The Inner-Core Asymmetric Rainfall Structure Of Landfalling Tropical Cyclones In Southern China

In this study, the 3-D temporal variation of inner-core rainfall asymmetric structure in landfalling tropical cyclones, and the relative contribution of three external factors (vertical shear, storm translation, and different surface characteristics between land and sea) to the above have been investigated using coastal Doppler radars in Southern China.10 landfalling cases during 2008 and 2013 have been chosen in total. Results of Fourier analysis show that the wavenumber-1 asymmetry dominates the asymmetric structure during TCs making landfall. Samples are divided into two stages by the characteristics of boundary layer surface under the eyewall-stage-I for sea surface and stage-II for land surface.Results reveal that in mid-to-strong sheared environment (>=5 ms-1), heavy rainfall is mainly located in downshear-left quadrant (DL) throughout the whole period. Convective-scale cells are mostly triggered in downshear-right (DR) quadrant and maturing in DL quadrant, then weakening in upshear side. Storm translation only slightly shifts the rainfall pattern to the front of moving. While in weak sheared (<5ms-1) environment, storm translation becomes dominating.In stage-I, heavy rainfall locates at front-right (front) quadrant in the eyewall (inner rainbands), and weakest in rear-right quadrant. As tropical cyclones approach the coast, the occurrence of land surface would force a low-level convergence and updraft which would enhance the rain rate to the downwind (mean reflectivity increases 2-5 dBZ). Moreover, the updraft is vertically restricted because of lacking enough energy due to frictional dissipation especially in inner rainbands. In mid-to-strong sheared environment, convective feature in DL enhances (the low-level maxima reflectivity and echo top increase by 2 dBZ and 1 km respectively) yet the azimuthal distribution of rainfall pattern remains in both eyewall and inner rainbands. However, in weak-sheared environment, rainfall enhances to the front (left of front) in the eyewall (inner rainbands), and the cloud top decreases significantly by 2-4 km. Thus, vertical shear is the dominate factor in determining rainfall structure in inner-core region of tropical cyclones, yet storm translation and different surface characteristic between land and sea are the two factors merely modifying rainfall distribution. In weak sheared environment, motion effect becomes essential. In addition, convective cells develop lower than that in mid-to-strong sheared environment. Previous related researches find that convections are stronger in rear quadrants relative to the coastline, and hypothesis that the difference between land and sea is the main cause. However, this paper put forward a new way to see landfalling tropical cyclones in vertical, which find it is more essential for modifying than shaping the rainfall distribution by triggering new updrafts at on-shore-wind quadrant above the interface between land and sea in landfall processes. This paper merely analyzes 10 tropical cyclones, especially in weak sheared one only 3 cases are included. Thus, our results are needed to be examined by more cases or high-resolution simulations.