Impacts of Urbanization on Extreme Weather: Long-Term WRF Simulations at Cloud Resolving Scale over Eastern USA

Although, the urban heat island (UHI) effect over cities due to land use land cover changes (LULC) and anthropogenic heat (AH) is well recognized, its association with the heat extremes and severe thunderstorms at climatic scale is not well quantified. In this study, the impact of urbanization is investigated using the Weather Research and Forecasting (WRF) model simulations at 4 km resolution. For this purpose, three different scenarios are simulated for 6 consecutive warm seasons (2008-2013) and compared: (1) present-day urban LULC and AH; (2) present-day urban LULC but no AH; and (3) background LULC without cities or AH. High-resolution nightlight and AH data are used for accurate representation of urban surface and anthropogenic forcing in our simulations. Results show that the daily mean UHI values are in the range of 2-5 ^OC. About 20% of the magnitude of midday UHI values are contributed from AH fluxes. Further, we calculated the Stedman’s heat stress index metric over 35 cities located in the region. On an average, the annual number of extreme (moderate) heat-stress days over these cities are enhanced by 8-10 days (13-15 days) due to urbanization. At the same time, we also found that the daily mean UHI values are higher by ~ 1^oC during heat wave period compared to normal days, highlighting that urbanization exaggerates heat-stress during heatwaves over this region. Interestingly, urbanization is found to decrease the number of days with severe thunderstorm environment by 2-12 days/summer by decreasing the near surface moisture and thus suppressing convection. However, this association between urbanization and severe storm environment is influenced by background climate.