Heating processes over the Asiatic landmass prior to the first transition of the summer monsoon
Hiroaki Ueda (1)
(1) University of Tsukuba
Using the GAME (GEWEX Asian Monsoon Experiment) 4DDA upper-air data, the large-scale
heat source (Q1) and moisture sink (Q2) 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 Ć-coordinate, since the analysis
area includes high-elevating mountains.
Over the western Plateau in May, there is a deep layer of heating occupying the whole
troposphere with the maximum value exceeding 3K 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 Wm-2 over the western Tibetan
Plateau are accompanied by a moisture sink of about half that (60 Wm-2). These results
indicate that the latent heat release associated with the 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 to the heat source indicating
that the dominance of moist process 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 with
the western and southwestern slopes of the western Plateau, while there is salient subsidence
motion over the eastern Plateau. The analyses of the static stability and the lifting
condensation level indicate that the release of latent heat relevant to the moist convection is
a dominant factor for the tropospheric heating after the monsoon onset. @ While the pre-
monsoon period (May) is composed of both the convective rainfall and the 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 gweth processes due to
condensation heating and gdryh processes associated with the sensible heat flux from the
elevated mountain surface.