The Moisture Budget over the Northeastern Arabian Sea during Premonsoon and Monsoon Onset, 1979

Michael R. Howland Atmospheric Sciences, University of Wisconsin, Milwaukee, WI 53211

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Dhirendra N. Sikdar Atmospheric Sciences, University of Wisconsin, Milwaukee, WI 53211

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Abstract

Moisture budgets are calculated for premonsoon and monsoon onset conditions in the northeastern Arabian Sea during summer 1979 from kinematic analysis of aircraft dropsonde, ship and island radiosonde, and satellite-derived winds. Dramatic changes are observed between the premonsoon and monsoon onset mean kinematic and moisture fields. Specific humidity increased as much as 5 g kg−1 in much of the middle troposphere between 29 May and 17 June 1979. This is apparently due to deep convection during the monsoon onset period and mid-level advection of moisture during the premonsoon period. Flux of moisture through the budget boundaries is comparable to previous estimates for the Arabian Sea. It is shown that the loss of moisture through cirrus outflow accounts for only 1–3% of the total budget flux. Evaporation from the sea surface is 3 to 4 times higher during the onset period and was greatest south of 12°N. Maps of precipitation as a residual of the moisture budget computations agree remarkably well with convective features seen in satellite imagery. During the monsoon onset period, rainfall averaged about 1 mm h−1 over the entire budget area. In order to test the validity of the combined data base and moisture budget computation two independent estimates of precipitation were made using a Krishnamurti et al. parameterization scheme and the Stout et al. satellite technique. Both showed good agreement to the budget results.

Abstract

Moisture budgets are calculated for premonsoon and monsoon onset conditions in the northeastern Arabian Sea during summer 1979 from kinematic analysis of aircraft dropsonde, ship and island radiosonde, and satellite-derived winds. Dramatic changes are observed between the premonsoon and monsoon onset mean kinematic and moisture fields. Specific humidity increased as much as 5 g kg−1 in much of the middle troposphere between 29 May and 17 June 1979. This is apparently due to deep convection during the monsoon onset period and mid-level advection of moisture during the premonsoon period. Flux of moisture through the budget boundaries is comparable to previous estimates for the Arabian Sea. It is shown that the loss of moisture through cirrus outflow accounts for only 1–3% of the total budget flux. Evaporation from the sea surface is 3 to 4 times higher during the onset period and was greatest south of 12°N. Maps of precipitation as a residual of the moisture budget computations agree remarkably well with convective features seen in satellite imagery. During the monsoon onset period, rainfall averaged about 1 mm h−1 over the entire budget area. In order to test the validity of the combined data base and moisture budget computation two independent estimates of precipitation were made using a Krishnamurti et al. parameterization scheme and the Stout et al. satellite technique. Both showed good agreement to the budget results.

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