Cloud cover analysis with METEOSAT-5 during INDOEX

During the Indian Ocean Experiment (INDOEX), METEOSAT-5 positioned at 63 degreesE 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 (January-March 1999), daily maps of the cloud cover type are built for 0730 and 0900 UTC. The occurrence frequency of clear sky, low-and high-level cloud cover is examined. These frequencies are compared to the International Satellite Cloud Climatology Program (ISCCP) D1 data set for the period 1984 to 1994. 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. Near the coast of India, clear sky is as frequent as 80 to 100%. The Intertropical Convergence Zone, characterized by the occurrence frequency of high-level clouds greater than 30%, spreads from Indonesia to North Madagascar. Near Indonesia, high-level cloud cover occurs more than 55% of the time. In the south subtropics, low cloud cover is the most frequent. In the eastern part the occurrence frequency reaches 80%. This percentage decreases along the western side of the ocean where low clouds break up. Between the African coast and Madagascar, high-level clouds are frequent. The mean spatial features found are in agreement with the ISCCP climatology, except for the eastern part of the south subtropics. A regional comparison shows the difficulty of making the analysis of interannual variations of cloud cover obtained from various cloud cover retrievals applied to different satellite data sets. This difficulty arises from the nonneglectable percentage of satellite pixels which can contain some very small low clouds.