Convective Clouds over the Bay of Bengal

Abstract

The behavior of convective activity over the Bay of Bengal during the 1988 and 1999 monsoon seasons is examined using 3-hourly satellite infrared data. More organized convective activity, spreading farther south into the bay, occurred in 1988 than in 1999. A distinct spatial grouping of convective systems by size is found. The east side of the bay experiences most of the rainfall over water, and here the convective systems are relatively small, short lived, and frequent. At the northwest side of the bay near most of the land-based rainfall, convective activity is organized into much larger and longer-lived systems. The diurnal cycle of all the systems over the bay, regardless of size, shows a 0600 local time (LT) maximum in very cold cloud tops (infrared brightness temperature <210 K), with genesis occurring between 2100 and 0300 LT (2100 LT for the larger, longer-lived systems). The cloud systems dissipate after sunrise, with the larger systems lasting until the afternoon. The land?water interface is important for the convection genesis and thereby affects the spatial distribution of convection. Offshore nocturnal convection begins near shore, with later convection occurring farther out over the bay and attaining a larger size. The preference for nocturnal initiation times differs markedly from the afternoon initiation times typical of the tropical western Pacific Ocean, but the time of maximum cloud area extent and dissipation are similar. The strength of the diurnal cycle varies greatly with location, with the northwest side of the bay experiencing both the highest amount of very cold cloudiness and the strongest diurnal cycle. The Joint Air?Sea Monsoon Interaction Experiment (JASMINE) research cruise experienced the only multiday sequence of large, diurnally repeating, southward moving disturbances at 11°N, 89°E in the 2 yr examined, but both the convective diurnal cycle and propagation direction were consistent with climatology. An interesting aspect of the convective life cycle is that systems often have a southward component to their motion, with the most common propagation direction over the entire bay being to the southwest. This can occur even when the larger-scale movement is toward the northwest (such as is typical for cyclones at the northern end of the bay).