Topographic Shading in the WRF Model with the Immersed Boundary Method: Implementation, Validation, and Application to Complex Terrain

In mountainous terrain, the diurnal variations of the surface sensible heat flux can lead to thermally-driven upslope and downslope flows during the daytime and nighttime, respectively. Topographic shading may influence these flows by limiting incoming solar radiation and creating large spatiotemporal inhomogeneities in the surface sensible heat flux. It is especially important to include topographic shading in high-resolution models of thermally-driven slope flows because of the effects it may have on flow development. In this work, the topographic shading algorithm in the Weather Research and Forecasting (WRF) model is coupled with the immersed boundary method (IBM). The IBM was originally implemented in WRF by Lundquist et al. (2010, 2012), and removes the restrictions on terrain slope that are associated with WRF's traditional terrain-following vertical coordinate, permitting high-resolution atmospheric simulations over complex terrain. The topographic shading implementation is validated by comparing shadow location, land-surface fluxes, and temperature and velocity fields between idealized WRF simulations both with and without the IBM. Following validation, the topographic shading implementation is tested in a realistic simulation of Granite Mountain, Utah, where topographic shading is known to affect downslope flow development in the evening (Fernando et al. 2015). The horizontal resolution of the model is 50 m and the vertical resolution is approximately 10 m near the surface, which would be infeasible in a standard WRF model with terrain-following coordinates due to large local slope values reaching roughly 55 degrees. The development of downslope flows after local sunset in the model is discussed and shown to be consistent with field observations from the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) project.