A Large-Eddy Simulation Study of Turbulent Flow Over Multiscale Topography

• Published in: Boundary-layer meteorology Vol. 141; no. 2; pp. 201 - 217
• Main Authors: Wan, Feng, Porté-Agel, Fernando
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Netherlands 01.11.2011; Springer; Springer Nature B.V
• Subjects: Analysis; Atmospheric boundary layer; Atmospheric models; Atmospheric Protection/Air Quality Control/Air Pollution; Atmospheric Sciences; Boundaries; Boundary layers; Computer simulation; Convection, turbulence, diffusion. Boundary layer structure and dynamics; Earth and Environmental Science; Earth Sciences; Earth, ocean, space; Elevation; Exact sciences and technology; External geophysics; Landscape evolution; Landscapes; Laws, regulations and rules; Marine; Mathematical models; Meteorology; Planetary boundary layer; Roughness; Topography; Turbulent flow; Wind; Multiscale complex terrain; Subgrid-scale topography; Large-eddy simulation; Scale invariance; Lagrangian model; topography; turbulent flow; digital simulation; Variance; Scaling law; parametrization; Large eddy simulation; roughness; Complex terrain; Atmospheric boundary layer; Spectral density; Dynamic model; Power law; Power spectrum
• ISSN: 0006-8314; 1573-1472
• DOI: 10.1007/s10546-011-9648-7

Most natural landscapes are characterized by multiscale (often multifractal) topography with well-known scale-invariance properties. For example, the spectral density of landscape elevation fields is often found to have a power-law scaling behaviour (with a −2 slope on a log–log scale) over a wide span of spatial scales, typically ranging from tens of kilometres down to a few metres. Even though the effect of topography on the atmospheric boundary layer (ABL) has been the subject of numerous studies, few have focussed on multiscale topography. In this study, large-eddy simulation (LES) is used to investigate boundary-layer flow over multiscale topography, and guide the development of parametrizations needed to represent the effects of subgrid-scale (SGS) topography in numerical models of ABL flow. Particular emphasis is placed on the formulation of an effective roughness used to account for the increased aerodynamic roughness associated with SGS topography. The LES code uses the scale-dependent Lagrangian dynamic SGS model for the turbulent stresses and a terrain-following coordinate transformation to explicitly resolve the effects of the topography at scales larger than the LES resolution. The terrain used in the simulations is generated using a restricted solid-on-solid landscape evolution model, and it is characterized by a −2 slope of the elevation power spectrum. Results from simulations performed using elevation fields band-pass filtered at different spatial resolutions indicate a clear linear relation between the square of the effective roughness and the variance of elevation.