PNNL

Abstract

Urban landscapes modify cloud formation and convection through complex thermodynamic and aerodynamic processes, yet their influence under different synoptic regimes remains poorly understood. This study investigates the impact of the Houston metropolitan land cover on summertime cloud cover and convective cell characteristics using a combination of satellite observations, radar data, and high-resolution convection-permitting modeling. We isolate urban effects by comparing simulations with realistic urban land cover against hypothetical scenarios where all urban areas are replaced by rural vegetation. Results reveal that Houston’s land cover consistently enhances cloud fraction and convective activity relative to surrounding rural areas, with the magnitude of the enhancement depending on the large-scale meteorological forcing. Under weakly forced conditions, enhanced surface heat flux primarily contributes to driving low-level convergence and vertical ascent, leading to over 8% cloud fraction increase between 2–6 km over the city. Under strongly forced conditions, urban influences manifest differently depending on cloud type. For non-convective clouds, typically occurring at night and in the morning, the city acts as a barrier that decelerates and lifts moist southerly inflow, increasing low cloud cover by up to 8% over the urban core, while decreasing it by 2–5% downwind the city. For convective clouds often developing in the afternoon, both synoptic ascent and urbanization modulate moisture redistribution and cloud structure, producing modest cloud enhancement over the city and slight suppression over the downwind area. Urbanization exerts small changes in the intensity of the convective cells, however, significantly decreases their duration and traveling distance. This work highlights the importance of accounting for land surface heterogeneity in modeling clouds and precipitation and demonstrates that urban impacts on clouds are highly regime-dependent.