The Convective System Area Expansion over Amazonia and Its Relationships with Convective System Life Duration and High-Level Wind Divergence

Abstract

The relationships between the initial area expansion rate of tropical convective systems and their total life duration are analyzed during the period of the Wet Season Amazon Mesoscale Campaign/Large-Scale Biosphere?Atmosphere (WETAMC/LBA) experiment over tropical South America, using an objective tracking of convective systems during their life cycle from infrared Geostationary Operational Environmental Satellite (GOES) images. The results show that it is possible to estimate the probable lifetime of a convective system, within certain error bars, considering only its initial area expansion. This result shows that the initial area expansion could be used as a predictor of the life cycle of convective systems. The area expansion is also a good indicator of convective activity such as the diurnal cycle of convection. Over the southwest Amazon, the maximum area expansion occurs close to the time of maximum precipitation and about 4 h before the maximum cold cloud fraction at the same threshold (235 K). Also, the hypothesis that the area expansion, and hence the convective activity, impacts the high-level wind divergence has been investigated using satellite wind observations. It is found that the wind divergence fields derived are able to describe the large-scale patterns but are not able to capture the small-scale features. The diurnal cycle of the high-level wind divergence generally shows a flat response over tropical South America, although a coherent but not significant signal is observed over the WETAMC/LBA area. It is shown that the area of the cloud shield of convective systems varies not only in association with the upper-level wind divergence but also with the condensation?evaporation process. The increase of area in this initial stage is mainly due to the condensation process. During the ensuing mature stage, the upper-air wind divergence also contributes to the expansion.