Heterogeneous precipitation for homogeneous clouds between buildings
The obstacle-resolving atmospheric model MITRAS was expanded to investigate rain heterogeneities within the urban canopy. For this boundary conditions of cloud and rain water content at obstacle surfaces (roof, vertical walls) were introduced that consider at each grid cell boundary advection, diffusion, and sedimentation. At obstacle surfaces, there is no advection through the walls. For the diffusion close to obstacle surfaces, a specific building surface flux is calculated and used. The rain water reaching a roof by sedimentation is assumed to be collected on the roof. Thus, the simulation of the rain distribution at the local scale is possible.
Figure 1: Homogeneous cloud over a city.
As a proof of concept, a low homogeneous cloud (fig. 1) is assumed over the area surrounding Hamburg City Hall (red dot). The resulting pattern of the instant rain rate reveals small-scale heterogeneities (fig. 2) with maximum differences of more than 10 mm/h induced by the heterogeneous building structure and land use of the inner city
Figure 2: Rain rate over the city at ground.
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