Background wind speeds outweigh urban heat islands in downwind precipitation enhancement by cities

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. Based on the traditional framework, UHI creates thermal instability, enhancing convective activities, particularly in downwind areas of cities. Surprisingly, global observational results show that the downwind enhancement of precipitation 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 by cities, with the background wind speed playing a more important role than the UHI effect. Moreover, the spatiotemporal characteristics of downwind precipitation enhancement can be well explained by background wind speed, which is generally stronger in high-latitude regions than in low-latitude regions, and stronger in winter than in summer. 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. Therefore, the competition between thermodynamic and dynamic factors makes background wind speed a key variable in explaining the urban downwind enhancement of precipitation.