Urban built-up expansion induced accelerated surface cooling modulates fog genesis over Delhi

Severe wintertime fog frequently affects Delhi National Capital Rregion (NCR), causing extreme visibility reduction and exacerbating air quality and transportation disruptions. Fog formation over the Indo-Gangetic Plain is commonly linked to high aerosol loading, elevated humidity, and favourable synoptic conditions; however, the influence of rapid urban expansion and land-use driven surface processes driven by micro-meteorology remains unexplored. Urban built-up surface possesses distinct radiative and thermal properties compared to surrounding agricultural land, potentially modifying nocturnal cooling, near-surface microclimate and thus vertical structure of fog.

This study investigates the influence of urban expansion over the last three decades on the surface energy balance and its feedback on different stages of urban fog genesis to delineate the aerosol induced effects across the extensive urban built-up landmass of the Delhi–NCR region. High-resolution (9–3–1 km) numerical simulations are conducted using the WRF-ARW/Chem model, employing historical (1992) and latest (2024) urban land-use datasets to isolate the impact of urban expansion and to examine the coupled effects of aerosols, radiation and microphysics. The model reasonably captures the spatial and temporal evolution of observed fog events.

Results show that urban built-up areas in Delhi–NCR have expanded by over 100% in recent decades, primarily replacing irrigated cropland and vegetation, thereby altering surface radiative-thermal properties and intensifying the urban heat island effect resulting in distinct impact and effect of surface cooling after sunset. Consequently, during dense fog events, fog onset over urban areas is delayed and dissipation occurs earlier than over surrounding agricultural regions, leading to reduced liquid water content across the fog life cycle, both spatially and vertically. In contrast, radiative fog event exhibits an increase in LWC during the fog evolution, with a pronounced enhancement in the lower fog layers and a simultaneous reduction in the upper fog layers. This vertical redistribution of LWC is consistently reproduced in urban sensitivity simulations. Furthermore, WRF-Chem simulations reveal stronger LWC increases (>0.5 g kg^-1) during fog initiation and continues to enhance in the upper fog layers throughout the event, while LWC decreases (< -0.4 g kg^-1) in the lower layers during fog development and dissipation.

Overall, the results demonstrate that urban expansion influences fog initiation and dissipation but also its vertical structure and microphysical characteristics through combined thermal and chemical feedback. The results highlight an underexplored pathway linking urbanization and aerosol feedback, surface thermal dynamics, and atmospheric chemistry in fog genesis.