In this project we aim to investigate how to bring atmospheric forecasts closer to where people live.
In Europe, the population living in urban areas is about 75% and up to 91% of Dutch citizens live in urban areas. Given the dense population, cities are generally high-impact areas and vulnerable to extreme weather. On the other hand, cities themselves also influence atmospheric conditions, especially temperature but also humidity, clouds and even precipitation. Studying atmospheric processes within a city requires a step change in model resolution, which is not as straightforward as changing the model’s grid length. The model physics of HARMONIE (KNMI’s operational weather and regional climate model) have been designed to run at grid spacings of several kilometres. Going to sub-km grid sizes, needed for detailed urban studies, some parameterisations are no longer valid (e.g., turbulence, shallow convection, urban).
The largest component of this work is to improve the representation of buildings in HARMONIE. Currently, buildings are represented to be below the surface. We have worked to ‘lift up’ the buildings so they can interact directly with the atmosphere (Figure 1). Early results show an improvement in both wind and temperature when including this. Especially when using the correct properties of the urban surface.
