Characteristics of West African and East Atlantic Cloud Clusters: A Survey from GATE

Abstract

This article describes the size, intensity, trajectory, lifetime and distribution of the GATE cloud clusters of West Africa and the eastern Atlantic Ocean and relates their distribution to the summer climate of the region. SMS-1 infrared and visible 3 h pictures for 85 days of GATE, starting 27 June 1974, were used. It was found that over 500 clusters occurred. Size averaged 2 ? 105 km2; lifetime, one day. Although both were highly variable, in general, lifetime increased with maximum size. The clusters occurred in a band oriented west-southwest to east-northeast over the ocean and eastward over land. Nodes were observed at intervals of 5?7° along the axis of maximum frequency of occurrence. Clusters at all latitudes moved generally westward, having straighter tracks and faster speeds over land. From July to September the axis of the cluster band shifted northward 100?300 km, and tended to split over the ocean. Clusters on the northern flank of the band were associated with African easterly waves, especially during Phase III; however, most of the clusters of GATE occurred to the south of the surface pressure trough and surface confluence, apparently in association with convergence within the mean low-level monsoon circulation.