A Preliminary Analysis of Features and Causes of the Snow Storm Event over the Southern Areas of China in January 2008

Four successive freezing rain/heavy snowfall processes occurred in the southern part of China from 11 January to 2 February 2008 (named "0801 Southern Snow Disaster" hereafter), during which a largescale blocking circulation lasted for a long time over the mid-high latitudes of the Euro-Asian continent. This severe event is featured with a broad spatial scale, strong intensity, long duration, and serious damage.During the event, the blocking situation in the mid-high latitudes maintained quasi-sationary, but weather systems in the lower latitudes were active. Abundant water vapor was supplied, and favorable weather conditions for ice storms were formed over the large areas across the southern part of China. The results in this paper demonstrate that the significant factors responsible for the abnormal atmospheric circulation and this severe event include: 1) the very active Arctic Oscillation (AO), which helped the permanent maintenance of the planetary-scale waves; 2) the continuous transfer of negative vorticity from the upstream region around 50°E into the blocking area, which caused the blocking situation reinforced repeatedly and sustained for a long time; and 3) the active air currents south of the Tibetan Plateau, which ensured abundant moisture supply to the southern areas of China. The 0801 Southern Snow Disaster was accompanied by extremely severe icing. In this paper, the data from Cloud-Profile Radar onboard the satellite CloudSat are used to study the dynamic and microphysical features of this event. The results show that there existed a melting layer between 2 and 4 km, and ice particles could be found above this layer and in the layer near the ground surface. Surface temperature kept between -4℃and 0℃ with relative humidity over 90%, which provided the descending supercooled waterdrops with favorable synoptic and physical conditions to form glaze and ice at the surface via freezing,deposition and/or accretion. Causes of the event might be, as a whole, traced back to the planetary-scale systems. The study on the polar vortex anomaly in this paper reveals that changes in the polar vortex in the stratosphere preceded those in the troposphere, especially in early December 2007, while the intensification of the polar vortex in the troposphere delayed dramatically until middle January and early February of 2008. This implies that changes in the polar vortex in the stratosphere may be a precursor of the ensuing severe event and a meaningful clue for extended forecasts of such a disaster.