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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.

[1] Analysis of the 41-year (1958-1998) reanalysis wind data from National Centers for Environmental PredictionNational Center for Atmospheric Research (NCEP-NCAR) and European Centre for Medium-Range Weather Forecasts (ECMWF) suggest that zonal extent of the Tropical Easterly Jet (TEJ) has reduced between 1960s and 1990s particularly over the Atlantic and African regions. The decrease in NCEP-NCAR reanalysis is more than the decrease in ECMWF reanalysis. The decrease is about 41% and 20% at 100 hPa and 52% and 30% at 200 hPa in NCEP-NCAR reanalysis and ECMWF reanalysis respectively. It is argued that the weakening and disappearing of TEJ over the Atlantic and African regions and prolonged drought conditions over the Sahel region during the past 4 -5 decades may be associated with each other.

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An algorithm for night time fog detection using satellite data was used to study 2009 and 2010 fog episodes over the North Indian plains. The algorithm employs a bispectral thresholding technique involving brightness temperature difference (BTD) between two spectral channels: 3.9 and 10.75 µm, combining radiative transfer simulations and satellite data. The prolonged fog episode of January 2010 was analysed in detail using this algorithm in conjunction with the meteorological parameters and it was found that upper tropospheric long waves passed over this region during the fog episode. These waves affect the circulation close to the surface and appear to have played an important role in the formation and persistence of fog during 2010. The present thresholding method can be used to monitor night time fog over the Indian subcontinent on an operational basis using the forthcoming geostationary satellite INSAT-3D.