A case study of a derecho storm in dry, high-shear environment

The present study examines the origin and environmental conditions of the severe convective windstorm on September 17, 2017, which affected several countries in the central and southeastern Europe, above all Serbia and Romania. The large area of the damage swath (at least 500 km long) and high wind gusts (up to 40 m/s) would classify this event as a derecho or at least as a storm very similar to derechos (with respect to newer definition proposals). Small-scale bow echoes were found in areas with highest reported wind gusts, and some thunderstorms within the storm-producing convective system were probably supercells. The existence of high wind shear and storm rotation could be also related to the significant rightward deflection of the system with respect to the mean wind and propagation of other thunderstorms and systems observed on that day. In contrary to many other known derecho events, this storm propagated toward a very dry airmass exhibiting only low or moderate convective available potential energy (CAPE) values. This is shown by soundings, ECMWF model outputs, and vertical profiles from the IASI L2 satellite sounder. Several convective parameters (e.g. CAPE, downdraft CAPE, derecho composite parameter, 0-3-km relative humidity, 0-6-km shear) were evaluated and compared with proximity soundings of other described European derechos or with the available climatology. The possibility of a balance between the cold pool-generated horizontal vorticity and the environmental shear is also discussed. It is concluded that identification of low-level humidity sources (with aid of storm-relative wind vectors or streamlines) can be important in forecasting of thunderstorm systems moving toward an airmass, which is seemingly too dry for development and maintenance of deep convection. It is also shown that due to low CAPE values, some composite parameters would not indicate favourable conditions for a long-lived convective system. The lack of radiosonde observations can be partially supplemented by data from the IASI L2 sounder, which profiles can be largely different from model forecasts, showing much drier air in the mid- and upper troposphere in this case. It is concluded that due to the absence of strong synoptic forcing and larger pressure gradient at surface, convective processes played major role in the windstorm development. The presence of high temperature lapse rates at low- and mid-levels, high wind shear and unusually dry pre-storm airmass could be considered as the most important signatures related to the storm severity