Climatology and Large-Scale Dynamics of High-Shear, Low-CAPE Derechos in Germany

Based on an 18-year climatology an average of two derechos affect Germany each year. The seasonal distribution peaks in the summer months, however, cold-season events contribute to 38 % of the total derecho number. The latter typically form in north-westerly flow which is characterized by very strong vertical wind shear and low CAPE. In Germany, cold-season derechos reach a similar intensity as derechos in high CAPE environments. We used proximity soundings and simulations of the high-resolution COSMO model to analyse large local variations of CAPE frequently observed close to cold-season derechos. We found a rapid increase of the local lapse rate in the pre-derecho environment. According to a calculation of the local lapse rate tendency equation, stretching at low levels significantly contributes to the observed lapse rate increase. This stretching occurred downstream of upper-level fronts or PV intrusions that were present close to all cold-season derechos. Based on the results of this work, we present a conceptual model that explains why the observed cold-season derecho situations can be favourable for the development of high-wind producing mesovortices. Finally, we propose to keep the term derecho also for the cold season.