Turbulence and Vertical Fluxes in the Stable Atmospheric Boundary Layer. Part II: A Novel Mixing-Length Model

2013 ◽  
Vol 70 (6) ◽  
pp. 1528-1542 ◽  
Author(s):  
Jing Huang ◽  
Elie Bou-Zeid ◽  
Jean-Christophe Golaz
Keyword(s):  
Large Eddy Simulations ◽  
Mixing Length ◽  
New Model ◽  
Vertical Fluxes ◽  
Stable Conditions ◽  
Large Eddy ◽  
Single Column ◽  
Single Column Model ◽  
Stable Atmospheric Boundary Layer ◽  
Neutral Conditions

Abstract This is the second part of a study about turbulence and vertical fluxes in the stable atmospheric boundary layer. Based on a suite of large-eddy simulations in Part I where the effects of stability on the turbulent structures and kinetic energy are investigated, first-order parameterization schemes are assessed and tested in the Geophysical Fluid Dynamics Laboratory (GFDL)’s single-column model. The applicability of the gradient-flux hypothesis is first examined and it is found that stable conditions are favorable for that hypothesis. However, the concept of introducing a stability correction function fm as a multiplicative factor into the mixing length used under neutral conditions lN is shown to be problematic because fm computed a priori from large-eddy simulations tends not to be a universal function of stability. With this observation, a novel mixing-length model is proposed, which conforms to large-eddy simulation results much better under stable conditions and converges to the classic model under neutral conditions. Test cases imposing steady as well as unsteady forcings are developed to evaluate the performance of the new model. It is found that the new model exhibits robust performance as the stability strength is changed, while other models are sensitive to changes in stability. For cases with unsteady forcings, which are very rarely simulated or tested, the results of the single-column model and large-eddy simulations are also closer when the new model is used, compared to the other models. However, unsteady cases are much more challenging for the turbulence closure formulations than cases with steady surface forcing.

scholarly journals ANÁLISE DA PERFORMANCE DE UM MODELO DE COLUNA SIMPLES UTILIZANDO DIFERENTES PARAMETRIZAÇÕES PARA A TAXA DISSIPAÇÃO VISCOSA DE ENERGIA CINÉTICA TURBULENTA

2016 ◽  
Vol 38 ◽  
pp. 394
Author(s):  
Janete Gonçalves Lira ◽  
Eduardo Stüker ◽  
Jean Jonathan Schuster ◽  
Cristiano Henrique Schuster ◽  
Daniel Michelon dos Santos ◽  
...  
Keyword(s):  
Numerical Models ◽  
Atmospheric Flow ◽  
Stable Regime ◽  
Stable Conditions ◽  
Single Column ◽  
Weather And Climate ◽  
Single Column Model ◽  
Stable Atmospheric Boundary Layer ◽  
Weakly Stable ◽  
Turbulence Formulations

The proper description of the atmospheric flow in the stable atmospheric boundary layer is one of the most complex tasks to be performed by numerical models of weather and climate prediction. Most of the parameterizations used in these models are based on the statistical theory of turbulence in their conception. However, this theory is valid only for a flow in which turbulence is homogeneous, isotropic and stationary, a conditions that are not commonly found overnight. Basically, the nighttime flow can be classified in two major regimes: very stable, where turbulence is almost entirely suppressed; and weakly stable regime, with intense turbulent mixing. The transition between these regimes is known as atmospheric coupling, and it can happens a lot of times along the same night. In this work, we implemented a single column model with turbulence closure 1.5, in three different configurations and three different turbulence formulations. In general, the model results show that, all the configurations are able to reproduce the average characteristics of the flow in the weakly stable conditions.

scholarly journals Large-Eddy Simulation and Single-Column Modeling of Thermally Stratified Wind Turbine Arrays for Fully Developed, Stationary Atmospheric Conditions

2015 ◽  
Vol 32 (6) ◽  
pp. 1144-1162 ◽  
Author(s):  
Adrian Sescu ◽  
Charles Meneveau
Keyword(s):  
Thermal Stratification ◽  
Layer Structure ◽  
Potential Temperature ◽  
Surface Fluxes ◽  
Heat Fluxes ◽  
Shear Stresses ◽  
Column Model ◽  
Large Eddy ◽  
Single Column ◽  
Single Column Model

