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Propagation Characteristics of Thunderstorms in Southern Germany

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InstitutfurPhysik der Atmosphare Report No. 97 Propagation characteristics of thunderstorms in southern Germany by Martin Hagen, Blasius Bartenschlager, Ullrich Finke I Diese Berichtsserie des Instituts fur Physik der Atmosphare enthalt Veroffentlichungen, die zu einem spateren Zeitpunkt an anderer Stelle erscheinen sollen, sowie spezielle Einzelergebnisse und erganzende Materialien, die von allgemeinem interesse sind. This series of reports of the Institute of Atmopsheric Physics comprises preprints of publications, specific results and complementary material, which may be of a more general interest. Herausgeber Ulrich Schumann Deutsches Zentrum ftir Luft- und Raumfahrt e.V. (DLR) Institut fur Physik der Atmosphare Redaktion Ute Lob Anschrift DLR - Oberpfaffenhofen D-82234 Wessling Germany Telefon +49-8153 28 1797 Telefax +49-8153 28 1841 WWW http://www.op.dlr.de/ipa/ E-Mail [email protected] Oberpfaffenhofen Mai 1998 ISSN 0943-4771 DISCLAIMER 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
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