Motion characteristics of thunderstorms in southern Germany

Transcription

1 Meteorol. Appl. 6, (1999) Motion characteristics of thunderstorms in southern Germany Martin Hagen, Blasius Bartenschlager, Deutsches Zentrum für Luft- und Raumfahrt, Institut für Physik der Atmosphäre, Oberpfaffenhofen, D Weßling, Germany Ullrich Finke, Universität Hannover, Institut für Meteorologie und Klimaforschung, Herrenhäuser Straße 2, D Hannover, Germany The motion of thunderstorms in southern Germany was investigated. The thunderstorms were observed by a lightning position system during the summer months of the years On average every second day thunderstorms were observed somewhere in southern Germany. In general thunderstorms approached from westerly and south-westerly directions. The average speed was 13 m s 1. No significant relation between the occurrence of thunderstorms and 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 eight weather patterns based on the low-level flow in southern Germany showed that thunderstorms are likely when the flow has a westerly (43%) or easterly direction (20%). Three distinct groups of different lighting patterns could be identified: stationary, moving thunderstorms and thunderstorm lines. The convective available potential energy (CAPE) and the wind shear were retrieved from radio soundings from München and Stuttgart. On average CAPE was 583 J kg 1 for stationary thunderstorms, 701 J kg 1 for moving thunderstorms and 876 J kg 1 for thunderstorm lines. The corresponding average bulk Richardson numbers were 37, 22 and 21. The steering level was found to be at about 6 km m.s.l. However, it should be noted that in most cases the soundings do not completely describe the local environment of thunderstorms, since radio soundings are only available twice a day. 1. Introduction Thunderstorms during the summer are a frequent weather phenomenon in southern Europe. They make an essential contribution to the total precipitation in southern Germany. Thunderstorms are connected with lightning strikes, heavy precipitation, hail and strong turbulence, and thus they have a considerable potential for endangering and damaging people and property. For that reason knowledge of the development and motion of thunderstorms is of great interest and represents a central problem in nowcasting and short-range forecasting. The object of this paper is to show how the motion characteristics and the organisation of thunderstorms in southern Germany is related to environmental and synoptic conditions. For this purpose a great number of thunderstorm days ( ) in southern Germany have been investigated. A lightning location system operated by the Badenwerk AG and the Bayernwerk AG in southern Germany has been used for the determination and classification of thunderstorm days. Lightning is an unambiguous indicator for thunderstorms in the state of maturity. By means of a phenomenological classification based on different patterns and structures of the spatial lightning distribution, the thunderstorms have been divided into three types. These phenomenological types of thunderstorm have then been related to environmental and synoptic conditions. Only a few related studies of thunderstorms in Central Europe are known. The analysis by Pelz (1984) is based on synoptic observations (1893 until 1907) and gives the spatial distribution of thunderstorm days in Germany. More recent studies have been performed with radar data by Höller (1994) and Finke & Hauf (1996a,b) using lightning observations. Houze et al. (1993) and Schiesser et al. (1995) investigated the mesoscale structure of severe precipitation systems in Switzerland. Höller (1994) showed how the mesoscale organisation of thunderstorms in southern Germany is related to the occurrence of hail. On the basis of radar observations, thunderstorms were classified as single cells, multicell storms and supercell storms. Their mesoscale organisation was classified as isolated storms, clusters, line-oriented storms and squall lines. Further details will be given in section 2. The analysis of polarimetric radar data showed that hail was observed on 61% of the thunderstorm days. 227

