拉萨地区夏季夜间雷暴的物理量指数分析

Physical Index Analysis of Summer Night Thunderstorms in Lhasa Area

青藏高原的大范围热力、动力作用,致使高原地区空气不稳定,也导致高原夏季雷暴活动非常频繁,成为北半球同纬度地带雷暴数最多的地区。利用2004—2009年常规地面观测资料和探空资料,对拉萨地区夏季夜间雷暴的时间分布特征和背景场特征的分析结果显示:1)拉萨地区的雷暴主要集中在14:00—23:00,其中17:00—20:00雷暴次数最多,超过总雷暴次数的60%。2)从20时整层大气特性上看,20:00—02:00时(次日)发生雷暴时,整层大气的湿度、假相当位温(θse)比不发生雷暴时的大,在400 h Pa层差别最明显;两者的温度差异并不明显。分析抬升凝结高度(LCL)、400 h Pa的θse、修正的KI、SI指数和300 h Pa到400 h Pa的平均垂直风切变(MS)对雷暴的指示作用,结果显示:1)各指数都能较好地区分雷暴日与非雷暴日。2)各指数的TSS预报评分由高到低依次为:400 h Pa的θse>352.1 K,TSS评分为0.401;修正的KI>15.7℃,TSS评分为0.373;修正的SI<3.20℃,TSS评分为0.288;LCL>441.4 h Pa,TSS评分为0.276;MS<1.87 m·s-1·km-1,TSS评分为0.201。400 h Pa的θse和修正的KI指数对拉萨夜间雷暴具有较好的指示意义。与稳定度有关的参数比与风切变有关的参数的指示意义好。

The large scale thermal and dynamic effects of Qinghai Tibet plateau result in the unstable air and frequent summer thunderstorm activities in plateau area. The area of Qinghai Tibet plateau becomes the most thunderstorms area in the same latitude zone in the northern hemisphere. Using the surface observational and radiosonde data from 2004--2009, this paper analyzed temporal distribution and background characteristics of summer night thunderstorms in Lhasa area. The results showed as follows. 1 ） Thunderstorms in Lhasa area mainly occurred during 14：00--23：00, while the period of 17：00--20：00 had the most numbers with more than 60% of the total thunderstorms. 2） In terms of the characteristics of the whole layer atmosphere, when the thunderstorms occurred during 20：00---02：00, the layer＇s humidi- ty ratio and pseudo-equivalent temperature （θse） at 20：00 was higher than that without thunderstorms, especially at 400 hPa layer. But there was no obvious temperature difference between them. The indicative functions of some indexes to thunderstorm were also analyzed, including the lifting condensation level （LCL） , θse in the 400 hPa layer, modified KI, SI indexes and average vertical wind shear index（MS） between 300 hPa to 400 hPa. The analysis results showed as follows. 1 ） Each index could well distinguish the thunderstorm days and non-thunderstorm days. 2） TSS score of each forecast index from high to low was followings ： θse 〉 352.1 K in the 400 hPa layer, with 0. 401 TSS score ; the modified KI 〉 15.7 ℃, TSS score was0.373; the modified SI〈3.20 ℃, TSS score was 0.288; LCL 〉441.4 hPa, TSS score was 0. 276 ; MS 〈 1.87 m · s ^- 1 · km ^- 1, TSS score of 0. 201. θse in the 400 hPa layer and modified KI index had better indication to night thunderstorm in Lhasa area. Stability parameters had better indicative significance than wind shear parameters to night thunderstorm.