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

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 surface-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 7.9×106 m3·s-1, equivalent to around 38 times of the enclosed 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7×106 m3·s-1) in the SASM break period and weak transfer (air volume flux of 6.6×106 m3·s-1) in the SASM active period; 3) the SASM can possibly affect the Himalayan surface-to-air exchange mainly by changing the local radiation conditions.