Assessing the local- impacts of heat advection on urban heat islands in Kolkata Metropolitan Area

Abstract

The urban heat advection (UHA) is an important meteorological process that modulates the air temperature (TA) in downwind rural areas around cities by forming heat plume in the urban boundary layer (UBL) (above roof level). This study argues that the effects of UHA also exists in the urban canopy layer (UCL) (from the ground to roof level) to impact the urban heat island (UHI) locally. Moreover, the nature and intensity of UHA depend on dynamic properties of the atmosphere including wind speed (WS) and wind direction (WD), and the nature of land-atmosphere interactions in the urban environment. Studies investigating UHA phenomena in the UCL and its impacts on the local UHIs are inadequate, but such studies are potential for providing insight into those local thermal anomalies which cannot be attributed only to onsite urban surface properties (geometry and materials). The present study uses hourly weather data (June 2019–December 2020) obtained from 9 fixed weather monitoring stations in the Kolkata Metropolitan Area (KMA) to empirically recognise the influences of WS and WD, as the proxy of UHA, on the spatio-temporal variability of UCL-UHI. The study also adopts numerical simulations of microclimate with ENVI-met for exploration of local scale UHA phenomena explicitly. Results highlight that the upwind land surface characteristics of a location have considerable influences in modifying UHI due to UHA in urban microclimatic environment. The downwind areas of impervious and pervious surfaces are influenced by the warmer and cooler air advection, respectively, and create TA contrasts by about 0.95 °C and 1.62 °C during the day and night-time, respectively. In calm and clear weather, WS imposes asymmetrical impacts on TA under different UHA situations. For warmer air advection, the TA increased with WS till a limiting value of ~2.5 m/s, while for the cooler air advection, significant lowering of TA continued till WS was set to ~5 m/s and beyond this critical value the response of TA to the WS was insignificant for both the cool and warm air advections.