Effect of offshore troughs on the South India erratic summer monsoon rainfall in June 2017 |
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| Affiliation: | 1. Department of Geography, Tokyo Metropolitan University, Hachioji-shi, Tokyo, Japan;2. Centre for Electromagnetics, National Aerospace Laboratories, Council of Scientific and Industrial Research, Kodihalli, Old Airport Road, Bengaluru, Karnataka, India;1. Dept. of Earth and Planetary Physics of Schmidt’s Institute of the Earth’s Physics, Russian Academy of Sciences, 10 Bolshaya Gruzinskaya, Moscow, 123995, Russia;2. School of Geosciences, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Ramat Aviv, 6997801, Tel Aviv, Israel;3. Azerbaijan State Oil and Industry University, 20 Azadlig Ave., Baku, AZ1010, Azerbaijan;4. School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Ramat Aviv, 6997801, Tel Aviv, Israel;1. Laboratory for Climate Studies & CMA-NJU Joint Laboratory for Climate Prediction Studies, National Climate Center, China Meteorological Administration, Beijing, China;2. State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing, China;3. Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science & Technology, Nanjing, China;4. National Centre for Atmospheric Science and Department of Meteorology, University of Reading, Reading, UK;1. Key Laboratory of Meteorological Disaster, Ministry of Education (KLME)/Joint International Research Laboratory of Climate and Environmental Change (ILCEC)/Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters (CIC-FEMD), Nanjing University of Information Science and Technology, Nanjing, 210044, China;2. Joint Innovation Center for Modern Forestry Studies, College of Biology and Environment, Nanjing Forestry University, Nanjing, 210037, China;3. Department of Atmospheric and Environmental Sciences, University at Albany, State University of New York, Albany, NY, 12222, USA |
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| Abstract: | The onset and advance of southwest monsoon are accompanied by the appearance of the offshore trough along the southwest coast of India. This offshore trough escorts a deluge of rainfall to the southwest coast, and sometimes rainfall band moves eastward further into south India. These broad observations were noticed during the summer monsoon of June 2017. Meteorological agencies and media had reported a huge amount of rainfall over the southwest coast of India during the month. But, in the far interior of south India, rainfall was less. Due to the less rainfall, water resources depleted, which affected local farmers and common man of south India. The confused views of the common man on southwest coast rainfall could be due to lack of understanding related to various factors affecting rainfall over the same region. This article is an endeavor to address the preliminary understanding of the southwest coast rainfall during June 2017, with more stress on offshore troughs. The study begins with area-averaged rainfall statistics over south, southwest, and southeast India by employing satellite and rain gauge merged rainfall datasets. Area averaged analysis revealed offshore trough contributed 80 % of rainfall over the South West India, 68 % over South East India, contributing to an overall 75 % over south India in 2017. To identify offshore trough position and strength in the reanalysis and model simulations, a new method called VSV (Vertical Shear of Vorticity) method was introduced. The computed offshore troughs were categorized into Active, Normal, and Feeble based on the strength of meridional gradient of mean sea level pressure and 850 hPa horizontal winds. The contribution due to each category of the offshore trough over different sub-regions was investigated to find out the effect of the offshore trough to total rainfall. Dynamic and thermodynamic features of these categories of the offshore trough were investigated by using proxies like equivalent potential temperature and moisture flux convergence. We found that during active offshore trough an eastward propagation of rain bands persists, which was explained by using moisture flux convergence and equivalent potential temperature at different levels of the atmosphere. |
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| Keywords: | Summer monsoon Rainfall Offshore trough Moisture flux convergence Equivalent potential temperature |
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