Cloud cover analysis with METEOSAT-5 during INDOEX

During the Indian Ocean Experiment (INDOEX), METEOSAT-5 positioned at 63 de greesE provided observation of the visible and infrared radiance field over the Indian Ocean. A cloud classification process using the dynamic cluster method is applied to these data. For the 3 months of the experiment (Janua ry-March 1999), daily maps of the cloud cover type are built for 0730 and 0 900 UTC. The occurrence frequency of clear sky, low-and high-level cloud co ver is examined. These frequencies are compared to the International Satell ite Cloud Climatology Program (ISCCP) D1 data set for the period 1984 to 19 94. The Indian Ocean region can be classified in three zones. In the north subtropics, clear sky and small cumulus occur at least 90% of the time. Nea r the coast of India, clear sky is as frequent as 80 to 100%. The Intertrop ical Convergence Zone, characterized by the occurrence frequency of high-le vel clouds greater than 30%, spreads from Indonesia to North Madagascar. Ne ar Indonesia, high-level cloud cover occurs more than 55% of the time. In t he south subtropics, low cloud cover is the most frequent. In the eastern p art the occurrence frequency reaches 80%. This percentage decreases along t he western side of the ocean where low clouds break up. Between the African coast and Madagascar, high-level clouds are frequent. The mean spatial fea tures found are in agreement with the ISCCP climatology, except for the eas tern part of the south subtropics. A regional comparison shows the difficul ty of making the analysis of interannual variations of cloud cover obtained from various cloud cover retrievals applied to different satellite data se ts. This difficulty arises from the nonneglectable percentage of satellite pixels which can contain some very small low clouds.