青藏高原雷暴天气层结特征分析

Analyses on Atmospheric Stratification Characteristics of Thunderstorms over Qinghai-Xizang Plateau

青藏高原那曲地区夏季雷暴活动相当频繁,这种雷暴主要是受地形的影响,在地形的热力和动力作用下形成雷暴,但强度不大,最大反射率一般不超过 40 dBz,相对云顶高度可伸展到10.0～ 13.0 km,强弱雷暴差别不大.雷暴持续时间大约为30 min左右,主要发生在13:00～19:00(北京时,下同)之间,峰值出现在16:00左右.此外,在晚上也有弱对流,最大反射率约为20 dBz.高原雷暴天气层结具有与平原雷暴完全不同的特征,一般为整层弱不稳定,高度可以伸展到100 hPa,整层不稳定能量不大,强雷暴CAPE值平均为782 J·kg-1,弱雷暴约为406 J·kg-1,分布较均匀,不出现能量特别大的不稳定层次.近地层相对湿度有'逆湿'现象,厚度约1～2 km,平均为60%～80%(雨季后).无论是强雷暴天气还是弱雷暴天气都具有上述相似层结.这种层结可触发对流,发展高度很高,但强度不大,能量较小.这种特殊层结揭示了高原雷暴的特殊结构.雷暴的闪电频数可以表征雷暴发展强度,通常可以建立闪电频数与雷暴单一参量(云顶高度)之间的统计关系式,从而可以利用测量闪电频数来预报雷暴的强弱,但上述关系对于高原雷暴并不适用,必须建立闪电频数与多参量之间的综合关系.

The thunderstorm activity is very frequent in summer in Naqu district of Qinghai-Xizang Plateau. It occurs in the consequence of thermal and dynamic effects topographically. The storms are usually not very severe, with the maximum radar reflectivity not exceeding 40 dBz in general. The relative cloud top can extend to 10. 0-13.0 kin, with little differences between severe and weak thunderstorms. The thunderstorm occurs usually at 13.00-19：00 Beijing time, the peak value appears at 16：00 or so, and the duration is about 30 minutes. Weak convection occurs sometimes in nighttime with maximum reflectivity of about 20 dBz. The atmospheric stratification of the Plateau storm is totally different from that of plain storm. The thunderstorm on plateau is usually instable on whole layers, up to the height of 100 hPa, and with low instable energy. The CAPE value of severe thunderstorm is about 782 J · kg^-1 on average and that of weak 406 J · kg^-1 approximately. The CAPE is homogeneously distributed without instable stratification .of large energy. The inverse humidity occurs near ground with thickness about 1-2 km and the value of 60%-80% on average （after the rainy season）. The atmospheric stratification forementioned is similar for both the heavy and weak thunderstorms; it can stimulate convection with large developing height but little intensity and energy. This special stratification revealed the particular structure of thunderstorms on plateau. The developing intensity of thunderstorm is characterized by its lightning frequency. Usually a statistical dependence can be established between the lightning frequency and the singular parameter of thunderstorms such as the height of cloud top; whether the thunderstorm is heavy or weak can be predicted by measuring the lightning frequency. However it is unsuitable for thunderstorms on plateau; a synthetic relation has to be established between the lightning frequency and the multi-parameters of thunderstorm.