A Composite Life Cycle of Nonsquall Mesoscale Convective Systems over the Tropical Ocean. Part I: Kinematic Fields

Abstract

The wind fields associated with cloud clusters observed during the Global Atmospheric Research Program's Atlantic Tropical Experiment (GATE) are investigated. A compositing procedure is devised to isolate the cluster circulations. Satellite-observed cloud cover estimates by Cox and Griffith form the basis for the identification and classification of clusters and for the determination of their life cycles. The compositing criteria focus on the upper-tropospheric portions of anvil clouds that are a prominent feature of cloud clusters. The compositing procedure is applied to a set of objectively analyzed upper-air winds for Phase 3 of GATE prepared by K. V. Ooyama and J.-H. Chu. The results show that slow-moving cloud clusters tend to form in regions of relatively small vertical wind shear and that the shear at the cluster center decreases during the cluster life cycle. Squall clusters, on the other hand, have significantly larger lower-tropospheric shear. Changes in the total horizontal wind field in the middle and lower troposphere during cluster evolution appear to be primarily due to the advance of easterly waves relative to the slower-moving clusters. However, the horizontal divergence field is centered on the cluster during its lifetime. The maximum value of the upper-tropospheric divergence at the cluster center lags the maximum boundary layer convergence by 3?6 hours. During the mature and dissipating stages, a layer of convergence develops in the middle troposphere. The vertical motion diagnosed in the growing, mature and dissipating stages of the clusters is found to be qualitatively similar to the life cycle hypothesized for the smaller-scale individual mesoscale precipitation features that are found within the clusters. Strong upward motion develops in the upper troposphere from the growing to the mature stage; in the lower troposphere, the upward velocities decrease dramatically from the mature to the dissipating stage. The cluster-scale vertical motion in the mature stage of the slow-moving clusters is compared with large-scale values in the trough of a composite easterly wave, with Phase-3 averaged vertical velocities, and with vertical motion estimates in squall lines. In general, vertical motion increases rapidly with decreasing space and time scales. The squall and nonsquall vertical motion profiles are qualitatively similar.