scholarly journals Monin–Obukhov Similarity Functions for the Structure Parameters of Temperature and Humidity in the Unstable Surface Layer: Results from High-Resolution Large-Eddy Simulations

2014 ◽  
Vol 71 (2) ◽  
pp. 716-733 ◽  
Author(s):  
Björn Maronga
Keyword(s):  
Surface Layer ◽  
Free Convection ◽  
Measurement Data ◽  
Surface Fluxes ◽  
Large Eddy Simulations ◽  
Structure Parameters ◽  
Similarity Functions ◽  
Dry Air ◽  
Large Eddy ◽  
Temperature And Humidity

Abstract Large-eddy simulations (LESs) of free-convective to near-neutral boundary layers are used to investigate the surface-layer turbulence. The article focuses on the Monin–Obukhov similarity theory (MOST) relationships that relate the structure parameters of temperature and humidity to the surface fluxes of sensible and latent heat, respectively. Moreover, the applicability of local free convection (LFC) similarity scaling is studied. The LES data suggest that the MOST function for is universal. It is shown to be within the range of the functions proposed from measurement data. It is found that follows MOST if entrainment of dry air from the free atmosphere is sufficiently small. In this case the similarity functions for and are identical. If entrainment is significant, dissimilarity between the transport of sensible heat and moisture is observed and no longer follows MOST. In the free-convection limit the LFC similarity functions should collapse to universal constants. The LES data suggest values around 2.7, which is in agreement with the value proposed in the literature. As for MOST, the LFC similarity constant for becomes nonuniversal if entrainment of dry air is significant. It is shown that LFC scaling is applicable even if shear production of turbulence is moderately high.

scholarly journals On the Formulation and Universality of Monin–Obukhov Similarity Functions for Mean Gradients and Standard Deviations in the Unstable Surface Layer: Results from Surface-Layer-Resolving Large-Eddy Simulations

2017 ◽  
Vol 74 (4) ◽  
pp. 989-1010 ◽  
Author(s):  
Björn Maronga ◽  
Joachim Reuder
Keyword(s):  
Surface Layer ◽  
Standard Deviation ◽  
Large Eddy Simulations ◽  
Specific Humidity ◽  
Horizontal Wind ◽  
Convective Conditions ◽  
Similarity Functions ◽  
Large Eddy ◽  
The Mean ◽  
Standard Deviations

Abstract Surface-layer-resolving large-eddy simulations (LESs) of free-convective to near-neutral boundary layers are used to study Monin–Obukhov similarity theory (MOST) functions. The LES dataset, previously used for the analysis of MOST relationships for structure parameters, is extended for the mean vertical gradients and standard deviations of potential temperature, specific humidity, and wind. Also, local-free-convection (LFC) similarity is studied. The LES data suggest that the MOST functions for mean gradients are universal and unique. The data for the mean gradient of the horizontal wind display significant scatter, while the gradients of temperature and humidity vary considerably less. The LES results suggest that this scatter is mostly related to a transition from MOST to LFC scaling when approaching free-convective conditions and that it is associated with a change of the slope of the similarity functions toward the expected value from LFC scaling. Overall, the data show slightly, but consistent, steeper slopes of the similarity functions than suggested in literature. The MOST functions for standard deviations appear to be unique and universal when the entrainment from the free atmosphere into the boundary layer is sufficiently small. If entrainment becomes significant, however, we find that the standard deviation of humidity no longer follows MOST. Under free-convective conditions, the similarity functions should reduce to universal constants (LFC scaling). This is supported by the LES data, showing only little scatter, but displaying a systematic height dependence of these constants. Like for MOST, the LFC similarity constant for the standard deviation of specific humidity becomes nonuniversal when the entrainment of dry air reaches significant levels.

scholarly journals Similarity Theory in the Surface Layer of Large-Eddy Simulations of the Wind-, Wave-, and Buoyancy-Forced Southern Ocean

2019 ◽  
Vol 49 (8) ◽  
pp. 2165-2187 ◽  
Author(s):  
William G. Large ◽  
Edward G. Patton ◽  
Alice K. DuVivier ◽  
Peter P. Sullivan ◽  
Leonel Romero
Keyword(s):  
Surface Layer ◽  
Southern Ocean ◽  
Wind Wave ◽  
Similarity Theory ◽  
Large Eddy Simulations ◽  
Stokes Drift ◽  
Total Production ◽  
Similarity Functions ◽  
Wide Range ◽  
Large Eddy

