Overlap Properties of Clouds Generated by a Cloud Resolving Models

摘要

The overlap properties of approx. 850 snapshots of convective cloud fields generated by a Cloud Resolving Model are studied and compared with previously published results based on cloud radar observations. Total cloud fraction is overestimated by the random overlap assumption and underestimated by the maximum overlap assumption, as well as two standard implementations of the combined maximum/random overlap assumption. When the overlap of two layers is examined as a function of vertical separation distance, the value of the parameter a measuring the relative weight of maximum (a=1) and random (a=O) overlap decreases in such a way that only layers less than 1 km apart can be considered maximally overlapped, while layers more than 5 km apart are essentially randomly overlapped. The decrease of a with separation distance is best expressed by a power law; exponential behaviour is, however, still a good approximation for separation distances up to 5 km.