COARE simulations with the mesoscale model MM5: various sensitivities to physical parameterizations

A major difficulty in simulating a realistic tropical atmosphere with a mesoscale model lies in the great sensitivity of model results to the treatment of subgrid scale physical processes. This study investigates how various physical parameterizations (convection, radiation and boundary layer schemes) affect a 10 day period simulation over the COARE area. The experiments are performed with the MM5 Modeling System Version 3 over a domain extending from 10S to 10N and 130E to 180E, with a basic horizontal resolution of 45 km. The time period (92/12/17 to 92/12/27) coincides with a Westerly Wind Burst. The model initial and boundary conditions are derived from the ECMWF analysis. This study focusses on the convective features and mean budgets as simulated and observed over the COARE IFA.

The precipitation patterns as simulated with three convection schemes (Betts-Miller, Grell and Kain-Fritsch schemes) are quite different. The convective and stratiform contributions to the total rainfall rates also largely depend on the convection scheme that is used. The 10-day IFA mean rainfall rate however are relatively close. The realism of simulated surface precipitation fields have been further investigated with the help of radar rainfall products, together with an evaluation of the cloud cover as inferred from radiative fluxes. This analysis reveals that the cloud cover is usually underestimated when convective rainfall is significant, whereas it agrees much better with observed for experiments where most of the rainfall occurs on a resolved scale. On the basis of radar data, showing that a large fraction of surface rainfall occurs at a convective scale, this finding suggests a lack of interaction between the convective activity and the cloud cover. Surprisingly, the atmospheric thermodynamical mean states for these various experiments are relatively close to each other, although the contributions from the convective and radiative processes to these mean states show different vertical structures. At the same time, the wind field shows a sensitivity to the convection scheme that is used, through an interplay of complex mechanisms.

Precipitations patterns are not notably modified by changing the radiation scheme (standard radiation scheme, RRTM or CCM2 radiation) or the boundary layer parameterization. Changing these parameterizations however lead to differences of the mean thermodynamical profiles in the same range as those previously found for various convection schemes.