The Sensitivity of the ECMWF Model to the Parameterization of Evaporation from the Tropical Oceans

Stimulated by the results of a simple SST anomaly experiment with the ECMWF forecast model, a study was carried out to examine the model parameterization of evaporation from the tropical oceans. In earlier versions of the model, these fluxes were parameterized with neutral transfer coefficients in accordance with the Charnock relation with equal coefficients for momentum, heat, and moisture. Stability correction was applied using Monin–Obukhov theory. This parameterization resulted in an extremely weak coupling between atmosphere and ocean at wind speeds below 5 m s−1. The transfer coefficients for heat and moisture have now been modified for low wind speeds to bring them in accordance with the empirical scaling law for free convection. It is shown that these revisions to the transfer coefficients at very low wind speeds (