A new method to identify and explain sources of precipitation modification, illustrated for the western Netherlands

Urban areas can influence local precipitation patterns and amounts. Although earlier studies have focused on the magnitude of precipitation modification by cities, this study develops a method to identify the source areas of precipitation rather than the location where the precipitation falls. We illustrate this for the western part of the Netherlands for 2017–2023. Precipitation anomalies are located using radar-derived precipitation data and traced back through the Lucas-Kanade Optical Flow technique to their source areas by following the motion of the precipitation for each rainfall event. Using machine-learning techniques, the influence of hypothesized causes—urban heat, surface roughness, air pollution, and their interactions—on precipitation anomalies is evaluated. We find that urban and industrial areas generally enhance downwind precipitation, while large rural areas weaken it. Nearly 30% of the study area frequently generates enhanced precipitation downwind, with an average increase of 16.1% of the total precipitation. The strongest signal for the initiation of precipitation modification is present near the industrial areas of IJmuiden and the Rotterdam Harbor, and around the relatively large urban areas of The Hague and Utrecht. Large sparsely built areas like the Green Heart generally reduce precipitation downwind. This confirms the hypothesis that urban and industrial areas can enhance precipitation, while rural areas tend to weaken it. The identified source areas of precipitation modification are consistent across different seasons. We also find that surface roughness and concentrations (SO2 is used as a proxy for air pollution) show a positive correlation with precipitation anomalies. Air pollution appears the most influential predictor of precipitation anomalies in this study, followed by the interaction between air pollution and urban heat, and then surface roughness. Compound effects show a lower contribution than individual variables. This methodology provides a novel perspective on precipitation modification, offering a foundation to refine assumptions about source locations and improve understanding of the mechanisms driving the effect.

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van derGraaff, J. & Steeneveld, G.-J. (2026) A new method to identify and explain sources of precipitation modification, illustrated for the western Netherlands. Quarterly Journal of the Royal Meteorological Society, e70173. Available from: https://doi.org/10.1002/qj.70173