Tropical Cyclone Rainfall Characteristics as Determined from a Satellite Passive Microwave Radiometer

Abstract

Data from the Nimbus-5 F-Electrically Scanning Microwave Radiometer (ESMR-5) have been used to calculate latent heat release (LHR) and other rainfall parameters for over 70 satelite observations of 21 tropical cyclones during 1973, 1974 and 1975 in the tropical North Pacific Ocean. The results indicate that the ESMR-5 measurements can be useful in determining the rainfall characteristics of these storms and appear to be potentially useful in monitoring as well as predicting their intensity. The ESMR-5 derived total tropical cyclone rainfall estimates agree favorably with previous estimates for both the disturbance and typhoon stages. The mean typhoon rainfall rate (1.9 mm h?1) is approximately twice that of disturbances (1.1 mm h?1). Case studies suggest that tropical cyclone intensification is indicated by the increase in the ESMR-5 derived LHR, the increase in the relative contribution of the heavier rain rates (≥5 mm h?1) to the total storm rainfall, and the decrease in the radius of maximum rain rate from the cyclone center. It also appears evident from these case studies that by monitoring the trend of increasing LHR the first indication of tropical cyclone intensification may be obtained 1?2 days prior to the tropical cyclone reaching storm stage and often prior to the first reconnaissance aircraft observation. Further, the time of the maximum intensity of the tropical cyclone lags by 1?2 days the time of maximum LHR. The statistics of the western Pacific tropical cyclones confirm the case study results in that tropical cyclone intensity can be monitored from ESMR-5 derived rainfall parameters. As the mean tropical cyclone intensifies from disturbance to typhoon stage the average LHR increases steadily. The mean relative contribution of the heavier rate (≥5 mm?1) to the total storm rainfall increased from 0.24 at depression stage to 0.33 at storm stage and finally to 0.39 at typhoon stage. The radial distance of the maximum rain rate from the center decreases with intensification while the azimuthal distribution indicates a slight preference for maximum rain rate in the right half of the composite storm at all stages. The study also indicates that eastern Pacific hurricanes have less LHR, are more compact, and have less intense rainfall than western Pacific typhoons.