Quantification of Shear-relative Precipitation Asymmetries of Tropical Cyclones in Different Intensity Change Stages

Previous studies (Lonfat et al. 2004; Chen et al. 2006) have demonstrated that the Fourier wavenumber decomposition method is a very useful tool to quantify Tropical Cyclone (TC) precipitation asymmetries. These studies analyzed three years of Tropical Rainfall Measuring Mission (TRMM) 2A12 rainfall data and showed the effects of storm motion and vertical wind shear on Fourier wavenumber-1 rainfall asymmetry for different intensity storms in different TC-prone basins. Prediction of TC intensity change, especially rapid intensification (RI), has been a challenge for forecasters. Recently, Kieper and Jiang (2012) have shown that a symmetric precipitative ring feature in the microwave satellite image is an early indicator of RI. It is necessary to quantify the precipitation asymmetries relative to shear direction in order to better understand the relationship between RI and the degree of precipitation asymmetry.

This study is focusing on quantifying TC rainfall asymmetries using the Fourier wavenumber decomposition method for different future 24-h storm intensity change stages, including RI, slowly intensifying, neutral, and weakening. Fourteen years (1998-2011) of TRMM Microwave Imager (TMI) data is used. A total of 1786 TMI overpasses with storm intensity between tropical storm and category 2 hurricane with favorable environmental conditions is selected. The RI category is divided into RI initial and RI continuing subcategories based on whether the storm is at the beginning of RI stage or has already undergone RI for 12 or more hours prior to the TRMM overpass. Based on energy contribution analysis of different wavenumbers, we found that higher wavenumbers are also important in contributing to the total perturbation energy. Therefore, the dominant wavenumber 1 through 6 asymmetries of surface rainrate relative to shear will be examined. Three rainfall intensity categories are classified including light, moderate, and heavy rain. The shear-relative distributions of light, moderate, and heavy rain frequency occurrence will also be examined for different intensity change stages.