Abstract
AbstractIntense 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.
Highlights
The most damaging wind hazards over Europe are associated with intense wintertime extratropical cyclones, which in addition can cause heavy precipitation and flooding (Catto, 2016)
Strong winds in extratropical cyclones are mainly caused by low-level jets associated with conveyor belts accompanying the fronts, namely the warm conveyor belt jet or warm jet and the cold conveyor belt jet or cold jet, as well as frontal convection (Hewson and Neu, 2015; Earl et al, 2017)
Since sting jet (SJ) can cause damaging wind speeds and are challenging to forecast accurately, it is important to fully understand this phenomenon and to predict it sufficiently well, even more so as the potential of SJ storms may increase with climate change (Knippertz et al, 2018; Martínez-Alvarado et al, 2018)
Summary
The most damaging wind hazards over Europe are associated with intense wintertime extratropical cyclones, which in addition can cause heavy precipitation and flooding (Catto, 2016). Schultz and Sienkiewicz (2013) claim that frontal dynamics are more important than small-scale moist processes and that an SJ is related to frontolysis at the tip of the bent-back front (see Slater et al, 2015; 2017; Coronel et al, 2016) These results suggest mixed contributions and some case-to-case variability in SJ dynamics. Most SJ case-studies are dedicated to cyclones over the North Atlantic and the British Isles with some of these storms crossing northern Europe later in their life cycle (e.g. Christian in October 2013: Browning et al, 2015; Pantillon et al, 2018). Using grid spacings down to 1.6 km and up to 90 vertical levels allows an adequate representation of orography and small-scale processes such as mesoscale and convective instabilities In these simulations, the SJ is identified on the basis of descending high-wind trajectories.
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