2 M Hagen, B Bartenschlager and U Finke Finke & Hauf (1996a) statistically evaluated the tracks of moving thunderstorms by using observations from a lightning network for a three-year time period ( ). According to the lightning patterns the thunderstorms have been divided into three classes. More details can be found in section 3. Single-cell storms were observed on 37%, storm tracks on 57% and fronts on 6% of the thunderstorm days. A frequency distribution of storm tracks shows that for 56% of all cases the storms approach from the southwest or west. The mean speed is 47 km h 1. The most frequently observed lifetime of storms is 4 hours. Storm tracks longer than 250 km can be found on an average of 18 days a year. Characteristic properties of thunderstorms, like the life cycle and the type of organisation, depend principally on environmental conditions. Numerical studies by Weisman & Klemp (1982, 1984) show typical values of environmental buoyancy and wind shear for the different types of thunderstorm organisation. The bulk Richardson number was shown to be a suitable indicator for the storm type: low values are observed in the surroundings of supercells, and higher values are observed with multicell storms. For several years thunderstorms in Switzerland have been investigated and environmental parameters have been calculated to separate heavy hail thunderstorms from other thunderstorms. It was found that for thunderstorm days with significant hail damage the mean value of CAPE (convective available potential energy; see section 2) is 656 J kg 1 and for the heaviest hailstorms the mean value is 876 J kg 1 (Schiesser et al., 1997). No attempt was made to investigate the great variety of indices which are used to describe the probability of thunderstorm occurrence. Huntrieser et al. (1997) tested several indices for their usefulness in Switzerland. Their results show that CAPE is one of the successful indices to be used for forecasting thunderstorms. The classification of thunderstorms, their initiation and the environmental conditions are outlined in section 2. Section 3 describes the database and the processing of the lightning data, and section 4 gives the results of this study. 2. Types of thunderstorm and environmental conditions The development of specific storm characteristics, such as the organisation of the cells, life cycle, or propagation velocity, depends mainly on the atmospheric environment conditions. This environment is described by the synoptic state of the atmosphere and, in more detail, by the vertical profile of the wind, temperature and humidity Organisation of thunderstorms Höller (1994) summarised the types of thunderstorm in southern Germany on the basis of the storm s scale and the mesoscale organisation. On the basis of the storm s scale three basic types can be identified (after Foote, 1985): Single cells. The complete cycle of precipitation development and fallout takes place in one cell within roughly half an hour. Multicell storms. All stages of development coexist with each one forming a part of the same storm system. Those systems can last for a few hours. Supercell storms. The storm is formed by a large quasi-stationary cell. All the different stages take place in the same cell but at different locations. Supercell storms can last up to 12 hours. This classification requires knowledge of the dynamical and microphysical structure of the storm. Alternatively, phenomenological approaches classify the storms on their appearance as seen by an observation system. On the basis of the mesoscale shape of radar echoes the following mesoscale organisations are observed in southern Germany (Höller, 1994; after Houze et al., 1990): Isolated storms. Thunderstorms exist in isolation and are not embedded in common reflectivity contours. Clusters or complexes of storms. Thunderstorms are closely grouped together. A cluster does not have a preferred direction of alignment. Line-oriented storms. Isolated storms or storms within a cluster are aligned. Squall lines. These are a special form of line-oriented systems. Squall lines are often characterised by a leading line of heavy convection followed by a region of widespread stratiform precipitation. In principle there is no direct relation between the two kinds of classification. Except for the isolated storms, the mesoscale organisations require long-living multicell or supercell storms. For this study we will use a scheme which focuses on properties like lifetime and motion of lightning clusters. This methodology is described in section 3.3. During the summer, thunderstorms are normally initiated by lifting of unstable air. These thunderstorms are termed airmass storms. If the air is only potentially unstable, additional forced lifting is necessary. Initial lifting can be forced by orography or approaching cold fronts. Finke & Hauf (1996 b) show the great spatial variability of the annual lightning distribution in southern Germany.

