Impact of increasing urbanization on heatwaves in Indian cities

Urbanization alters local climates and exacerbates urban heat islands. Understanding and addressing the impacts of urbanization on regional high impact weather systems is critical. This study examines the feedback loop between urbanization and heatwaves (HWs) in inland and coastal Indian cities of Hyderabad and Bhubaneswar which have been profoundly affected by urbanization and temperature extremes. Observational analysis reveals that during the pre-monsoon season, daytime and nighttime air temperature anomalies, and the frequency of 90th percentile days, have increased by ~0.35°C and ~3 days for Hyderabad, and by ~0.2°C, and ~6 days for Bhubaneswar in the last two decades (2001–2020) relative to the previous decades (1981–2000). Analysis of  land-use land-cover (LULC) datasets shows a dramatic urban expansion by ~13 and ~11 times in Hyderabad and Bhubaneswar, respectively, between 1993 and 2019. Numerical experiments with the Weather Research and Forecasting model were undertaken considering hectometer spatial resolution (~500 m) and a lower boundary conditions representing the 1993 and 2019 LULC. The impact of urbanization on temperature changes and HWs in particular were analyzed. The HW simulations indicate that urbanization significantly enhances air and surface temperatures by ~4–6°C, particularly during nighttime rather than daytime. Urbanization effects are discerned in surface temperatures at night by 1–2°C relative to  air temperatures. Unlike nighttime, urbanization showed a negative or little influence on air and surface temperatures during the daytime. In contrast to surface and air temperatures, increased urbanization runs indicated enhanced regional soil temperature by ~5°C more during the daytime than at nighttime. The rise in nighttime air and surface temperatures is due to an increase in surface sensible heat fluxes by ~40–50 W/m² in urban areas. The influence of urbanization on nighttime temperatures emphasizes the necessity for cool housing and engineering recommendations in urbanized regions of India.