Influence of urban land cover dynamics on pre-monsoonal precipitation in Kolkata, India

Studies continue to link urbanization (land cover, aerosols) with regional hydroclimate variability. Urbanization is rapidly occurring globally, particularly in developing countries. Relatively few studies have investigated urban effects on precipitation in India. Further, most precipitation related studies for India focus on monsoonal rainfall. However, pre-monsoonal periods (March to May) account for 12-14% of the annual cumulative rainfall in eastern India. The majority of pre-monsoonal rainfall (PMR) is convective and caused by mesoscale forcing, which may include urban effects.

The objectives of this study were to: (1) delineate urban land cover change using historic cartography and remote sensing data; (2) analyze historical rainfall data and quantify trends in the spatio-temporal PMR climatology of the area; (3) explore potential correlations between urban land cover growth and PMR trends; (4) to 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 pre-, current, and 2025 urban land cover scenarios using the WRF-NOAH mesoscale model. The methodology is a synergistic blend of historical cartographic analysis, satellite remote sensing, urban growth modeling, and numerical weather modeling. Preliminary statistical analyses suggest that PMR has increased at urban stations in Kolkata in over the past 50 to 60 years (e.g., period of rapid urbanization); while there is no discernible precipitation trend in rural stations, the Ganghetic plain, or India itself. Growth model results suggest continued urbanization along rail lines so it is plausible to expect future changes in Kolkata's PMR as a function of such changes alone.