Observed impact of fragmented urban areas on downwind precipitation

The impact of urban areas on precipitation is widely accepted, with increased precipitation often observed over and downwind of city centers. However, this urban impact on precipitation is still not fully understood, and uncertainties remain regarding the underlying mechanisms as well as the role of background climate and city morphology (e.g. city size, density, and surface characteristics). Since most existing studies focus on large, isolated cities, the impact of fragmented urban areas, characterized by midsize cities and urban sprawl, remains uncertain.

This study investigates the impact of fragmented urban areas on downwind precipitation using a dense hourly to daily frequency rain-gauge network in Belgium. To account for the fragmented urban morphology, an innovative method was applied that assigns each observation an urban or rural label based on the fraction of urban land cover in the upwind area. The results indicate a decrease in precipitation frequency downwind of urban areas, while a significant increase in precipitation intensity is observed during summer. Furthermore, the dependence of the urban impact on season, time of day, and wind direction suggests that convection plays an important role. The study also highlights the added value of hourly observations in revealing the characteristics of the urban precipitation impact.