally thunderstorms are initiated by lifting of unstable air. Those thunderstorms are 2.3 Stratification of the airmass termed as air-mass storms. If the air is only potential unstable additional forced lifting is necessary. Initial A necessary condition for thunderstorm development, lifting can be forced by orography or approaching independent of the mechanism of initiation, is a poten­ frontal storms. tial unstable stratification of the atmosphere (e.g. Weisman and Klemp, 1982).

2.2 Synoptic state of the atmosphere The stratification of airmasses can be determined from radio soundings. In order to have a simple access to The synoptic state of the atmosphere describes the the probability and strength of thunderstorm develop­ environment as the source of the whole system. Hess ment a great number of indices do exist (e.g. Huntrie- and Brezowsky, and later continued by Gerstengarbe ser et al., 1997). However, for the calculation of these and Werner (1993), divided the synoptic weather indices usually data only from a few pressure levels pattern in Europe in 29 large scale weather pattern are used. (Grosswetterlagen), depending on the large scale flow pattern in Central Europe. The Grosswetterlagen are To estimate the kind and strength of expected thunder­ defined by the direction of the surface flow and the storms, a quantity is chosen that characterises the state flow at 500 hPa in Central Europe considering the of the atmosphere in a more extensively way. An inte­ origin of the airmasses and the pressure field over gral measure of the lability of the atmospheric stratifi­ Europe and the Northern Atlantic. It should be noted cation is the buoyancy energy CAPE (convective that during the same Grosswetterlage the weather available potential energy). The buoyancy determines itself at a location can change considerable. It is ex­ the acceleration of air volumes and gives a measure of pected that distinct types of thunderstorms will only be thunderstorm intensity. CAPE corresponds to the spe-

Grosswetterlage low- Grosswetterlage low- (large scale weather pattern) level (large scale weather pattern) level flow flow Zonal circulation patterns Meridonal circulation patterns westerly, anticycl. WA W northerly, anticycl. NA N westerly, cyclonic WZ W northerly, cylonic NZ N westerly, southern ws W high, North Atlantic, anticycl. HNA W westerly, angular ww NW high, North Atlantic, cyclonic HNZ SW Mixed circulation patterns high, British Isles HB NE south-westerly, anticyc. SWA sw trough, central Europe TRM NW south-westerly, cyclonic SWZ w north-easterly, anticyc. NEA NE north-westerly, anticyc. NWA NW north-easterly, cyclonic NEZ NE north-westerly, cyclonic NWZ NW high, Scandinavia, anticyc. HFA E high, central Europe HM E high, Scandinavia, cyclonic HFZ E bridge, central Europe BM E high, north Scandinavia, anticycl. HNFA SE low, central Europe TM W high, north Scandinavia, cyclonic HNFZ E south-easterly, anticycl. SEA SE south-easterly, cyclonic SEZ SE southerly, anticycl. SA SW southerly, cyclonic SZ SW low, British Isles TB W trough, western Europe TRW W

Table 1. The 29 Grosswetterlagen (large scale weather pattern) after Hess and Brezowsky, their abbreviation and a re­ duced classification according to the low level flow in southern Germany.

3 cific energy which is released during the lifting of an 6000 500 air volume with unit mass from its level of free con ­ j u(z) ■ p(z) • dz J u(z) • p(z) • dz vection (LFC) to the level of neutral buoyancy (NB): — _ 0______0______11 ~ 6000 500 NB J p(z) • dz | p(z) • dz 0(PLP -6r CAPE = g ■ J dz 0 0 ©,>[Z [J/kg] LFC and correspondingly for V , where A(z) is the vertical density profile and u(z), v(z) are the vertical profiles of where g is the earth acceleration, 0LP the potential the orthogonal wind components, and z is the height in temperature of the ascending air volume, and 0V the meters above ground. potential temperature of the environment. It can be assumed that the motion vector of moving CAPE is a good indicator for stability or lability of air thunderstorms is related to the wind profile. Moreover, masses. Positive values of buoyant energy indicate it can be assumed that the storms are directed by the unstable stratification and negative values are ob­ flow in a certain altitude range. This altitude is termed served at stable stratification. Houze et al. (1993) give as the steering level. In Section 4 we will compare the values between 340 and 2340 J/kg for mesoscale con ­ observed storm motion with the wind profile. vective systems in Switzerland.

The maximum updraft velocity in convective clouds is 2.5 Bulk Richardson number estimated by (neglecting losses due to friction and entrainment ) The Richardson number is a dimensionless measure for the stability of a dynamic system. It is defined by -yjl- CAPE the ratio of energy available for the vertical motion W max (buoyancy energy) and energy produced by vertical wind shear. Three definitions are commonly used It was shown by Haase-Straub et al. (1997) or Huntrie- (Stull, 1988). Here, the bulk Richardson number is ser et al. (1997) how variable the value of CAPE can used, because of its simple relation between CAPE and be, if different lifting levels are applied. The above shear: definition of CAPE uses dry adiabatic lifting from the surface conditions (temperature and mixing ratio) until CAPE condensation is reached.

