Cloud components of the Intertropical Convergence Zone

Abstract

This study explores the roles of different synoptic‐scale cloud systems in the making of the Intertropical Convergence Zone (ITCZ). Outgoing longwave radiation (OLR) data are used to represent both the mean ITCZ and cloud systems. An OLR threshold of 205 W m−2 is used to distinguish daily mean deep convective clouds from nondeep clouds. Zonally moving synoptic‐scale cloud systems are identified through a cloud‐tracking method. Mean OLR of the ITCZ is decomposed into five components: westward and eastward moving synoptic‐scale deep convective clouds, random (nonpropagating) deep convective clouds, nondeep clouds, and clear‐sky backgrounds. Hypothetical ITCZs, each composed of one of the cloud components and the clear‐sky background, are constructed and compared. The largest synoptic‐scale cloud constituents of the ITCZ are nondeep clouds and deep clouds with no persistent tendency of zonal propagation. Each of the two comprises about 40–45% of total cloud signals in the ITCZ. A weak ITCZ could exist solely because of westward propagating, synoptic‐scale, deep‐convective clouds. In reality, these westward propagating clouds contribute 25–40% of deep convective clouds and 10–20% of total clouds in the ITCZ. Contributions from eastward propagating synoptic‐scale clouds are much smaller (8% of the deep convective clouds and less than 5% of total clouds in the ITCZ). An ITCZ without any zonally propagating synoptic‐scale deep convective clouds would be diffused in latitude. The cloud compositions in the ITCZ and other tropical convective zones are compared. Their discrepancies indicate fundamental characteristics in large‐scale environments for the cloud components in different regions.