3 2.2. Synoptic state of the atmosphere The synoptic state of the atmosphere describes the environment in which thunderstorms develop. Hess and Brezowsky, and Gerstengarbe & Werner (1993), divided the synoptic weather pattern in Europe into 29 large-scale weather patterns (Grosswetterlagen), depending on the large-scale flow pattern in Central Europe. The Grosswetterlagen are defined by the direction of the surface flow and the flow at 500 hpa in Central Europe by considering the origin of the airmasses and the pressure field over Europe and the Northern Atlantic. It should be noted that during the same Grosswetterlage the weather itself at a location can change considerably. It is expected that distinct types of thunderstorm will only be observed in favourable Grosswetterlagen. The Grosswetterlagen and a reduced classification (see section 3.2) are listed in Table 1. For the characterisation of the storm environment we use the convective available potential energy (CAPE), wind shear energy and the bulk Richardson number. These quantities are defined in the following sections Stratification of the airmass A necessary condition for thunderstorm development independent of the mechanism of initiation is a potentially unstable stratification of the atmosphere (e.g. Weisman & Klemp, 1982). The stratification of airmasses can be determined from radio soundings. In order to have an easily accessible indication of the probability and strength of thunderstorm development a great number of indices have been developed (e.g. Huntrieser et al., 1997). However, for the calculation of these indices usually data from only a few pressure levels are used. Motion characteristics of thunderstorms in southern Germany CAPE is a good indicator for the stability and lability of air masses. Positive values of buoyant energy indicate unstable stratification and negative values stable stratification. Houze et al. (1993) give values between 340 and 2340 J kg 1 for mesoscale convective systems in Switzerland. The maximum updraft velocity in convective clouds is estimated by (neglecting losses due to friction and entrainment): It was shown by Haase-Straub et al. (1997) and Huntrieser et al. (1997) how variable the value of CAPE can be, if different lifting levels are applied. The above definition of CAPE uses dry adiabatic lifting from the surface conditions (temperature and mixing ratio) until condensation is reached Vertical wind profile w = 2 max CAPE The vertical wind shear has a great influence on the organisation of convection (Weisman & Klemp, 1982). The characteristic structures of single, multicell and supercell storms are mainly caused by a different structure of the wind profile. For the formation of multicells or supercells a distinctly stronger wind shear at lower levels is necessary than for single cells. The vertical wind shear can be characterised by a density weighted shear energy (Weisman & Klemp, 1984; Moller et al., 1994). The density weighted (kinetic) shear energy is defined by: 2 2 ( ) E = 1 sh u + v 2 To estimate the kind and strength of expected thunderstorms, a quantity is chosen that characterises the state of the atmosphere in a more extensive way. An integral measure of the lability of the atmospheric stratification is the buoyancy energy CAPE (convective available potential energy). The buoyancy determines the acceleration of air parcels and gives a measure of thunderstorm intensity. CAPE corresponds to the specific energy which is released during the lifting of an air parcel with unit mass from its level of free convection (LFC) to the level of neutral buoyancy (NB): NB CAPE = g Θ Θ LP U d z ΘU LFC [J/kg] where g is the acceleration due to gravity, Θ LP the potential temperature of the ascending air volume and Θ U the potential temperature of the environment. with u and v representing the density weighted shear velocities: u = 6000 uz ( ) ρ( z)dz uz ( ) ρ( z)dz ρ( z)dz ρ( z)dz and correspondingly for v. Here ρ(z) is the vertical density profile, u(z), v(z) are the vertical profiles of the orthogonal wind components and z is the height in metres above ground Bulk Richardson number 0 The Richardson number is a dimensionless measure of the stability of a dynamic system. It is defined by the ratio of energy available for the vertical motion (buoy

4 M Hagen, B Bartenschlager and U Finke Table 1. The 29 Grosswetterlagen (large-scale weather patterns) and their abbreviations, after Hess and Brezowsky, and a reduced classification according to the low-level flow in southern Germany Grosswetterlage Abbreviation Low-level flow Zonal circulation patterns westerly, anticyclonic WA W westerly, cyclonic WZ W westerly, southern WS W westerly, angular WW NW Mixed circulation patterns south-westerly, anticyclonic SWA SW south-westerly, cyclonic SWZ W north-westerly, anticyclonic NWA NW north-westerly, cyclonic NWZ NW high, central Europe HM E bridge, central Europe BM E low, central Europe TM W Meridional circulation patterns northerly, anticyclonic NA N northerly, cylonic NZ N high, North Atlantic, anticyclonic HNA W high, North Atlantic, cyclonic HNZ SW