This study addresses the tropical cyclone (TC) rain structure using the tropical rainfall measuring mission (TRMM) passive microwave imager (TMI). Characterizing the TC rainfall structure and understanding the physical mechanisms responsible for the observed distributions are fundamental issues for quantitative precipitation forecast (QPF). Addressing these issues requires an extensive dataset, as the TC rainfall variability is very large. Before TRMM, global observations with fine enough resolution were not available for a detailed study of the precipitation.

From January 1998 to December 2000, 260 storms developed worldwide, for which TRMM has provided more than 2100 instantaneous observations. We partition this dataset as a function of the storm intensity and oceanic basin. For each sub dataset, we derive azimuthal mean rain rates (R) in 10 km annuli around the storm center. We determine the precipitation asymmetry in storm motion relative coordinates and in vertical wind shear relative coordinates. We compute the asymmetry location and amplitude in 50km annuli around the center, using a first order Fourier analysis based on the quadrant average rain rates. We consider the shear between the 200 and 850mb winds.

The azimuthal averages of rainfall vary with the intensity and from basin to basin. Peak R is about 12 mm/hr for category three and higher systems, and decreases to 7 mm/hr for lower hurricane intensities and to 3 mm/hr for tropical storm intensity. Rainfall distributions around TC show significant asymmetries, both with intensity and between basins. In the motion-relative referential, the rainfall asymmetry is found in front of the storm. The asymmetry shifts from front-left to front-right quadrant with TC intensity. Larger asymmetries are observed in the TC outer core. The asymmetry amplitude varies with intensity. Lower intensity systems are more asymmetric. In the shear-relative referential, the asymmetries are located down-shear, mainly to the left. The asymmetry rotates anticlockwise with increasing intensity. Larger asymmetry amplitudes are observed in the shear referential. Our study provides a quantitative description of the global TC precipitation, which is of fundamental interest for both climatology applications and improvements in QPF.