AbstractMonin–Obukhov similarity theory is applied to the surface layer of large-eddy simulations (LES) of deep Southern Ocean boundary layers. Observations from the Southern Ocean Flux Station provide a wide range of wind, buoyancy, and wave (Stokes drift) forcing. Two No-Stokes LES are used to determine the extent of the ocean surface layer and to adapt the nondimensional momentum and buoyancy gradients, as functions of the stability parameter. Stokes-forced LES are used to modify this parameter for wave effects, then to formulate dependencies of Stokes similarity functions on a Stokes parameter ξ. To account for wind-wave misalignment, the dimensional analysis is extended with two independent variables, namely, the production of turbulent kinetic energy in the surface layer due to Stokes shear and the total production, so that their ratio gives ξ. Stokes forcing is shown to reduce vertical shear more than stratification, and to enhance viscosity and diffusivity by factors up to 5.8 and 4.0, respectively, such that the Prandtl number can exceed unity. A practical parameterization is developed for ξ in terms of the meteorological forcing plus a Stokes drift profile, so that the Stokes and stability similarity functions can be combined to give turbulent velocity scales. These scales for both viscosity and diffusivity are evaluated against the LES, and the correlations are nearly 0.97. The benefit of calculating Stokes drift profiles from directional wave spectra is demonstrated by similarly evaluating three alternatives.

Sensitivity studies with different formulations for similarity functions used in surface layer scheme, over the tropical region, in a climate model

2020 ◽  
Author(s):  
Prabhakar Namdev ◽  
Maithili Sharan ◽  
Saroj Kanta Mishra
Keyword(s):  
Boundary Layer ◽  
Surface Layer ◽  
Climate Model ◽  
Stable Stratification ◽  
Turbulent Fluxes ◽  
Surface Fluxes ◽  
Tropical Region ◽  
Similarity Functions ◽  
Unstable Stratification ◽  
Stable Conditions

<p>The lowest portion of the planetary boundary layer (PBL), where the turbulent fluxes are assumed to be constant, is known as the atmospheric surface layer (ASL). Within the surface layer, the surface exchange processes play an important role in land-atmosphere interaction. Thus, a precise formulation of the surface fluxes is crucial to ensure an adequate atmospheric evolution by numerical models. The Monin–Obukhov Similarity Theory (MOST) is a widely used framework to estimate the surface turbulent fluxes within the surface layer. MOST uses similarity functions of momentum (φ<sub>m</sub>) and heat (φ<sub>h</sub>) for non-dimensional wind and temperature profiles. Over the years, various formulations for these similarity functions have been proposed by the researchers ranging from linear to non-linear forms. These formulations have limitations in the weak wind, stable, and unstable atmospheric conditions. In the surface layer scheme currently available in the Community Atmosphere Model version 5 (CAM5.0), the stable and unstable regimes are divided into four parts, and the corresponding similarity functions are the functions proposed by Kader and Yaglom (1990) for strong unstable stratification, by Businger et al. (1971) for weak unstable stratification, functions by Dyer (1974) for weak stable stratification, and for moderate to strongly stable stratification, the functions from Holtslag et al. (1990) have been utilized. The criteria used for this classification are somewhat ad-hoc, and there is an abrupt transition between different regimes encountered.            </p><p>       In the present study, an effort has been made to implement the similarity functions proposed by Grachev et al. (2007) for stable conditions and Fairall et al. (1996) for unstable conditions in the surface layer scheme of Community Land Model (CLM) for CAM5.0. In the modified version, the similarity functions for unstable conditions are a combination of commonly used Paulson type expressions for near-neutral stratification and an expression proposed by Carl et al. (1973) that takes in to account highly convective conditions. Similarly, the formulation proposed by Grachev et al., for stable conditions, can cover a wider range of stable stratifications. The simulations with CAM5 model using the existing and modified version of surface layer scheme have been performed with 1° resolution for ten years, and the impact of modified functions on the simulation of various important near-surface variables over the tropical region is analyzed. It is found that the scheme with modified functions improving the simulation of surface variables as compared with the existing scheme over the tropical region. In addition, the limitations arbitrarily imposed on particular variables in the existing surface layer scheme can be eliminated or suppressed by using these modified functions.  </p><p>References:</p><p>Fairall CW, Bradley EF, Rogers DP, Edson JB, Young GS (1996) Bulk parameterization of air-sea fluxes for tropical ocean global atmosphere coupled-ocean atmosphere response experiment. J Geophys Res 101(C2):3747–3764</p><p>Grachev, A.A., Andreas, E.L., Fairall, C.W., Guest, P.S. and Persson, P.O.G. (2007a) SHEBA: flux–profile relationships in stable atmospheric boundary layer. Boundary-Layer Meteorology,124, 315–333.</p><p>Keywords:</p><p>Boundary layer, Turbulence, Climate Model, Surface Fluxes</p>

Optimizing large-eddy simulations for investigating the energy-balance closure problem at typical flux measurement heights

2020 ◽  
Author(s):  
Luise Wanner ◽  
Frederik De Roo ◽  
Matthias Mauder
Keyword(s):  
Energy Balance ◽  
Eddy Covariance ◽  
Surface Fluxes ◽  
Heat Fluxes ◽  
Large Eddy Simulations ◽  
Scale Model ◽  
Energy Balance Closure ◽  
Balance Study ◽  
Large Eddy ◽  
Secondary Circulations

