喜马拉雅山地区地气间物质交换及其与南亚夏季风的联系

The Air Mass Exchange between the Surface and Free Atmosphere and Its Relationship with the South Asian Summer Monsoon

喜马拉雅山是青藏高原的重要组成部分,是大型山地的代表。该地区陡峭的地形和复杂的地表状态在强烈太阳辐射条件下形成了特殊的局地环流系统。该特殊局地环流系统所导致的地气交换过程可能不同于平坦地形和其他山区。喜马拉雅山毗邻南亚季风区,该地区的地气交换过程也可能受到南亚季风活动的影响。为正确认识喜马拉雅山地区的地气交换过程,利用2006年夏季喜马拉雅山北侧绒布河谷地区获得大气观测资料,结合同期的大气环流资料,对绒布河谷地区地气间的物质交换进行研究。结果表明:1)该地区的地气间物质交换以午后至次日凌晨强烈的向下输送为主;观测期间平均向下输送强度为7.9×106m3.s-1,相当于每天将整个封闭河谷内大气置换约38次;2)该地区地气间的物质输送与南亚夏季风存在密切关系,南亚夏季风活动弱(季风中断期)则物质交换量大(9.7×106m3.s-1),季风活动强(季风活跃期)则物质交换量小(6.6×106m3.s-1);3)南亚夏季风可能主要通过改变局地大气辐射状况进而影响该地区地气间的物质交换过程。

The Himalayas is an important area of the Tibetan Plateau, and is also a typical large-scale mountain region. The strong solar irradiance under the steep topography and complex terrains of the Himalayas introduces a distinguished local circulation system. The distinct mountain system could cause a different surface-to-air exchange process from that over the plateau flat areas and other mountain regions. Besides, the Himalayas is adjacent to the South Asian monsoon regions, the air mass exchange between the surface and free atmosphere could also be affected by the monsoon evolution. In this study, the surfaee-to-air mass exchange in the Rongbuk Valley on the north side of the Himalayas is analyzed, based on the observational and large scale grid data in June 2006. Results show that： 1） The surface-to-air exchange is dominated by the strong downward transfer from afternoon to early morning. The averaged downward transfer of the air volume flux is 7.9×10^6m3·S-1, equivalent to around 38 times of the en- closed valley volume; 2） the surface-to-air mass exchange is closely related to the South Asian summer monsoon （SASM） activities, with strong air mass transfer （air volume flux of 9.7×10^6m3·S-1 ） in the SASM break period and weak transfer （air volume flux of ×10^6m3·S-1） in the SASM active period; 3） the SASM can possibly af- fect the Himalayan surface-to-air exchange mainly by changing the local radiation conditions.