A diagnostic study on heavy rainfall induced by typhoon utor (2013) in South China: 1. Rainfall asymmetry at landfall

Abstract

After striking the Philippines on 11 August 2013, Typhoon Utor made its second landfall over southern South China on 14 August and brought heavy rainfall, causing widespread damages after its landfall. The heavy rainfall associated with Utor includes three main episodes: rainfall at landfall, postlandfall rainfall in the inner core region, and postlandfall rainfall induced by the intensification of southwesterly monsoon flow. In part 1 the rainfall at landfall is analyzed and diagnosed using the surface mesonet observations and the high-resolution European Centre for Medium-Range Weather Forecasting Interim reanalysis (ERA-Interim) data. Results show that when Utor approached and made landfall over the southern coast of South China, it was embedded in a deep-layer (200-850 hPa) easterly northeasterly vertical wind shear (VWS) environment with heavy rainfall mostly occurring downshear left in its southwestern quadrant. Under a favorable environment with moderate convective available potential energy and moderate low-level VWS, a surface cold pool was found in the west-southwestern quadrants of Utor triggered by the middle-lower tropospheric dry air intrusion from inland to the north. Both frontogenesis and quasi-balanced, ageostrophic Q-vector diagnostics demonstrate that the quasi-balanced forcing near the surface cold cool contributed to the enhancement of convection in the west-southwestern quadrants of Utor. It is concluded that the environmental VWS and the consequent surface cold pool dynamics play an important role in rainfall asymmetry in Utor when it made landfall over the southern coast of South China.