Quantification of Precipitation Asymmetries of Tropical Cyclones Using 16-Year TRMM Observations

Abstract

Both low and higher wavenumber asymmetries of tropical cyclone (TC) precipitation are quantified from 16 years of Tropical Rainfall Measuring Mission (TRMM) microwave imager rainfall estimates using Fourier decomposition. The energy spectrum of rainfall perturbation and impacts of the storm motion and vertical wind shear to the rainfall asymmetry are analyzed. It is found that the wavenumber 1 perturbation only contributes 37% of the total perturbation energy of total TC precipitation, while over 70% of the total perturbation energy is from the sum of wavenumbers 1 through 6. Most of the total precipitation asymmetry is from the heavy precipitation, and contributions for light and moderate precipitation types are small. The maximum motion-relative precipitation asymmetry is generally located down motion but shifts cyclonically after adding wavenumber 2–6 components to wavenumber 1. The asymmetry index of wavenumber 1 and 1–6 asymmetries relative to the vertical wind shear is about 2 to 3 times as large as that relative to the storm motion. The vertical wind shear is a more important factor than the storm motion in producing precipitation asymmetry, especially for the wavenumber 1. A dominant downshear left wavenumber-1 asymmetry maximum is found regardless of the shear vector orientation to the TC motion, the magnitude of the shear value, and the speed of TC motion. However, the maximum asymmetry of wavenumber 1–6 asymmetry becomes downshear right for the subgroup with low shear, fast motion, and the shear vector left to the motion.