A synergistic study of urban land cover dynamics-premonsoonal rainfall relationships in Kolkata, India using climatological, remotely-sensed, and modeling methodologies
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Empirical and modeling studies show that increasing population in cities around the world and related anthropogenic activities have an impact on coupled human-natural systems. The focus of this study is the city of Kolkata, which is located in the eastern part of the India. The Kolkata megalopolis had an initial population of 100,000 in 1735 and has grown to 14.79 million in 2007, reflecting largely uncontrolled growth over the past 300 years, especially after India’s independence in 1947. Such population growth has been accompanied by intensive conversion of natural land cover to impervious surfaces. The objectives of this study were to: (1) delineate urban land cover change in Kolkata using historic cartography and remote sensing data; (2) analyze historical rainfall data and quantify trends in the spatio-temporal premonsoonal rainfall (PMR) climatology of the area; (3) explore potential associations between urban land cover growth and PMR trends; (4) project future growth of Kolkata in the next 25 years using the CA Markov urban growth model; and (5) investigate possible mechanisms associated with possible urban rainfall effects in and around Kolkata, under different urban land cover scenarios in a coupled atmosphere-land surface modeling system. The methodology is a synergistic blend of historical cartographic analysis, satellite remote sensing, urban growth modeling, and numerical weather modeling. Statistical analyses suggest that PMR amounts have increased at urban stations in Kolkata over the past 50 to 60 years (i.e., the period of rapid urbanization); while there is no discernible precipitation trend in rural stations, the Gangetic plain, or India itself. Growth model results suggest continued urbanization along lines of communication and transportation, so it is plausible to expect future changes in Kolkata’s PMR as a function of such changes alone. A novel set of coupled atmosphere-land surface modeling experiments explored the interactions between urban land cover and the well-known “Norwester” mesoscale storm systems common to the premonsoon study period. Results clearly establish a linkage between urban land cover and precipitation evolution. Further, the results suggest that trends in urbanization patterns and tendencies will further modify precipitation distributions in the future. Analyses indicate that augmentation of land cover affects precipitation processes through alteration of surface convergence, sensible-latent heat flux, and boundary layer thermodynamic processes. Overall, the results highlight that “climate change” in the Kolkata region may be a function of urbanization and land cover change in addition to broader forcing related to greenhouse gases.