<p>The eddy-covariance method generally underestimates sensible and latent heat fluxes, resulting in an energy-balance gap from 10 % to even 30 % across sites worldwide. In contrast to single-tower eddy-covariance measurements, large-eddy simulations (LES) provide information on a 3D array of grid points and can capture atmospheric processes such as secondary circulations on all relevant scales, which makes them a powerful tool to investigate this problem. In order to compare LES results to field measurements at 20 m height from the CHEESEHEAD (Chequamegon Heterogeneous Ecosystem Energy-balance Study Enabled by a High-density Extensive Array of Detectors) campaign, a LES-setup that provides comparability to the measurements at these low levels is necessary. However, former LES studies have shown that the energy balance is almost closed near the surface, which does not reflect the energy-balance gap in measurements. One possible reason might be the common use of prescribed surface fluxes that cannot adapt to changes in surface temperature and moisture, which would allow for the self-reinforcement of secondary circulations. Therefore, we set up an idealized study, in which we compare the performance of the land-surface and plant-canopy models implemented in PALM to the use of prescribed surface fluxes above homogeneous forest and grassland ecosystems under different atmospheric conditions with respect to realistic energy-balance closure behavior. Furthermore, we evaluate the performance of a dynamic subgrid-scale model, as well as an alternative to the Monin-Obukhov similarity theory (Banerjee et al. 2015, Q. J. R. Met. Soc.).</p>

Large-eddy simulations of convection initiation over heterogeneous, low terrain

2022 ◽  
Keyword(s):  
Kinetic Energy ◽  
Free Convection ◽  
Deep Convection ◽  
Physical Space ◽  
Peak Height ◽  
Spectral Shape ◽  
Large Eddy Simulations ◽  
Power Spectral ◽  
Large Eddy ◽  
Convection Initiation

Abstract Large-eddy simulations are conducted to investigate and physically interpret the impacts of heterogeneous, low terrain on deep-convection initiation (CI). The simulations are based on a case of shallow-to-deep convective transition over the Amazon River basin, and use idealized terrains with varying levels of ruggedness. The terrain is designed by specifying its power-spectral shape in wavenumber space, inverting to physical space assuming random phases for all wave modes, and scaling the terrain to have a peak height of 200 m. For the case in question, these modest terrain fields expedite CI by up to 2-3 h, largely due to the impacts of the terrain on the size of, and subcloud support for, incipient cumuli. Terrain-induced circulations enhance subcloud kinetic energy on the mesoscale, which is realized as wider and longer-lived subcloud circulations. When the updraft branches of these circulations breach the level of free convection, they initiate wider and more persistent cumuli that subsequently undergo less entrainment-induced cloud dilution and detrainment-induced mass loss. As a result, the clouds become more vigorous and penetrate deeper into the troposphere. Larger-scale terrains are more effective than smaller-scale terrains in promoting CI because they induce larger enhancements in both the width and the persistence of subcloud updrafts.

The Use of Large-Eddy Simulations in Lagrangian Particle Dispersion Models

2004 ◽  
Vol 61 (23) ◽  
pp. 2877-2887 ◽  
Author(s):  
Jeffrey C. Weil ◽  
Peter P. Sullivan ◽  
Chin-Hoh Moeng
Keyword(s):  
Boundary Layer ◽  
Surface Layer ◽  
Dispersion Model ◽  
Particle Dispersion ◽  
Large Eddy Simulations ◽  
Surface Source ◽  
Mean Velocity ◽  
Convective Boundary ◽  
Large Eddy ◽  
Better Than

Abstract A Lagrangian dispersion model driven by velocity fields from large-eddy simulations (LESs) is presented for passive particle dispersion in the planetary boundary layer (PBL). In this combined LES–Lagrangian stochastic model (LSM), the total velocity is divided into resolved or filtered and unresolved or subfilter-scale (SFS) velocities. The random SFS velocity is modeled using an adaptation of Thomson's LSM in which the ensemble-mean velocity and velocity variances are replaced by the resolved velocity and SFS variances, respectively. The random SFS velocity forcing has an amplitude determined by the SFS fraction of the total turbulent kinetic energy (TKE); the fraction is about 0.15 in the bulk of the simulated convective boundary layer (CBL) used here and reaches values as large as 0.31 and 0.37 in the surface layer and entrainment layer, respectively. For the proposed LES–LSM, the modeled crosswind-integrated concentration (CWIC) fields are in good agreement with the 1) surface-layer similarity (SLS) theory for a surface source in the CBL and 2) convection tank measurements of the CWIC for an elevated release in the CBL surface layer. The second comparison includes the modeled evolution of the vertical profile shape with downstream distance, which shows the attainment of an elevated CWIC maximum and a vertically well-mixed CWIC far downstream, in agreement with the tank data. For the proposed model, the agreement with the tank data and SLS theory is better than that obtained with an earlier model in which the SFS fraction of the TKE is assumed to be 1, and significantly better than a model that neglects the SFS velocities altogether.

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