Dynamics of sting-jet storm Egon over continental Europe: Impact of surface properties and model resolution

Eisenstein, Lea ; Pantillon, Florian ; Knippertz, Peter

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<p align="justify">Intense Shapiro-Keyser cyclones are often accompanied by a sting jet (SJ), an air stream that descends from the cloud head into the frontal-fracture region and can cause extreme surface gusts. Previous case-studies have concentrated on the North Atlantic and the British Isles. Here we present the first-ever detailed analysis of an SJ over continental Europe and investigate the influence of topography on its dynamical evolution based on observations and high-resolution simulations using the ICOsahedral Nonhydrostatic model (ICON). Windstorm Egon intensified over the English Channel and then tracked from northern France to Poland on 12-13 January 2017, causing gusts of almost 150 km·h−1 and important damage. ICON reproduces the storm dynamics, although it delays the explosive deepening, shifts the track southward over Belgium and Germany and underestimates gusts over land. Storm characteristics show weak sensitivity to varying grid spacing between 1.6 and 6.5 km, while switching off the convection parametrization at 3.3 km grid spacing improves correlations with surface observations but deteriorates the mean error. Trajectories reveal typical SJ characteristics such as mid-level descent, strong acceleration and conditional symmetric and other mesoscale instabilities, while evaporative cooling is stronger than in previous cases from the literature, preventing drying during descent. The SJ identification and the occurrence of mesoscale instabilities depend considerably on model resolution, convective parametrization, output frequency and employed thresholds for trajectory selection. Sensitivity experiments with modified surface characteristics show that the combined effects of warm-air blocking by the Alps, higher roughness over land and reduced surface fluxes cause Egon to fill more quickly and to move on a faster, more northern track across Germany. While the SJ response is complex, showing some compensating effects, surface gusts strongly increase when roughness is reduced. These results suggest that weather forecasters in continental Europe should be more aware of the potential risks associated with SJs.</p>
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