A Study of the Rain Distribution in Tropical Cyclones Using TRMM/TMI

The goals of this study are (1) to improve our understanding of tropical cyclone (TC) precipitation fields by developing a climatology of rainfall distribution and vertical profiles of hydrometeors and heating in TCs around the globe, and (2) to develop methodologies to validate operational and research model forecasts of TC rainfall distribution and structure. Surface rain estimates from the NASA Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) were used to study the global TC rainfall distribution. From November 1997 to December 1998, 87 storms were observed, providing approximately 1200 events, ranging from tropical storm intensity to category 4 hurricanes. At the time of the satellite measurement, approximately 70 % of the events were tropical storms, 25 % category 1-2 hurricanes and 5 % category 3 or higher. Although the sample size for major hurricanes is smaller than other categories, we have more than 50 observations, which is an order of magnitude larger than previous studies. For each event, we derive mean rain rates in 10 km rings around the TC center. Averaging these distributions produces rates up to 5 mm h^-1 close to the center. Rain rates decrease rapidly below 1 mm h^-1 by 200 km radius. We examine the rain rate distribution as a function of the storm intensity and geographical location. Close to the storm center, the mean rates are greater than 15 mm h^-1 for category 3 and higher hurricanes, and about 3 mm h^-1 for tropical storms. Rain distributions show strong asymmetries in the rain patterns between the different basins. We also calculate the latent heat release in 1 and 2 degree radial rings. Values are in the order of 10¹⁴ Watts, in agreement with previous studies by Rodgers and Adler (1980), Marks (1985). We are in the process of comparing our TC rain rate distributions with a high-resolution model simulations. Our study provides a wide description of hurricane precipitation distributions, which is of fundamental use as a climatological basis, notably for simulation purpose, and gives a feedback for TRMM algorithm improvement as well.