2.4 Vertical wind profile Richardson numbers less than 1 indicate development of turbulence, values greater 1 point to decreasing The vertical wind shear has a great influence on the turbulence. Weisman and Klemp (1982) have found organisation of convection (Weisman and Klemp, for the middle west of the USA, that at values between 1982). The characteristic structures of single, multi- 15 and 35 development of supercells takes place. Val­ and supercell storms are mainly caused by a different ues greater than 35 conditions are favourable for the structure of the wind profile. For the formation of development of multicells. multi- or supercells a distinct stronger wind shear in lower levels is necessary than for single cells. 3 Database The vertical wind shear can be characterised by a density weighted shear energy (Weisman and Klemp, 1984; Moller et al., 1994). The density weighted 3.1 Area of investigation (kinetic) shear energy is defined by Thunderstorms in southern Germany have been inves­ tigated. The investigation area extends in west-east- direction between the mountain ridges of the Schwarzwald and the Bayerischer Wald, and in north- with U and v representing the density weighted south-direction between the river Main and the Alps. shear velocities: Its dimension is about 350 x 350 km2. The orography is inhomogeneous. Therefore, orographic effects can play a great part in the development, formation and motion of thunderstorms.

4 3.2 Synoptic and sounding data

The daily classification of the Grosswetterlagen is available from the Europaische Wetterbericht issued by the Deutscher Wetterdienst.

For statistical considerations it seems more efficient to comprise the 29 classes to a smaller number. There ­

fore, we grouped the weather pattern by Hess and Munchen Brezowsky into 8 classes describing the surface or low-level wind direction in southern Germany. Also the local effects due to the Alps as a barrier to the south were considered (see Table 1). This modified system of classes preserves mainly the origin of the airmass, however, the information on the curvature of 1.5.1993 12 16 20 24 UTC the flow (cyclonic, anticyclonic) is lost. This reduced number of pattern does better reflect the local flow in the region of interest.

The dynamic and thermodynamic parameters are de­ termined from radio soundings at Munchen and Stutt­ gart. Both stations are within the investigation area, however, the low spatial (app. 200 km between both) and temporal density (only twice a day at 00 and 12 UTC) of soundings remains a great problem. Espe­ cially because meteorological phenomena are investi­ gated which do not seldom occur in the meso-y-scale and have a duration of only a few hours.

The sounding at 12 UTC was preferred, even though storms often occur in the late afternoon and can last

until the early morning. The maximum storm activity 5.8.1993 is observed between 14 and 20 UTC (Finke and Hauf, 12 16 20 24 UTC 1996 b). The sounding of Munchen was preferred because it represents the more homogeneous and larger part of the investigation area. If this sounding was not available or the storms were only observed in the northern part the sounding from Stuttgart was used. Numberg ;

3.3 Classification of thunderstorms by their lightning pattern Munchen Since 1992 a lightning positioning and tracking system (LPATS) covers southern Germany. It is operated by the two power-supply companies Badenwerk AG, Karlsruhe, and Bayernwerk AG, Munchen (Hoffman and von Rheinbaben, 1991; Fister et al., 1994). With this system about 70% of the cloud-to-ground light ­ 25.6.1994 12 16 20 24 UTC ning flashes can be detected. The accuracy of the lo­ calisation is 1 km and the temporal resolution is 15 ms.

The use of lightning location data for thunderstorm Figure 1. Examples for thunderstorm days in southern monitoring is advantageous due to its continuous Germany. Lines indicate cities, rivers and international availability in time with nearly uniform spatial cover­ borders. Grey-level code indicates the time of the day. a) age. stationary thunderstorms, b) moving thunderstorms and c) thunderstorm lines.

5 Animations of the spatial lightning patterns are avail­ comparison with the classification for this study and an able for the years 1992 to 1996. Examples of images independent one (years 1992 through 1994) by Finke are shown in Figure 1. Grey-level codes stand for the and Hauf (1996 a) shows a good agreement. time of the day. Three different forms of organisation can be distinguished. The examples have (by reason of For moving thunderstorms the direction and speed of clearness) model character and are not representing the motion was determined. However, the lightning pat­ majority of the thunderstorm days. terns are not always rectilinear. Long moving storms may change their direction with time. Due to storm - The first class are small clusters of lightning po­ splitting and merging the storm direction and speed sitions that seem not to be organised (Figure 1 can vary. a). They appear almost simultaneously in many parts of the observation area. The distinguishing property is a short life time (shorter than one hour ) and an almost stationary cluster position. 4 Results These thunderstorms will be termed "stationary thunderstorms". 4.1 Distribution of storm types

- The second group are lightning events, which A total of 699 days were analysed (May to September follow along a line (Figure 1 b). This indicates in 1992 to 1996). At 53% of all days anywhere in the lightning generated in thunderstorms which are investigation area thunderstorms have been observed moving and mark a track. These line structures (Figure 2). The minimum was 46% in 1995, the are very different in width, length and velocity. maximum 65% in 1994. On average stationary thun­ One storm track can have broader and thinner derstorms occur on 46%, moving thunderstorms on sections. A ratio between length and width of 44% and thunderstorm lines on 10% of the thunder­ minimum 3, and a minimum length of 50 km was storm days, respectively. required. Thunderstorms describing such tracks will be termed "moving thunderstorms". Figure 3 shows the distribution of the motion direction for moving thunderstorms. In more than 63% of the - The third class are lightning aligned along a line cases the storms arrived from the sector between almost perpendicular to the moving direction south-west and west with a maximum at about 240°. (Figure 1 c). These thunderstorms are supposed All other directions are seldom observed. A small, but to be related to frontal systems, however, only significant secondary maximum is around south­ the visual appearance is considered. The crite­ easterly directions. Thunderstorm lines arrived exclu­ rion for differentiation to the second class is, that sively from south to west directions with a distinct the patterns are more broad then long (seen in maximum at west. moving direction). Also, during the motion a clear defined front line can be identified. Their The speed of the moving storms varies between 9 m/s width exceeds at least 100 km. Thunderstorms of and 18 m/s (Figure 4) with a large scatter. A mean ve­ this class will be termed "thunderstorm lines". locity of 13 m/s can be determined. In a few cases ve­ locities higher than 21 m/s were observed. These high It has to be pointed out that this classification of thun­ velocities are not related to the motion of air masses, but derstorms is a pure phenomenological one on the base correspond to the phase speed of the leading front of the of lightning signatures. A priori there is no relation to storm. For most of the thunderstorm days the speed the classification based on environmental conditions listed in Section 2. 100%

