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Formation Mechanism of the Snowstorm over Beijing in Early Winter of 2009
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摘要: 利用多种新观测资料，对2009年11月1日北京出现的60年来降雪量最大的初雪过程的发生、发展特征进行分析，并探讨了暴雪形成机制。结果表明：此次初冬暴雪过程是在500 hPa东亚大槽斜压发展、低层锋区较强的背景下，由华北锢囚锋强迫所致。该锢囚锋是贝加尔湖南下冷空气在华北燕山和太行山地形影响下变为低层东西两股冷空气相向挤压的产物；伴随锢囚锋的形成，在北京西南37°~40°N之间形成狭窄水汽输送通道，为锢囚锋降水提供必需的水汽条件。锢囚锋区结构浅薄，主要存在于850 hPa层以下，为垂直东倾的冷式锢囚；东倾结构决定了降水落区，北京恰处于地面锢囚锋东侧的锢囚锋区中；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，故暴雪是降雪时间长、累积量大的结果。另外，北京雨转雪的发生是因降雪前近地面层气温下降较快接近冰点的缘故，降温主要源于雪前降雨在近地面层蒸发冷却的贡献；低层东路冷空气的平流作用则是降雪期间近地面气温维持较低的主因。Abstract: Using a variety of high spatial and temporal resolution observation data, an in-depth observation analysis is carried out on the formation and development mechanism of a snowstorm case happened on 1 Nov 2009 over Beijing and notable by its nature of the most early winter snowstorm in the past 60 years in Beijing. It shows that the snowstorm occurs under the favorable large scale atmospheric conditions. The enhanced 500 hPa trough over East Asian region together with the low level occluded front in Taihang and Yan mountain areas of North China provided strong forcing for the snowstorm. The occluded front is a terrain-driven cold-style occluded front system due to the encounter of the low level (below 1500 m) northwest to southeast cold air with the Taihang and Yan mountains. The low level cold air turns its original moving direction into west, and results in the moisture feeding to the occluded front zone from two different directions (the Bohai Sea and the edge of the water vapor-rich region in southern China).It shows that the snowstorm is the result of the low level occluded front, and precipitation distribution of 1101 process in 2009 is decided by the eastward-tilting structure of the occluded front, and Beijing is just located in the favorable front zone (the east side to the top of surface occluded front). Because vertical structure (below 1500 m) of the occluded front is shallower than the classic occluded front developed from the typical frontal cyclone, the vertical motion of the snowstorm case is not very strong. Therefore, its precipitation distributes evenly with time but lasts about 15 hours (rain first then it turns into snow) with large amount of the accumulated precipitation. The observation analysis based on wind profile suggests that the occluded front weakens from its bottom to top, which is caused by the cold air invading to the mature occluded front zone starting from layers below 500—800 m and then extending upward.Furthermore, the diagnosis shows that the precipitation transits from rain to snow because the surface air temperature decreases to near freezing point rapidly. The mechanisms for the temperature drop are not the same in different stages of precipitation. Evaporative cooling of the rainfall is the main contributor to the temperature drop before snowfall (from 00:00 to 08:00 on 1 November), while the low-level cold air advection plays the key role for maintaining lower air temperature during the whole snowfall period from 08:00 to 14:00 on 1 November in 2009.
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Key words:
- occluded front;
- snowstorm;
- rain to snow;
- formation mechanism
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图 1 “1101”过程的降水分布特征 (a) 2009年11月1日08:00—14:00北京范围的降雪分布图，(b) 2009年11月1日石景山站的降水量、地面温度的时间序列图
Fig. 1 The precipitation distribution of 1101 process
(a) the precipitation from 08:00 to 14:00 on 1 Nov 2009 in Beijing, (b) time series of precipitation, surface temperature on 1 Nov 2009 at Shijingshan Station
图 2 2009年10月31日20:00—11月1日20:00我国中东部的降水分布 (单位：mm)