high, British Isles HB NE trough, central Europe TRM NW north-easterly, anticyclonic NEA NE north-easterly, cyclonic NEZ NE high, Scandinavia, anticyclonic HFA E high, Scandinavia, cyclonic HFZ E high, north Scandinavia, anticyclonic HNFA SE high, north Scandinavia, cyclonic HNFZ E south-easterly, anticyclonic SEA SE south-easterly, cyclonic SEZ SE southerly, anticyclonic SA SW southerly, cyclonic SZ SW low, British Isles TB W trough, western Europe TRW W ancy energy) and energy produced by vertical wind shear. Three definitions are commonly used (Stull, 1988). Here, the bulk Richardson number is used, because of its simple relation between CAPE and shear: CAPE Ri = E Richardson numbers less than 1 indicate the development of turbulence, whereas values greater than 1 point to decreasing turbulence. Weisman & Klemp (1982) have found for the mid-west of the USA that development of supercells takes place at values between 15 and 35 and that at values greater than 35 conditions are favourable for the development of multicells Steering level and motion of thunderstorms sh it can be assumed that the storms are directed by the flow in a certain altitude range. This altitude is termed the steering level. However, this is not completely true, because thunderstorms develop and decay. It is necessary to distinguish between the propagation and translation vector. Propagation refers to the direction in which new cells develop, whilst translation refers to the movement of the individual cells. The sum of both vectors describes the motion of the system. This direction is termed the motion direction in this paper. Further on it is observed that storms sometimes split with one cell decaying relatively fast. Depending on the shear, multicell storms can develop on either side of the old one. The observed track of lightning flashes is the final result of all the described processes superimposed upon orographic effects. Thus, the wind at the steering level gives the most likely motion vector of a storm or system. It can be assumed that the motion vector of moving thunderstorms is related to the wind profile. Moreover, 230

5 3. Database 3.1. Area of investigation Thunderstorms in southern Germany have been investigated. The investigation area extends in a west east direction between the mountain ridges of the Schwarzwald and the Bayerischer Wald, and in a north south direction between the River Main and the Alps (Figure 1). Its dimension is about km 2. The orography is inhomogeneous. Therefore, orographic effects can play an important part in the development, formation and motion of thunderstorms Synoptic and sounding data Motion characteristics of thunderstorms in southern Germany of the airmass, but the information on the curvature of the flow (cyclonic, anticyclonic) is lost. This reduced number of patterns better reflects the local flow in the region of interest. The dynamic and thermodynamic parameters are determined from radio soundings at München and Stuttgart. Both stations are within the investigation area. However, the low spatial resolution (approximately 200 km between the two) and temporal density (only twice a day at 00 and 12 UTC) of soundings remains a great problem. This is especially so because meteorological phenomena are investigated which do not often occur on the meso-γ-scale and have a duration of only a few hours. The daily classification of the Grosswetterlagen is available from the Europäische Wetterbericht issued by the Deutscher Wetterdienst. For statistical considerations it seems more efficient to reduce the 29 classes to a smaller number. Therefore, we grouped the weather pattern by Hess and Brezowsky into eight classes describing the surface or low-level wind direction in southern Germany (see Table 1). The low-level wind was determined from the typical examples provided by Gerstengarbe & Werner (1993). Also the local effects on the flow owing to the Alps being a barrier to the south were considered. This modified system of classes preserves mainly the origin 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 is observed between 14 and 20 UTC (Finke & Hauf, 1996b). Only from the 12 UTC sounding can the actual CAPE values, as defined in section 2.3, be computed. Also, storms during the night are mainly driven by internal processes which might not be covered by a single sounding. The sounding of München was preferred because it represents the more orographically homogeneous and larger part of the investigation area. If this sounding was not available or the storms were only observed in the northern part (about north of 49 N), the sounding from Stuttgart was used. Figure 1. Map of Southern Germany and the surrounding countries. Grey-shades indicate the height of the terrain. Levels are 50, 300, 500, 700, 1000, 2000, 3000 and 3500 metres above mean sea level. 231

