锋面北侧冷气团中连续降雹环境场特征及成因

Environment Characteristics and Causes of a Continuous Hail Fall Event Occurred Within the Cold Air Mass to the North of a Cold Front

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。

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-A- sian, 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 air- flow 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~C 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~C, and the temperature lapse rate is about 0.74~C/100 m. The height of the 0~C 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, A0sez00-500 is 2--6~C mostly. The minus ba- roclinic term of moist potential vorticity increases absolutely, which induces the southward expansion of the negative MVP value area. Meanwhile, MVPI, 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 insta- bility 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 tempera- ture variation or a preceding temperature trough. There is a melting layer in 700--850 hPa, and the thiek- hess of upflow is small, therefore, the diameters of this kind of hail are less than 10 ram, few of them are 10--20 mm. The potential forecast index and criterion of this kind of hail failing area includes strong horizontal temperature frontal area, △θse700-500〉0℃ or T700-500≥16℃, axis of 700 hPa is more than 20 m·s-1 south- west airflow, area of strong 700--850 hPa vertical wind shear and 500 hPa shortwave trough.