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Center for Nonlinear Studies |
Mesoscale models, such as the Weather Research and Forecasting (WRF) model, are increasingly used for high resolution simulations, particularly in complex terrain. Terrain-following coordinates employed by mesoscale models are mapped to the terrain, which simplifies the application of lower boundary conditions. The signature of the topography causes distortion of computational cells in the transformed coordinate system. With steep terrain or high computational resolution, the distortion can be extreme in the near surface region, and often extends as high as the troposphere. This grid skewness introduces numerical truncation errors, which can significantly degrade the accuracy of the solution. This loss of fidelity is demonstrated using a test case of a scalar cloud advecting over highly variable topography.
Use of an alternative gridding technique, known as an immersed boundary method, alleviates coordinate transformation errors and eliminates restrictions on terrain slope which currently limit mesoscale models to slowly varying terrain. Simulations are presented for canonical cases with shallow terrain slopes, and comparisons between simulations with the native terrain-following coordinates and those using the immersed boundary method show excellent agreement. Additionally, surface fluxes of heat and moisture complicate treatment of the immersed boundary. Realistic surface forcing can be included at the immersed boundary by atmospheric physics parameterizations, which are modified to include the effects of the immersed terrain.
Using the immersed boundary method, WRF is capable of simulating highly complex terrain,
as demonstrated by validation cases and preliminary simulations of flow over urban
terrain. In order to demonstrate the ability of the IBM-WRF code to simulate atmospheric
flow around complex structures, we will present results from building resolving simulations
in urban environments. Specifically, flow and dispersion are demonstrated for terrain in the
Mock Urban Setting Test (MUST) and for the Joint Urban 2003 field campaign in Oklahoma
City.
Host: Xylar Asay-Davis