The role of polygonal eyewalls in rapid intensification of Typhoon Megi (2010)

Abstract

High-resolution numerical experiments for Typhoon Megi (2010) in the western North Pacific are conducted using the Advanced Research version of the Weather Research and Forecasting (WRF) Model to understand the mechanisms of rapid intensification (RI). With a dynamically initialized vortex, sensitivity experiments are carried out focusing on the planetary boundary layer (PBL) and the following microphysics schemes: WRF single-moment 6-class (WSM6) and WRF double-moment 6-class (WDM6) microphysics with Yonsei University, Mellor-Yamada-Janjic (MYJ), and Mellor-Yamada-Nakanishi-Niino (MYNN) 2.5-level (MN2.5) and 3.0-level (MN3) PBL schemes. The largest differences are found between WSM6-MN3 and WDM6-MN3, and we therefore examine RI mechanisms based on the results of these experiments. Prior to RI, WDM6-MN3 shows a drier environment and stronger downdrafts in the lower troposphere than in WSM6-MN3. As a result, during the RI period, WSM6-MN3 (WDM6-MN3) significantly intensifies with the minimum sea level pressure decreasing by 51 (29) hPa and the maximum surface wind increasing by 28 (12) m s -1 in 24 h. In both experiments, the maximum values of surface heat fluxes, potential vorticity (PV), radial absolute angular momentum advection, inertial stability, supergradient wind, and convective bursts inside the radius of maximum winds are frequently observed at each vertex of polygonal eyewalls in the lower troposphere. In particular, WSM6-MN3 exhibits more convective cells inside the inner-core region, a more persistent and thicker polygonal eyewall in the lower troposphere, and a more robust vertical structure of hydrometeors and vertical velocity than WDM6-MN3. This study suggests that within the inner-core region, polygonal eyewalls like WSM6-MN3 provide favorable conditions for RI.