Mesoscale boundary layer clouds structures as observed during the semaphore campaign

First results on a comprehensive study of Marine Atmospheric Boundary Layer (MABL) clouds during the SEMAPHORE campaign are given. The low level cloud s structures are determined from a dynamic cluster analysis applied to four channels of the AVHRR satellite radiometer over five days. To introduce ou r future objectives and methods, a tentative synoptic structure analysis of the clouds is presented using a Global Circulation Model (GCM) assimilated on the intensive zone of measurements. It is found that the GCM cloudiness is not strictly related to the observations, but reveals some correct tren ds, confirmed by an analysis of the low level ARPEGE velocity convergence; it is thus inferred that the GCM modelling of the mean MABL contains enough information to explain at least partially the observed clouds field. (C) 1 999 Elsevier Science Ltd. All rights reserved.