A Satellite Observational Study of Precipitation Characteristics in Western North Pacific Tropical Cyclones

Abstract

Special Sensor Microwave/Imager (SSM/I) observations were used to examine spatial and temporal changes in the precipitation characteristics for western North Pacific tropical cyclones that reached storm stage or greater during 1987-92. The second version of the Goddard scattering algorithm, that employed the 85-GHz brightness temperatures to measure rain rate, provided an analysis of the tropical cyclone precipitation distribution in greater detail, while the numerous SSM/I observations helped to better define the relationship between the tropical cyclones? spatial and temporal distribution of precipitation and the systems intensity, intensity change, radiational forcing, and mean monthly sea surface temperatures (SSTs). The two SSM/Is flown since 1992 also helped to provide a more detailed analysis of the evolution of the tropical cyclone inner-core diabatic heating. Similar to the SSM/I-observed 1987?89 western North Atlantic tropical cyclones, the SSM/I observations of the western North Pacific tropical cyclones revealed that the more intense systems had higher rain rates and greater areal distribution of rain. In addition, the heaviest rain rates were found nearer to the center of all the tropical cyclones. However, western North Pacific typhoons were found to have heavier azimuthally averaged rain rates and a greater contribution from the heavier rain within the inner core (i.e., within 111 km of the center) than the western North Atlantic hurricanes. The SSM/I observations of the western North Pacific tropical cyclones also suggested the following: 1) there appears to be a diurnal variation in the tropical cyclone precipitation (i.e., morning maximum and an evening minimum) except in the inner-core regions of systems that are at storm stage and greater; 2) the maximum rain rate that a tropical cyclone can produce in the inner-core region is dictated by SSTs with maximum rain rates occurring at SSTs greater than 29°C; 3) the large changes in the tropical cyclone inner-core rain rate (latent heat release) help to initiate and maintain periods of tropical cyclone intensification; and 4) the intensity of these tropical cyclones become more responsive to rain-rate changes as the tropical cyclones become more intense.