Analysis on the Long-Lasting Freezing Rain and Snowstorm Event at the Beginning of 2008

In this paper, main characteristics of the long-lasting freezing rain and snowstorm event in southern China at the beginning of 2008, features of the related atmospheric circulation and the causes thereof are analyzed. During the event, patterns of the atmospheric circulation stayed stable; the polar vortex located in the northern part of the Eastern Hemisphere was strong with little movement; the cold front from the polar region and the active warm air mass from the tropical ocean confronted each other for a long time; the blocking high to the west of Baikal remained strong and steady; the trough over central and western Asia maintained its position for quite long with a group of little troughs splitting from it frequently; the dominant wind at 700 hPa was southwesterly while shears and vortexes at 850 hPa developed continually, providing the necessary low-level convergence for subsequent precipitation. Meanwhile, in the mid troposphere, eddies were generated over the Tibetan Plateau and positive vorticity disturbances in the Sichuan Basin propagated eastward to the coastal regions of eastern China. The western Pacific subtropical high was intensive with westward and northward migrations. The subtropical frontal zone was puissant and the north-south temperature gradient was large. Quasi-stationary fronts over South China and the Yunnan-Guizhou Plateau remained stable. Warm air masses over the tropical ocean were active, so was the trough in the southern branch of the westerlies over the Bay of Bengal. There were four episodes associated with this event. The first one was featured with the interaction of strong cold and warm air, while the other three with the quasi-stationary fronts over South China and the Yunnan-Guizhou Plateau as well as vigorous penetration of cold air from the north. The existence of the inversion layer and the thick melting layer were one of the main reasons for the long-lasting freezing rains.The main reason for the snowstorms was that the positive vorticity over the Sichuan Basin propagated eastward to the coastal regions of eastern China. Abundant water vapor and intense updraft also favored the heavy snows.