Large-Scale Aspects of Deep Convective Activity over the GATE Area

Abstract

Using digital IR data obtained by SMS I satellite, the large-scale behavior of convective activity was investigated over the tropical Atlantic within the GATE area. Spectral analysis has revealed the existence of two remarkable periodicities which show good association with the large-scale atmospheric field, namely, a 4-5 day mode and a diurnal mode. The area of the enhanced convection associated with the 4-5 day mode moves westward from Africa through the tropical Atlantic. Its mean phase speed turned out to be about 7° day-1 and the mean wavelength about 30° in longitude. The vertical structure of the corresponding disturbance, was investigated by the time-composite technique applied to upper air data over the GATE A/B area. The result shows that the enhancement of convection is associated with deep upward motion throughout the troposphere and southerly (northerly) winds in the lower (upper) troposphere. It also shows that the enhancement is accompanied by moistening in the cloud layer, while drying appears in the subcloud layer. As for the diurnal mode, composited variation shows the maximum convective activity in the late afternoon over the GATE A/B area. Horizontal distributions of the composited anomalies revealed that the late afternoon maxima have a zonal distribution roughly along 10°N rather than the confined one in the vicinity of West Africa. It also revealed the coexistence of the region which shows the early morning maximum. The latter takes place to the south of the former one over the middle Atlantic. Diurnal components of the large-scale kinematic and thermal fields were also examined over the A/B area. During the daytime, the vertical motion shows upward anomalies and the temperature shows distinct warm anomalies in the upper troposphere. As for the moisture variation, the atmosphere tends to show dry anomalies near local noon and moist anomalies near midnight.