An Empirical Scaling Analysis of Heat and Momentum Cospectra Above the Surface Friction Layer in a Convective Boundary Layer

2018 ◽  
Vol 170 (2) ◽  
pp. 257-284 ◽  
Author(s):  
Subharthi Chowdhuri ◽  
Keith G. McNaughton ◽  
Thara V. Prabha
Keyword(s):  
Boundary Layer ◽  
Convective Boundary Layer ◽  
Surface Friction ◽  
Scaling Analysis ◽  
Convective Boundary ◽  
Friction Layer

scholarly journals Scaling properties of velocity and temperature spectra above the surface friction layer in a convective atmospheric boundary layer

2007 ◽  
Vol 14 (3) ◽  
pp. 257-271 ◽  
Author(s):  
K. G. McNaughton ◽  
R. J. Clement ◽  
J. B. Moncrieff
Keyword(s):  
Boundary Layer ◽  
Kinetic Energy ◽  
Surface Friction ◽  
Inertial Subrange ◽  
Turbulence Energy ◽  
Convective Boundary ◽  
Friction Layer ◽  
Thermal Structures ◽  
Temperature Spectra ◽  
Velocity Spectra

Abstract. We report velocity and temperature spectra measured at nine levels from 1.42 meters up to 25.7 m over a smooth playa in Western Utah. Data are from highly convective conditions when the magnitude of the Obukhov length (our proxy for the depth of the surface friction layer) was less than 2 m. Our results are somewhat similar to the results reported from the Minnesota experiment of Kaimal et al. (1976), but show significant differences in detail. Our velocity spectra show no evidence of buoyant production of kinetic energy at at the scale of the thermal structures. We interpret our velocity spectra to be the result of outer eddies interacting with the ground, not "local free convection". We observe that velocity spectra represent the spectral distribution of the kinetic energy of the turbulence, so we use energy scales based on total turbulence energy in the convective boundary layer (CBL) to collapse our spectra. For the horizontal velocity spectra this scale is (zi εo)2/3, where zi is inversion height and εo is the dissipation rate in the bulk CBL. This scale functionally replaces the Deardorff convective velocity scale. Vertical motions are blocked by the ground, so the outer eddies most effective in creating vertical motions come from the inertial subrange of the outer turbulence. We deduce that the appropriate scale for the peak region of the vertical velocity spectra is (z εo)2/3 where z is height above ground. Deviations from perfect spectral collapse under these scalings at large and small wavenumbers are explained in terms of the energy transport and the eddy structures of the flow. We find that the peaks of the temperature spectra collapse when wavenumbers are scaled using (z1/2 zi1/2). That is, the lengths of the thermal structures depend on both the lengths of the transporting eddies, ~9z, and the progressive aggregation of the plumes with height into the larger-scale structures of the CBL. This aggregation depends, in top-down fashion, on zi. The whole system is therefore highly organized, with even the smallest structures conforming to the overall requirements of the whole flow.

scholarly journals Dynamics of Sheared Convective Boundary Layer Entrainment. Part II: Evaluation of Bulk Model Predictions of Entrainment Flux

2006 ◽  
Vol 63 (4) ◽  
pp. 1179-1199 ◽  
Author(s):  
Robert J. Conzemius ◽  
Evgeni Fedorovich
Keyword(s):  
Boundary Layer ◽  
Convective Boundary Layer ◽  
Wind Shear ◽  
Flux Ratio ◽  
Surface Friction ◽  
Ratio Test ◽  
Convective Boundary ◽  
Surface Shear ◽  
Entrainment Zone ◽  
Bulk Model

Abstract Several bulk model–based entrainment parameterizations for the atmospheric convective boundary layer (CBL) with wind shear are reviewed and tested against large-eddy simulation (LES) data to evaluate their ability to model one of the basic integral parameters of convective entrainment—the entrainment flux ratio. Test results indicate that many of these parameterizations fail to correctly reproduce entrainment flux in the presence of strong shear because they underestimate the dissipation of turbulence kinetic energy (TKE) produced by shear in the entrainment zone. It is also found that surface shear generation of TKE may be neglected in the entrainment parameterization because it is largely balanced by dissipation. However, the surface friction has an indirect effect on the entrainment through the modification of momentum distribution in the mixed layer and regulation of shear across the entrainment zone. Because of this effect, parameterizations that take into account the surface friction velocity but exclude entrainment zone shear may sufficiently describe entrainment when wind shear in the free atmosphere above the CBL is small. In this case, the surface shear acts as a proxy for the entrainment zone shear. Such parameterizations can be most useful if applied in situations where atmospheric data are insufficient for calculating entrainment zone shear. The importance of modeling a Richardson-number-limited, finite-depth entrainment zone is evidenced by the relatively accurate entrainment flux predictions by models that explicitly account for effects of entrainment zone shear, but predictions by these models are often adversely affected by the underestimation of TKE dissipation in the entrainment zone.

A Numerical Model for Dispersion in a Convective Boundary Layer

1993 ◽  
Vol 50 (22) ◽  
pp. 3775-3789 ◽  
Author(s):  
Nizam U. Ahmed ◽  
Steven K. Chai ◽  
James W. Telford
Keyword(s):  
Boundary Layer ◽  
Numerical Model ◽  
Convective Boundary Layer ◽  
Convective Boundary
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