Delineating Intra-Urban Air-sheds using High-Resolution WRF Modelling: Insights from Ahmedabad, India

Understanding intra-urban air-shed dynamics is increasingly critical for effective air quality management. While regional-scale air-shed characterisation has been widely studied, comparatively limited attention has been given to air-shed delineation at the city scale, where pollutant transport pathways, stagnation zones and recirculation patterns exert direct influence on population exposure. Identifying prominent urban air-sheds and quantifying their dependence on key meteorological drivers such as wind speed, wind direction, thermal structure and humidity can substantially improve the predictive understanding of pollutant accumulation hotspots.

With this motivation, the present study investigates the air-shed behaviour of Ahmedabad, a densely populated and industrially active city in western India having a population of around 8.2 million and spread across 464 sq km. The Weather Research and Forecasting (WRF) model was configured for high-resolution urban simulations and validated for two contrasting seasons: a five-day period in May representing hot, dry summer conditions and a five-day period in winter characterised by lower boundary-layer heights and reduced dispersion. Simulated fields of wind speed, wind direction, ambient temperature and relative humidity were generated at hourly intervals and evaluated against available meteorological observations. Using these validated simulations, the study attempts to delineate intra-city air-sheds by analysing dominant flow regimes, seasonal shifts in ventilation and stagnation patterns and the sensitivity of air-shed boundaries to changes in meteorological parameters.

The insights derived from this analysis hold significant implications for urban regulators and civic administrators. A refined understanding of air-shed structure can support targeted emission control strategies, optimise the spatial prioritisation of air action plans, and ultimately contribute to improving public health outcomes for city residents.