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.

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[1] This paper studies climatically important low clouds (with cloud top below 680 hPa) over the Arabian Sea using the Kalpana-1 satellite data and International Satellite Cloud Climatology Project cloud data. Characteristics and possible mechanisms behind low-cloud formation in the summer monsoon season are presented. The dominant lower tropospheric circulation over the Arabian Sea during the summer season is the monsoon low-level jet (LLJ). Low clouds are predominantly found on the exit portion of the LLJ with decelerating winds. We postulate that the moisture carried by the LLJ from the entrance region with accelerating winds converge at the exit region with decelerating winds. This low-level convergence (observed~925 hPa level) may be conducive for moisture convergence, parcel uplifting, and low-cloud formation. These clouds are unable to grow vertically due to the presence of lower tropospheric thermal inversion. Change in the spatial extent of low-level convergence is found to influence the spatial coverage of the low clouds. It is found that low cloud cover is inversely related to the Indian summer monsoon activity. It is speculated that the changes in the strength of the lower tropospheric stability and spatial extent of convergence may be possible causes behind the observed inverse relation between low cloud cover and monsoon activity. Based on Clouds and the Earth's Radiant Energy System data, we find that these low clouds exert a peak cooling of about À55 Wm À2 .

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Examination of the Global Precipitation Climatology Project (GPCP) and Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) rainfall datasets, both available for the past 31 years (1979-2009), suggests that rainfall has increased considerably over the premonsoon rainfall belts of the Bay of Bengal (BoB) and Southeast Arabian Sea (SEA) during May. The rainfall rate has increased by 83.9% (94.5%) over SEA and 54.6% (19.6%) over BoB in the GPCP (CMAP) dataset during the study period. During the study period, the atmospheric temperature has increased over north/northwestern parts of the Indian summer monsoon region in May, which has possibly caused more rain by enhancing the meridional pressure gradient.