Eddy kinetic energy redistribution within windstorms Klaus and Friedhelm

Abstract

The formation of lower‐tropospheric wind speed maxima is analyzed during the mature stage of two distinct windstorms using ERA‐Interim reanalysis datasets and performing an eddy kinetic energy (EKE) budget. Both storms developed according to the Shapiro–Keyser conceptual model and crossed the large‐scale low‐frequency jet from its warm‐air towards its cold‐air side. The formation of strong wind regions are shown to depend on the position of the storms relative to the large‐scale jet axis, which confirm theoretical results of a companion study. As long as the storms are travelling south of the low‐frequency jet or close to the jet axis, the most intense EKE maxima as well as the total kinetic energy maxima are located in the warm sector of the surface cyclones on their southeastern side. As soon as the surface cyclones move to the north of the low‐frequency jet, EKE is cyclonically redistributed in the lower troposphere, first to the northnorthwest of the cyclone's centre, and then to the southwest along the bent‐back warm fronts. At this later stage, EKE, which is generated by baroclinic conversion in the mid‐troposphere, is redistributed downwards by the vertical ageostrophic geopotential fluxes before being further redistributed southwestwards in the lower troposphere by the ageostrophic geopotential fluxes. This EKE redistribution led to the formation of a low‐level westerly jet to the south of each cyclone centre behind the cold front. These common features between the two storms happened in spite of differences in their shape and environment.