A Statistical study on axial asymmetry in tropical cyclone rainfall using satellite data

Tropical cyclones form and develop over tropical oceans where few ground observation networks are available. There are, therefore, strong needs for satellite observations to study tropical cyclones in the early development stage based on a global, long-term record. Tropical Rainfall Measurement Mission (TRMM) satellite data, available for more than 15 years, are analyzed in this study with focus on TC rainfall asymmetry and its relationship to the vertical wind shear and moving direction. Data analyzed include TRMM/3B42 data for rainfall and JRA25/JCDAS data for vertical wind shear between 200 and 850 hPa. The time resolution is 3 hours, and horizontal resolution is 0.25 degrees. TC centers and moving direction are determined from the Best Track provided from JTWC/NHC. The time period for this study is 10 years from 1998 to 2007. Rainfall within 500 km from the storm center is averaged for each 10 degrees azimuthal bin to examine rainfall asymmetry relative to storm motion in each 6 hour bin. Six major oceanic basins are compared to each other. Rainfall asymmetry is similar between Northwest Pacific Ocean (NWPac) and North Atlantic Ocean (NAtl). For first few days after TC genesis, rain is relatively intense around the moving direction. After around the 2nd day, however, a positive anomaly appears on the left side of storm motion in NWPac while it appears on the right side of storm motion in NAtl basin. The amplitude of positive anomaly increases about a week after genesis. Although NWPac and NAtl have TCs with similar storm tracks and are both located off the East Coast of continents, the distribution of positive anomaly is different. Rainfall asymmetry relative to shear direction is also examined. Rain is relatively intense on the down-shear side throughout the evolution in NAtl, while rainfall asymmetry is weak from the first to 3rd day in NWPac. Factors responsible for the regional differences in rainfall asymmetry will be also discussed.