Understanding the relationship between tropical cyclone (TC) intensity and rainfall distribution is key for improving quantitative precipitation forecast (QPF). Rainfall distribution around a TC is affected by various factors such as environmental wind shear and TC translation speed. Until recently, our knowledge was limited by a lack of global observations of TCs. Data from the Tropical Rainfall Measuring Mission (TRMM) can help to overcome this issue.

In this study, TRMM surface precipitation estimates are used to quantify rainfall asymmetry in TCs, globally. From January 1998 to December 2000, more than 250 storms were observed worldwide, providing approximately 2200 instantaneous observations of TC precipitation structure. To examine the relationship between storm intensity and rainfall distribution, we stratify the dataset by intensity, from tropical depression to category 5 hurricane. For each individual observation, we derive azymuthal mean rain rates (R) in 10 km rings around the storm center. We then determine precipitation asymmetries in storm relative coordinates, by computing normalized R in each quadrant for all rings. Distributions for individual observations are averaged together for observations of comparable intensity and same oceanic basin.

Azymuthal averages of rainfall vary with intensity and from basin to basin. Peak R is about 12 mmhr^-1 for category three and higher systems, decreases to 7 mmhr^-1 for lower hurricane intensities and to 3 mmhr^-1 for tropical storm intensity. Rainfall distributions around TC show significant asymmetry, both with intensity and between basins. For the 2200 observations averaged together, the rainfall asymmetry is located in the front-right quadrant. The location of the asymmetry shifts clockwise with TC intensity. Magnitude of asymmetry vary with intensity as well. Category 1 and 2 systems show large magnitudes, while stronger systems are more symmetric. Our study provides a quantitative description of global TC precipitations, which is of fondamental interest for climatological applications.