COMPASS 2025-01-08: Sophia Brumer, LAERO, Toulouse, France
"On Sub-Mesoscale Air-Sea Interactions in Extratropical Windstorms"
Windstorms associated with extratropical cyclones are destructive natural hazards. We are interested in elucidating the processes involved in the formation of near-surface extreme winds, and my focus is on wave and wave-breaking related processes. Though crucial for their societal impact, these processes are not well understood and too small-scale to be explicitly represented in numerical weather prediction models. Waves modulate air-sea exchanges, mix the upper ocean, and inject sea spray into the atmosphere when breaking. Air-sea fluxes of enthalpy and momentum greatly influence the dynamics of the marine atmospheric boundary layer (MABL). Waves increase the surface roughness but sea spray loading may act as a buffer layer reducing drag and stabilizing the MABL. Larger droplets increase air-sea enthalpy and decrease momentum transfers, thus promoting the intensification of tropical cyclones, but what of extratropical cyclones?
In this talk, I will give an overview of ongoing and planned work at the Laboratoire d'Aérologie (LAERO) in Toulouse, France, on sub-mesoscale ocean-wave-atmosphere interactions in extratropical cyclones. Ongoing work will be illustrated through three case studies: 1) the Mediterranean cyclone Adrian, where sub-mesoscale wind rolls show strong sensitivity to air-sea fluxes; 2) the North Atlantic storm Alex, where wave coupling influences mesoscale jets and the downward momentum transport; and 3) the cold wake producing medicane (Mediterranean Hurricane) Ianos, where the ocean induces a negative feedback similar to that seen in certain tropical cyclones.
Future work aims at establishing a coherent air-sea coupled framework for numerical weather predictions, which includes the impact of waves on roughness, of sea spray on the MABL, and takes into account relative alignment between the wind and wave systems. For this purpose, we are designing realistic coupled simulations with horizontal resolutions approaching those of Large Eddy Simulations. These will allow gauging the scale of impacts of non-resolved and poorly constrained processes, such as sea spray generation and subsequent heat and momentum exchanges within the MABL. Field measurements needed to evaluate these simulations will include the NAWDIC field campaign, which will sample North Atlantic storms over the winter of 2025/2026. Future campaigns in the Mediterranean / Ionian Sea and the south Indian Ocean are also under consideration.
