Diurnal Variations and Modulation by Easterly Waves of the Size Distribution of Convective Cloud Clusters over West Africa and the Atlantic Ocean

Abstract

Using Metecosat satellite data in the atmospheric infrared window, the authors study short time-scale fluctuations of the size distribution of tropical convective cloud clusters for July to September 1989. A cluster at a given brightness-temperature threshold (TIR) is defined as the area covered by adjacent cloudy pixels with brightness temperature lower than the threshold. The clusters are classified according to the area they cover and the position of their center of mass. Over land regions of West Africa, the size distribution undergoes a very coherent diurnal behavior with development of small cells between noon and 1500 LST that later grow or merge into larger clusters. Over the Atlantic Ocean, the highest cloudiness has a weak maximum extent in early morning, while cloudiness at lower levels (but with infrared brightness temperature TIR < 253 K) is more extended in the afternoon. This diurnal behavior is primarily due to large cloud clusters (r > 100 km at TIR = 218 K), suggesting that the diurnal variation over the ocean results mainly from internal variations of large convective systems and not from the initiation of convection at a given hour of the day. This is confirmed by the analysis of 15 large convective systems propagating over the ocean. In agreement with previous studies, we find that the high cloud cover is maximum within the trough of easterly waves. At midlevel (TIR = 253 K), these waves modulate mostly the number of clusters with radii larger than 200 km. At colder levels (TIR = 218 K), while the wave modulates the number of clusters at all sizes, the clusters are organized at larger scale within the trough. The cluster size also depends on the wave amplitude with larger mean cluster size when the amplitude is larger. These results show that over the ITCZ, the trough phase of the wave more promotes the development of large clusters than it favors the initial stage of the convection.