Application of the WRF-IBM model to mountainous terrain

The Weather Research and Forecasting model (WRF) is being used over increasingly complex terrain at higher grid resolutions. As the grid resolution increases, so does the resolved terrain slope, posing a challenge to the traditional terrain-following coordinates used by WRF and other mesoscale models. An immersed boundary method (IBM) was recently implemented into WRF (Lundquist et al. 2010, 2012), to alleviate numerical errors associated with steep terrain slopes. The IBM uses a Cartesian grid with the terrain boundary “immersed” within the grid. Boundary conditions are set through interpolation procedures for grid cells intersected by the immersed surface. The WRF-IBM model has been previously tested for urban flows using a no-slip bottom boundary condition.

In this work, we extend the existing WRF-IBM model to include a wall model in accordance to Monin-Obukhov (M-O) similarity theory. This enables simulations over complex terrain where a no-slip condition is not appropriate. Validation test cases include pressure driven flow over flat terrain and idealized small amplitude Gaussian hills. These test cases will provide a proof of concept and verify the implementation of the new boundary condition. Further simulations will be generated with real, complex terrain, and validated with field data. We will investigate nighttime down-slope flow over Granite Mountain for the MATERHORN (Mountain Terrain Atmospheric Modeling and Observation) program. Initialization from reanalysis meteorology data (NAM) and real high-resolution topography data will be used. Horizontal resolution down to ~30 m is used with semi-idealized lateral boundary conditions. Initial results for the MATERHORN program will be presented here in order to study the evolution of the slope flow.