A Large Eddy Simulation Study on Atmospheric Flows over Multi-scale Terrain

Using Cloud Model, version 1 (CM1; Bryan et al. 2003) as large eddy simulation (LES), we examine atmospheric flows over multi-scale terrain. The multi-scale terrain will be analytically prescribed following the finding of some previous studies that landscapes present multi-scale self-similar properties through a wide range of scales, from about 100 km to 100 m, which is represented with the Fourier spectral slope of K^-2 (where K is wave number; e.g., Huang and Turcotte 1989; Passalacqua et al. 2006; Wan and Porte-Agel 2011). On a one-dimensional, sinusoidal shape terrain, of which wavelength is designed to be comparable to the greater dimension of the LES domain, smaller-scale terrain fluctuations will be superimposed. Following the K^-2 slope, the multi-scale terrain will be prescribed. The longest wavelength of the multi-scale terrain will be the greater LES dimension of 36 km, and the smallest wavelength be the Nyquist wavelength of the LES grid spacing of 200 m. The Froude number (F = U/NL, where U is mean horizontal wind speed, L mountain width, N buoyancy frequency), the ratio of natural wavelength to mountain width, will be controlled with different background atmospheric condition and mountain shape. Under the varying Froude number across the critical value of 1, the multi-scale terrain effects will be evaluated in terms of meso- and micro-scale processes. Based on the evaluation, we will discuss the value of higher-resolution terrain data for a more accurate simulation of atmospheric flows over complex, multi-scale terrain.