6 M Hagen, B Bartenschlager and U Finke 3.3. Classification of thunderstorms by their lightning pattern Since 1992 a lightning positioning and tracking system (LPATS) has covered southern Germany. It is operated by the two power-supply companies Badenwerk AG, Karlsruhe, and Bayernwerk AG, München (Hoffman & von Rheinbaben, 1991; Fister et al., 1994). According to the literature about 70% of the cloud-to-ground lightning flashes can be detected with an LPATS system. This might vary from case to case, but it should not affect this study considerable. The accuracy of the localisation is 1 km and the temporal resolution is 15 ms. The use of lightning location data for thunderstorm monitoring is advantageous owing to its continuous availability in time with nearly uniform spatial coverage. Animations of the spatial lightning patterns are available for the years Examples of images are shown in Figure 2. Grey-level codes stand for the time of day. Three different forms of organisation can be distinguished. The examples have (by reason of clearness) model character and do not represent the majority of thunderstorm days. The first class are small clusters of lightning positions that seem not to be organised (Figure 2(a)). They appear almost simultaneously in many parts of the observation area. The distinguishing property is a short lifetime (shorter than one hour) and an almost stationary cluster position. These thunderstorms will be termed stationary thunderstorms. This group was called single-cell storms by Finke & Hauf (1996a). The second group are lightning events, which follow along a line (Figure 2(b)). This indicates lightning generated in thunderstorms which are moving and mark a track. These line structures are very different in width, length and velocity. One storm track can have broader and thinner sections. A ratio between length and width of minimum 3 and a minimum length of 50 km was required. Thunderstorms describing such tracks will be termed moving thunderstorms. Finke & Hauf (1996a) termed storms of this kind storm tracks or long tracks. The third class are lightning events aligned along a line almost perpendicular to the direction of motion (Figure 2(c)). These thunderstorms are supposed to be related to frontal systems; however, only their visual appearance is considered. The criterion for differentiation from the second class is that the patterns are more broad than long (seen in the direction of motion). Also, during the motion a clearly defined front line can be identified. Their width exceeds at least 100 km. Thunderstorms of this class will be termed thunderstorm lines. This 232 Figure 2. Examples of thunderstorm days in southern Germany. Lines indicate cities, rivers and international borders. Grey-level code indicates the time of the day. (a) Stationary thunderstorms, (b) moving thunderstorms and (c) thunderstorm lines.

7 group was called lightning fronts or squall lines by Finke & Hauf (1996a). It has to be pointed out that this classification of thunderstorms is a purely phenomenological one based on lightning signatures. A priori there is no relation to the classification based on the environmental conditions listed in section 2. Motion characteristics of thunderstorms in southern Germany On the basis of this phenomenology all thunderstorm events during summer (from May until September) from 1992 to 1996 have been assigned by manual inspection to one of the three groups. A day was regarded as a thunderstorm day if at least 100 lightning flashes were observed with the system in the investigation area. Only a few days have fewer lightning observations. The observed events have to last at least 20 minutes. Shorter events or only a few observed flashes of lightning do not allow for a classification of the thunderstorm type. Also, only one thunderstorm in the investigation area is not representative of the whole area. This decision level will influence the number of days with stationary thunderstorms to some minor degree. 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 class was counted. If there was a significant change during the day, the day was divided into two independent thunderstorm events. A comparison with the classification for this study and an independent one by Finke & Hauf (1996a) shows good agreement. For moving thunderstorms the direction and speed of motion were determined. However, the lightning patterns are not always rectilinear. Long moving storms may change their direction with time. Owing to storm splitting and merging, the storm direction and speed can vary. The speed and the direction of the leading front of moving thunderstorms were estimated for systems travelling more than 150 km. For most of the thunderstorm days the speed was nearly the same for all moving storms. The speed variations with time and geographical location were less than 3 m s 1. Figure 3. Annual frequency distribution of days with and without thunderstorms during May September. Figure 4 shows the distribution of the direction of motion of moving thunderstorms. In more than 63% of the cases the storms arrived from the sector between south-west and west, with a maximum at about 240. All other directions are seldom observed. A small but significant secondary maximum is around south-easterly directions. Thunderstorm lines arrived exclusively from south to west directions with a distinct maximum from the west. The