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.
Considering basic types of urban land cover land use, we then set up idealized large-eddy simulations (LESs) for varying urban morphological types with different building heights and building plan area fractions under free convective conditions. LES results suggest that cloud patterns closely correspond to the vertical transport of moisture. Clouds over urban-rural interface are controlled by the urban breeze circulation (UBC); while clouds over urban core are controlled by vertical velocity variance. Thus, urban morphologies affect cloud formation via two plausible mechanisms: the roughness-induced convergence at the urban-rural interface increases intensity of UBC while roughness as momentum sinks reduces vertical turbulent transport at the urban core. Two conceptual models are proposed to explain the variation of vertical velocity scales with morphologies. Observations across 44 major US cities corroborate LES results: cloud enhancement increases with street-canyon aspect ratio and decreases with building density. This study demonstrates the importance of capturing vertical motions modified by urban roughness for boundary-layer parameterizations. It also underscores the value of synthesizing long-term observations across distinct cities to understand the effect of urban form on boundary-layer processes.