Interaction of Low-Level Flow with the Western Ghat Mountains and Offshore Convection in the Summer Monsoon

Abstract

Seven-year averaged values of percent frequency of occurrence of highly reflective cloud for the months June, July, and August indicate that offshore convection is a major component of the cloudiness of the southwest monsoon. Principal areas of convection occur off of the western coats of India, Burma, Thailand, and the Philippines. This study concentrates on the area upstream of the Western Ghats Mountains of India. Analysis of a special boundary layer mission flown during the WMO/ICSU Summer Monsoon Experiment leads us to believe that partial deceleration of the monsoon flow by upstream blocking effects of the mountains initiates and maintains a vertical and horizontal motion field that could support the observed convection. Data obtained on this mission allow a large-scale momentum budget computation for the subcloud layer, which shows pressure deceleration to be significant. The budget, dominated by advection, predicts an increase of average wind speed which is observed. The pressure deceleration result is further explored by applying an idealized monsoon flow to an analytical, nonliner, two-dimensional mountain-flow interaction model using a smoothed profile of the Western Ghats Mountains. The model qualitatively agrees with aircraft observations taken in the subcloud layer, and predicts large vertical wind shears over the coastal area and mountain crest which would inhibit deep convection. These shears are confirmed by earlier observations. When the lifting predicted by the model is applied to mean dropwindsonde soundings, well upstream of the coast, for days with and without offshore convection, deep convection is predicted for the mean sounding associated with offshore convection. The mean sounding for days without deep convection shows more offshore lifting is needed to produce convection; even if the lifting were applied, the convection would not be very deep due to a cooler surface layer and a dry layer above the boundary layer which may have originated from the desert areas to the west and/or upper tropospheric downward motion. We conclude that the mountains, though not very high, play an important role in overall monsoon convection for India. It is suggested that, given the climatic character of offshore monsoon convection, interaction of the low-level flow with the western coastal mountains of India, Burma, Thailand, and the Philippines should be considered a factor in monsoon climatology.