An Observational Study of the Upper-Tropospheric Vorticity Fields in GATE Cloud Clusters

Abstract

A large-amplitude asymmetric vorticity couplet has been observed in the upper troposphere near a lame cloud cluster in GATE (GARP Atlantic Tropical Experiment) on 5 September 1974. The couplet consists of a cyclonic center to the north of the cluster and an anticyclonic center to the south. The couplet is a result of the deceleration of the upper-tropospheric easterly winds in a region near the center of the cluster. These features also appear on a composite of large slow-moving clusters occurring during Phase 3 of GATE. Vorticity budget analysis shows that the couplets are produced by subcluster-scale process and, to a lesser degree, by cluster-scale twisting. On the basis of this finding and on the basis of the three-dimensional structure of horizontal momentum fields, it is suggested that a part of the deceleration producing the couplet is a result of momentum redistribution by subcluster-scale circulations (such as cumulonimbi or mesoscale cloud lines). The composite wind fields of the large slow-moving cloud clusters during Phase 3 of GATE are found to be similar to horizontal composites made within easterly wave phase categories near the wave trough.