AbstractEffects of atmospheric thermal stratification on the asymptotic behavior of very large wind farms are studied using large-eddy simulations (LES) and a single-column model for vertical distributions of horizontally averaged field variables. To facilitate comparisons between LES and column modeling based on Monin–Obukhov similarity theory, the LES are performed under idealized conditions of statistical stationarity in time and fully developed conditions in space. A suite of simulations are performed for different thermal stratification levels and the results are used to evaluate horizontally averaged vertical profiles of velocity, potential temperature, vertical turbulent momentum, and heat flux. Both LES and the model show that the stratification significantly affects the atmospheric boundary layer structure, its height, and the surface fluxes. However, the effects of the wind farm on surface heat fluxes are found to be relatively small in both LES and the single-column model. The surface fluxes are the result of two opposing trends: an increase of mixing in wakes and a decrease in mixing in the region below the turbines due to reduced momentum fluxes there for neutral and unstable cases, or relatively unchanged shear stresses below the turbines in the stable cases. For the considered cases, the balance of these trends yields a slight increase in surface flux magnitude for the stable and near-neutral unstable cases, and a very small decrease in flux magnitude for the strongly unstable cases. Moreover, thermal stratification is found to have a negligible effect on the roughness scale as deduced from the single-column model, consistent with the expectations of separation of scale.

scholarly journals Nested Mesoscale Large-Eddy Simulations with WRF: Performance in Real Test Cases

2012 ◽  
Vol 13 (5) ◽  
pp. 1421-1441 ◽  
Author(s):  
Charles Talbot ◽  
Elie Bou-Zeid ◽  
Jim Smith
Keyword(s):  
Small Scale ◽  
Meteorological Forcing ◽  
The Real ◽  
Eddy Simulation ◽  
Stable Conditions ◽  
Large Eddy ◽  
Local Properties ◽  
Fine Resolution ◽  
Neutral Conditions ◽  
Les Model

Abstract This paper assesses the performance of the Weather Research and Forecasting Model (WRF) as a tool for multiscale atmospheric simulations. Tests are performed in real and idealized cases with multiple configurations and with resolutions ranging from the mesoscale (gridcell size ~10 km) for the real cases to local scales (gridcell size ~50 m) for both real and idealized cases. All idealized simulations and the finest real-case simulations use the turbulence-resolving large-eddy simulation mode of WRF (WRF-LES). Tests in neutral conditions and with idealized forcing are first performed to assess the model’s sensitivity to grid resolutions and subgrid-scale parameterizations and to optimize the setup of the real cases. An increase in horizontal model resolution is found to be more beneficial than an increase in vertical resolution. WRF-LES is then tested, using extensive observational data, in real-world cases over complex terrain through nested simulations in which the mesoscale domains drive the LES domains. Analysis of the mesoscale simulations indicates that the data needed to force the largest simulated domain and to initialize surface parameters have the strongest influence on the results. Similarly, LES model fields are primarily influenced by their mesoscale meteorological forcing. As a result, the nesting of LES models down to a 50-m resolution does not improve all aspects of hydrometeorological predictions. Advantages of using fine-resolution LES are noted at nighttime (under stable conditions) and over heterogeneous surfaces when local properties are required or when resolving small-scale surface features is desirable.

scholarly journals A Theoretical Analysis of Mixing Length for Atmospheric Models From Micro to Large Scales

2021 ◽  
Vol 8 ◽  
Author(s):  
Rachel Honnert ◽  
Valéry Masson ◽  
Christine Lac ◽  
Tim Nagel
Keyword(s):  
Boundary Layer ◽  
Isotropic Turbulence ◽  
Atmospheric Model ◽  
Large Eddy Simulations ◽  
Mixing Length ◽  
Gray Zone ◽  
Convective Boundary ◽  
Homogeneity Assumption ◽  
Large Eddy ◽  
The Right

A new mixing length adapted to the constraints of the hectometric-scale gray zone of turbulence for neutral and convective boundary layers is proposed. It combines a mixing length for mesoscale simulations, where the turbulence is fully subgrid and a mixing length for Large-Eddy Simulations, where the coarsest turbulent eddies are explicitly resolved. The mixing length is built for isotropic turbulence schemes, as well as schemes using the horizontal homogeneity assumption. This mixing length is tested over three boundary layer cases: a free convective case, a neutral case and a cold air outbreak case. The later combines turbulence from thermal and dynamical origins as well as presence of clouds. With this new mixing length, the turbulence scheme produces the right proportion between subgrid and resolved turbulent exchanges in Large Eddy Simulations, in the gray zone and at the mesoscale. This opens the way of using a single mixing length whatever the grid mesh of the atmospheric model, the evolution stage or the depth of the boundary layer.

scholarly journals The Extrapolation of Near-Surface Wind Speeds under Stable Stratification Using an Equilibrium-Based Single-Column Model Approach

2016 ◽  
Vol 55 (4) ◽  
pp. 923-943 ◽  
Author(s):  
Michael Optis ◽  
Adam Monahan
Keyword(s):  
Wind Profile ◽  
Stable Stratification ◽  
Internal Boundary ◽  
Near Surface ◽  
Column Model ◽  
Wind Speeds ◽  
Stable Conditions ◽  
Single Column ◽  
Equilibrium Approach ◽  
Single Column Model

