No abstract

ABSTRACT

The differences in the thermodynamic structure of the atmosphere over India and the adjoining areas between active and weak monsoon (southwest) periods in the various meteorological sub-divisions are presented.                  The mean dry bulb and dew point remperatures and equivalent potentioal temperatures during active and weak monsoon were calculated at all the radiosonde stations in India and neighbourhood and studied. The results show that whatever be the monsoon activity, there is no significant change in the dry bulb temperature at any level and the moisture content in the lower tropospheric levels remains high without any appreciable variation. The main change is in the moisture content in the mid-tropospherica levels. The implication of these results in the vertical circulation associated with active and weak monsoon and in the radiation budget are brought out, since the vigorous ascending air in the areas of strong monsoon ultimately subsides slowly over the regions of weak monsoon. The region of the seasonal monsoon trough has been indentified as the area of maximum total energy both in active and weak monsoon is least. This is also the area where convective instability is present upto greater heights than elsewhere. The estimated heights of cb tops are highest in the region of the seasonal monsoon trough. It is also shown that there is no reversal of virtual temperature gradient over the northwest India in the midtropospheric levels during the SW monsoon season.

The results of 66 upper air soundings made over India during 1963 and 1964 with Suomi-Kuhn radiometersondes show that, although significant day-to-day and seasonal variations in the upward, downward, and net infrared radiative fluxes in the atmosphere over Poona occur, the mean values for the winter, summer, and monsoon months show remarkable features in common: an increase in the net upward flux with height, with a maximum at about 12 km, and a decrease at the tropopause followed by an incr.ease in the stratosphere. Steady values of the order of 0.35 ly/min are reached at about 25 km in the winter and summer seasons when the sky is clear, in good agreement with satellite observations of net infrared flux over the tropics. During the cloudy monscon months the net flux is much less, 0.22 ly/min, and the net loss to space is reduced by over 50%. Mean radiative cooling in the troposphere is of the order of I to 2øC/day up to 200 mb, with pronounced maximums at 3, 6, and 10 km and a marked warming just below the tropopause. Radiative cooling in the troposphere during the clear seasons is about 50% more than that during the cloudy months except over cloud decks. The effect of dust in the lower layers is conspicuous during the summer months. Tropospheric warming immediately below the tropopause is largest during the monsoon and least during winter. Stratospheric cooling is large and variable and in the mean about 5 times that in the troposphere. RADIATIVEFLUXES OVER INDIA 4533 426, 1960. Goody, R. M., Atmospheric Radiation, I, Theoretical Basis, Clarendon Press, Oxford, 271, 1964. Houghton, J. G., On the annual heat balance of the northern hemisphere, J. Meteorol., 11, 1-9, 1954. House, F. G., and J. R. Blankenship, Applications of infrared measurements in meteorology, Tech. Rept. 157, Air Weather Service, Scott AFB, Ill., 1961. Kuhn, P.M., Accuracy of the airborne economical radiometer, Monthly Weather Rev., 89, 285-288, 1961. Kuhn, P.M., and V. E. Suomi, Infrared radiometer soundings on a synoptic scale, J. Geophys. Res., 65, 3669-3677, 1960. Kuhn, P.M., V. E. Suomi, and G. C. Darkow, Soundings of terrestrial radiation flux over Wisconsin, Monthly Weather Rev., 87, 129-135, 1959. London, J. A., A study of the atmospheric heat balance, Department of Meteorology, New York University, New York, 1957. Mani, A., and O. Chacko, Measurements of solar radiation and atmospheric turbidity with strSm pyrheliometers at Poona and Delhi during the IGY, Indian J. Meteorol. Geophys., 14, 270-282, 1963a. Mani, A., and O. Chacko, Studies of nocturnal radiation at Poona and Delhi, Indian J. Meteorol. Geophys., 14, 196-204, 1963b. Riehl, H., Radiation measurements over the Caribbean during the autumn of 1960, J. Geophys. Res., 67, 3935-3942, 1962. RSnicke, G., •ber Messungen des Verlaufs der Warmestrahlungsbilanz in der freien Atmosphare in San Salvador, Z. Meteorol., 15, 8-12, 1961. A direct measurement of infrared radiation divergence to 160 mb, Quart. J. Roy. Meteorol.

Radar observations of monsoon rain over Poona on the lee-side of the Western Ghats have frequently shown the presence of 'melting bands'. On the windward side on the west coast however, the rain echoes generally appear much below the freezing level. The paper describes an unusual observation of a 'melting band' noticed at Alibag on the west coast of the peninsula during the southwest monsoon.