Based on this phenomenology all thunderstorm events during summer (respectively from May until Septem­ ber) from 1992 to 1996 have been assigned by manual inspection to one of the three groups. A day was re­ garded as a thunderstorm day, if at least 100 lightning flashes have been observed in the investigation area. The observed events have to last at least 20 minutes. Sometimes two (very rarely three) groups exist for the same time in the investigation area. Also mixed forms are possible. In these cases the dominating, respecti­ vely the most obvious class was counted. If there was □ stationary ll moving ■ lines D no storms a significant change during the day, the day was di­ Figure 2. Annual frequency distribution of days with and vided into two independent "thunderstorm events". A without thunderstorms during May to September.

6 30

o o o o o o o o o o O o CO CO O) CM in CO i— N o CO T- CM CM CM CO CO 3 6 9 12 15 18 21 24 27 30 approaching direction (deg.) speed (m/s) Figure 3. Frequency distribution of the direction from Figure 4. Frequency distribution of the motion velocity of which moving thunderstorms approach. moving thunderstorms. was nearly the same for all storms. The speed varia­ with low thunderstorm probability (app. 30%) are the tions with time and geographical location were less pattern HB and NZ. Higher probability (70 to 80%) of than 3 m/s. The relation between the storm velocity thunderstorms is observed at the patterns SWZ, BM, vector and the wind profile is discussed below in section HNZ, HNFA and TRW. At three weather pattern 4.6. (NWZ, HNA, SEA) thunderstorms are appearing with a probability of 80 to 100%. However, these weather pattern are seldom. Thunderstorm lines have been 4.2 Weatherpattern and thunderstorm observed at 15 weather pattern only, whereas, no clear types separation between stationary and moving thunder­ storms can be observed. In Figure 5 the relative and absolute frequency of days without thunderstorms and days with the different Figure 6 shows a clearer view. Here the reduced num­ types of thunderstorms relative to the Grosswetterla- ber of weather pattern (cf. Section 3) considering the gen (weather pattern) are shown. It should be noticed low-level flow in southern Germany was used. Thun­ that thunderstorms have been observed at nearly all 29 derstorms are observed at all patterns, the ratio be­ Grosswetterlagen. Only six Grosswetterlagen were tween stationary and moving thunderstorms is fairly not observed during summer time. Weather pattern equal. Thunderstorm occurrence is concentrated at the

z r_ XX large scale weather pattern (Grosswetterlagen)

□ stationary ■ moving ■ lines Dno storms

large scale weather pattern (Grosswetterlagen)

Figure 5. Absolute (top) and relative (bottom) frequency of days with thunderstorms relative to the 29 Grosswetterlagen (weather pattern) after Hess and Brezowski. U stands for undefined weather pattern.

7 w 400 -§ 300 stationary thunderstorms 2 200 #100 %20 r = 0 N NE E SE S SW W NW U weather pattern with respect to low-level flow

CAPE (J/kg)

£ moving thunderstorms Z20 ill I 5

Figure 6. Absolute (top) and relative (bottom) frequency of 200 800 1400 2000 2600 days with thunderstorms relative to the weather pattern CAPE (J/kg) defined by the low-level flow in southern Germany. U stands for undefined weather pattern. classes "west" and "east". Despite that moving thun­ derstorms and thunderstorm lines normally approach from the west the surface flow in southern Germany is often observed to be easterly ahead of large convective systems (Meischner et al., 1991; Holier et al., 1994; Haase-Straub et al., 1997). This indicates the conver­ gence and the vertical shear associated with those systems. During the rare occurrence of southerly flow Figure 7. Frequency distribution of the convective available only stationary thunderstorms are observed. This is potential energy /CAPE). due to the descending dry foehn flow over the Alps in which heavy thunderstorms are unlikely. occurred later on. This may explain the large scatter of observed CAPE values.

