Impact of urbanization on boundary-layer parameters and mesoscale circulations over tropical coastal city, Chennai

Chennai, the fourth largest metropolitan and coastal city of India, has undergone rapid urbanization in the last 2 decades. In this work, the impact of urbanization on the boundary-layer parameters and mesoscale circulations over Chennai is studied using a high-resolution (1 km) Weather Research and Forecast (WRF) model. Simulations are performed for four fair-weather cases (08–11 May 2015: 25–28 April 2015: 21–24 May 2014: 25–28 May 2015) for two scenarios (i) less urbanization corresponding to 1993 (NOURB) and (ii) extensive urbanization (URB) corresponding to 2013. Simulations show strong Urban Heat Island (UHI) effect with warm and dry conditions during the late night and early morning hours due to extensive urbanization. Simulations show significant modification of air temperatures, winds, energy fluxes, PBL height and ventilation coefficient due to increased drag during daytime and enhanced buoyancy during night due to extensive urbanization. The increased buoyancy in URB during the late night and early morning weakened the nocturnal stable boundary layer and slightly enhanced the wind speed and ventilation coefficient relative to NOURB. The strong drag effect in URB reduced the wind speed, caused delay in the horizontal intrusion of sea breeze and also led to stronger convergence and vertical motion in the frontal zone during day time.