AbstractClassical approaches to modeling the near-surface (i.e., below 200 m) wind profile are equilibrium based (i.e., no time evolution) and either lack a physical basis or are based on surface-layer physics. In this study, the limits of the equilibrium approach in stable stratification are further tested by applying the method within a more physically comprehensive single-column model (SCM) framework. The SCM considered here is a highly idealized momentum and temperature budget model that uses a range of different parameterizations of turbulent fluxes. A 10-yr observational dataset obtained from the 213-m Cabauw tower in the Netherlands is used to drive the SCM and to assess model performance. Results from this study demonstrate several limitations of this SCM-based equilibrium approach. The existence of two physically meaningful equilibrium solutions for a given value of the surface turbulent temperature flux (used as a lower boundary in the SCM) generally results in either a tendency to underestimate stratification or the breakdown of the model because of runaway cooling and collapsed turbulence. Different representations of the geostrophic wind profile accounting for baroclinic effects caused by the strong land–sea temperature gradient at Cabauw are shown to have only a modest influence on the mean wind profile. The local internal boundary layer (IBL) at Cabauw results in a strong tendency for the SCM to overestimate wind speeds in weakly to moderately stable conditions. In very stable conditions (where the IBL influence was low), the equilibrium approach remained limited because of its inability to account for time-evolving phenomena such as the inertial oscillation and the low-level jet.

Vertical Variations of Mixing Lengths under Neutral and Stable Conditions during CASES-99

2011 ◽  
Vol 50 (10) ◽  
pp. 2030-2041 ◽  
Author(s):  
Jielun Sun
Keyword(s):  
Similarity Theory ◽  
Mixing Length ◽  
Vertical Layer ◽  
Surface Exchange ◽  
Exchange Study ◽  
Von Karman ◽  
Stable Conditions ◽  
Vertical Variations ◽  
Von Kármán ◽  
Neutral Conditions

AbstractAn investigation on vertical variations of the mixing lengths for momentum and heat under neutral and stable conditions was conducted using the data collected from the Cooperative Atmosphere–Surface Exchange Study in 1999 (CASES-99). By comparing κz with the mixing lengths under neutral conditions calculated using the observations from CASES-99, the vertical layer where the Monin–Obukhov similarity theory (MOST) is valid was identified. Here κ is the von Kármán constant and z is the height above the ground. On average, MOST is approximately valid between 0.5 and 10 m. Above the layer, the observed mixing lengths under neutral conditions are smaller than the MOST κz and can be approximately described by Blackadar’s mixing length, κz/[1 + (κz/l∞)], with l∞ = 15 m for up to z ~ 20 m for the mixing length for momentum and up to the highest observation height for the mixing length for heat. Above ~20 m, the mixing length for momentum approaches a constant. Both MOST κz and Blackadar’s formula systematically overestimate the mixing length for momentum above ~20 m, leading to overestimates of turbulence.

Radiation Fog. Part II: Large-Eddy Simulations in Very Stable Conditions

2011 ◽  
Vol 139 (2) ◽  
pp. 193-224 ◽  
Author(s):  
Aurore Porson ◽  
Jeremy Price ◽  
Adrian Lock ◽  
Peter Clark
Keyword(s):  
Large Eddy Simulations ◽  
Radiation Fog ◽  
Stable Conditions ◽  
Large Eddy

scholarly journals A Unified Eddy-Diffusivity/Mass-Flux Approach for Modeling Atmospheric Convection

2019 ◽  
Vol 76 (8) ◽  
pp. 2505-2537 ◽  
Author(s):  
Kay Suselj ◽  
Marcin J. Kurowski ◽  
Joao Teixeira
Keyword(s):  
Mass Flux ◽  
Deep Convection ◽  
A Priori ◽  
Eddy Diffusivity ◽  
Moist Convection ◽  
Near Surface ◽  
Cold Pools ◽  
Large Eddy ◽  
Single Column ◽  
Single Column Model

Abstract A fully unified parameterization of boundary layer and moist convection (shallow and deep) is presented. The new parameterization is based on the stochastic multiplume eddy-diffusivity/mass-flux (EDMF) approach, which distinguishes between convective plumes and nonconvective mixing. The convective plumes represent both surface-forced updrafts and evaporatively driven downdrafts. The type of convection (i.e., dry, shallow, or deep) represented by the updrafts is not defined a priori, but rather depends on the near-surface updraft properties and the stochastic interactions between the plumes and the environment through lateral entrainment. Consequently, some updrafts may contribute only to the nonlocal transport within the subcloud layer, while others may condense and form shallow or even deep convection. Such a formulation is void of trigger functions and additional closures typical of modular parameterizations. The updrafts are coupled to relatively simple warm-, mixed-, and ice-phase microphysics. Each precipitating updraft forms a downdraft driven by the evaporation of detrained precipitation. The downdrafts control the development of cold pools near the surface that can invigorate convection. The new parameterization is tested in a single-column model against large-eddy simulations (LESs) for cases representing weakly precipitating marine convection and the diurnal cycle of continental deep convection. The results of these EDMF experiments compare well with the LES reference simulations. In particular, the transitions between the different dominant convection regimes are realistically simulated.

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