3A.2 Implementation and evaluation of a new drag and length scale parameterization in the multilayer urban canopy parameterization of WRF

Simon-Moral, Andres, Jose-Luis Santiago, Alberto Martilli, CIEMAT, and Scott Krayenhoff, University of British Columbia

A new parameterization for the drag coefficient (Cd) and the length scales involved in the turbulence transport and dissipation is introduced in the multilayer urban canopy parameterization implemented in WRF. This parameterization has been derived from microscale CFD simulations over array of cubes with different layouts. These results show a dependence of the Cd on the sheltering (distance between buildings in the direction parallel to the wind), and channelling (distance between buildings in the direction orthogonal to the wind). The new scheme distinguishes between configurations with the same packing density, but different sheltering and channelling. Moreover, the turbulent length scales in the canopy and just above, are controlled by the building packing density. Different WRF 3D simulations over idealized cities with different morphological parameters are carried out in order to evaluate the impact of the microscale features parameterized by the new scheme. The results show different values for the wind and Turbulent Kinetic Energy inside the canopy depending on the value of the Cd. Also, the new scheme increases vertical diffusion and the height of the planetary boundary layer, compared to the old one, reducing the variability of the temperature in the urban canopy up to 0.5-1 K.