Fig. 2 The precipitation distribution over the central and east of China from 20:00 31 Oct to 20:00 1 Nov in 2009(unit:mm)
图 3 “1101”过程环流形式特征 (a)2009年10月31日20：00的500 hPa高度场 (实线，单位：dagpm)、温度场 (虚线，单位：℃)，(b)850 hPa高度场 (实线，单位：dagpm)、温度场 (虚线，单位：℃)、比湿场 (阴影) 和风场，(c)2009年11月1日08:00的实况地面填图及锋面演变
(实线为海平面气压，单位:hPa；圆圈内为降雪集中区域)
Fig. 3 The circulation characteristics of 1101 process (a)500 hPa geopotential height (solid line, unit:dagpm) and temperature (dashed line, unit:℃) at 20:00 31 Oct 2009, (b)850 hPa geopotential height (solid line, unit:dagpm), temperature (dashed line, unit:℃), specific humidity (shaded) and wind at 20:00 31 Oct 2009, (c) the surface observation plots and front evolution at 08:00 1 Nov 2009 (solid line: sea level pressure, unit:hPa; black circle: the concentrated area of snow)
图 4 2009年10月31日—11月1日1030 hPa海平面气压等值线和蒙古高压的时间演变图
(绿色实线内为降雪区位置；黑色实线为地形等高线；棕色虚线内为地形相对高值区)
Fig. 4 Time evolution of sea level pressure isoline of 1030 hPa and Mongolia high from 31 Oct to 1 Nov in 2009
(black line is for the topographic contour; within the green line is snowfall area; within the brown dashed line is the area with relatively high topography)
图 5 2009年“1101”过程水汽分布特征
(a)11月1日02:00的925 hPa比湿与温度平流的叠加 (实线为温度平流，单位：10-5℃·s-1；阴影为比湿)，(b)11月1日02:00的925 hPa的水汽通量矢量合成、水汽通量的模以及水汽通量散度的叠加图 (阴影为水汽通量散度；等值线为水汽通量的模，单位：g·hPa-1·cm-1·s-1；箭头为水汽通量的矢量合成), (c)37°N, 114.5°E (北京以南) 的水平风场的时间高度序列图
Fig. 5 The water vapor distribution of 1101 process in 2009
(a) specific humidity (shaded) and thermal advection (solid line, unit:10-5℃·s-1) of 925 hPa at 02:00 1 Nov 2009, (b) composite vector (arrow), modulus (contour, unit:g·hPa-1·cm-1·s-1) and divergence (shaded) of 925 hPa moisture flux at 02:00 1 Nov 2009, (c) cross section of horizontal wind through the time-height plane at 37°N, 114.5°E (south of Beijing)
图 6 2009年11月1日02:00沿图 4中的棕色实线所做的各要素剖面图
(a) 温度平流 (等值线，单位：10-5 ℃·s-1) 与垂直速度 (阴影)，(b) 位温 (等值线，单位：K) 与uv投影与w风场的叠加 (矢量)
Fig. 6 Vertical sections along the brown line in Fig. 4 at 02:00 1 Nov 2009
(a) thermal advection (contour, unit: 10-5 ℃·s-1) and vertical velocity (shaded), (b) potential temperature (contour, unit:K) and the projection of horizontal wind combined with vertical wind (vector)
图 7 北京市观象台在2009年11月1日02:00的1.5°仰角雷达反射率因子 (a) 及沿图 7a中黑色实线的剖面 (b)
Fig. 7 Radar reflectivity at 1.5° elevation (a) and vertical sections along the black line (b) at 02:00 1 Nov 2009 of Beijing Weather Observatory
图 8 2009年11月1日站点风场特征
(a) 海淀气象站的风廓线图，(b) 北京市观象台站1.5°仰角的径向速度图
Fig. 8 The wind field characteristics of stations (a) wind profile at Haidian Station, (b) radar PPI velocity of 1.5° at 10:00 1 Nov 2009 of Beijing Weather Observatory
图 9 式 (1) 中各项对975 hPa和950 hPa 6 h变温的贡献
(实线为∂T/∂t项；点划线为平流项 (-V·∇hT-ω·∂T/∂P)；虚线为dT/dt项)
Fig. 9 The contribution of every item in formula 1 to 6 h temperature fluctuation in 975 hPa and 950 hPa
(solid line is ∂T/∂t; dotted-dashad line is-V·∇hT-ω·∂T/∂P; dashed line is dT/dt)
图 10 海淀气象站地面半小时变温的时间序列图
(断线为“1101”过程对应的时间段，2009年10月31日15:00—11月1日15:00；实线为选取的无降水对比时间段，2009年10月27日15:00—28日15:00与10月28日15:00—29日15:00的平均；方框Ⅰ和Ⅱ分别对应的是降雨和降雪发生时间)
Fig. 10 Time series of surface half-hour temperature variation at Haidian Station
(broken line is for the period of 1101 process from 15:00 31 Oct to 15:00 1 Nov in 2009; solid line is for the comparision period of no precipitation which is the average of 15:00 27 Oct to 15:00 28 Oct in 2009 and from 15:00 28 Oct to 15:00 29 Oct in 2009; box Ⅰ and box Ⅱ denote the periods of rainfall and snowfall, respectively)
图 11 北京市观象台的自动站降水量、相对湿度、零度层高度以及微波辐射计观测的不同高度的温度时间序列图
Fig. 11 Time series of precipitation, relative humidity of AWS and the time series of the 0℃ level height, temperature at different height of microwave radiometer observations in Beijing Weather Observatory
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