Large-eddy simulation of the evolving stable boundary layer over flat terrain.

摘要

The goal of this research is to improve our ability to realistically simulate the stable boundary layer (SBL). The scientific objectives are: (1) to characterize features of the evolving SBL structure for a range of meteorological conditions (wind speed and surface cooling), (2) to simulate realistically the transfer of energy between resolved and subgrid scales, and (3) to apply results to improve simulation of dispersion in the SBL. A large-eddy simulation (LES) approach with a dynamic, mixed subgrid-scale (SGS) turbulence model is used.; The several SBLs simulated illustrate the key role of mechanical turbulence supported by the geostrophic forcing, and the lesser competing effects of turbulence damping by buoyancy that develops in response to the surface cooling. A forcing effects of surface buoyancy flux and geostrophic wind are strongly correlated with the bulk stability of the resulting SBL. The SGS model allows for backscatter (upscale transfer) of turbulent kinetic energy (TKE) and thermal energy. The TKE backscatter is dominated by the interaction of the streamwise velocity component with the wall-normal shear stress. The thermal backscatter occurs during ejections of cool surface air, associated with the action of coherent structures in the flow.; The simulation of episodes of enhanced turbulence is made possible by the inclusion of energy backscatter. These episodes are associated with the breakdown of large-scale wave-like activity. The implications for dispersion in the SBL are demonstrated by releasing marker particles in LES-generated wind fields for an SBL with an enhanced turbulence event. The effect of the enhanced turbulence is to spread the plume over a larger volume in response to (1) mixing due to the increased small scale turbulence and (2) differential advection after the mixing begins due to the presence of a strong vertical gradient in horizontal wind direction. Eddy diffusivities are estimated directly from LES fields. These values agree surprisingly well with estimates from algorithms used in practical dispersion models for the undisturbed SBL. However, the practical models cannot capture effects of enhanced turbulence on eddy diffusion during episodes.