A STUDY OF A 2-DIMENSIONAL CLOUDINESS TRANSITION DURING A COLD AIR OUTBREAK EVENT

A two-dimensional version of a hydrostatic mesoscale model with a partial cloudiness scheme is used to study a cold air outbreak event during MASEX (Mesoscale Air-Sea EXchange experiment). The model produces a weak mesoscale circulation which is slightly advected offshore and simulates well an observed cloudiness transition zone of 80 km where the cloud cover ranges between 0 and 100%. It is shown that the cloud top entrainment instability criterion can explain the observed cloudiness phenomenon i.e., the transition from a cumulus field to a solid stratocumulus cloud layer. The usefulness of the partial cloudiness scheme is demonstrated by comparing the results to those obtained with a simple "all or nothing" condensation scheme. A model sensitivity study shows that the sea surface temperature and the horizontal advection speed control the surface heat and moisture fluxes and so explain the structure and horizontal extent of the cloudiness transition zone.