Orographic Influence of the Tibetan Plateau on the Asiatic Winter Monsoon Circulation Part II. Diurnal Variations1

Abstract

Some of the characteristic features of diurnal variations over and around the Tibetan Plateau were investigated by using twice-daily wind, temperature and geopotential height data at eight standard pressure levels for the period from 1 December 1978 through 28 February 1979. These standard pressure level data were then used to compute twice-daily pressure, temperature and wind data over the smoothed topographic surface determined by Berkofsky and Bertoni (1955). At the surface, winter mean 12-hr difference, (12-00 GMT) wind vectors are directed northward and flow up the southeastern slope of the Himalayan massif. The higher elevations of the Tibetan Plateau are characterized by convergent surface wind differences, and positive (negative) 12-hr difference surface temperatures (pressures). Equatorward of about 25°N over the Arabian Sea, Bay of Bengal and the South China Sea, (1) vertical profiles of winter mean 12-hr temperature differences exhibit wavelike fluctuations with amplitudes (wavelengths) on the order of 0.5°(5-10km), and (2) hodographs of wind difference vectors clearly display clockwise rotation with height. These features resemble the structures of planetary-scale "vertically propagating" tidal oscillations. Over the Tibetan Plateau, (1) winter mean temperature differences decrease gradually with increasing height which is an indication of deep penetration of the diabatic processes (eddy sensible heat fluxes and longwave emission) operating within the planetary boundary layer, and (2) 12-hr difference wind holographs vary markedly from one geographical location to another, implying large local orographic influence. Near Assam, a pronounced meridionally-oriented, diurnal vertical circulation is induced with terrain slope and high-low land contrasts as a controlling influence. Here, diurnal upward motions forced by up-slope southerly winds are associated with positive 12-hr difference temperatures throughout the troposphere, indicating diurnal variations of local kinetic energy generation. These conversion processes are far more pronounced at 12 GMT (early evening) than at 00 GMT (early morning). In the lower (upper) troposphere equatorward of about 30°N, horizontal sensible heat fluxes due to winter mean diurnal oscillations are down (up) the gradient of the winter mean temperature fields. At 300mb over central India, the magnitude of sensible heat flux convergence associated with winter mean diurnal variations corresponds to a rate of tempera ture change of about 2°C per 10 days.