Impact of a sharp, small-scale SST front on the marine atmospheric boundary layer on the Iroise Sea: Analysis from a hectometric simulation

Impact d'un front SST net et de petite échelle sur la couche limite atmosphérique marine en mer d'Iroise : Analyse à partir d'une simulation hectométrique

Redelsperger, J.-L. ; Bouin, Marie-Noëlle ; Pianezze, Joris ; Garnier, V. ; Marié, L.

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<p align=justify>The impact of a sharp, small-scale sea-surface temperature (SST) front located in the Iroise Sea off the northwestern coast of France on the marine atmospheric boundary layer (MABL) is investigated using a realistic very high-resolution numerical simulation. The modelling configuration is based on a two-way nesting of the atmospheric model Meso-NH over three embedded domains, with horizontal resolution of 100 m in the central domain. The outer mesoscale domain is initialized and forced on its lateral boundaries with the operational analyses of Météo-France. A high-resolution, realistic SST field from the Model for Application at Regional Scale (MARS) 3D is used as surface forcing. This configuration has been carefully designed to accurately represent this weak-wind situation highly inhomogeneous in space and time, with wind flowing from the warm to the cold side of the front. The impacts include a slight modification of the thermal structure of the MABL, and a strong and rapid decrease of the wind speed, wind stress and turbulent heat fluxes. Conversely to previously published work, the coupling of the wind stress with the SST is largely related to atmospheric stability effects rather than to the change of the surface wind speed. This is attributed to the conjunction of the weak wind speed around 3 m s−1 and of the narrowness of the front resulting in relatively strong advection. These characteristics explain the momentum budget with strong change of the turbulent mixing over a shorter length-scale than the pressure adjustment mechanism and the Coriolis term. The deceleration and counter-clockwise veering of the near-surface wind over colder water is consistent with previous work. Its attribution to turbulent mixing via a thermal stratification change in the context of a realistic study of a sharp front is a novel result.</p>
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