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Environment Characteristics and Causes of a Continuous Hail Fall Event Occurred Within the Cold Air Mass to the North of a Cold Front
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摘要: 对2009年2月24日—3月5日我国南方锋面北侧冷气团中连续冰雹天气过程的环流形势和环境场特征及形成机理进行分析。此次冰雹过程是发生在我国南方典型的连阴雨天气形势背景下，欧亚中高纬度地区为两槽一脊形势，副热带高压偏强、中南半岛为低槽区，青藏高原有5次短波槽东移，中层700 hPa暖湿气流势力强盛。中层强西南暖湿气流在强锋区 (冷垫) 上抬升，形成我国南方典型的高架雷暴。高架雷暴的发生与中低层强温度锋区、中层700 hPa不低于20 m·s-1的西南急流、强风垂直切变、对流层中层较大的温度直减率、较低的0℃层高度 (4 km以下) 有关。Abstract: A continuous hail fall process occurred over Southern China during 24 February—5 March of 2009 is analyzed. This process happens under the background of typical continuous rain weather in spring over Southern China. During this period, a steady two-trough and one-ridge pattern maintains over the Euro-Asian, the stronger subtropical high is further north and west, the Indo-China Peninsula is occupied by a low trough, five short wave troughs move eastward from the Tibet Plateau, and 700 hPa warm-moist airflow is strong. The hail fall occurs within the cold air mass, 300—600 km north of the surface cold front. The vertical distribution of moisture presents dry over 500 hPa and wet (with the relative humidity more than 80%) under 500 hPa. The strong frontal zone in the middle and low levels exists between 25°—30°N of 850 hPa, with the temperature difference between south and north (frontal intensity) more than 14℃ per 5-latitude distances. There is a significant temperature inversion between 700—850 hPa. The 700 hPa southwest jet (≥20 m·s-1) area ranges more than 1000 km. The 700—850 hPa vertical wind shear is strong, with the vector difference reaching 16—20 m·s-1. Averagely, the temperature difference of 500—700 hPa exceeds 16℃, and the temperature lapse rate is about 0.74℃/100 m. The height of the 0℃ layer is very low (below 3 km). 500—700 hPa weak convective instability and symmetric instability may be the causes which result in the formation of this kind of hail. During the hail fall process, Δθse700-500 is 2—6℃ mostly. The minus baroclinic term of moist potential vorticity increases absolutely, which induces the southward expansion of the negative MVP value area. Meanwhile, MVP1, MVP2 and MVP present the variations that form or strengthen the latitudinal frontal zone. The hail fall happens on the south side of frontal zone, which is close to the negative MVP value area. The mechanism of this kind of hail formation is that the strong southwest airflow in middle troposphere lifts in the strong frontal zone, and induces the convective instability and symmetric instability. When trough passes over the strong frontal zone, the inclination of frontal surface becomes steep, the ascending motion strengthens, which form the typical elevated thunderstorm, and the ice embryo grows to hail in middle troposphere. This kind of trough often presents minus temperature variation or a preceding temperature trough. There is a melting layer in 700—850 hPa, and the thickness of upflow is small, therefore, the diameters of this kind of hail are less than 10 mm, few of them are 10—20 mm. The potential forecast index and criterion of this kind of hail falling area includes strong horizontal temperature frontal area, Δθse700-500 > 0℃ or T700-500≥16℃, axis of 700 hPa is more than 20 m·s-1 southwest airflow, area of strong 700-850 hPa vertical wind shear and 500 hPa shortwave trough.