speed of the moving storms varies mainly between 9 m s 1 and 18 m s 1 (Figure 5) with a large scatter; the mean speed is 13 m s 1. In a few cases speeds higher than 21 m s 1 were observed. These high speeds are not related to the motion of air masses but correspond to the phase speed of the leading front of the storm. The relation between the storm velocity vector and the wind profile is discussed below in section Weather pattern and thunderstorm types In Figure 6 the relative and absolute frequency of days without thunderstorms and days with the different types of thunderstorm relative to the Grosswetterlagen 4. Results 4.1. Distribution of storm types A total of 699 days were analysed (May September in ). On 53% of all days thunderstorms have been observed somewhere in the investigation area (Figure 3). The minimum was 46% in 1995 and the maximum 65% in On average, stationary thunderstorms occur on 46%, moving thunderstorms on 44 % and thunderstorm lines on 10% of thunderstorm days. Figure 4. Frequency distribution of the direction from which moving thunderstorms approach. 233

8 M Hagen, B Bartenschlager and U Finke whereas no clear separation between stationary and moving thunderstorms can be observed. Figure 5. Frequency distribution of the speed of moving thunderstorms. (weather pattern) are shown. It should be noticed that thunderstorms have been observed for nearly all 29 Grosswetterlagen. Only six Grosswetterlagen were not observed during summer time. Weather patterns with low thunderstorm probability (approximately 30%) are the patterns HB and NZ. A higher probability (70 80%) of thunderstorms is observed with the SWZ, BM, HNZ, HNFA and TRW patterns. For three weather patterns (NWZ, HNA, SEA) thunderstorms appear with a probability of %. However, these weather patterns seldom occur. Thunderstorm lines have been observed for only 15 weather patterns, Figure 7 shows a much clearer view of the impact of the weather pattern. Here the reduced number of weather patterns (cf. section 3) based on the low-level flow in southern Germany was used. Thunderstorms are observed for all patterns; the ratio between stationary and moving thunderstorms is fairly uniform. Thunderstorm occurrence is concentrated in the classes west and east. Even though moving thunderstorms 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; Höller et al., 1994; Haase- Straub et al., 1997). This behaviour indicates the convergence and the vertical shear associated with those systems. During the rare occurrence of southerly flow only stationary thunderstorms are observed. This is due to the descending dry foehn flow over the Alps, in which heavy thunderstorms are unlikely Relation between stratification and thunderstorm types In Figure 8 the frequency distribution of the convective available potential energy (CAPE) is shown for the three types of thunderstorm. The computation of Figure 6. Absolute (top) and relative (bottom) frequency of days with thunderstorms relative to the 29 Grosswetterlagen (weather patterns), after Hess and Brezowski. 234

9 Motion characteristics of thunderstorms in southern Germany Figure 7. Absolute (top) and relative (bottom) frequency of days with thunderstorms relative to the weather pattern defined by the low-level flow in southern Germany. CAPE is based on the temperature and the humidity at the surface observed at the 12 UTC sounding. For all three types the values of CAPE are of the same order. CAPE values up to 2800 J kg 1 are observed for single events (e.g. Haase-Straub, 1997). The mean values of CAPE are 583 J kg 1 for stationary thunderstorms, 701 J kg 1 for moving thunderstorms and 876 J kg 1 for thunderstorm lines. According to Houze et al. (1993) the values of CAPE for convective systems with medium intensity are at least 2000 J kg 1. This holds for single events, but the majority of the observations give lower CAPE values. In several cases the soundings do not exactly describe the storm environment. Also, sometimes the 12 UTC sounding of München or Stuttgart is not representative for the airmass in which the storms occurred later on. This may explain the large scatter of observed CAPE values. A priori there is no relation between CAPE and the observed storm types. However, high CAPE values favour the development of more highly organised and longer-living storms. The average values of CAPE are in agreement with those found in Switzerland (Schiesser et al., 1995, 1997). They report mean values of 656 J kg 1 for days with hail damage. An average CAPE of 938 J kg 1 was observed during the days with most hail damage by line-oriented thunderstorms. This corresponds to our classification of thunderstorm lines (average CAPE of 876 J kg 1 ). The difference between the CAPEs for days without storms and days with stationary storms is small. This underlines the fact that high CAPE is a necessary condition for deep convection but not always sufficient. Thunderstorms do not develop even for high CAPE values if the energy which is necessary to reach the level of free convection (CIN) is too high and not provided by forced lifting. The