【作者】 叶晨；

【导师】 王建捷； 陈敏；

【作者基本信息】 中国气象科学研究院 ， 气象学， 2011， 硕士

【摘要】 2009年10月下旬末到11月中旬的初冬季节,北京发生了连续数场强降雪,历史罕见。特别是,11月1日和11月9日两次降雪过程均达到暴雪量级,11月1日的暴雪是北京过去60年以来雪量最大的冬季初雪,且初雪发生时间比常年提前了28天,降水性质由雨转为雪的变化成为预报员始料未及的预报失败点;而9日的暴雪过程则出现短时雪强骤增并伴随雷电现象。利用北京地区高时空分辨率的地面自动站资料、微波辐射计资料、风廓线数据、多普勒雷达数据、BJ-RUC数值预报资料以及6小时一次的NCEP再分析资料等,对2009年11月上旬影响北京地区特别是北京城区的这两次典型暴雪过程,即11月1日(简称“1101”过程)和11月9日(简称“1109”过程),发生发展的环境条件和形成机制进行了细致的分析和对比,得到如下主要结论:1,“1101”和“1109”两次暴雪过程降雪强度大,持续时间也较长,对于北京地区而言,两场降雪均集中在中部与南部地区,城区为降雪中心区。然而,两次过程也存在明显区别:首先在两次过程相应时间的雪线以北,由于所受锋面系统的差异,雪区形状明显不同,分别为块状和带状,同时“1101”过程的降雪过程较为稳定,降雪强度均匀,而“1109”过程中有明显的降雪集中时段,并伴有打雷现象。2,两场暴雪过程,发生在东亚低槽冷空气南下早于和势力强于历史同期的气候背景条件下:2009年11月上半月,500hPa环流经向度明显强于常年,且东亚地区的槽较气候平均而言显著偏西;在东亚对流层中低层的40N-70N区域,冷空气较历史同期明显偏强,低层的冷空气活动更为频繁,从而有利于低槽冷空气影响北京和周边地区。3,在上述气候背景条件下,两次过程在大尺度环流形势上尽管有一些相似之处,如500hPa上均处于东亚低槽活动区内,低层有较强的锋区存在,位置在40°N至45°N之间等,但细节差异明显。包括:(1)500hPa上槽区内槽线的位置、形状不同。“1101”过程500hPa上受南北两支槽的影响,“1109”过程500hPa上则主要受准横槽(东亚低槽)的影响,华北东北地区始终受东亚低槽槽前偏西锋区控制。(2)850hPa上两次过程不仅高低压配置具有明显的不同,而且低层风环流和高度场/地面气压场也差异明显。“1101”过程850hPa高压前部北风与温度线几乎成垂直交角而“1109”过程风向(偏东风)与温度线几乎平行。(3)动力条件及其变化有差异。“1101”过程在降雪发生前存在显著的低层辐合,高层辐散的有利配置,同时配合有低层正涡度高层负涡度,由此产生了较强的上升运动,“1109”过程的大尺度动力条件并没有“1101”过程优越,但利用BJ-RUC(高分辨率快速循环同化预报模式系统)资料诊断发现,在降雪集中时段的动力条件还是较为理想的,辐合中心和正涡度区的强度均明显的强于其他的时次。(4)水汽来源存在差异。“1101”降雪过程的水汽来源于同一层次上的两个方向,水汽通道位于850hPa和其下的对流层低层;“1109”过程的水汽输送则来自于不同层次上的两个方向,700hPa上的西风带系统所携带的水汽加之850hPa上小股由渤海输送而来的水汽共同为“1109”过程的发生提供必要的水汽条件。4,针对“1101”暴雪过程产生机制的研究分析表明,此次暴雪过程是在500hPa东亚低槽斜压发展的背景下,在对流层低层冷暖空气剧烈交绥区,由华北特殊地形影响下的锢囚锋强迫所致:(1)伴随锢囚锋的形成,在北京西南形成狭窄水汽输送通道,连接华北锢囚锋区和来自两个不同方向的水汽输送汇合区,为锢囚锋的发生发展提供了必需的水汽条件;(2)锢囚锋区结构“浅薄”,且该锢囚锋的减弱与东路冷平流契入成熟锢囚锋区有关,冷平流从500-800m以下层契入后向上扩展,使得锢囚锋结构自下向上减弱消失;(3)该地形锢囚锋的东倾结构决定了降水落区位置,北京恰好处于地面锢囚锋东侧的锢囚锋区之中,故在锢囚锋整个生命阶段对应有降水;而锢囚锋浅薄的垂直结构则决定了其强迫抬升运动并不深厚,故北京暴雪是降雪持续时间较长、累积量大所形成的结果;(4)北京雨转雪的发生是因降雪前(即1日00时-08时)近地面层气温下降较快接近冰点的缘故,而这期间的降温主要源于雪前降雨在近地面层的蒸发冷却贡献;低层东路冷空气的平流作用则是降雪期间(即1日08时-14时)北京近地面气温维持较低的主因。5,针对“1109”暴雪过程产生的成因分析表明,此次暴雪过程是一次典型的冷锋影响下的降雪过程,其中对流层低层的中尺度扰动和对称不稳定共同促成了雪强的短时激增:(1)“1109”过程存在有非常明显的降雪集中时段,并且雪量在短时的较大增长主要发生在北京城区及以南的带状区域内(39.8N-40.2N);对应降雪强度的激增(利用在9日23时左右),北京城区地面自动站观测到气温和变温骤降,相对湿度突升的现象;在降雪密集的短时间段里,卫星云图上可见河北中部以及北京的中南部有非常明显的中尺度“带状”云系的发展,相应在多普勒天气雷达的反射率因子强度(接近40dbz)、垂直发展高度(达到7-8km)、强回波持续时间(约2小时)等方面亦表现出清楚的中尺度特征。(2)对地面连续自动风场观测分析发现,在雷电以及雪量激增发生前2小时左右,北京城区附近出现一条准东西走向的中-β尺度辐合线,维持至22:40分左右,暗示强降雪的发生与近地面层这一中-β尺度系统的生成有关。(3)基于观测和3km分辨率快速循环同化预报系统BJ-RUC预报结果的综合诊断显示,“1109”过程雪强的激增是对称不稳定机制所致。由于11月09日22:00-23:30左右,北京上空700hPa附近出现西南风小扰动,改变了相应层次温湿条件,继而在局部垂直层内(大约750-500hPa间)出现了对称不稳定;该对称不稳定的出现,为已经在沿天气尺度冷锋倾斜爬升的气流提供了自由倾斜向上发展的条件和能量,使得气流沿着冷锋加速上升,上升运动非常强劲,从而导致了地面雪量的短时激增并使雪伴雷电成为可能。更多还原

【Abstract】 From late October to early November in Beijing in 2009,there are several snowfall, rarely seen in history. In particular, Nov. 1 and Nov. 9 during the two reached the blizzard snow magnitude, the snowfall in Nov 1 is the largest amount of snow in winter snow over the past 60 years in Beijing, and snow occurs earlier than normal time 28 days, the nature of precipitation changes from rain turned into snow is the unexpected point of failure for the forecast forecaster; and