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Key words:
- cold air mass;
- hail fall;
- frontal zone;
- elevated thunderstorm
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图 1 2009年2月24日20：00—3月4日20：00平均500 hPa高度 (实线，单位：dagpm) 与24 h变高 (虚线；单位：dagpm)(a) 和平均海平面气压 (实线，单位：hPa)、平均10 m风 (风羽)(b)
Fig. 1 The averaged 500 hPa height (solid lines, unit: dagpm) with 24 h height-change line (dashed line, unit: dagpm)(a) and the sea level pressure (solid line, unit: hPa) with 10 m wind (barb)(b) from 24 Feb 2009 to 4 Mar 2009
图 2 2009年2月26日 (a) 和3月1日 (b) 冰雹落区 (绿色) 天气系统配置图
(灰色：925 hPa切变和急流；红色：850 hPa切变和急流; 棕色：700 hPa切变和急流; 紫色：200 hPa急流; 急流后端的数值为急流核风速大小)
Fig. 2 Hailfall areas (green) on 26 Feb 2009(a) and 1 March 2009(b) with weather system configuration diagram
(gray: 925 hPa shear and jet; red: 850 hPa shear and jet; brown: 700 hPa shear and jet; purple: 200 hPa jet; figures denotes the velocity)
图 3 2009年2—3月25°N，110°E处风和相对湿度 (阴影) 随时间的演变 (a) 及2009年2月4日08：00—3月4日20：00 850 hPa平均风场和平均温度 (等值线，单位：℃)、平均相对湿度场 (阴影)(b)
Fig. 3 Time series diagrams for the wind at 25°N, 110°E and the relative humidity (the shaded area) during February—March 2009(a), and 850 hPa average wind and average temperature (contour, unit: ℃), average relative humidity (the shaded area) from 0800 BT 14 February to 2000 BT 4 March in 2009(b)
图 4 2009年2月26日20：00贵阳站 (a) 和长沙站 (b) 探空
Fig. 4 Soundings of Guiyang Station (a) and Changsha Station (b) at 2000 BT on 26 February 2009
图 5 2009年2月24日—3月4日20：00 700 hPa与500 hPa温度差 (单位：℃)(a) 和850 hPa与700 hPa风矢量差 (单位：m·s-1)(b)
Fig. 5 Temperature difference between 700 hPa and 500 hPa (unit: ℃)(a) and the value of wind vector difference between 850 hPa and 700 hPa (unit: m·s-1)(b) from 2000 BT 24 February to 2000 BT 4 March in 2009
图 6 2009年2—3月700 hPa与500 hPa θse差值分布 (单位：℃)
Fig. 6 Difference of θse between 700 hPa and 500 hPa during February—March in 2009(unit:℃)
图 7 2009年2月26日—3月3日MPV2分布 (单位：PVU)
Fig. 7 Distribution of MPV2 from 26 February to 3 March in 2009(unit: PVU)
图 8 2009年2月24日20：00—3月4日20：00平均θse沿110°E剖面 (单位：℃)(a) 及25°~30°N平均垂直速度沿110°E随时间分布 (单位：Pa·s-1)(b)
Fig. 8 Cross section of averaged θse (unit: ℃) along 110°E (a) and time series of 25°—30°N averaged vertical velocity (unit: Pa·s-1) along 110°E (b) from 2000 BT 24 February to 2000 BT 3 March in 2009
图 9 2009年2月26日14：00(a) 和2月26日20：00(b)θse(黑线，单位：℃)、垂直速度 (阴影)、流场 (流线) 沿110°E剖面
Fig. 9 Cross sections of θse(contour, unit: ℃), vertical velocity (shaded), and the flow field (stream line) at 1400 BT 26 February (a) and 2000 BT 26 February (b) in 2009 along 110°E
表 1 2009年2月24日—3月5日降雹站数
Table 1 The hailfalls observed from 24 February to 5 March in 2009
省份 02-24 02-25 02-26 02-27 02-28 03-01 03-02 03-03 03-04 03-05 贵州 1(14) 11(9) 7(8) 6(6) 18(8) 4(20) 湖南 1(3) 3(8) 10(6) 12(8) 2(3) 8(7) 3(4) 33(12) 3(4) 1(7) 湖北 1(3) 4(5) 4(5) 1(2) 江西 3(4) 3(3) 注：括号中为最大冰雹直径，单位:mm。 
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