Background Wind Speeds Outweigh Urban Heat Islands in Downwind Precipitation Enhancement by Cities
Geophysical Research Letters, 2025 · doi:10.1029/2024gl114142
Abstract
Ample evidence shows that cities can enhance precipitation in the downwind region due to the urban heat island (UHI) effect and the high momentum roughness of urban land. Surprisingly, global observational results show that the downwind enhancement of precipitation caused by large metropolitan areas is weaker under conditions of stronger surface UHIs. This is because stronger UHIs tend to be associated with lower background wind speeds, while the downwind enhancement of precipitation is stronger with higher background wind speeds. These results suggest a competition between thermodynamic and dynamic factors in regulating the downwind enhancement of precipitation, with the background wind speed playing a more important role than the UHI effect. By considering the urban‐rural difference in momentum roughness length, a simple model is utilized to qualitatively explain the link between the downwind enhancement of precipitation and background wind speed.
Plain-language summary
Traditionally, more heat (i.e., thermodynamic factor) and increased roughness (i.e., dynamic factor) in cities can lead to greater upward movement of air, which promotes precipitation, particularly in the downwind areas of cities. Although both mechanisms have been studied, their interactions under varying background wind speeds have not yet been explored. Our global results show that the downwind enhancement of precipitation caused by large metropolitan areas is weaker with stronger urban heat island effects but stronger with higher background wind speeds, suggesting a competition between thermodynamic and dynamic factors and background wind speeds outweighing urban heat islands in enhancing precipitation downwind of cities. This work provides new insights into the link between urban surface characteristics and regional hydroclimates.
Key points
- Background wind speeds and urban heat islands (UHIs) are correlated and seem to exert opposing influences on downwind precipitation enhancement
- Background wind speeds can better explain the spatiotemporal variations of downwind precipitation enhancement than UHIs
- An increase in background wind speed, coupled with urban‐rural momentum roughness length difference, leads to stronger air convergence
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