Characteristics of low clouds over the Arabian Sea

Abstract

AbstractThis 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 −55 Wm−2.