Improving the representation of surface exchanges between the urban canopy and the surface layer in a mesoscale atmospheric model

Exchanges of momentum, heat, and moisture between the various surfaces and the atmosphere can significantly affect the state of the low-level atmosphere known as the surface boundary layer (SBL). Since most human activities are confined within the SBL, a realistic representation of the relevant physical processes at the surface is important. The present work is focused on improving the representation of the surface processes in the Global Environmental Multiscale (GEM) atmospheric model pertinent to the exchanges between the urban canopy and the atmosphere. The approach uses the Town Energy Balance (TEB) scheme to determine the temperatures of urban canopy elements (i.e., roofs, roads and walls) for each model grid cell. These elements are allowed to directly interact with GEM's vertical levels, which are now positioned inside the urban canopy. Both dynamical and thermodynamical effects of the urban canopy are directly parameterized instead of being represented by surface turbulent fluxes between the urban canopy and a first atmospheric level typically located tens of meters above the canopy. In the new approach, the vertical turbulent diffusion process is also affected through modifications of the turbulent kinetic energy and the mixing length. Detailed test results on the improvement achieved using the proposed new approach will be presented at the conference, based on the comparison of the model output with observations from the Joint Urban 2003 Experiment which occurred in Oklahoma City.