Analysis of microphysical features in the inner core of Super Typhoon Meranti （2016） based on dual-polarization radar observations

Xiamen S-band dual-polarization radar observations of super Typhoon Meranti (2016) prior to and during its landfall provide essential insights into the microphysical characteristics of tropical cyclone (TC) inner core. The study yields the following results. (1) The distribution of Meranti's inner core precipitation exhibited an obvious asymmetric feature. The flash heavy rain was mainly located in the front of TC movement direction. Correspondingly, extreme flash rain was concentrated on the left side of TC movement direction and shifted from the front-left quadrant to the rear-left quadrant. (2) The maximum values of polarization parameters such as the reflectivity (Z_H), differential reflectivity (Z_DR) and specific differential phase (K_DP) are found to have asymmetric horizontal distributions within a radius of about 30 km from the TC center. The horizontal structure of the TC inner core was influenced by topography and underwent changes as Meranti made landfall. The eyewall gradually contracted inward with increased polarization parameters, and the high polarization parameters shifted from left to right of the TC movement direction. (3) The front quadrants of the eyewall exhibited a polarimetric radar signature of hydrometeor size sorting, which marked by a K_DP increase region located downwind of a Z_DR increase region, both overlapping high Z_H. The Z_DR column suggested the occurrence of a persistent updraft in the front quadrant of the eyewall, where large precipitation particles were produced and fell downstream, forming a slanted high Z_DR region, whereas smaller particles had a larger advection distance, accompanied by a high K_DP region. (4) The melting-layer altitude in the TC inner core was displaced upward as the radial distance from the TC center decreased, which reveals the warm core structure of the TC. Below the melting layer, Z_H, Z_DR and K_DP increased significantly toward the surface, indicating that the precipitation of Meranti's inner core was generally dominated by warm rain processes. (5) The spatial distribution of precipitation particles within different quadrants of TC eyewall resulted in different characteristics of surface precipitation. The eyewall front-of-motion quadrants had a greater probability of high Z_DR, while the left quadrant more frequently had high K_DP. The larger denser precipitation particles produced extremely high precipitation efficiency in these quadrants. Regions of Z_DR and K_DP intensification corresponded to flash heavy rain in the surface. However, the area of large K_DP provided a better indicator of the location of extreme flash rain. (6) The topography in southern Fujian had an enhanced effect on the eyewall precipitation of Meranti during landfall. The low-level convergence induced by local terrain caused enhancements in polarization parameters such as Z_DR and K_DP. The larger denser precipitation particles frequently appeared there, promoting sustained and intensified heavy precipitation.