Observational study of the impact of Belgian urbanization on downwind precipitation

It is widely known that cities influence their local climate, although their impact on precipitation is less well understood. Several studies on large cities show an increase in mean precipitation over and downwind of the city. Due to the distinct urban situation in Belgium, characterized by a fragmented urban landscape with midsize cities and urban sprawl, the impact of Belgian urbanization on precipitation is unknown.

In our study we aim to identify the impact of Belgian urbanization on downwind precipitation amounts, based on daily precipitation measurements from the dense rain-gauge network of the Royal Meteorological Institute of Belgium. Due to the unique urban situation, a non-conventional methodology is required to analyze the observations. In contrast to the widely utilized city-centered approach, in which the mean precipitation amounts are calculated for the different wind regions around the city, we use an observation-centered approach (Daniels et al., 2016). In this alternative method, each precipitation measurement receives an urban or rural label based on the percentage of urban land cover in the upwind region relative to the measurement location. The upwind region is defined by the main wind direction at 100m height from the ERA5 reanalysis, and the land cover is based on the local climate zones.

The results are analyzed by comparing the mean precipitation amount between urban and rural labeled measurements. This analysis is performed for each season and main wind direction separately, to differentiate between weather types. In addition to the methodology of this study, some preliminary results will be presented.

Daniels, E. E., Lenderink, G., Hutjes, R. W. A., & Holtslag, A. A. M. (2016). Observed urban effects on precipitation along the Dutch West coast. International Journal of Climatology, 36(4).