High Resolution Numerical Study of Atmospheric Response to SST Front: Realistic LES Case Study over the Iroise Sea

The Ushant SST front is due to a contrast between the stratified surface layer present in summer offshore the Iroise Sea (NW France) and the strong tidal mixing on the coastal area. SST gradients of 4°C over 3 to 5 km are usually present south of the Ushant Island between May and early October. The location, shape and magnitude of this SST front moving on a subdiurnal time period with the tidal currents is well reproduced by the oceanic MARS 3D model, as shown by comparisons with in situ and satellite observations.

Previous studies of the effects of sharp, midlatitude SST fronts on the marine atmospheric boundary layer (MABL) are mostly based on observations, analytical models or idealized Large Eddy Simulations (LES). They show that several effects related to the SST gradient play competing roles in establishing a new balance of the MABL, namely: the difference in the height of the MABL between warm and cold side; the difference in turbulent mixing; the presence of a shallow internal boundary layer on the cold side; the low-level horizontal pressure gradient; the difference in the Coriolis force. The relative impact of these effects depends on the characteristics of the SST gradient and on the synoptic conditions, especially low-level wind speed and atmospheric stability.

In this study we use the non-hydrostatic atmospheric model Meso-NH with three embedded domains 2-way grid nested with each other to realistically simulate an observed situation with a weak-wind warm-to-cold flow. The initial and boundary conditions of the coarsest domain of 500 x 500 km and 2.5 km horizontal resolution come from the AROME operational analyses of Meteo-France. The three-domain simulation we use enables the innermost domain to be run with a LES horizontal resolution of 100 m and 100 vertical levels, 30 of them being in the first 600 m above sea level. All the domains are forced at the surface by the 2.5-km SST field simulated by the oceanic MARS 3D model, updated every hour. The outputs of this simulation are validated using observations, and used to investigate the effects of the SST front on the MABL and the respective impact of the horizontal pressure gradient, of the stability in the warm and cold sides, and of the MABL height variations between the two compartments. We show that significant differences on the potential temperature, atmospheric stability and heat fluxes are obtained between the two sides. The surface flow coming from the warm side is also significantly slowing down over the cold side, due to the pressure gradient effect and the formation of an internal boundary layer.