Effects of vertical wind shear and tropical cyclone motion on asymmetric distribution of convective clouds in TCs over the western North Pacific

The FY-2 geostationary satellite black body temperature (TBB) data and NCEP global reanalysis data were used to investigate the asymmetric distribution of convective clouds (DCC) in the outer region of tropical cyclones (TCs) over the western North Pacific during 2005-2012. It is demonstrated that the environmental vertical wind shear (VWS) and TC motion are closely related to the DCC in TCs. When VWS is weak (< 5 m/s), the DCC is located to the right front quadrant of the TC path in the lower troposphere due to friction-induced asymmetric boundary layer convergence. With increases in the VWS, the main zone of DCC changes from the right to the left quadrant of the TC path. This is because the VWS becomes the dominating factor leading to TC's convection asymmetric distribution and the convection is concentrated to the left of the downshear. On one hand, how large the convections deviate from the downshear is subject to the counter-clockwise flow of TC; on the other hand, it is related to the strength of VWS. The results also show that the direction of the VWS corresponds well to the heavy TBB's onewave asymmetry. When a TC is evolving in a strong VWS, TBB's one wave asymmetry becomes more obvious. Meanwhile, convections in the outer spiral rainbands region tend to the left of the downshear. For a weakened TC, the effect of the VWS is not distinct compared with that for a stronger one.