The Large-Scale Circulation and Heat Sources over the Tibetan Plateau and Surrounding Areas during the Early Summer of 1979. Part II: Heat and Moisture Budgets

Abstract

The large-scale heat and moisture budgets over the Tibetan Plateau and surrounding area during a 40-day period from late May to early July 1979 are studied using the FGGE Level II-b data. During this period the general circulation over East Asia underwent a distinct seasonal change characterizing the onset of the summer monsoon circulation. The analyses of the horizontal distributions of the vertically integrated heat source and moisture sink reveal the major heat source regions and their different degrees of association with precipitation. The 40-day mean distributions show intense heat sources of 150?300 W m?2 with moisture sinks of nearly equal magnitude over the Assam?Bengal region and in a broad belt extending over the China Plain along the Mei-yü front. The heat source of ?100?150 W m?21 over the eastern Tibetan Plateau is accompanied by a moisture sink with a magnitude about half as large. The heat sources over the western Plateau and the Takla Makan Desert are not accompanied by appreciable moisture sinks. The heat sources over the Plateau are pronounced in the upper troposphere. The mean heating rate of ?3 K day?1m in the 200?500 mb layer above the eastern Plateau is as intense as that over the Assam?Bengal region. Examination of the daily vertical profiles of the areal mean heat source and moisture sink shows distinct differences in the heating process and its time change over the western Plateau and adjacent areas (Region I), the eastern Plateau (Region II), the Yangtze River (Region III) and the Assam?Bengal region (Region IV). In Regions I and II heating in the troposphere is seen prior to the onset of the summer rains. The heating in Region II intensifies with the progress of the rain season. The heating in Region III is primarily due to frontal rains and that in Region IV is due to highly convective monsoonal rains. In Region I the mean sensible heat flux at the surface during the pre-onset period is ?170 W m?2, while in Region II the mean sensible heat flux for the total period is ?105 W m?2. The corresponding values of the condensation heating are ?10 and ?70 W m?2, respectively. The estimated mean evaporation rates on the Plateau for the total period are ?1 mm day?1. These estimates compare well with the June mean values obtained by Yeh and Gao from surface observations. There is a pronounced diurnal change of the surface air temperature on the Plateau, and a deep mixed layer with very high potential temperature is observed in the evening (1200 GMT). It is suggested that dry thermal convection originating near the heated surface in the afternoon hours is responsible for the deep tropospheric heating over the Plateau during the pre-onset phase. Above the eastern Plateau this heating mechanism is replaced by the condensation heating associated with cumulus convection after the onset of the summer rains.