THE SENSITIVITY OF A LAM MODEL TO INCLUSION OF CLOUD FRACTION IN AN EXPLICIT REPRESENTATION OF CONVECTION

This study presents some results on the sensitivity of a regional mode l (LAHM/Geophysical Fluid Dynamics Laboratory, CIMA), to the inclusion of cloud fraction in an explicit representation of convection. To acc omplish this objective, four different synoptic situations were select ed, all of them associated with heavy precipitation over Argentina. Se veral experiments were carried out, one of them was with doubled verti cal resolution. Precipitation fields obtained with and without the pro posed cloud fraction formulation were very similar, thus suggesting th at the model is not as sensitive as expected. An analysis of individua l terms in the water vapor specific humidity prognostic equation was a lso performed in order to understand the detected low sensitivity. It is believed that, under the selected horizontal and vertical resolutio n, only stratiform precipitation can be efficiently captured by the mo del and the related precipitation fields are weakly modified by the ch osen cloud fraction profile. (C) 1998 Elsevier Science B.V. All rights reserved.