stratification and humidity of the boundary layer has a strong influence on the computation of CAPE. To reduce the effects of the boundary layer, the CAPE was recalculated using mean values of temperature and dew-point within the lowest 400 m of the sounding. The results are similar to the values given for the complete sounding. The average value of CAPE is 155 J kg 1 for stationary thunderstorms 201 J kg 1 for moving thunderstorms and 306 J kg 1 for thunderstorm lines. As expected, the values are lower than those shown above, but the relative differences are higher Vertical wind shear and thunderstorm types Figure 9 shows the frequency distribution of densityweighted wind shear for all three classes of thunderstorms. Wind shear is the same order of magnitude for all three classes. Low values prevail for stationary thunderstorms, with a mean value of 5.7 m s 1. For days with moving thunderstorms and thunderstorm lines the mean shear was 7.2 and 7.3 m s 1, respectively. The corresponding mean values of density-weighted shear energy are 16, 26 and 27 J kg 1. For both moving types of thunderstorms higher values of shear are observed. 235

10 M Hagen, B Bartenschlager and U Finke Figure 8. Frequency distribution of the convective available potential energy (CAPE) for the three classes of thunderstorms. Figure 9. Frequency distribution of the density-weighted wind shear for the three classes of thunderstorms. It is concluded that at low shear the short-living stationary thunderstorms prevail, whereas stronger shear favours long-living moving thunderstorms. Owing to the size of the investigation area and the change of the weather situation during one day, two or three different thunderstorm structures sometimes have been observed at the same time or during the course of a day. In such situations, besides a dominating thunderstorm type there are also less organised thunderstorms (i.e. moving thunderstorms and stationary thunderstorms). Thus only the class with stationary thunderstorms is homogeneous. Subsequently in mixed storm cases the mean values of shear for moving thunderstorms and thunderstorm lines is underestimated Bulk Richardson number and thunderstorm types The frequency distribution of the Richardson number for the three thunderstorm types is shown in Figure 10. The mean values of the Richardson number for these distributions are 37 for stationary thunderstorms, 22 for moving thunderstorms and 21 for thunderstorm lines. However, the scatter is large and the differences between the mean values are certainly not significant. 236 Numerical modelling studies by Weisman & Klemp (1984) showed that Richardson numbers between 15 and 35 are indicative of the development of supercell storms; values higher than 35 promote multicell storms. Again it should be noted that soundings are not always representative of the environment in which the observed systems occurred. Therefore the comparison with idealised numerical simulations should only be done to describe the relative location within the parameter space Steering level of moving thunderstorms For moving thunderstorms the speed and direction were compared to the wind profile from the soundings. Direction and speed were compared independently. Here we will present the magnitude of the difference vector between the motion vector and the wind vector at the respective heights. The observed storm speeds are in best agreement with the wind speed at heights around 3000 m and 5500 m. Lower-level winds are weaker than the storm speed; upper-troposphere winds are higher than the storm speed. The directions match best with heights around 3000 m. At lower levels the wind vector is to the left of

11 Motion characteristics of thunderstorms in southern Germany Figure 10. Frequency distribution of the bulk Richardson number (Ri) for the three classes of thunderstorms. Figure 11. Frequency distribution of height levels where the magnitude of the difference vector between the wind direction at the respective height and the motion vector is a minimum. the motion vector, at higher levels to the right. However, at higher levels the differences are around 5. Figure 11 shows the frequency distribution of the heights where the magnitude of the difference vector between the motion vector and the wind vector was minimum. Again the heights around 3000 m and 6000 m dominate. Figure 12 shows the magnitude of the difference vector. This is in agreement with Figure 11, except that the gap at 4000 m is not so obvious. 