4.3 Relation between stratification and A priori there is no relation between CAPE and the thunderstorm types observed storm types. However, high CAPE values favour the development of higher organised and long living storms. In Figure 7 the frequency distribution of the convec­ tive available potential energy (CAPE) is shown for The average values of CAPE are in agreement with the three types of thunderstorms. The computation of those found in Switzerland (Schiesser et al., 1995 and CAPE is based on the temperature and the humidity at 1997). They report mean values of 656 J/kg for days the surface observed at the 12 UTC sounding. For all with hail damage. An average CAPE of 938 J/kg was three types the values of CAPE are in the same order. observed during the days with the heaviest hail damage CAPE values up to 2800 J/kg have been observed for by line oriented thunderstorms. This is corresponding to single events (e.g. Haase-Straub, 1997). our classification of thunderstorm lines (average CAPE of 876J/kg). The mean values for the three frequency distributions range from 583 J/kg for stationary thunderstorms, 701 The difference between the CAPE for days without J/kg for moving thunderstorms and 876 J/kg for thun­ storms and days with stationary storms is small. This derstorm lines. According to Houze et al. (1993) the underlines the fact that high CAPE is a necessary con ­ values of CAPE for convective systems with medium dition for deep convection but not always sufficient. intensity are at least 2000 J/kg. This holds for single Thunderstorms do not develop even for high CAPE events, the majority of the observations gives lower values, if the energy which is necessary to reach the CAPE values. In several cases the soundings do not level of free convection (CIN) is too high and not exactly describe the storm environment. Also, some­ provided by forced lifting. times the noon soundings of Munchen or Stuttgart are not representative for the airmass in which the storms The stratification and humidity of the boundary layer has a strong influence on the computation of CAPE.

8 40 :------>, stationary thunderstorms -8 30 -m

1 4 7 10 13 16 0 50 100 150 200 250 shear energy (J/kg) Richardson number

25 r moving thunderstorms

o 20 h ! <5 15 *-

7 10 12 shear energy (J/kg) 100 150 200 250 Richardson number

thunderstorm lines

1 4 7 10 13 16 0 50 100 150 200 250 shear energy (J/kg) Richardson number

Figure 8. Frequency distribution of the density weighted Figure 9. Frequency distribution of the bulk Richardson wind shear for the three classes of thunderstorms. number fRi). To reduce the effects of the boundary layer, addition ­ It is concluded that at low shear short-living stationary ally mean values of temperature and dew-point within thunderstorms are prevailing, whereas stronger shear the lowest 400 m of the sounding were used for the favours long-living moving thunderstorms. computation of CAPE. The average value of CAPE is now 155 J/kg for stationary thunderstorms, 201 J/kg Due to the size of the investigation area and the for moving thunderstorms, and 306 J/kg for thunder­ change of the weather situation during one day two or storm lines, respectively. As expected, the values are three different thunderstorm structures sometimes have lower than those shown above, but the relative differ­ been observed at the same time or during the course of ences are higher. a day. In such situations, besides a dominating thun­ derstorm type also less organised thunderstorms are existent (i.e. moving thunderstorms and stationary 4.4 Vertical wind shearand thunder­ thunderstorms). Thus only the class with stationary storm types thunderstorms is homogeneous. Subsequently in mixed storm cases the mean values of shear for moving thun­ derstorms and thunderstorm lines is underestimated. Figure 8 shows the frequency distribution of density weighted wind shear for all three classes of thunder­ storms. Wind shear is in the same order of magnitude for all three classes. Low values are prevailing for 4.5 Bulk Richardson numberand thun­ stationary thunderstorms, the mean value is 5.7 m/s. At derstorm types days with moving thunderstorms and thunderstorm lines the mean shear was 7.2 and 7.3 m/s, respectively. The frequency distribution of the Richardson number The corresponding mean values of density weighted for the three thunderstorm types is shown in Figure 9. shear energy are 16, 26 and 27 J/kg, respectively. For The mean value of the distribution is 152 for station ­ both moving types of thunderstorms higher values of ary thunderstorms, 80 for moving thunderstorms, and shear are observed. 52 for thunderstorm lines. However, the scatter is large and the differences between the mean values are cer­ tainly not significant.

9 12 S 10000 - S 11 ■ HI 8000 -

7000 r

6000

E 5000

4000 L

3000 )

2000 -

magnitude of difference vector (m/s) Figure 11. Mean value of the magnitude of the difference vector between the wind direction at the respective height 0 5 10 15 20 and the motion vector. number of days Figure 10. Frequency distribution of height levels where the higher levels to the right. However, at higher levels the magnitude of the difference vector between the wind direc­ differences are around 5°. tion at the respective height and the motion vector is mini- mum. Figure 10 shows the frequency distribution of the heights were the magnitude of the difference vector Numerical modelling studies by Weisman and Klemp between the motion vector and the wind vector was (1984) showed that Richardson numbers between 15 minimum. Again the heights around 3000 m and 6000 and 35 are indicative for the development of supercell m dominate. Figure 11 shows the magnitude of the storms; values higher than 35 promote multicellular difference vector. This is in agreement with Figure 10, storms. Again it should be noted that soundings are not except that the gap at 4000 m is not so obvious. always representative for the environment in which the observed systems occurred. Therefore the comparison When considering the steering level of a moving thun­ with idealised numerical simulations should only be derstorm it is implicitly assumed that thunderstorms done to describe the relative location within the pa­ move with the wind and do not develop. However, this rameter space. is not the truth. It is necessary to distinguish between the propagation and translation vector. Propagation stands for the direction in which new cells develop, 4.6 Steering level of moving thunder­ translation stands for the movement of the individual storms cells. The sum of both vectors describe the motion of the system. This direction is termed motion direction For the moving thunderstorms the speed and direction in this paper. Further on it is observed that storms were compared against the wind profile from the sometimes can split with one cell decaying relatively soundings. Direction and speed were compared inde­ fast. Depending on the shear, multicell storms can pendently. Here we will present the magnitude of the develop on either side of the old one. The observed difference vector between the motion vector and the track of lightning flashes is the final result of all the wind vector at the respective heights. described processes. Thus, the wind at the steering level gives the most likely motion vector of a storm or The observed storm speed are in best agreement with system. the wind speed in the heights around 3000 m and in the heights around 5500 m. Lower level winds are weaker than the storm speed, upper troposphere winds are higher than the storm speed. The directions match best in the heights around 3000 m. At lower levels the wind vector is to the left of the motion vector, at

