The morphology of tropical rainfall systems and their hydrological significance

This work analyses rainfall systems over Eastern São Paulo State, Brazil. Measurements of the São Paulo Weather Radar (SPWR) within a 240-km radius were used to analyze the different types of rainfall systems and their respective frequencies between 1998 and 2003. The rainfall systems were grouped in five categories, namely, isolated convection, squall line, cold front, rain band and sea breeze. Each type is illustrated along with its main dynamic and thermodynamic characteristics. Convective and stratiform systems show both an annual cycle in frequency lagged by four months. Convective and stratiform systems are dominant in the warm and cold season, respectively. The morphology and time evolution of each category also depend on the topography, associated circulation and source of moisture. Specifically over the Alto Tietê River Basin, the most populated watershed in Brazil, major floods tend to be associated with long lived stratiform rainfall systems produced by synoptic features and remote sources of moisture. Furthermore, most flash floods tend to be linked to convection induced by the heat island of the Metropolitan Area of São Paulo (MASP) and the sea breeze circulation. Furthermore, the observed dominance of isolated convection between 1998 and 2003 is consistent with the depletion of water in most reservoirs within the Alto Tietê Basin.