Revisiting Sea State Dependent Surface Fluxes Parameterisations in a Fully Coupled Forecast System with Emphasis on Tropical Cyclone

The global analyses and medium range forecasts from the European Centre for Medium range Weather Forecasts rely on a state of the art atmospheric model. In order to best represent the momentum exchange at the surface of the oceans, it is tightly coupled to an ocean wave model. Recently, progress has been made to include an ocean model as part of its medium range forecasting system.

In this context, sea state (waves) effects on Upper Ocean mixing and dynamics were successfully added to the system. The first operational implementation of this system was with the ensemble prediction system and work in under way to implement the same system into the operational high resolution suite. Tropical cyclone prediction with increased resolution has generally become quite reliable. Nevertheless, systematic biases still exist. The benefit of adding the active coupling with the ocean has been successfully demonstrated.

Because the feedback from the ocean can be significant, it is only in the fully coupled system that the formulations for heat, moisture and momentum fluxes can be revisited. Experimental evidences point to a sea state/wind dependency of the heat and moisture fluxes. Following an extension of the wind wave generation theory, we have tested a sea state dependent parametrisation for the roughness length scales for heat and humidity. Furthermore, under very strong wind forcing, there are evidences that the current parametrisation of the sea state dependent momentum flux should be modified to respect physical constrains on the wave spectral steepness. We propose a simple adaptation of the current scheme.