10 5 Summary and conclusions The direction of moving thunderstorms is mainly con ­ trolled by the flow in heights around 3 or 6 km m.s.I. We emphasise that this steering level is the result of Thunderstorms are frequently observed in southern translation, propagation, and processes like splitting of Germany during the summer period. The analysis of the cells. lightning data showed that thunderstorms occur in average every second day anywhere in southern Ger­ The analysis of dynamic and thermodynamic parame­ many. In general thunderstorms approach from west­ ters suggests that the phenomenological classification erly and south-westerly directions. The analysis is in of stationary thunderstorms corresponds to the basic good agreement with the observations by Finke and type of a single cell. Those cells are short living and Hauf (1996 a). do not show long tracks. Moving thunderstorms corre­ spond to multicell or supercell storms. These storms The observed lightning patterns could be classified can exist for several hours, which requires strong low- into three different groups, namely stationary thunder­ level wind shear. Due to the prevailing higher winds in storms, moving thunderstorms and thunderstorm lines. the middle troposphere these kind of storms are mov­ This phenomenological classification is a priori not ing. Finally, the group of thunderstorm lines corre­ related to the classification based on the storm scale sponds to thunderstorms with a mesoscale organisation (Foote, 1985). described by line-oriented storms or squall lines. This relation between the phenomenological classification No significant relation between thunderstorm occur­ of lightning patterns and the basic cell types is in rence and the Grosswetterlagen was found, nor was it agreement with the observed Richardson numbers and possible to give an indicator for the observed thunder­ the CAPE values. Increasing of CAPE and decreasing storm types. The hypothesis was that thunderstorms of the Richardson number denote a higher degree of develop only in favourable weather situations. How­ organisation. The observed Richardson numbers are ever, thunderstorms where observed during nearly all larger than those found by Weisman and Klemp of the 29 pattern. It can be concluded that the descrip­ (1984). tion of the synoptic state of the atmosphere by Grosswetterlagen is not suitable for the investigation Main shortcomings of this study are the great variety of thunderstorms. A reduction to 8 pattern based on of lightning patterns, the great orographic inhomogen- the low-level flow in southern Germany did not show ity of the investigation area, and the low spatial and much improvement. However, this classification indi­ temporal separation between individual radio sound­ cates that about 20% of the thunderstorms occur when ings. The classification of the lightning patterns re­ the low-level flow indicates easterly flow. This is in quires compromises when different types are occurring contrast to the prevailing direction from which thun­ during the same time, or no clear assignment is possi­ derstorms arrive. It can be explained by the conver­ ble. The orographic structure in southern Germany gence and strong wind shear in the lowest levels, with the mountain ridges of the Schwarzwald, which is enforced by orographic effects due to the Schwabische and Frankische Alb, the Bayerische Alps. Examples of storm development in this envi­ Wald, and the Alps as well as the valley of the river ronment are given by Meischner et al. (1991) or Donau has a strong influence on the initiation and Haase-Straub et al. (1997). motion of thunderstorms. This influence can hardly be excluded from the overall picture of the observed The dynamic and thermodynamic conditions at a thun­ lightning patterns. Finally, the radio soundings from derstorm day have been analysed from radio soundings Miinchen and Stuttgart at 12 UTC (only from these at Miinchen and Stuttgart. Despite radio soundings are ones CAPE can be computed) can not completely not always representative for the thunderstorm envi­ describe the thunderstorm environment. This is namely ronment it was found that higher wind shear favours because thunderstorms occur mostly in the late after­ moving thunderstorms or thunderstorm lines. High noon or in the evening and, they are not always repre­ CAPE values are observed when moving thunder­ sentative for the environment in which the observed storms or thunderstorm lines are present. The latter is systems occurred. in agreement with the observations of Schiesser et al. (1995 and 1997) in Switzerland. Stationary storms This study emphasises that the behaviour of a complex develop at low wind speeds and low shear, which system like a thunderstorm can hardly be described by results in high Richardson numbers. As shown above a few characteristic numbers. A great uncertainty will moving thunderstorms and thunderstorm lines have remain when it is attempted to forecast the future de­ similar shear values but they differ in CAPE. Like the velopment of a thunderstorm based on a few environ­ results presented by Weisman and Klemp (1982 and ment parameters. 1984) lower Richardson numbers are observed for higher organised convection systems.

