2008年1月中国南方低温雨雪冰冻天气特征及其天气动力学成因的初步分析

A preliminary analysis of features and causes of the snow storm event over the Southern China in January 2008

中文关键词:  灾害性天气，低温雨雪冰冻，特征，成因

英文关键词:Disasterous weather, Low temperature freezing rain/heavy snowfall, Characteristic features, Causes of formation

中文摘要:

      在欧亚大陆中高纬度长期维持阻塞形势的背景下，2008年1月11日至2月2日中国南方连续经历了4 次低温雨雪冰冻天气过程（简称“0801南方雪灾”）。这次强天气事件过程范围广、强度大、持续时间长且灾害极为严重。其天气学特征表现为中高纬阻塞形势稳定少变，低纬系统活跃确保水汽输送，以及南方部分地区满足冰雪风暴形成的有利天气条件等。研究表明，导致大气环流异常从而促使这次低温雨雪冰冻强天气事件出现的主要因素包括：（1）北极涛动（AO）的异常活跃，有利于行星尺度波动的稳定维持；（2）阻塞上游50°N区域有极强的负涡度平流持续输送到阻塞区，使濒临崩溃的阻塞形势得以重新加强，从而使阻塞形势长时间稳定维持；（3）青藏高原以南低纬地区南支气流活跃，确保中国南方充沛的水汽来源；以及（4）长期存在有利于冰雪风暴生成发展的天气 动力 物理学条件等。 “0801南方雪灾”事件一个突出的特征就是冰冻现象极为严重。文中借助新型卫星CloudSat的星载云廓线雷达（CPR）资料对这次事件中典型云系进行天气 动力 物理学分析，揭示出西南暖湿气流沿锋面爬升，形成界限分明上下交替的两个冷暖气团，冷气团较浅薄；在2—4 km高度存在一个融化层，冰水不仅存在融化层之上，在近地面层亦含有丰富的冰粒子。 结合常规观测资料分析发现，在此期间中国南方大部地区中层（大约在850—700 hPa）存在温度大约在0—4 ℃的逆温层，地面气温大致维持在-4—0 ℃且相对湿度在90％以上，分析 表明，此次大范围冰冻灾害天气是由于冻结、凝华和冰雾粒子的附着增长等物理过程共同作用的结果。“0801南方雪灾”事件持续时间较长，事件本身作为一个整体其成因可追溯到行星尺度系统。研究极涡异常及其随时间变化的结果表明，平流层极涡变化比对流层的超前，尤其是该事件前期平流层极涡进入12月后就趋于加强，而对流层的极涡加强则明显滞后，并且直到1月中至2月初才快速加强；这意味着平流层蕴含着对流层极涡变化及伴随的强天气事件的先期信号，这可能是中长期预报的一个值得深入研究的方向。

英文摘要:

      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.