Cloud Clusters and Superclusters over the Oceanic Warm Pool

Abstract

Infrared satellite images of the oceanic warm-pool region (8O°E-160°W) have been objectively processed to reveal tropical ?cloud clusters? with temperature colder than a given threshold. Cloud clusters span a somewhat lognormal distribution of sizes. The cloudiness per unit size interval peaks at small size, but half of the very cold (<208 K) cloudiness is contributed by cloud clusters greater than 20 000 km2in size, while half of the moderately cold (<235 K) cloudiness is contributed by cloud clusters greater than 100 000 km2 in size. The diurnal cycle of cold cloudiness is primarily a sun-synchronous process within large and giant clusters, not a modulation of populations of isolated convective clouds. Deep convection in these clusters peaks before dawn and decreases through the morning; then the moderately cold cloud area expands suddenly in the afternoon. Over the maritime continent, an additional diurnal cycle of small clusters is present, with in afternoon pulse of convection over land and a lesser peak in small-cluster convection over the surrounding seas at night. The eastward-propagating intraseasonal variation (ISV) is apparent in fractional cold cloudiness integrated across the entire tropical latitude belt. The ISV modulates cloud clusters of all sizes, but larger clusters are proportionately more affected than smaller clusters. Cloud clusters have been tracked in time to reveal ?time clusters?which spatially overlap from one frame of imagery to the next. In some cases, convection is so gregarious and interconnected that these time clusters last for more than two days. These cases are called ?superclusters?. Although they may exist at any given instant as several distinct cloud clusters, these superclusters are apparently real physical entities as defined by space-time continuity of very cold-topped cloud (proxy for precipitation) area.