11 Acknowledgements Huntrieser, H., H.H. Schiesser, W. Schmid, and A. Waldvogel, 1997: Comparison of traditional and We gratefully acknowledge the provision of the newly developed thunderstorm indices for Switzer­ LPATS data by the Bayernwerk AG, Munchen. The land. Wea. Forecasting, 12, 108-123. contribution of Ulli Finke was supported by the Bay- Meischner, P.F., V.N. Bringi, D. Heimann and H. erisches Staatsministerium fur Landesentwicklung und Holler, 1991: A squall line in southern Germany: Umweltfragen and is part of the Bavarian climate kinematics and precipitation formation as deduced research program BayFORKLIM. The radio soundings by advanced polarimetric and Doppler radar meas­ were kindly provided by the Deutscher Wetterdienst, urements. Mon. Wea. Rev., 119, 678-701. Offenbach. Moller A.R., C.A. Doswell, M.P. Foster and G.R. Woodall, 1994: The operational recognition of su­ percell thunderstorm environments and storm struc­ References tures. Wea. Forecasting, 9, 327-347. Pelz, J., 1984: Die geographische Verteilung der Tage Finke, U. and T. Hauf, 1996 a: An observational study mit Gewitter in Mitteleuropa. Beilage zur Berliner on propagation and lifetime of convective storms in Wetterkarte, 48 (12), 32 pp. central Europe based on lightning data. 7th Confer ­ Schiesser, H.H., R.A. Houze Jr. and H. Huntrieser, ence on Mesoscale Processes, September 9-13, 1995: The mesoscale structure of severe precipita­ 1996, Reading, Unites Kingdom, 611-612. tion systems in Switzerland. Mon. Wea. Rev., 123, Finke, U. and T. Hauf, 1996 b: The characteristics of 2070-2097. lightning occurrence in southern Germany., Beitr. Schiesser H. H., Waldvogel A., Schmid W. and Wil- Phys. Atmosph., 69, 361-374. lemse S., 1997: Klimatologie der Stiirme und Sturm- Fister, V. , H. von Rheinbaben and T. Zundl, 1994: systeme anhand von Radar- und Schadendaten. Analysis of the 1992 and 1993 lightning data in SchluBbericht NFP 31, vdf Hochschulverlag an der South Germany, 22nd International Conference on ETH Zurich, 130 pp. Lightning Protection, Budapest - Hungary, 6 pp. Stull, R. B., 1988: An introduction to Boundary Layer Foote, G.B., 1985: Aspects of cumulonimbus classifi­ Meteorology. Kluwer Academic Publishers, Dor­ cation relevant to the hail problem. J. Rech. Atmos., drecht, 175-177. 19, 61-74. Weisman M.L. and J.B. Klemp, 1982: The depend­ Gerstengarbe, F.-W., and P C. Werner, 1993: Katalog ence of numerically simulated convective storms on der GroBwetterlagen Europas nach Paul Hess und vertical wind shear and buoyancy. Mon. Wea. Rev., Helmuth Brezowsky 1881-1992. Bericht des Deut- 110, 504-520. schert Wetterdienstes, 113, 249 pp. Weisman M.L. and J.B. Klemp, 1984: The structure Haase-Straub, S.P., M. Hagen, T. Hauf, D. Heimann, and classification of numerically simulated convec­ M. Peristeri and R.K. Smith, 1997: The squall line tive storms in directionally varying wind shears. of 21 July 1992 in southern Germany: An observa­ Mon. Wea. Rev., 112, 2479-2498. tional case study. Beitr. Phys. Atmosph., 70, 147- 165. Hoffmann, E. and H. von Rheinbaben, 1991: Ein Blit- zortungssystem fur die Elektrizitatsversorgung, Me- teorol. Rdsch., 43, 112-118. Holler, H., 1994: Mesoscale organisation and hailfall characteristics of deep convection in southern Ger­ many. Beitr. Phys. Atmosph., 67, 219-234. Holler, H., V. N. Bringi, J. Hubbert, M. Hagen and P. F. Meischner, 1994: Life cycle and precipitation formation in a hybrid-type hailstorm revealed by polarimetric and Doppler radar measurements. J. Atmos. Sci., 51, 2500-2522. Houze, R.A., B.F. Smull and P. Dodge, 1990: Mesos­ cale organisation of springtime rainstorms in Okla­ homa. Mon. Wea. Rev., 118, 613-654. Houze, R.A., Jr., W. Schmid, R.G. Fovell and H.H. Schiesser, 1993: Hailstorms in Switzerland: Left movers, right movers, and false hooks. Mon. Wea. Rev., 121,3345-3370.

12 List of Reports

Report No. 1-10 1993 Report No. 11 -23 1994 Report No. 24-45 1995

Report No. 46 Model-estimated mircrowave emissions from rain systems for remote sensing January 1996 applications Mikhail T. Smirnov and Peter F. Meischner published in: J. Geophys. Res. 101, 29479-29489,1996.

Report No. 47 Verification of statistical-dynamical downscaling in the Alpine region January1996 Ursula Fuentes and Dietrich Heimann published in: Climate Res. 7, 151-168, 1996.

Report No. 48 Mesoscale surface-wind characteristics and gravity-wave potential during cross- February 1996 Alpine airflow Dietrich Heimann published in: Meteorol. Atmos. Phys. 62, 49-70,1997.

Report No. 49 The characteristics of lightning occurrence in southern Germany February 1996 Ullrich Finke and Thomas Hauf published in: Beitr. Phys. Amosph. 69, 361-374,1996.

Report No. 50 Contributions of aircraft emissions to the atmospheric NOx content February 1996 Ines Kohler, Robert Sausen and Robert Reinberger published in: Atmos. Environ. 31,1801-1818,1997.

