Shear-motion Combined Effects on Tropical Cyclone Low-wavenumber Precipitation Asymmetry

Bob Houze has made significant contributions in understanding clouds and precipitations in tropical cyclones especially by using Tropical Rainfall Measuring Mission (TRMM) data. This study is inspired by his studies (Hence and Houze 2011, 2012). Fourier wavenumber decomposition method has been demonstrated as a very useful tool to quantify Tropical Cyclone (TC) precipitation asymmetries. Previous studies showed that the wavenumber-1 rainfall asymmetry in response to the effects of storm motion and vertical wind shear for different intensity storms in different TC-prone basins (Lonfat et al. 2004; Chen et al. 2006). Based on the theoretical studies of Shapiro (1983) and Terwey and Montgomery (2002) and the observational study of Reasor et al. (2009), low-wavenumbers besides wavenumber-1 also have contributions to TC asymmetries.

As a complementary study to Lonfat et al. (2004) and Chen et al. (2006), this study will not only quantify the TC rainfall wavenumber-1 asymmetry, but also the contributions to asymmetry from wavenumbers 2 through 6. Using the Fourier wavenumber decomposition method, the goal is to understand how rainfall asymmetries respond to varying storm motion and environmental vertical wind shear for different intensity storms in different basins. Sixteen years (1998-2013) of TRMM Microwave Imager (TMI) data, totaling 3542 TMI overpasses of TCs, will be used. Composite results will show wavenumber-1 and wavenumber-2 through -6 asymmetries for total, light, moderate, and heavy rainfall distributions in six TC-prone basins including Atlantic Ocean, East Pacific Ocean, Northwest Pacific Ocean, North Indian Ocean, South Indian Ocean, and South Pacific Ocean. The analysis will differentiate intensity categories with slow motion, fast motion, weak shear, moderate shear, and strong shear. According to shear and motion directions, TC overpasses will also be categorized into four categories including that the shear direction is same to, opposite to, to the left of, and to the right of motion direction. Both shear-relative and motion relative composites will be examined.