5. Summary and conclusions Thunderstorms are frequently observed in southern Germany during summer. The analysis of lightning data showed that thunderstorms occur on average every second day somewhere in southern Germany. In general, thunderstorms approach from westerly and south-westerly directions. The analysis is in good agreement with the observations by Finke & Hauf (1996a). The observed lighting patterns could be classified into three different groups: stationary thunderstorms, moving thunderstorms and thunderstorm lines. This phenomenological classification is a priori not related to the classification based on the storm scale (Foote, 1985). No significant relation between thunderstorm occurrence and the Grosswetterlagen was found, nor was it possible to give an indicator for the observed thunderstorm types. The hypothesis was that thunderstorms develop only in favoured weather situations. Surprisingly thunderstorms were observed during nearly all of the 29 patterns. It can be concluded that the description of the synoptic state of the atmosphere by Grosswetterlagen is not suitable for the investigation of thunderstorms. A reduction to eight patterns based on the low-level flow in southern Germany did not show much improvement. However, the classification based on the low-level flow indicates that about 20% of the thunderstorms occur when the low-level flow is easterly. This is in contrast to the prevailing direction from which thunderstorms arrive. It can be explained by the convergence and strong wind shear in the lowest levels, which is enforced by orographic effects due to the Alps. Examples of storm development in this environment are given by Meischner et al. (1991) and Haase-Straub et al. (1997). 237

12 M Hagen, B Bartenschlager and U Finke The analysis of dynamic and thermodynamic parameters suggests that the phenomenological classification of stationary thunderstorms corresponds to the basic type of a single cell. Those cells are short lived and do not have long tracks. Similarly, moving thunderstorms correspond to multicell or supercell storms. Multicell and supercell storms can exist for several hours; this requires strong low-level wind shear. And, owing to the prevailing higher winds in the middle troposphere, those storms are moving. Finally, the group of thunderstorm lines corresponds to thunderstorms with a mesoscale organisation producing line-oriented storms or squall lines. This relation between the phenomenological classification of lightning patterns and the basic cell types is in agreement with the observed Richardson numbers and the CAPE values. The observed Richardson numbers are similar to those found by Weisman & Klemp (1984). Figure 12. Mean value of the magnitude of the difference vector between the wind direction at the specified height and the motion vector. The dynamic and thermodynamic conditions of a thunderstorm day have been analysed from radio soundings at München and Stuttgart. Despite radio soundings not always being representative of the thunderstorm environment, it was found that higher wind shear favours moving thunderstorms or thunderstorm lines. High CAPE values are observed when moving thunderstorms or thunderstorm lines are present. The latter is in agreement with the observations of Schiesser et al. (1995, 1997) in Switzerland. The highest Richardson numbers are observed for stationary storms; the lowest values are observed for thunderstorm lines. Stationary storms develop at low wind speeds and low shear, which results in high Richardson numbers. As shown above, moving thunderstorms and thunderstorm lines have similar shear values but they differ in their CAPE. In accordance with the results presented by Weisman & Klemp (1982, 1984), lower Richardson numbers are observed for more highly organised convection systems. The direction of moving thunderstorms is mainly controlled by the flow at heights around 3 and 6 km m.s.l. We emphasise that this steering level is the result of translation, propagation and processes like splitting of the cells. 238 The main shortcomings of this study are the great variety of lighting patterns, the orographic inhomogeneity of the investigation area and the low spatial and temporal separation between individual radio soundings. The classification of the lighting patterns requires compromises when different types occur at the same time, or no clear assignment is possible. The orographic structure in southern Germany with the mountain ridges of the Schwarzwald, Schwäbische and Fränkische Alb, the Bayerische Wald and the Alps as well as the valley of the River Danube has a strong influence on the initiation and motion of thunderstorms. This influence can hardly be excluded from the overall picture of the observed lightning patterns. Finally, the radio soundings from München and Stuttgart at 12 UTC (only from these can CAPE be computed) cannot completely describe the thunderstorm environment, mainly because thunderstorms occur mostly in the late afternoon or in the evening, and they are not always representative of the environment in which the observed systems occurred. This study emphasises that the behaviour of a complex system like a thunderstorm can hardly be described by a few characteristic numbers. Great uncertainty will remain when it is attempted to forecast the future development of a thunderstorm based on a few environmental parameters. Acknowledgements We gratefully acknowledge the provision of the LPATS data by the Bayernwerk AG, München. The contribution of Ulli Finke was supported by the Bayerisches Staatsministerium für Landesentwicklung und Umweltfragen and is part of the Bavarian climate research program BayFORKLIM.

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