A preliminary analysis of features and causes of the snow storm event over the Southern China in January 2008
A preliminary analysis of features and causes of the snow storm event over the Southern China in January 2008
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作者：
Xu Xiangde
2008年1月11日至2月2日，在欧亚大陆中高纬度地区长期持续阻塞的背景下，中国南部地区经历了4次连续的冻雨/暴雪过程（以下称为“0801南方雪灾”）。此次灾害性天气事件具有范围广、强度强、持续时间长、灾情严重等特点。在这次事件中，高纬度地区的阻塞情况保持准稳定，变化不大，低纬度地区的天气系统非常活跃，可以保证充足的水汽供应，在华南广大地区具备了形成冰暴的有利天气条件等。时间和空间大尺度事件本身形成的原因，作为一个整体，可以追溯到行星尺度系统。极地涡旋异常及其随时间变化的研究结果表明，平流层极地涡旋的变化先于对流层，特别是平流层极地涡旋在事件发生前的12月初开始逐渐增强，而对流层极地涡旋的增强明显滞后，在1月中旬至2月初期间迅速增强。这意味着对流层极地涡旋的变化和与之相关的恶劣天气事件的迹象可能包含在平流层内，这对改进10-30天的延长预报具有重要意义。“0801南方雪灾”持续时间较长，结冰严重。利用CloudSat卫星上的云廓线雷达资料，对该事件期间的典型云系进行了天气-动力-物理分析。结果表明，沿锋面爬升的西南暖湿气流与锋面下方较薄的冷空气形成了一个由两个气团组成的结构，两个气团之间有明显的分界线。在2 ~ 4 km之间存在一个融化层，在这一层之上和近地面的层中都可以发现结冰颗粒。另一方面，常规气象资料显示，此次事件期间华南大部分地区对流层中下层（850 ~ 700 hPa）存在0 ~ 4℃的温度层。同时，相应的表面温度一般保持在-4 ~ 0℃之间，相对湿度在90%以上，这为下落的过冷水滴形成严重釉面并通过冻结、沉积和/或吸积在表面结冰提供了非常有利的天气物理条件。
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 characteristical 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 wide region within the southern China and so forth.The cause of formation of the temporally 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 on 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 currents climbing along the front form a structure consisting of two air masses 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 ground.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 waterdrops 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.