An Evaluation of Boundary Conditions for Modeling Urban Boundary Layers

Numerical modeling of the urban boundary layer is complicated by the need to describe airflow patterns outside of the computational domain. These patterns have an impact on how successfully the simulation is able to model the turbulence associated with the urban boundary layer. This talk presents experiments with model boundary conditions for simulations which were done to support two observational programs involving the Salt Lake City basin. The Chemical/Biological Non-proliferation Program (CBNP) is concerned with the effects of buildings on influencing dispersion patterns in urban environments. The Vertical Transport and Mixing Program (VTMX) is investigating mixing mechanisms in the stable boundary layer and how they are influenced by the channeling caused by drainage flows or by obstacles such as building complexes.

Mixing in the Salt Lake City is multi-scale in scope with drainage flows from the nearby mountains interacting with boundary layer motions whose scale is reduced due to the strong stratification in the nocturnal stable boundary layer. We are investigating four approaches for including the effects of this drainage flow on a boundary layer simulation: 1) use nudging and open boundary conditions to influence the boundary layer winds with a mesoscale mean wind and velocity gradient, 2) using periodic boundary conditions and nudging to influence the boundary layer winds with a mesoscale mean, 3) use approach 1 but with a random perturbation on inflow to simulate the effect of embedded turbulent motions in the mesoscale flow, 4) approach 1 but with perturbations derived from an LES model to better represent turbulent motions embedded in the mesoscale flow. Each approach will be evaluated on how it alters the dynamics of the boundary layer simulation and its effects on the dispersion patterns of advected tracers.