Application of a dynamic drag model in LES of atmospheric boundary layer flow over fluvial-like, fractal topography

The elevation field of evolved fluvial topographies are known to exhibit a power-law energy spectrum with exponent -2 frequently reported. The ABL's response to such topographies is important in a variety of microscale meteorology problems. When simulating ABL flow over these topographies with LES, the computational mesh resolution is likely to exceed the finest length scale of the topography, thus requiring a model for surface momentum fluxes associated with the unresolved topography. Recently, Anderson and Meneveau, 2011: JFM 679 288—314 have presented a dynamic approach to describing these SGS drag effects. In the present study, we apply the dynamic approach in LES of flow over numerically-constructed evolved fluvial topography built with a modified Kardar-Parisi-Zhang equation (Passalacqua et al., 2006: WRR 42 WOD611, Anderson et al., 2012: BLM, Accepted). These topographies possess the fractal-like channel network and the anisotropic features often found in real terrains. The dynamic model is shown to lead to accurate flow predictions when the surface height distributions exhibit power-law scaling (scale-invariance) in the prevalent mean flow direction.