P3.8 Large-eddy‭ ‬simulation of the stable boundary layer with a focus on low-level jet and boundary layer depth

Monday, 2 August 2010
Castle Peak Ballroom (Keystone Resort)
Bowen Zhou, University of California, Berkeley, Berkeley, CA; and F. K. Chow

Large-Eddy Simulation‭ (‬LES‭) ‬of the stably stratified atmospheric boundary layer is performed using an explicit filtering and reconstruction approach with a finite difference method.‭ ‬Turbulent stresses are split into the Resolvable Subfilter-Scale‭ (‬RSFS‭) ‬stresses and Subgrid-Scale‭ (‬SGS‭) ‬stresses.‭ ‬The former are recovered from a velocity reconstruction approach,‭ ‬and the latter are represented by a dynamic eddy-viscosity model.‭ Subfilter-Scale (SFS) dissipation and backscatter ‬events are statistically characterized, showing the ability of the RSFS/SGS framework to allow for energy exchange from small to large scales. A Low-Level Jet‭ (‬LLJ‭) ‬develops in the simulations,‭ ‬and wind shear in the jet is responsible for generating additional‭ ‬turbulent‭ ‬kinetic‭ ‬energy‭ (‬TKE‭) ‬around the top of the boundary layer.‭ ‬This elevated TKE is unexpected based on previous modeling results with traditional turbulence closures,‭ ‬but is in agreement with field observations under certain conditions.‭ ‬Boundary layer depth is an important parameter in the‭ ‬stable‭ ‬boundary‭ ‬layer‭ (‬SBL‭) ‬simulations.‭ ‬Unlike neutral or convective cases,‭ ‬where boundary layer depth is usually defined by the residual inversion layer,‭ ‬the SBL depth is formed in the simulation due‭ ‬to‭ ‬a self-developed over-capping inversion layer.‭ ‬The SBL depth is mainly affected by the strength of stratification,‭ ‬geostrophic wind forcing,‭ ‬and surface roughness.‭ ‬Many previous studies in the literature have shown a strong dependence of SBL depth on‭ ‬the‭ ‬turbulence model,‭ ‬and grid resolution.‭ ‬Higher grid resolution usually means a general decrease‭ ‬in‭ ‬boundary layer depth‭ ‬and‭ ‬an‭ ‬increase‭ ‬in‭ ‬jet strength.‭ ‬A grid refinement study using the reconstruction model is performed to compare with conventional eddy viscosity closures.‭
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