Report No. 51 Boundary layer budgets over the NOPEX area March 1996 Michael Freeh, Patrick Samuelsson, Michael Tjemstrom and Anne M. Jochum

Report No. 52 Colors of contrails from fuels with different sulfur contents March 1996 Klaus Gierens and Ulrich Schumann published in: J. Geophys. Res. 101,16731-16736,1996.

Report No. 53 Cyclonic activity in a warmer climate March 1996 Frank Lunkeit, Michael Ponater, Robert Sausen, Martin Sogalla, Uwe Ulbrich and Martin Windelband published in: Beitr. Phys. Atmoph. 69, 393-407,1996.

Report No. 54 On the scattering behaviour of bullet rosette and bullet shaped ice crystals April 1996 Bernhard Strauss published in: Ann. Geophys. 14, 566-573, 1996.

Report No. 55 A note on the application of linear wave theory at a critical level April 1996 Andreas Dornbrack and Carmen J. Nappo published in: Boundary LayerMeteorol. 82, 399-416,1997.

Report No. 56 A reexamination of the derivation of the equilibrium supersaturation curve for soluble April 1996 particles Paul Konopka published in: J. Atmos. Sci. 53, 3157-3163, 1996.

Report No. 57 A global climatology of the tropopause height based on ECMWF-analyses May 1996 Thomas Reichler, Martin Dameris, Robert Sausen and Dieter Nodorp

i Report No. 58 Wake dynamics and exhaust distribution behind cruising aircraft May 1996 Thomas Gerz and Thorsten Ehret published in: AGARD CP-584, 35.1-35.12,1996.

Report No. 59 In situ observations of airtraffic emission signatures in the north atlantic flight July 1996 corridor Hans Schlager, Paul Konopka, Peter Schulte, Ulrich Schumann, Helmut Ziereis, Frank Arnold, Matthias Klemm, Donald E. Hagen, Philip D. Whitefied and Joelle Ovarlez published in: J. Geophys. Res. 102,10739 —10750,1997.

Report No. 60 LINOX - Wissenschafts- und Operationsplan July 1996 Hartmut Holler

Report No. 61 A three-dimensional model for calculating reflection functions of inhomogeneous and July 1996 orographically structured natural landscapes Werner Thomas published in: Remote Sens. Environ. 59, 44-63,1997.

Report No. 62 Large-eddy simulation of turbulence in the free atmosphere and behind aircraft July 1996 Ulrich Schumann, Andreas Dornbrack, Tilman Durbeck and Thomas Gerz published in: Fluid Dyn. Res. 20,1-10,1997.

Report No. 63 Dispersion of aircraft exhausts in the free atmosphere August 1996 Tilman Durbeck and Thomas Gerz published in: J. Geophys. Res. 101,26007-26015,1996.

Report No. 64 Estimation of boundary-layer humidity fluxes and statistics from airborne DIAL August 1996 Christoph Kiemle, Gerhard Ehret, Andreas Giez, Kenneth J. Davis, Donald H. Lenschow and Steven P. Oncley published in: J. Geophys. Res. 102, 29189-29203,1997.

Report No. 65 Time constants of temperature sensors for use in dropsondes September 1996 Rainer Stager, Heinz Ldbel and Harold L. Cole

Report No. 66 Turbulent mixing by breaking gravity waves September 1966 Andreas Dornbrack

Report No. 67 A modified mass-flux scheme for convection which maintains positive tracer September 1996 concentrations Sabine Brinkop and Robert Sausen published in: Beitr. Phys. Atmosph. 70, 245-248,1997.

Report No. 68 Contributions on the topic of impact of aircraft emissions upon the atmosphere October 1996 Ulrich Schumann (ed.) published in: Proc. of ‘Impact of Aircraft Emissions upon the Atmosphere', Intern. Colloquium, Paris, 15.-18.10.96, pp. 378.

Report No. 69 Stratospheric temperature anomalies and mountain waves: A three-dimensional October 1996 simulation using a multi-scale weather prediction model Martin Leutbecher and Hans Volkert published in: Geophys. Res. Lett. 23, 3329-3332,1996.

Report No. 70 NOx emission indices of subsonic wide-bodied jet aircraft at cruise altitude: In situ October 1996 measurements and predictions Peter Schulte, Hans Schlager, Ulrich Schumann, Steve L. Baughcum and Frank Deidewig published in: J. Geophys. Res. 102, 21431-21442,1997.

u Report No. 71 Determination of humidity and temperature fluctuations based on an evaluation of November 1996 MOZAIC data and application to a parametrization of persistent contrail coverage in general circulation models Klaus M. Gierens, Ulrich Schumann, Herman G.J. Smit, Manfred Helten and Gunther Zangl published in: Ann. Geophys. 15,1057-1066,1997.

Report No. 72 Surface UV from ERS-2/GOME and NOAA/AVHRR data: A case study November 1996 Ralf Meerkotter, Bruno Wissinger and Gunther Seckmeyer published in: Geophys. Res. Lett. 24,1939-1942,1997.

Report No. 73 Heat and moisture transport in the presence of cumulus cloud convection January 1997 Claudia Strodl and Anne M. Jochum

Report No. 74 Development of a chemistry module for GCMs: First results of a multi-annual March 1997 integration Benedikt Steil, Martin Dameris, Christoph Bruhl, Paul J. Crutzen, Volker Grewe, Michael Ponater and Robert Sausen published in: Ann. Geophys. 16, 205-228,1998.

