Modulation of Convective Activity by Large-scale Flow Patterns Observed in GATE

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Abstract
The primary goal of this study is to examine how rain events, basically of convective nature, observed during the GARP Atlantic Tropical Experiment (GATE) were modulated by large-scale flow patterns. The low-level convergence associated with the confluence of northeast tradewind and the cross-equatorial flow prevailed inside the A/B array during the entire experiment. The easterly waves, which were most pronounced in Phase III, moved westward from Africa and interacted with the equatorial confluence zone. The low-level convergence associated with the large-scale circulation resulting from the superposition of the equatorial confluence air flow and the African waves controlled the convective outbreaks in GATE. The wave-related surface meridional wind in Phase I was much weaker than in Phase III. As a consequence, the position of the surface confluence line changed significantly in the north-south direction with the passage of an African wave in Phase III, whereas it did not vary with easterly wave passage in Phase I. Generally, rain events occurred in the equatorial confluence zone, even when the position of the surface confluence line moved northward with the passage of the easterly waves. Enhanced precipitation in the equatorial confluence zone was observed ahead and in the vicinity of the wave troughs. However, some cloud clusters, most frequent in Phase III, formed several degrees south of the equatorial confluence zone. Clouds associated with these cloud clusters were generally rather shallow. Ekman pumping does not appear to have played a significant role in the formation of organized convective systems. For all three phases, the surface convergence fields agree remarkably well with the precipitation field. The agreement is best at a time lag of ∼5–10 h between the maxima of the surface convergence and the precipitation rate. Two local maxima in the phase mean vertical velocity fields were present in Phase III: one was located over the mean position of the equatorial confluence zone and the other ∼3.5° south of it. In contrast, them was only one broad upward motion region with a distinct water vapor maximum near the phase-mean equatorial confluence zone in Phase I. Thew features are consistent with the phase-mean rainfall rates estimated by Hudlow (1979); the precipitation areas in Phase I were concentrated in a narrow band oriented northeast-southwest, whereas they were latitudinally widespread in Phase III.

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