04/23/09

Deutsches Zentrum für Luft- und Raumfahrt e.V.in der Helmholtz-Gemeinschaft

Institut für Physik der Atmosphärehttp://www.dlr.de/ipa

Severe and long lasting thunderstorms pose a tremendous threat to property and life in Europe each year. Despite the rapid enhancement of models' resolution and the increase in calculating capacity, forecasting the initiation or the future tracks of thunderstorms remains difficult. In this study the violent F4 tornado at Hautmont, France on the 03rd of August 2008 and the regional tornado outbreak over southwest Poland on the 15th of August 2008 are analyzed. The efficiency of the model is assessed by means of the calculation of forecasting parameters for severe thunderstorms (e.g. convective available potential energy (CAPE), directional and speed shear, but also composite parameters like the significant tornado parameter (STP) or the supercell composite parameter (SCP)). Among other things, violent tornadoes can develop in low CAPE environments with a maximum of the instability distribution at lower levels or along boundaries, which can alter the local environment considerably.

Environmental conditions and radar observationsof organized tunderstormsHelge Tuschy1,2,3, Martin Hagen1, Georg J. Mayr 2

1) Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Physik der Atmosphäre (IPA), Oberpfaffenhofen, Germany2) University of Innsbruck, Innsbruck, Austria3) European Severe Storms Laboratory, ESSL

Contact: Helge.Tuschy@dlr.de

Motivation

Large-scale flow (COSMO-DE)

CAPE and shear (COSMO-DE)

Conclusion

Surface analysis/observations

Damage and weather phenomena

- intense directional and speed shear (SRH1km up to 300 m2/s2 and 0-6km bulk shear between 25 – 30 m/s , 12Z soundings)- wavy frontal boundary, serving as focus for scattered thunderstorm initiation. The passage of a weak short-wave increases thunderstorm coverage- very moist boundary layer with dewpoints in excess of 20°C all the way to east-central Poland- abundant CAPE release (MLCAPE 600-1600 J/kg, 12Z soundings)

background color: orange (13-16°C), red (16-19°C) and purple (19-22°C), sfc. dewpointlong wind barbs: wind at 850hPa(black) and 500hPa(orange), COSMO-DE (15Z)radar source: max.radar reflectivity composed from radars Brdy and Skalky (CHMI)

Geopotential heights (isohypses) at 300hPa, 2-4km lapse rates (colored) and wind field at 300hPa (wind barbs) (all 1500Z)

- elevated mixed layer (Spanish plume) advects to the northeast with steep mid-level lapse rates at the top of a moist boundary layer- numerous weak short waves are embedded in this southwest flow, serving as foci for convective initiation- the periphery downstream of the trough is strongly sheared at all levels

Hautmont (France), 03.08.08

SW Poland, 15.08.08

- progressive upper trough swings in from Ireland and United-Kingdom- more meridional aligned streamline pattern with an intense speed maximum, approaching from the west- overall, lapse rates are just moderately steepened with a pronounced minimum south of Luxembourg

Germany

Acknowledgement

The authors want to thank Meteo France, the Institut Royal Météorologique (KMI) and the Czech Hydrometeorological Institute (the Radar Department and the Satellite Department, CHMI) for providing the data.

France

Fig.1 Fig.2 Fig.3

Fig.4

Fig.1)- a tongue of near 100 J/kg surface based CAPE at lowest 3km evolved just ahead of the front- well mixed airmass over the North Sea (cold air advection over warm waters)Fig.2) (Hautmont at 50°N; cross section at 3.9°E)- very moist low-levels (3-4km above ground level)- dry air at mid-levels overspreads the region from the west combined with lift supportive for potential instability release

Fig.3) (white lines are SRH-3km, starting at 300 m2/s2)- impressive directional shear at lowest 1km (colored) and 3km

Fig.4) - intense speed shear at all levels, increasing from 15- 22m/s at the lowest km to 30m/s 0-6km bulk shear

- very thin CAPE profiles with a chance for some low-level CAPE release

copyright: www.strzelecopolski.pl copyright: www.tvn24.pl copyright: AP

- violent tornadoes can occur even in low instability environments, when most of the CAPE can be found at lower levels (e.g. 0-3km) and strong forcing/intense shear compensate limiting destabilization of the atmosphere- in both cases, boundary layer moisture was high, resulting in low lifted condensation levels, supportive for tornado development- both, directional and speed shear were exceptionally high for this time of the year and favored significant severe thunderstorm events with violent tornadoes being a distinct possibility- COSMO-DE did a good job in highlighting the environment, especially for the 15th of August 08. It placed the front a bit too far south on the Hautmont day, but still pointed to the possibility of organized thunderstorms and an attendant tornado threat

The significant tornado parameter (source: http://www.spc.noaa.gov/):

STP = (sbCAPE/1500 J kg-1) * ((2000-sbLCL)/1500 m) * (SRH1/100 m2 s-2) * (6BWD/20 m s-1)* ((200+sbCIN)/150 J kg-1)

sbCAPE = surface based CAPE CIN = convective inhibition SRH1 = storm relative helicity LCL = lifted condensation level BWD = bulk wind difference

Geopotential heights (isohypses) at 300hPa, 2-4km lapse rates (colored) and wind field at 300hPa (wind barbs) (all 2100Z)

- intense directional and speed shear (SRH1km up to 250 m2/s2 and 0-6km bulk shear between 25 – 30 m/s , 00Z soundings)

background color: surface dewpoints: orange (11-14°C), red (14-17°C), purple (17-20°C) and green 20°C and more.radar source: Meteo France - in courtesy of KERAUNOS

COSMO-DE forecast (1500UTC)

- MLCAPE of 500 – 1700 J/kg with peaks of near 2000 J/kg - strongest instability release in the favorable hail growth zone of -10°C to -30°C with limited low-level instability release- strongest winds are just about to enter the area of interest, e.g. E-Czech Republic and extreme SW- Poland with 0-6km bulk-shear increasing to near 30m/s. 0-1km shear is very strong just east of the front with 15-25m/s as the low-level jet strengthens - the STP peaks with 2-3 just at the border of SW-Poland, where tornadic thunderstorms evolved in the upcoming 1-2 hours.- combination of a quasi-stationary surface boundary and a diffuse surface pressure field generate a complex surface wind field pattern, variable both in strength and direction

copyright for both images : Bjoern Stumpf (WTINFO)Photograph: Philippe Frutier/AFP/Getty Images

- line echo wave pattern evolves along the southeastward moving cold front. Eastward racing and sharply bent upper trough keeps line going over northeast France

- by comparison between the model and the observation (e.g. sounding reports or surface data), COSMO-DE performed well regarding strength of the wind field and the quality of the boundary layer (the latter one is important in how good the model handles the instability forecast)