Complementarity between Large-Eddy Simulation and Synthetic-Aperture Radar observations to characterise surface wind in an extratropical cyclone

Abstract. While the large-scale dynamics of extratropical cyclones are well understood, the mechanisms by which fine-scale processes generate strong surface winds remain poorly understood. Storm Alex, located over the English Channel on 2 October 2020, featured a secluded warm front associated with enhanced surface wind speeds. Its fine-scale structure is investigated using Synthetic Aperture Radar (SAR) observations and three Meso-NH Large-Eddy Simulations (LES), with spectral and two-dimensional auto-correlation analyses applied to both SAR data and LES outputs. The results reveal energetic fine-scale structures responsible for locally enhanced surface wind in three high wind speed regions identified from SAR data. These fine-scale structures are wind streaks quasi-aligned along the mean wind direction, with a characteristic length scale of approximately 1 km, about twice the marine boundary layer height. They are reproduced by LES in two of the three regions, where they are due to the presence of boundary layer rolls in which the strongest surface winds are associated with downward transport of momentum from a low-level jet. In the third region, the absence of boundary layer rolls in LES may be related either to insufficient thermal instability or to excessively strong simulated wind shear, suggesting a strong sensitivity to surface fluxes and vertical stability for developing boundary layer rolls within extratropical cyclones.  Overall, this study demonstrates that combining LES and SAR observations provides a robust framework for accurately capturing, validating, and improving our understanding of the fine-scale processes associated with surface wind in an extratropical cyclone.