那曲市历年冰雹日数变化特征及冰雹、雷暴环境条件分析

Variation Characteristics of Hail Days and Analysis of Environmental Conditions of Hails and Thunderstorms in Nagchu City

利用那曲市7个国家气象观测站建站至2015年的冰雹日数资料,分析了那曲市冰雹日数时空分布特征及气候突变情况,并分析了2012—2016年色尼区单站冰雹日、雷雨日个例,揭示了色尼区冰雹、雷暴发生前的环境条件。结果表明:那曲市年平均冰雹日数呈减少趋势,减少趋势最明显的是申扎县,每10 a减少4. 9天,减少趋势最弱的为色尼区和班戈县,每10 a减少1. 9天,减少趋势均通过了0. 01的显著性检验;不同区域各代表站均显示减少趋势且出现突变现象;那曲市冰雹天气具有明显的季节特征,大多集中在6—9月;降雹时间多集中在14—20时,占全天的78%。对那曲市色尼区冰雹日和雷雨(无降雹)日个例分析表明,高原较强对流发生在0℃层高度低(多在1.5 km高度以下)、CAPE值小(多在500 J·kg^(-1)以下)、风垂直切变弱的环境下,与平原地区大冰雹环境差异显著,因而高原冰雹绝大部分为直径5 mm以下的小冰雹,严格意义上应称为霰;雷雨天气低层较雹日的更暖湿,其温度露点差略小于雹日的,露点比雹日的高,因而雹日抬升凝结高度高于雷雨日的。

The temporal and spatial distribution characteristics and climate abrupt conditions of hailstone days in Nagchu of Tibet Autonomous Region are investigated by hailstone days records at 7 national meteorological stations from construction station period to 2015. The environmental parameters of hailstorm and thunderstorm in Seni district are analyzed by the weather cases between 2012 and 2016. The results show as following. There is an overall decreasing trend in the hail days in Nagchu. The most obvious decreasing trend is in Shenzha county with a decreasing rate of 4.9 days per decade and the least decreasing trend is in Seni district and Ban’ge district with a decreasing rate of 1.9 days per decade. The decreasing trends both pass the significance test of 0.01.There are decreasing trends and abrupt phenomenon in all representative stations of different regions. There are obvious seasonal characteristics of hail weather in Nagchu city,most occurring during the period June September. Hail occurs mostly between 14：00 and 20：00,which accounts for 78% of hail in a day.The analysis of hail and thunderstorm days in Seni district,Nagchu city shows that the convective weather of Tibet occurs in the environmental conditions of low freezing level （under 1.5 km）,small CAPE （less than 500 J·kg -1 ） and weak vertical wind shear,which is significantly different from the environmental conditions of the hail in the plain area,so most of the hailstone in plateau is in small size （diameter below 5 mm）,which should be called graupel. It’s warmer and more moist in the rainstorm days than in the hail days,the difference between temperature and dew-point temperature are smaller,while dew-point temperature are higher in rainstorm days than in hail days,so the lifting condensation level are higher in hail days than in rainstorm days.