西藏那曲地区一次霰过程的大气边界层特征分析

Analysis of Atmospheric Boundary Layer Characteristics of a Graupel Process in Nagqu Region

本文利用探空气球加密观测资料和欧洲中心ERA-Interim 0.125°×0.125°再分析资料,对2016年8月29日午后降霰过程进行大气边界层特征分析,与同年8月26日典型晴天个例对比分析,结果表明:降霰过程前,温度0℃线随时间增加而升高,温度递减率分层现象显著,逆温层不明显,边界层多为对流不稳定层结;位温随高度增加而增加,随时间增加呈现5K·(2h)^(-1)的增加趋势;比湿随高度增加而减小,水汽含量较晴天更大;风速随高度呈多层次变化,近地层风速大于晴天同高度风速,边界层顶风速小于晴天边界层顶风速,风向始终以西风为主,随高度不存在大波动;降霰过程前云覆盖量大,云层厚度达4000m,存在复杂垂直运动,近地层为下沉运动,云层内为上升运动。综合以上可以看出那曲29日降霰过程前,08时边界层内存在明显过冷水,边界层顶波动极大,08时存在最大高度(3780m),10时为最低高度(850m)。位温随时间增加而上升,持续积累能量达6h,比湿大于晴天,边界层内风速大于晴天,且随高度变化不大,风向始终以西风为主,存在深厚的云系提供水汽,云内的上升运动和云下的下沉运动是促发霰过程的主要动力机制。

In this paper, the characteristics of atmospheric boundary layer for the afternoon graupel process on 29 August 2016 are analyzed by making full use of densified observation data of sounding balloon and reanalysis data from ECMWF＇s ERA-Interim 0.125°×0.125°and comparing to the typical sunny cases on 26 August 2016. The results show that the temperature zero line increases with time and the stratification of temperature lapse rate is remarkable. The inversion layer is not obvious and the boundary layer has more unstable convection stratifications. Potential temperature increases with height and it presents an in- creasing trend of 5 K · （2 h） ^-1 with time. Specific humidity decreases as height increases and the content of water vapor is greater than that in sunny days. Wind speed varies with height. The wind speed in the near surface layer is higher than that at the same altitude in sunny days and the top wind velocity of bound- ary layer is less than that of sunny boundary layer. The wind direction is always dominated by west wind and it does not fluctuate greatly with height. Cloud coverage before graupel is large and cloud thickness reached 4000 m, there are complex vertical motions. The surface layer is descending while clouds are ascending. There is obvious supercooled water in the boundary layer at 08.00 BT before graupel on 29 Au- gust in Nagqu, and the top of the boundary layer fluctuates enormously. The maximum height of 3780 m exists at 08.00 BT while the minimum height of 850 m exists at 10：00 BT. The potential temperature in- creases with time and the accumulated energy lasts for 6 h. The specific humidity is greater than that in sunny days. The wind speed in boundary layer is larger than that of the sunny days, varying little with altitude and the wind direction is always dominated by west wind. There is deep cloud system which provides moisture. The ascending motion in clouds and descending motion under the clouds are main dynamic mechanism to promote graupel.