the snowstorm in Nov 9 appears an explosion of snow accompanied by thunder and lightning phenomena. Using many high spatial-temporal resolution data of Beijing, such as auto-weather station (AWS) network data, Doppler radar data, wind profile data, and BJ-RUC Numerical simulation data ,a much in-depth comparative analysis and study are carried on two snowstorm cases in Beijing on 1th and 9th November 2009 are shown. It is revealed that:1, Both the 1101 snowstorm and the 1109 snowstorm have significant snowfall intensity,long duration,concentration in the central and southern regions and large snowfall in town. However, there are significant differences between the two processes: firstly, duing to the differences between the frontal systems, there are significantly different shapes of the snow area in the north of the snow line.Secondly,the 1101 snowstorm is relatively stable during the process of snow, having snowfall intensity of uniform , while the 1109 snowstorm has concentrated periods of snow, accompanied by thunder.2, These two snowfall that intervals of less than ten days,can not be exist without the better climatic background conditions:In early November 2009,the meridional circulation of 500hPa is significantly stronger than normal year,the position of trough in East Asia is significantly more west than climate average.Beijing is in the strong cold air region.Meanwhile, Whether in high or low,the cold air of early November 2009 is stronger than normal year,the activities of low-level cold air is , more frequent,contributing to the occurrence of snowfall.3, The same point on the large circulation of two snowfall is that they are both in the activities of East Asia trough and have strong front at low level.The differences about weather circulation and conditions include:(1) The positions and shapes of 500hPa trough are different. The 1101 snowstorm is affected by two troughs of 500hPa,while the major effect system of 1109 process in 500hPa is the trough of East Asian which is relatively stable,the north and the south China is always under the control of frontal zone before the trough of East Asian.(2) The differences between the two processes of 850hPa is not only the different configuration of low and high,but also the low-level wind circulation and the surface pressure field.(3) The significant convergence and positive vorticity in low level, and the significant divergence and negative vorticity in high level produce a continuous and strong upward motion in 1101 process before snow. Either the divergence or the vorticity in low and high level, the process in 1101 is more ideal than that in 1109. The Large-scale dynamic conditions in 1101 is superior. But using BJ-RUC data find that although the whole dynamic conditions of 1109 process is not good, but near the rainfall concentrated period ,the conditions is still more ideal, the intensity of convergence center and positive vorticity areas are obvious stronger than other times.