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

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, Δθ_se700-500 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, Δθ_se700-500 > 0℃ or T_700-500≥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.