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

Xiamen S-band dual-polarization radar observations were taken of super Typhoon Meranti (2016) prior to and during landfall, providing needed insight 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 asymmetry. The flash heavy rain was mainly located in the front of TC"s motion direction. Correspondingly, the extreme flash rain was concentrated on the left side of TC"s motion direction, and shifted from the front-left quadrant to the rear-left quadrant. (2) The maximum value of polarization parameters such as the reflectivity (Z_H), differential reflectivity (Z_DR) and specific differential phase (K_DP) were found to have asymmetric horizontal distribution within a radius of ~30km from the TC center. The horizontal structure of the TC inner core was influenced by topography and undergoes alterations, as Meranti made landfall. The eyewall gradually contracted inward with the increased of polarization parameters, and the high polarization parameters shifted from left to right of TC"s motion direction. (3) The front quadrants of the eyewall exhibited a polarimetric radar signature of hydrometeor size sorting, which marked by a K_DP enhancement region situated downwind of a Z_DR enhancement region, both overlapping the 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 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 results in different characteristics of surface precipitation. The eyewall front-of-motion quadrants had a greater probability of high Z_DR, while the left quadrant had high K_DP more frequently. The larger denser precipitation particles produced extremely high precipitation efficiency in these quadrants. Both Z_DR and K_DP intensification regions correspond to flash heavy rain in the surface. However, the high value area of K_DP provides a better indicator of the location of the extreme flash rain. (6) The topography of southern Fujian had an enhanced effect on the eyewall precipitation of Meranti during landfall. The low-level convergence induced by local terrain caused the enhancement in polarization parameters such as Z_DR and K_DP. The larger denser precipitation particles frequently appeared there, thereby promoting sustained and intensified heavy precipitation.