(4) Differences in moisture sources.The water vapor of 1101 snowfall comes from two directions of same level,which is located below 850hPa,but the water vapor of 1109 snowfall comes from two directions of different levels, the water vapor carried by the westerly system of 700hPa combines with the water vapor transported from the Bohai of 850hPa together provide the necessary moisture conditions for the occurrence of 1109 snowstorm.4, The analysis of the mechanism of 1101 snowfall shows that the snowstorm is forced by the occluded front from the north. (1) With the formulation of the occluded front,water vapor transport From two different directions converge between 35-40N in Southwest of Beijing,inducing a narrow channel of moisture transportation directly to the occluded front in North China .(2) The Structure of front is shallow and the weakening of the occluded front is related to the east of cold advection by the way of invading the mature occluded front. Firstly the cold advection from layers below 500-800m cuts in the occluded front, the occluded front structure weaken from the bottom and finally die away .(3) The occluded front’s precipitation area is decided by the east-dipping structure on this terrain, during its whole life stage (15h or so), there is a process of precipitation correspondingly in Beijing. The snowstorm of Beijing belongs to precipitation of occluded front and the snow is over with the disappearance of occluded front.(4) The process from rain to snow in Beijing is due to the rapidly decreasing and closing to the freezing point of air temperature near the surface. The decrease of air temperature before snow (from 0000 BT 1 to 0800 BT 1 November) is because of the rain by the means of evaporative cooling. While, the east of low-level cold air advection contributes to air temperature maintaining low near the surface in Beijing during the heavy snowfall (from 0800 BT 1 to 1400 BT 1 November).5, The analysis of the causes of 1109 snowfall shows that the heavy snow is forced by the typical cold front, associated with the mesoscale activities:(1) Using the AWS data, satellite images and radar data can reveal the mesoscale features on 1109 process. (2) according to AWS surface wind, there was a east-west directionβ-scale convergence line in the intersection between Daxing district and Beijing city before the lighting and explosion of snow about 2 hours ago.,the process lasts until about 22:40 pm.(3) According to the analysis of the stability conditions by BJ-RUC model data for "1109" process, it can be seen that the stable layer into neutral and pseudo-equivalent temperature changed shape above 700hPa.The process snows that contour slope of pseudo-equivalent temperature is greater than the contour slope of the absolute momentum and symmetric unstable conditions are met. Meanwhile, the wind climb along the front area to the symmetric instability area in order to getting the unstable energy. After that,a very strong upward motion emerges , so that the explosion of snow and thunder during snow becoming possible.更多还原