Seasonal contrasting features of heat and moisture budgets between the eastern and western Tibetan Plateau during the GAME IOP

摘要

Using the Global Energy and Water Cycle Experiment (GEWEX) Asian Monsoon Experiment (GAME) four-dimensional data assimilation (4DDA) upper-air data, the large-scale heat source (Q_1) and moisture sink (Q_2) over the western and eastern Tibetan Plateau are examined for a 4-month period from 1 May to 31 August 1998. The computations were performed on the sigma-pressure hybrid coordinate, named eta-coordinate, since the analysis area includes high-elevation mountains. Over the western Tibetan Plateauin May, there is a deep layer of heating occupying the whole troposphere with the maximum value exceeding 3 K day~(-1) around 400-600 hPa. The smaller magnitude of the apparent moisture sink is confined in the lower troposphere 1 km above the ground surface. Vertically integrated heat sources of 103 W m~(-2) over the western Tibetan Plateau are accompanied by a moisture sink of about half that (60 W m~(-2)). These results indicate that the latent heat release associated with condensation plays an important role in the total heating besides the sensible heat supply from the land surface. Later in July, the moisture sink over the eastern Tibetan Plateau nearly equals the heat source indicating the dominance of moist processes associated with summertime monsoon rains. The contrasting features of the heat source and moisture sink are closely related to the circulation fields. Throughout May and June, we observe strong upward motion along the western and southwestern slopes of the western Tibetan Plateau,while there is salient subsidence motion over the eastern Tibetan Plateau. The analyses of static stability and lifting condensation level indicate that the release of latent heat relevant to moist convection is a dominant factor for tropospheric heatingafter the monsoon onset, while the premonsoon period (May) is composed of both convective rainfall and dry thermal convection. Thus, the heating mechanism prior to the onset of the monsoon, especially over the western Tibetan Plateau, can be characterized by the hybrid nature of "wet" processes due to condensation heating and "dry" processes associated with the sensible heat flux from the elevated mountain surface.