Large Eddy Simulation and Study of the Urban Boundary Layer
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Abstract:
Based on a pseudo-spectral large eddy simulation (LES) model, an LES model with an anisotropy turbulent kinetic energy (TKE) closure model and an explicit multi-stage third-order Runge-Kutta scheme is established. The modeling and analysis show that the LES model can simulate the planetary boundary layer (PBL) with a uniform underlying surface under various stratifications very well. Then, similar to the description of a forest canopy, the drag term on momentum and the production term of TKE by subgrid city buildings are introduced into the LES equations to account for the area-averaged effect of the subgrid urban canopy elements and to simulate the meteorological fields of the urban boundary layer (UBL). Numerical experiments and comparison analysis show that: (1) the result from the LES of the UBL with a proposed formula for the drag coefficient is consistent and comparable with that from wind tunnel experiments and an urban subdomain scale model; (2) due to the effect of urban buildings, the wind velocity near the canopy is decreased, turbulence is intensified, TKE, variance, and momentum flux are increased, the momentum and heat flux at the top of the PBL are increased, and the development of the PBL is quickened; (3) the height of the roughness sublayer (RS) of the actual city buildings is the maximum building height (1.5-3 times the mean building height), and a constant flux layer (CFL) exists in the lower part of the UBL.Keywords:
Urban climatology
Large-Eddy Simulation
Momentum (technical analysis)
Roughness length
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Large-Eddy Simulation
Laminar sublayer
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In mesoscale numerical weather predictionmodel (MM4), original bulk boundary layer parameterizations was improved by two turbulence closure schemes level 3 and %E－e－ l%, so as to straightly output turbulence quantities in the mesoscale model. At the same time surface flux algorithm was also improved. Furthermore, we analyze turbulence quantities and boundary layer characteristic quantities on different underlying surfaces and study boundary layer structure characteristics in the mesoscale system. In this paper, we study surface flux, turbulence exchange coefficient, turbulent kinetic energy(q2), temperature fluctuation deviation( 2) and profiles of velocity and temperature. The results show that improved schemes can reflect characteristics of boundary layer well. For example, the sensible heat flux and the height of PBL have significant diurnal variation for various surfaces; the sensible heat flux over desert usually is larger than that over vegetation; the spatial and temporal variations of the eddy exchange coefficient are more close to the general law than original PBL scheme; usually the eddy kinetic energy increases with height near the surface, then decreases with height after the maximum value, is reached for unstable stratification but deceases with height monotonically for stable stratification the dissipation rate of the eddy kinetic energy and the variance of the fluctuating potential temperature usually decrease with height.
Potential temperature
Stratification (seeds)
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Based on a large eddy simulation (LES) model with anisotropy subgrid scale closure model and an explicit multi-stage third-order Runge-Kutta scheme, the drag term on momentum, the heat source term and the production term of turbulent kinetic energy (TKE) imposed by the canopy elements are introduced to the LES equations to simulate the meteorological field in the forest canopy and the forest boundary layer. The analysis of simulation results and the comparison with observational data show that the LES model simulates the turbulent flow in forest canopy and forest boundary layer very well. Further research indicates that, under unstable stratification, the turbulent structure of Kinking and Pairing in the dense forest canopy, together with the large eddy structure in forest boundary layer, forms the temperature ramp near the forest canopy.
Large-Eddy Simulation
Tree canopy
Stratification (seeds)
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Large-Eddy Simulation
Length scale
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Parametrization (atmospheric modeling)
Momentum (technical analysis)
Eddy diffusion
Large-Eddy Simulation
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Microscale chemistry
Large-Eddy Simulation
Mean flow
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In this study, a Large-Eddy Simulation model that is capable of resolving urban buildings was used to investigate the influence over turbulence statistics caused by the existence of urban geometry.In the convective boundary layer, the horizontal temperature variances from both flat and urban geometry are smaller than the field observation results. And the horizontal velocity variances in the urban ABL are slightly smaller than those for a flat throughout the whole range of height. In the surface layer, the results of the urban simulation are not in agreement with the results from observations. And that is partly caused by the disability of current SGS model.
Convective Boundary Layer
Large-Eddy Simulation
Urban climatology
Similarity (geometry)
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Turbulence Modeling
Momentum (technical analysis)
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Turbulence Modeling
Forcing (mathematics)
Large-Eddy Simulation
Surface layer
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Citations (507)