Exploring the effect of urban water surfaces on the urban heat island and induced rainfall patterns

Urban water surfaces, as a nature-based solution, play a crucial role in mitigating heat stress. While their impact on the thermal environment is well-documented, their influence on thermodynamic meteorological processes remains less understood. To bridge this knowledge gap, we conducted ensemble large-eddy simulations coupled with the Weather Research and Forecasting model, using idealized urban landscapes for both coastal and inland cities. We analyzed three recent extreme summer weather in North-West Europe: a hot day without rainfall, a hot day with convective rain, and a three-day compound event featuring a hot day followed by extreme rainfall. This study first examines the impact of urban surface water on the near-surface urban thermal environment during a hot day without rainfall. Subsequently, we investigate the effects of urban surface water on urban rainfall amounts and spatial patterns for the two rainfall events.

Results indicate that in a scenario where 30% of the urban surface is covered by water, the canyon-layer urban heat island (UHI) effect can be completely offset during the daytime on the spatial average. The urban water surface can reduce daytime air temperature by 0.39 °C on average under a heatwave event. However, it can intensify the UHI by approximately 0.13 °C and 0.32 °C for coastal and inland cities during the pre-dawn period.

Moreover, urban areas induce precipitation over both urban and rural regions, while urban water surfaces can mitigate this effect. Different placements of urban water surfaces can also cause temperature and rainfall anomalies throughout the thermodynamic process. The findings underscore the importance of strategic planning for urban water surfaces to mitigate heat stress and urban rainfall nuisance in future climates. This study provides significant insights into the bidirectional interaction between urban water surfaces and atmospheric hydro-meteorological processes, offering valuable directions for future research.