Understanding processes leading to surface gusts by modeling windstorms at very high resolution

Windstorms associated with extratropical cyclones are destructive natural hazards due to the resulting surface gusts mostly. Processes governing the formation of gusts are thus crucial for the societal impact of windstorms but are not well understood and too small scale to be explicitly represented in numerical weather prediction models. The ANR project WINDGUST aims to better understand the formation of wind gusts through innovative numerical simulations with the Meso-NH atmospheric model. Accurately modeling windstorms requires high resolution over a large domain to represent both fine-scale structures and mesoscale dynamics, as well as advanced physical parameterizations and coupling with surface models to capture complex interactions. Running such simulations is a computational challenge, as is the analysis of the resulting deluge of data.

First results are presented for the intense Mediterranean cyclone Adrian that hit Corsica on 29 October 2018 with gusts above 50 m/s and extended damages. While state-of-the-art meteorological simulations with kilometer-scale resolution are able to develop strong mesoscale winds associated with the cold conveyor belt of the cyclone, large-eddy simulations with hectometer-scale resolution are necessary to explicitly represent their boundary-layer organization. The latter reveal the presence of coherent structures as convective rolls align with the main wind direction over the warm Mediterranean sea. Lagrangian tracers computed online during the model integration highlight their crucial role in the downward transport of momentum to the surface. The characteristics of convective rolls depend on the horizontal grid spacing but are also strongly sensitive to the representation of surface fluxes over sea, which are poorly constrained under winds above 20–25 m/s. These results suggest a large uncertainty in numerical weather predictions of surface gusts using kilometer-scale models.

Future work will extend the methodology to Atlantic cyclones involving different mesoscale features and using full atmosphere-wave-ocean coupling in order to accurately represent air-sea interactions under windstorm conditions.