Influence of the Upper-Tropospheric Wind Shear upon Cloud Radiative Forcing in the Asian Monsoon Region

Abstract

Using the top-of-the-atmosphere radiative flux and cloud data from satellites, as well as atmospheric data from NCEP?NCAR reanalysis, this paper investigates the reason for the unusually large high-cloud amount in the Asian monsoon region during the summer monsoon season (June?September). Earlier studies attributed the large negative net cloud radiative forcing in the Asian monsoon region to the unusually large high-cloud amounts with high optical depth. Analysis during 1985?89 suggests that the unique upper-tropospheric easterly wind shear [tropical easterly jet (TEJ)], present over the Asian monsoon region during the summer monsoon season, may be responsible for the unusual increase in cloud amount. This strong wind shear sweeps the cloud tops and may be unfavorable for cloud growth beyond about 300 hPa. The spreading of cloud tops by wind may increase the high-cloud amount. A significant association is found between the high-cloud amount and the speed of the easterly jet. In addition, magnitudes of the shortwave, longwave, and net cloud radiative forcing also strongly depend upon the variations in the speed of TEJ.