GAFD Seminar: Jean-Raymond Bidlot (ECMWF)

The global analyses and medium range forecasts from the European Centre for Medium range Weather Forecasts (ECMWF) rely on a state-of-the-art atmospheric model. To best represent the momentum, heat and moisture exchange at the surface of the oceans, it is tightly coupled to an ocean wave model.  A coupled ocean circulation model is also included as part of the operational medium range, extended range and seasonal forecasting systems.

During the last three decades, the counterpart of the operational wave models has been the numerical weather prediction (NWP) models providing the required wind input information. A partial, but significant, coupling was achieved with an atmospheric–sea surface momentum exchange depending on the stage of development of the local wave field. NWP systems have also moved toward a more Earth system approach adding an active coupling with an ocean circulation component. Hence ocean waves can play an active role in the modelling of exchanges between atmosphere and ocean as well as to the internal dynamics of the upper ocean.

The modelling of ocean wind waves at global scale is done using a statistical approach in which the waves are represented by their distribution given by the 2-dimensional wave energy spectrum. The evolution of the spectrum is given by the action balance equation. Waves propagation is determined by water depth and ocean currents. In the open ocean, waves are generated by the action of the wind, interacts nonlinearly, and dissipate due to wave breaking. In shallow waters, waves are also influenced the ocean bottom characteristics and in the presence of sea ice, there are strong interactions between waves and sea ice.

The talk will review the state of coupling with waves in the ECMWF Earth System Model. The different physical processes affecting waves interact with different components of the Earth System Model. This raises many challenges in developing such a state-of-the-art system.