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Tropical cities are more susceptible to the suggested fall outs from projected global warming scenarios as they are located in the Torrid Zone and growing at rapid rates. Therefore, research on the mitigation of urban heat island (UHI) effects in tropical cities has attained much significance and increased immensely over recent years. The UHI mitigation strategies commonly used for temperate cities need to be examined in the tropical context since the mechanism of attaining a surface energy balance in the tropics is quite different from that in the mid-latitudes. The present paper evaluates the performance of four different mitigation strategies to counterbalance the impact of UHI phenomena for climate resilient adaptation in the Kolkata Metropolitan Area (KMA), India. This has been achieved by reproducing the study sites, selected from three different urban morphologies of open low-rise, compact low-rise and mid-rise residential areas, using ENVI-met V 4.0 and simulating the effects of different mitigation strategies-cool pavement, cool roof, added urban vegetation and cool city (a combination of the three former strategies), in reducing the UHI intensity. Simulation results show that at a diurnal scale during summer, the green city model performed best at neighborhood level to reduce air temperature (Ta) by 0.7 °C, 0.8 °C and 1.1 °C, whereas the cool city model was the most effective strategy to reduce physiologically equivalent temperature (PET) by 2.8°-3.1 °C, 2.2°-2.8 °C and 2.8°-2.9 °C in the mid-rise, compact low-rise and open low-rise residential areas, respectively. It was observed that (for all the built environment types) vegetation played the most significant role in determining surface energy balance in the study area, compared to cool roofs and cool pavements. This study also finds that irrespective of building environments, tropical cities are less sensitive to the selected strategies of UHI mitigation than their temperate counter parts, which can be attributed to the difference in magnitude of urbanness.

ABSTRACT:Empirical and modelling studies show that urbanization can have an impact on the environment. Relatively few studies have investigated urban effects on precipitation in India or other developing countries experiencing rapid urbanization. Furthermore, most precipitation-related studies for India focus on monsoonal rainfall. However, premonsoonal periods (March-May) account for 12-14% of the annual cumulative rainfall in eastern India. The majority of premonsoonal rainfall (PMR) is convective and caused by mesoscale forcing, which may include urban effects. In this study, the area under scrutiny is a large urban area in eastern India, Kolkata city. Herein, our goal was to (1) produce a comprehensive characterization of historical land cover dynamics associated with the Kolkata megalopolis, (2) provide a spatio-temporal climatology of PMR in the Kolkata region, and (3) identify possible associations between Kolkata's land cover and PMR. The analysis shows that the rate of change of urban land cover has increased by 50% compared to the period prior to India's independence in 1947. A multi-scalar time series analysis with Mann-Kendall statistics indicated statistically significant increasing trends in rainfall over the last 50 years for two Kolkata stations and a nearby downwind station. Furthermore, there was no significant trend for cumulative PMR in less urbanized stations, the country of India, or the East Gangetic region. This finding suggests that the anomaly of the three stations, showing increasing trends in PMR, could be the effect of urban land cover change.

Atmospheric blocking is a long standing structure stalled in the mid-troposphere which is often associated with extreme weather events such as droughts, heatwaves, flood and cold air outbreak. A striking atmospheric blocking is identified to persist over the US during 13-17 August 2007, exacerbating the existing drought over the Southeastern US. This pronounced blocking event not only intensified the concurrent drought conditions, but also led to a record-breaking heatwave over the Southeast of the US. The excessive heat observed during this heatwave is attributable to the subsidence-associated adiabatic warming as well as the dry-and-warm air advection over Alabama and the neighboring states. At the local scale, we choose Birmingham, AL, as the study area for exploring the blocking influence on urban heat island. Based on the remote sensing data, the surface (skin) urban heat island is found to be 8 • C in this area on the block-onset day. This provides partial evidences that the surface urban heat island intensity is likely amplified by the blocking-induced heat waves. The present work provides a unique case study in which blocking, drought, heatwave and urban heat island all occur concurrently, and interplay across a spectrum of spatial scales. We conclude that atmospheric blocking is capable of reinforcing droughts, initiating heatwaves, and probably amplifying the urban heat island intensity during the concurrent period.

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