藏北地区午后热对流天气过程特征

利用常规气象观测资料、NCEP及ERA-interim再分析资料，对那曲市2017~2019年6~8月发生的热对流降水进行统计分析，结果表明：近三年出现了27d的热对流天气，午后热对流降水最集中的时段是15~17时；出现热对流天气时最高气温（Tmax）、最高地温至少分别达到14.2℃、32℃，日最高气温、最高地温与08时的数值相比较，平均高出10.4℃、37.3℃；从对流温度（Tg）看，当Tmax-Tg处于（1.5~5.5）℃区间时，出现热对流的频率最高，当Tmax-Tg值<1℃或>8℃时，出现热对流的概率为0，并非Tmax-Tg值越大，热对流发生的概率就越大；水汽条件上，08时平均湿层在460hPa以下，具有上干下湿的特性；通过订正Tlog-P曲线得到，发生热对流时CAPE值最少要达到800J·kg⁻¹；从个例合成分析得出，对流发生前那曲市处于高压控制或晴空少云状态，午后有辐合上升运动，低层的辐合上升引起周围水汽的汇聚；水汽的来源是前期平流输送或是地表蒸发有待于进一步研究。

Characteristics of Afternoon Thermal Convective Weather Process in Northern Tibet

With the conventional meteorological observation data, NCEP and ERA-interim reanalysis data, the thermal convective precipitation in Nagqu City from June to August 2017~2019 is statistically analyzed. The results show that in the past three years, there have been 27 days of thermal convective weather, and the most concentrated period of thermal converction precipitation in the afternoon is from 15 to 17 o'clock. When thermal convection weather occurred, the maximum temperature (Tmax) and maximum ground temperature reached at least 14.2℃and 32℃ respectively. The daily maximum temperature and maximum surface temperature were 10.4℃ and 37.3℃ higher than those at 8:00 a.m. Considering the convection temperature (Tg), the frequency of thermal convection is the highest while the value of Tmax-Tg is between 1.5℃ and 5.5℃, and the probability of thermal convection is 0 when the value is less than 1℃ or greater than 8℃. Therefore, it is not that the larger the Tmax-Tg value is, the greater the probability of heat convection occurs. In terms of water vapor conditions, the average wet layer at 8:00 a.m. is below 460hPa, with the characteristics of being dry at the top and wet at the bottom. By correcting the Tlog-P curve, the CAPE value should reach at least 800J·Kg−1 when the thermal convection occurs. From the synthetic ananlysis of individual cases, it is concluded that Naqu City is in a state of high pressure control or clear sky with few clouds before the convection weather occurs, and there is convergence and upward movement in the afternoon. The convergence and upward movement of the lower level will result in the condensation of surrounding water vapor. The source of water vapor whether is derived from advection transport or surface evaporation remains to be further studied.