Wang Donghai, Liu Chongjian, Liu Ying, et al
Acta Meteorologica Sinica
The reason of ice-snow disaster
Disasterous weather, Low temperature freezing rain/ heavy snowfall , Characteristic features, Causes of formation
The four successive freezing rain/ heavy snowfall processes were undergone in the southern part of China from 11 January to 2 February 2008, under the background of the long-term lasting blocking in the middle-higher latitudes of the Euro-Asian continent (named “ 0801 Southern Snow-Disaster” hereafter).This severe weather event has the characteristic features of broad range, strong intensity, long duration and serious disaster .During this event the blocking situation in the higher latitudes is kept quasi-steady with minor changes, the weather systems in the lower latitudes are so active that the sufficient vapour supplies can be guaranteed, the favourable weather conditions to the formation of ice storm are met in the w ide region within the southern China and so forth. The cause of formation of the tempo rally and spatially large-scale event itself might be, as a whole, traced back to the planetary-scale systems. The results from the research, in the polar vortex anomaly and its change with time, show that changes in the polar vortex in the stratosphere precede those in the troposphere, especially the polar vortex in the stratospheres in early December before this event commenced to gradually strength while the intensification of the polar vortex in the troposphere delayed dramatically with its rapid strengthening in the period between the middle of January and early February .This implies that indications of changes in the polar vortex in the troposphere and the severe weather events associated might be contained within the stratoposphere, which is very meaningful to improving the 10-30 days' extended forecasts. The results demonstrate that the significant factors that lead to the unusual atmospheric circulation and thus this severe weather event include :(1)the very active Arctic Oscillation (AO)that benefits the permanent maintenance of the planetary-scale waves ;(2)transferring continuously of negative vorticity into the blocking area from its upstream around 50°N so as to cause repeatedly the blocking high on the verge of disintegration to reintensify with the result that the blocking conditions can be maintained for a long time ;(3)the active southern branch of currents in the lower latitudes south of the Tibet plateau that ensures the abundant vapour supplies to the southern China ;and (4)the favourable synoptic-dynamic-physical conditions to the formation and development of ice storm and so forth. ‘0801 Southern Snow-Disaster’ lasted for a longer period with an extremely severe icing .The data from the Cloud-Profile Radar o n the satellite CloudSat are used in order to make the synoptic-dynamic-physical analyses of the typical cloud series during this event . The results show that the warm and humid southwestern cur rents climbing along the front form a structure consisting of two air mass-es with a clear dividing line between them and the thinner cold air mass under the front .There exists a melting layer between 2 and 4 km and, icing particles can be found above this layer as well as in the layer near the g round .On the other hand, there are the layers with their temperature ranging from 0 to 4 ℃, in the middle-lower troposphere (850 -700 hPa)in the large part of the southern China during this event as suggested by the conventional meteorological data .At the same time , the corresponding surface temperature kept generally between -4 and 0 ℃ with the relative humidity over 90%, which provides the falling supercooled water drops with a very favourable synoptic-physical conditions to form the severe glaze and to ice up at the surface via freezing , deposition and/or accretion.