Report No. 75 A numerical study of aircraft wake induced ice cloud formation April 1997 Klaus M. Gierens and Johan Strom

Report No. 76 Dilution of aircraft exhaust plumes at cruise altitudes April 1997 Ulrich Schumann, Hans Schlager, Frank Arnold, Robert Baumann, Peter Haschberger and Otto Klemm

Report No. 77 Mesoscale forecasts of stratospheric mountain waves April 1997 Andreas Dombrack, Martin Leutbecher, Hans Volkert and Martin Wirth published in: Meteorol. Atmos. Phys. 5,117-126,1998

Report No. 78 Heterogeneous chemistry in aircraft wakes: Constraints for uptake coefficients May 1997 Bernd Karcher published in: J. Geophys. Res. 102,19.119-19.135,1997.

Report No. 79 Impact of aircraft Nox-emissions on tropospheric and stratospheric ozone May 1997 Part II: 3-D model results Martin Dameris, Volker Grewe, Ines Kohler, Robert Sausen, Christoph Bruhl, Jens-Uwe Grooss and Benedikt Steil

Report No. 80 Chemical conversion of aircraft emissions in the dispersing plume: June 1997 Calculation of effective emission indices Herbert Petry, Johannes Hendricks, Martin Mollhoff, Elmar Lippert, Andreas Meier, Adolf Ebel and Robert Sausen published in: J. Geophys. Res. 103, 5759-5772,1998.

Report No. 81 Observations and model calculations of B747 engine exhaust products at cruise June 1997 altitude and inferred initial OH emissions Hans G. Tremmel, Hans Schlager, Paul Konopka, Peter Schulte, Frank Arnold, Matthias Klemm and Bert Droste-Franke

Report No. 82 The impact of stratospheric dynamics and chemistry on Northern Hemisphere mid­ July 1997 latitude ozone loss Volker Grewe, Martin Dameris and Robert Sausen

Report No. 83 Heterogeneous PSC ozone loss during an ozone mini-hole July 1997 Volker Grewe and Martin Dameris published in: Geophys. Res. Lett. 24, 2503-2506,1997.

Ill Report No. 84 Coupled simulation of meteorological parameters and sound intensity in a narrow July 1997 valley Dietrich Heimann and Gunter Gross

Report No. 85 The role of entrainment in surface-atmosphere interactions over the boreal forests July 1997 Kenneth J. Davis, Donald H. Lenschow, Steven P. Oncley, Christoph Kiemle, Gerhard Ehret, Andreas Giez and Jakob Mann published in: J. Geophys. Res. 102, 29219-29230,1997.

Report No. 86 Jet engine exhaust aerosol characterization August 1997 Andreas Petzold and Franz P. Schroder published in: Aerosol Sci. Technol. 28, 62-76,1998.

Report No. 87 Near field measurements on contrail properties from fuels with different sulfur content August 1997 Andreas Petzold, Reinhold Busen, Franz P. Schroder, Robert Baumann, Marion Kuhn, Johan Strom, Donald E. Hagen, Phil D. Whitefield, Darrel Baumgardner, Frank Arnold, Stephan Borrmann and Ulrich Schumann published in: J. Geophys. Res. 102, 29867-29880,1997.

Report No. 88 Elemental composition and morphology of ice crystal residual particles in cirrus August 1997 clouds and contrails Andreas Petzold, Johan Strom, Sofia Ohlsson and Franz P. Schroder

Report No. 89 A diagnostic study of the global coverage by contrails September 1997 Part I: Present day climate revised version (March 1998) Robert Sausen, Klaus Gierens, Michael Ponater and Ulrich Schumann

Report No. 90 Physiochemistry of aircraft generated liquid aerosols, soot, and ice particles October 1997 -1. Model description Bernd Karcher

Report No. 91 Physiochemistry of aircraft generated liquid aerosols, soot, and ice particles October 1997 - II. Comparison with observations and sensitivity studies Bernd Karcher, Reinhold Busen, Arnold Petzold, Franz P. Schroder, Ulrich Schumann and Eric J. Jensen

Report No. 92 Operational detection of contrails from NOAA-AVHRR-data November 1997 Hermann Mannstein, Richard Meyerand Peter Wendling

Report No. 93 Particulate emissions of commercial jet aircraft under cruise conditions November 1997 Paul Konopka, Ulrich Schumann, Hans Schlager, Donald E. Hagen, Phil D. Whitefield and Joelle Ovarlez

Report No. 94 The mesoscale moisture variability and its impact on the energy transfer through the January 1998 boundary layer Michael Freeh

Report No. 95 Estimates of the climate response to aircraft emissions scenarios March 1998 Robert Sausen and Ulrich Schumann

Report No. 96 Wake vortex physics April 1998 Frank Holzapfel and Thomas Gerz

IV Report No. 97 Propagation characteristics of thunderstorms in southern Germany May 1998 Martin Hagen, Blasius Bartenschlager, Ullrich Finke

A complete list of reports can be found in report No. 80 or is available from the internet: http://www.op.dlr.de/ipa/pubs.html.

Einzelne Exemplare konnen unter der E-Mail-Adresse ‘[email protected]’ bestellt werden.

Single copies can be ordered under the email-address ‘[email protected]’

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