Large-eddy simulations and sensor networks to study flow and heat transport in urban canopies

We present results from large-eddy simulations of neutral atmospheric boundary-layer flow over a cluster of buildings surrounded by relatively flat terrain. The first investigated question is the effect of the level of building detail included in the numerical model. The simplest representation is found to give similar results to more refined representations for the mean flow, but not for turbulence. We then investigate the effect of the clustering of buildings and homes into small areas separated by surfaces of lower roughness, we look at the adjustment of the atmospheric surface layer as it flows from the smoother terrain to the built-up area. This transition has unexpected impacts on the flow; mainly, a zone of global backscatter (energy transfer from the turbulent eddies to the mean flow) is found at the upstream edge of the built-up area. We follow up on this study with an experimental investigation of the same building canopy (the campus of the École Polytechnique Fédérale de Lausanne), using a distributed network of 92 wireless weather stations combined with routine atmospheric profiling. We compare estimates of the surface roughness of the urban area obtained through the simulations and measurements. Subsequently, we estimate the sensible heat flux between the ground and the atmospheric surface layer for convective conditions using Monin-Obukhov flux-profile similarity relations. The results illustrate a good agreement between the sensible heat flux inferred from the thermal roughness length approach and independent measurements from a scintillometer located inside the urban canopy.