A METHODOLOGY STUDY OF THE VALIDATION OF CLOUDS IN GCMS USING ISCCP SATELLITE-OBSERVATIONS

The cloudiness fields simulated by a general circulation model and a v alidation using the International Satellite Cloud Climatology Project (ISCCP) satellite observations are presented. An adapted methodology i s developed, in which the issue of the sub-grid scale variability of t he cloud fields, and how it may affect the comparison exercise,is cons idered carefully. In particular different assumptions about the vertic al overlap of cloud layers are made, allowing us to reconstruct the cl oud distribution inside a model grid column. Carrying out an analysis directly comparable to that of ISCCP then becomes possible. The releva nce of this method is demonstrated by its application to the evaluatio n of the cloud schemes used in Laboratoire de Meteorologie Dynamique ( LMD) general circulation model. We compare cloud properties, such as c loud-top height and cloud optical thickness, analysed by ISCCP and sim ulated by the LMD GCM. The results show that a direct comparison of si mulated low cloudiness and that shown from satellites is not possible. They also reveal some model deficiencies concerning the cloud vertica l distribution. Some of these features depend little on the cloud over lap assumption and may reveal inadequate parameterisation of the bound ary layer mixing or the cloud water precipitation rate. High convectiv e clouds also appear to be too thick.