Diurnal Variation of Summer Convection Over West Africa and the Tropical Eastern Atlantic During 1974 and 1978

Richard J. Reed Department of Atmospheric Sciences. University of Washington Seattle 98195

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Kenneth D. Jaffe Department of Atmospheric Sciences. University of Washington Seattle 98195

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Abstract

Four-times daily infrared images from the geostationary satellite Meteosat- I are used to compare thediurnal variations of deep convection over West Africa and the eastern Atlantic during the summer of 1978with the variations observed by the SMS-l satellite during Phases II and III of GATE (28 July 19 September, 1974). A subjective method similar to that employed by McGarry and Reed (1978) in analyzing the 1974 data is used in estimating the degree of convective activity.

A close agreement is found between the behavior of the convection in the two years. In particular:1)Maximum convective activity in the area of the GATE ship hexagon occurred again in the lateafternoon and the amplitude of the convective cycle was again larger at the eastern edge of the outerhexagon than at the western edge.2)Closer to shore the peak activity occurred near noon, as in 1974, and in the immediate vicinity of theshore the amplitude of the cycle was again very small.3)Over the highlands of West Africa, explosive growth of convection was observed during the afternoon,as before, and the convective cloud cover reached its maximum extent in the late evening.4)In the lowlands to the north and northwest of the region of elevated terrain, the maximum was delayeduntil the hours from midnight to sunrise.

Plausible physical explanations are offered for the observed diurnal behavior over land, but the causesof the convective regimes over the ocean are obscure. The fact that the diurnal variation over the ocean islargest near but somewhat removed from the coast suggests that continental influences may affect theconvective cycle in the eastern Atlantic.

Abstract

Four-times daily infrared images from the geostationary satellite Meteosat- I are used to compare thediurnal variations of deep convection over West Africa and the eastern Atlantic during the summer of 1978with the variations observed by the SMS-l satellite during Phases II and III of GATE (28 July 19 September, 1974). A subjective method similar to that employed by McGarry and Reed (1978) in analyzing the 1974 data is used in estimating the degree of convective activity.

A close agreement is found between the behavior of the convection in the two years. In particular:1)Maximum convective activity in the area of the GATE ship hexagon occurred again in the lateafternoon and the amplitude of the convective cycle was again larger at the eastern edge of the outerhexagon than at the western edge.2)Closer to shore the peak activity occurred near noon, as in 1974, and in the immediate vicinity of theshore the amplitude of the cycle was again very small.3)Over the highlands of West Africa, explosive growth of convection was observed during the afternoon,as before, and the convective cloud cover reached its maximum extent in the late evening.4)In the lowlands to the north and northwest of the region of elevated terrain, the maximum was delayeduntil the hours from midnight to sunrise.

Plausible physical explanations are offered for the observed diurnal behavior over land, but the causesof the convective regimes over the ocean are obscure. The fact that the diurnal variation over the ocean islargest near but somewhat removed from the coast suggests that continental influences may affect theconvective cycle in the eastern Atlantic.

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