A Numerical Simulation of Microphysical Structure of Cloud Associated with the 2008 Winter Freezing Rain over Southern China

Abstract

 The cloud microphysical structure for a freezing rain event between Jan 11 and Feb 4, 2008, over southern China is studied using the 30 km-mesh Weather Research and Forecasting (WRF) model simulations with four different microphysics schemes and CloudSat satellite observations. This 3-week-long freezing rain event, centered in the domain of 108°-113°E and 25°-28°N, has been extraordinarily rare over southern China during the recent 50 years. Except the Lin scheme, the other three microphysics schemes (Morrison, WSM6, and Thompson) yield WRF simulations that capture the temporal and spatial distribution of precipitation and surface air temperature associated with the freezing rain event, albeit the modeled center of precipitation is slightly drifted northward in comparison with the observation. The WRF simulations highlight the importance of the following characteristics of an atmospheric vertical thermal structure for forming freezing rain: above-freezing temperature in the middle troposphere (600-850 hPa) and below-freezing temperature in the lower troposphere (below 850 hPa). The temporal evolution of cloud structure and icing processing from Jan 11 to Feb 4 is also well simulated by WRF, which can be divided into two phases: (a) abundant liquid water is found below 700 hPa from Jan 20 to 26, 2008, and (b) liquid and solid water content coexist, and there are mixed-phase clouds in the whole column of atmosphere from Jan 26 to Feb 1, in particular, solid water dominated at the higher level, while liquid water dominated at the lower level. Such a change of cloud structure represents the typical ice-phase mechanism for the freezing rain such as on Jan 28, 2008, over the Hunan province. Further evaluation with CloudSat observation over the Hunan province shows that the WRF model can capture the warm and refreezing layers near the surface, although it overestimates the solid water content.