Role of Clouds in the Urban Heat Island and Extreme Heat: Houston-Galveston Metropolitan Area Case

This study examines the influence of shallow cumulus clouds on the excessive summertime heat in the Houston-Galveston metropolitan area, a coastal urban area in the warm Southeast United States. Specifically, it aims to improve our understanding of how both the clouds and the relatively cool, moist afternoon sea breeze impact the Urban Heat Island (UHI) and Heat Index (HI). During the warm season, the afternoon sea breeze phenomenon in this coastal city acts as a natural air conditioner for city residents, facilitating the dispersion of moisture, heat, and pollutants. To investigate the relationship among urbanization, clouds, and land-sea interactions, we conducted cloud- and urban-resolving simulations at a 900 m grid resolution and perform simulation scenarios aiming to isolate urbanization, clouds and land-sea circulations. Results show that urbanization correlates with the presence of shallow cumulus clouds, higher cloud bases, and increased cloud duration over the Galveston-Houston region compared to rural areas. These urban clouds benefit from the enhanced moist static energy that is favored by intensifying vertical mixing and moisture flux convergence. Urbanization raises the mean HI while mitigating its afternoon HI high. We found that the urban circulation dome overwhelms the sensitivity of the sea breeze to the urbanization. Instead, the influence of urbanization on cloud enhancement emerges as a crucial pathway responsible for reducing the high afternoon HI values. Moreover, uncertainties in SSTs are closely linked to the sensitivities of land-sea circulations, which in turn modulate UHI and HI.