Doppler radar observation, CG lightning activity and aerial survey of a multiple downburst in southern Germany on 23 March 2001

Two downburst events from one thunderstorm are investigated, which occurred on 23 March 2001, in Germany's climatologic annual minimum of downburst activity. Observations by two Doppler radars are combined with hail reports, ground lightning detection and an aerial survey conducted after the event. The downburst-producing storm had formed at a synoptic convergence line within the warm sector of a cyclone. It had a remarkably high propagation speed of up to 31 m s−1 corresponding to the midtropospheric flow. Thus, by superposition with the storm motion, even two weak downbursts were sufficient to cause the observed damage of F1 and F2 intensity, respectively. While in its late stages, the storm was dynamically characterized by lower- and midtropospheric divergence; at about the time of the first downburst, a mesocyclonic vortex signature was verified. Aside from midtropospheric dry air entrainment, a thermodynamic explanation for the triggering of the two downbursts by melting of small hail according to recent findings by Atlas et al. [Atlas, D., Ulbrich, C.W., Williams, C.R., 2004. Physical origin of a wet microburst: observations and theory. J. Atmos. Sci. 61, 1186–1196] appears probable. Despite the lack of warnings to the public, the storm's potential for hail and strong straight-line winds was detected by the German weather service radar software CONRAD more than a half hour before the downbursts occurred

Book Reviews of Lin, Y.-L.: Mesoscale Dynamics

Buchbesprechung Lin, Y.-L., 2007: Mesoscale Dynamic

Reconstruction of near-surface tornado wind fields from forest damage

Tornado intensity is usually inferred from the damage produced. To foster postevent tornado intensity assessments, the authors present a model to reconstruct near-surface wind fields from forest damage patterns. By comparing the structure of observed and simulated damage patterns, essential parameters to describe a tornado near-surface wind field are derived, such as the ratio Gmax between circular and translational velocity, and the deflection angle a between peak wind and pressure gradient. The model consists of a wind field module following the Letzmann analytical tornado model and a tree module based on the mechanistic HWIND tree model to assess tree breakage. Using this method, the velocity components of the near-surface wind field, the track of the tornado center, and the spatial distribution of the Fujita scale along and across the damage path can be assessed. Necessary requirements to apply the model are knowledge of the tornado translation speed (e.g., from radar observations) and a detailed analysis of the forest damage patterns. One of the key findings of this analysis is that the maximum intensity of the tornado is determinable with an uncertainty of only (Gmax 1 1) times the variability of the usually well-known tornado translation speed. Further, if Letzmann�s model is applied and the translation speed of the tornado is known, the detailed tree model is unnecessary and could be replaced by an average critical velocity for stem breakage ycrit independent of the tree species. Under this framework, the F3 and F2 ratings of the tornadoes in Milosovice, Czech Republic, on 30 May 2001 and Castellcir, Spain, on 18 October 2006, respectively, could be verified. For the Milosovice event, the uncertainty in peak intensity was only 66.0 m s21. Additional information about the structure of the near-surface wind field in the tornado and several secondary vortices was also gained. Further, this model allows for distinguishing downburst damage patterns from those of tornadoes

Quantitative comparison of METEOSAT-based thunderstorm detektion and nowcastin with in-situ reports in the European Severe Weather Database (ESWD)

Severe thunderstorms constitute a major weather hazard in Europe, with an estimated total damage of € 5-8 billion each year. Yet a pan-European database of severe weather reports in a homogeneous data format has become available only recently: the European Severe Weather Database (ESWD). We demonstrate the large potential of ESWD applications for storm detection and forecast evaluation purposes and complement an earlier case-based study. The analysis of all warm-season (JJA) severe weather days in Europe in 2008 corroborated our earlier findings. There is a good agreement between ESWD reports and Cb-TRAM detected thunderstorms, even though no exact correspondence between ESWD reports and Cb-TRAM cells is required (e.g. due to storm morphology). Correspondingly, a large portion of ESWD reports regarded as misses by our strict in/out-of-Cb-TRAM-polygon criterion were still located close to a Cb-TRAM cell. Quantitatively, only the probability of detection (POD) can be evaluated due to the different characteristics of the two data sources. The POD for storm detection was 0.24 on average, with maximum values up to 0.58. The respective analysis for the 30 and 60 minutes nowcasts yielded average POD values of 0.11 and 0.08, respectively, with maximum values of POD exceeding 0.4 on 3 days for the 30 minutes nowcast and on one day for the 60 minutes nowcast

Derivation of physically motivated wind speed scales

A class of new wind speed scales is proposed in which the relevant scaling factors are derived from physical quantities like mass flux density, energy density (pressure), or energy flux density. Hence, they are called Energy- or E-scales, and can be applied to wind speeds of any intensity. It is shown that the Mach scale is a special case of an E-scale. Aside from its foundation in physical quantities which allow for a calibration of the scales, the E-scale concept can help to overcome the present plethora of scales for winds in the range from gale to hurricane intensity. A procedure to convert existing data based on the Fujita-scale or other scales (Saffir-Simpson, TORRO, Beaufort) to their corresponding E-scales is outlined. Even for the large US tornado record, the workload of conversion in case of an adoption of the E-scale would in principle remain manageable (if the necessary metadata to do so were available), as primarily the F5 events would have to be re-rated. Compared to damage scales like the �Enhanced Fujita� or EF-scale concept recently implemented in the USA, the E-scales are based on first principles. They can consistently be applied all over the world for the purpose of climatological homogeneity. To account for international variations in building characteristics, one should not adapt wind speed scale thresholds to certain national building characteristics. Instead, one worldwide applicable wind speed scale based on physical principles should rather be complemented by nationally-adapted damage descriptions. The E-scale concept can provide the basis for such a standardised wind speed scale

Use of the European Severe Weather Database to verify satllite-based storm detection or nowcasting

Severe thunderstorms constitute a major weather hazard in Europe, with an estimated total damage of € 5-8 billion each year. Yet a pan-European database of severe weather reports in a homogeneous data format has become available only recently: the European Severe Weather Database (ESWD). We demonstrate the large potential of ESWD applications for storm detection and forecast or nowcasting/warning verification purposes. The study of five warm-season severe weather days in Europe from 2007 and 2008 revealed that up to 47% of the ESWD reports were located exactly within the polygons detected by the Cb-TRAM algorithm for three different stages of deep moist convection. The cool-season case study of extratropical cyclone “Emma” on 1 March 2008 showed that low-topped winter thunderstorms can provide a challenge for satellite storm detection and nowcasting adapted to warm-season storms with high, cold cloud tops. However, this case also demonstrated how ESWD reports alone can still be valuable to identify the hazardous regions along the cold front of the cyclone

Downburst-producing thunderstorms in southern Germany: Radar analysis and predictability

Three days with downburst-producing thunderstorms during the VERTIKATOR intensive observation period in June and July 2002 are studied by means of the C-band polarisation diversity radar POLDIRAD and its network of three bistatic receivers.We present the first wind vector fields from a downburst measured by such a bistatic network. The polarimetric radar data allowed testing the recent hypothesis that a dominant trigger mechanism for wet downbursts might be the cooling due to melting of small hail or graupel in the storm, and we found some evidence for this process in the VERTIKATOR storms. This could be exploited by polarimetric radar nowcasting algorithms for downburst detection. The predictability of the downburst potential was further investigated from proximity soundings and their derived indices WINDEX as well as different formulations of GUSTEX. In particular, a new formulation of GUSTEX is proposed here which shows promising predictive skill for the VERTIKATOR cases and a number of other severe (and non-severe) situations from the same region in southern Germany