Observational Study of Heat Sources Over the Eastern Tibetan Plateau during the Summer Monsoon

Abstract

Spatial and temporal variations of the heat source over the eastern Tibetan Plateau are analyzed based on the FGGE Level II-b upper-air observation data during the Summer MONEX in 1979. The budget results of mass, heat and moisture obtained over the four divided subareas show that the upward mass circulation, large-scale apparent heat source and moisture sink predominate in the eastern Plateau as a whole, but there exist considerable spatial and temporal variations.Over the Plateau region higher than 3000m surface elevation, the maximum heating of about 4°C d-1 exists around 400mb and both the effects by the heat supply from the elevated surface and the latent heat release contribute nearly equally to the net total heating. It is suggested from the moisture budget analysis that the water vapor lost by the condensation over the Plateau is generally balanced by the surface evaporation and that the effect of moisture convergence by large-scale motions is rather small. On the contrary, the low-level southwesterly monsoon flow brings a lot of water vapor and heavy precipitation occurs over the southern part of the Plateau including the Assam region. Large amounts of the condensation heating are liberated in the whole troposphere with the maximum near 400mb. Small amounts of heating and relatively large amounts of heating are observed over the northern and eastern slopes of the Plateau, respectively.It is found that there exist large diurnal variations of the upward motion and the heat source over the eastern Tibetan Plateau except in its northern part. Larger upward motion and heat source are found at 1200GMT (-1800LT) than at 0000GMT (-0600LT). Amplitudes of the diurnal variations are 1mb h-1-2mb h-1 for the vertical p-velocity and 1°C d-1-2°C d-1 for the heat source, respectively. These diurnal variations over the Plateau might be generated by the intense convective activity enhanced by the solar heating absorbed on the elevated Plateau surface during the daytime.Total heating, precipitation and relative vorticity in the upper troposphere averaged over the eastern Plateau have long-period fluctuations with the periods of 10-15 days and -30 days. Large heating corresponds to large amounts of precipitation and strong upperlevel anticyclonic circulation. Time variations of the heat source and precipitation over the eastern Plateau show general out of phase relationships with the precipitation over the central India. The long-period fluctuations of the heat source over the Tibetan Plateau appear to be a part of the oscillation of the whole summer monsoon activity.