Impacts of Urbanization on Indian Summer Monsoon Rainfall

Rapid urbanisation all around the world is a matter of concern to the scientific community. The fast growing urban areas carries out huge anthropogenic activities that burdens natural environment and its resources like air-water quality and space, thus have different climatology to their rural surroundings. World Urbanization Prospects 2005 annual report described 20th century as witnessing a rapid urbanization of the world's population. Though urbanization is a worldwide phenomenon, it is especially prevalent in India, where urban areas have experienced an unprecedented rate of growth with level of urbanization increased from 17.23 % to 31.16% in year 1951 to 2011and the number of cities with population more than one million has grown from 5 to 53 over the same time. We take up an observational study to understand influence of urbanisation on mesoscale circulations and resulting convection, thus nature of precipitation around urban areas. The spatially distributed analysis of gridded daily precipitation data over the country is carried out to identify nature of trends in selected statistics of Indian summer monsoon precipitation and examine its association with urban land cover to have an impact on precipitation statistics. We evaluate explicit changes around urban land use in context of 40 large Indian urban areas. Further we assess local-urban climatic signals in the point level rainfall observations with model based analysis of two nearby locations under similar climatic conditions but differing largely in terms of urbanisation. The results of gridded data analysis indicate an overall tendency towards decrease in mean precipitation however, rainfall activities are enhanced around urban areas across different climate zones of the country. Though trends observed in selected climatic parameters revealed great degree of spatial inter variability in selected precipitation statistics over the country, they accounts a greater degree of inclination for occurrence under regions of urban influence. Examination of urbanization influence on heavy rainfall climatology carried out through point scale experiment with statistical framework of quantile based regression for the most populated city of India Mumbai, in pair with a nearby non-urban area Alibaug also point toward sensitivity of extreme rainfall events to the local land use under urbanisation. Overall the study indicate influence of urbanisation over amendments in conventional regional rainfall pattern to a convinced extent and illustrate that even if only a small percentage of land covers urban areas they may play a key role to alter the hydrology at local and regional scales. The study highlights need of further investigation in terms of quantifying the impact and estimation of associated uncertainties in form of detailed theoretical and numerical studies for India to more clearly highlight the role that urbanisation plays in precipitation enhancement of Indian monsoon rainfall in order to make better assessment of urban planning, water resources management and urban flooding.