In the subtropics, there are three significant subtropical frontal zones or subtropical convergence zones (STCZs) ; the Baiu frontal zone, the South Pacific convergence zone (SPCZ), and the South Atlantic convergence zone (SACZ). We studied rainfall activity of the STCZs based on satellite observations and generation mechanisms of the STCZs by numerical experiment using atmospheric general circulation model.
Firstly, we investigated the large-scale rainfall activity of the STCZs and its seasonal variation using two datasets of monthly precipitation ; one is provided by GPCP (Global Precipitation Climatology Project) based on satellite IR and Microwave observations and rain-gage observations at ground surface, and the other is derived from TRMM (Tropical Rainfall Measuring Mission)-PR(precipitation radar) observations. We found that zonal-averaged rainfall intensity increases during warm seasons over both land and ocean due to seasonal intensification of the STCZs in summer. This seasona
l variation is contrasted to that of mid-latitude precipitation over ocean which is intensified during cold seasons.
Secondary, we studied on a meso-scale rainfall system developed over ocean near the Baiu front based on TRMM multi-sensor observations by PR, TMI (TRMM Microwave Imager), and VIRS (Visible and Infrared Scanner). We showed fine 3D structure of the rainfall system by PR observation, close relationship between cloud distribution and rainfall activity by VIRS and PR observations, etc.Such analysis for maritime rainfall systems has been imposible without TRMM observations and our results stress the usefulness of TRMM multi-sensor observations for studying meso-scale rainfall systems.
Finally, we studied the roles of atmospheric heat sources for generating the STCZs by numerical experiments using an aqua-planet GCM.We found that the STCZs are maintained under strong influence of a locally confined atmospheric heat source in the tropics, which imitates the tropical monsoons in the real atmosphere, and that strong rainfall along the STCZs also contributes to intensify the STCZs' rainfall by modifying the circulation around the STCZs. Less