Local cloud enhancement in cities depends on urban morphology

Previous studies have noted that cities enhance cloud cover, but the mechanisms of urban morphological types on cloud formation remain elusive. Observations of cloud climatology from 44 major U.S. cities show that cloud enhancement increases with the street-canyon aspect ratio and decreases with building density. To explain these observations, we conducted numerical experiments using urban morphology-resolving large-eddy simulations. Results show that urban morphology influences cloud formation through two mechanisms: taller buildings intensify urban-breeze circulations at the urban-rural interface, while denser buildings, acting as momentum sinks, reduce vertical turbulent transport at the urban core. These vertical motions modify the transport of moisture in the urban atmospheric boundary layer, causing different cloud amounts across different urban morphology. This study for the first time establishes a mechanistic link between urban form and cloud enhancement. The results also provide a basis for city-specific boundary-layer convective parameterizations in large-scale weather and climate models.