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

Abstract

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.

Ship- and island-based atmospheric soundings from the 2020 EUREC<sup>4</sup>A field campaign

“…On the other hand, the height of the mixed layer, that can be defined as near homogeneous potential temperature layers close to the surface, tends to be deeper over the cold side than over the warm side. This contrasts with results obtained over stronger SST fronts from observation (Ablain et al, 2014) and modeling studies (e.g., Kilpatrick et al, 2013;Redelsperger et al, 2019) and suggests that the lower atmosphere does not solely respond to the SST gradient. Over the cold side, wind speed tends to decrease with altitude ( Fig.…”

Towards kilometer-scale ocean–atmosphere–wave coupled forecast: a case study on a Mediterranean heavy precipitation event

“…Oceanic modeling is now able to accurately represent such structures and their time evolution, provided the resolution of the simulation is fine enough (e.g., Gula et al, 2014). SST fronts for instance can significantly impact the atmospheric conditions (Small et al, 2008), low-level flow (Redelsperger et al, 2019) and convergence (Meroni et al, 2020) independently of strong-impact weather events. The feasibility of using tri-coupled configurations like the one developed in the present study for a reasonable computing cost opens the way to a more explicit representation of the surface heterogeneities at sea, of their time evolution, and of their impact on the atmosphere for high-resolution deterministic operational NWP.…”

Towards kilometer-scale ocean-atmosphere-wave coupled forecast: a case study on a Mediterranean heavy precipitation event

“…Gula et al, 2014). SST fronts for instance can significantly impact the atmospheric conditions (Small et al, 2008), low-level flow (Redelsperger et al, 2019) and convergence (Meroni et al, 2020) independently of strong-impact weather events. The feasibility of using tri-coupled configurations like the one developed in the present study for a reasonable computing cost opens the way to a more explicit representation of the surface heterogeneities at sea, of their time evolution, and of their impact on the atmosphere for high-resolution deterministic operational NWP.…”