scholarly journals Bulk Models of the Sheared Convective Boundary Layer: Evaluation through Large Eddy Simulations

2007 ◽  
Vol 64 (3) ◽  
pp. 786-807 ◽  
Author(s):  
Robert Conzemius ◽  
Evgeni Fedorovich
Keyword(s):  
Boundary Layer ◽  
Convective Boundary Layer ◽  
Entrainment Rate ◽  
Convective Boundary ◽  
Surface Shear ◽  
Large Eddy ◽  
Entrainment Zone ◽  
Sheared Convective Boundary Layer ◽  
Bulk Model ◽  
Approach Zero

Abstract A set of first-order model (FOM) equations, describing the sheared convective boundary layer (CBL) evolution, is derived. The model output is compared with predictions of the zero-order bulk model (ZOM) for the same CBL type. Large eddy simulation (LES) data are employed to test both models. The results show an advantage of the FOM over the ZOM in the prediction of entrainment, but in many CBL cases, the predictions by the two models are fairly close. Despite its relative simplicity, the ZOM is able to quantify the effects of shear production and dissipation in an integral sense—as long as the constants describing the integral dissipation of shear- and buoyancy-produced turbulence kinetic energy (TKE) are prescribed appropriately and the shear is weak enough that the denominator of the ZOM entrainment equation does not approach zero, causing a numerical instability in the solutions. Overall, the FOM better predicts the entrainment rate due to its ability to avoid this instability. Also, the FOM in a more physically consistent manner reproduces the sheared CBL entrainment zone, whose depth is controlled by a balance among shear generation, buoyancy consumption, and dissipation of TKE. Such balance is manifested by nearly constant values of Richardson numbers observed in the entrainment zone of simulated sheared CBLs. Conducted model tests support the conclusion that the surface shear generation of TKE and its corresponding dissipation, as well as the nonstationary terms, can be omitted from the integral TKE balance equation.

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.

scholarly journals Lagrangian Particle Dispersion Modeling of the Fumigation Process Using Large-Eddy Simulation

2005 ◽  
Vol 62 (6) ◽  
pp. 1932-1946 ◽  
Author(s):  
Si-Wan Kim ◽  
Chin-Hoh Moeng ◽  
Jeffrey C. Weil ◽  
Mary C. Barth
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Convective Boundary Layer ◽  
Particle Dispersion ◽  
Lagrangian Particle ◽  
Convective Boundary ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Entrainment Zone ◽  
The Mean

Abstract A Lagrangian particle dispersion model (LPDM) is used to study fumigation of pollutants in and above the entrainment zone into a growing convective boundary layer. Probability density functions of particle location with height and time are calculated from particle trajectories driven by the sum of the resolved-scale velocity from a large-eddy simulation (LES) model and the stochastic subgrid-scale (SGS) velocity. The crosswind-integrated concentration (CWIC) fields show good agreement with water tank experimental data. A comparison of the LPDM output with an Eulerian diffusion model output based on the same LES flow shows qualitative agreement with each other except that a greater overshoot maximum of the ground-level concentration occurs in the Eulerian model. The dimensionless CWICs near the surface for sources located above the entrainment zone collapse to a nearly universal curve provided that the profiles are time shifted, where the shift depends on the source heights. The dimensionless CWICs for sources located within the entrainment zone show a different behavior. Thus, fumigation from sources above the entrainment zone and within the entrainment zone should be treated separately. An examination of the application of Taylor’s translation hypothesis to the fumigation process showed the importance of using the mean boundary layer wind speed as a function of time rather than the initial mean boundary layer wind speed, because the mean boundary layer wind speed decreases as the simulation proceeds. The LPDM using LES is capable of accurately simulating fumigation of particles into the convective boundary layer. This technique provides more computationally efficient simulations than Eulerian models.

scholarly journals The Two-Layer Structure of the Entrainment Zone in the Convective Boundary Layer

2014 ◽  
Vol 71 (6) ◽  
pp. 1935-1955 ◽  
Author(s):  
Jade Rachele Garcia ◽  
Juan Pedro Mellado
Keyword(s):  
Boundary Layer ◽  
Convective Boundary Layer ◽  
Layer Structure ◽  
Finite Thickness ◽  
Buoyancy Frequency ◽  
Length Scale ◽  
Entrainment Rate ◽  
Entrainment Ratio ◽  
Convective Boundary ◽  
Entrainment Zone

Abstract The entrainment zone (EZ) of a dry, shear-free convective boundary layer growing into a linearly stratified fluid is studied by means of direct numerical simulation. The scale separation between the boundary layer thickness and the Kolmogorov length scale is shown to be sufficient to observe Reynolds number similarity in the statistics of interest during the equilibrium entrainment regime. Contrary to previous considerations, the vertical structure of the entrainment zone is found to be better described by the superposition of two sublayers: 1) an upper EZ sublayer that is dominated by overshooting thermals and is characterized by a penetration depth that scales with the ratio of the convective velocity and the buoyancy frequency of the free troposphere and 2) a lower EZ sublayer that is dominated by troughs of mixed fluid and is characterized by the integral length scale of the mixed layer. Correspondingly, different buoyancy scales are identified. The consequences of this multiplicity of scales on the entrainment rate parameters are evaluated directly, without resorting to any bulk model, through an exact relation among the mean entrainment rate, the local buoyancy increment, and both the turbulent and the finite-thickness contributions to the entrainment ratio A measured at the height of minimum buoyancy flux. The smaller turbulent contribution to A that is usually observed for relatively thick EZs is found to be compensated by the smaller local buoyancy increment instead of by the finite-thickness contribution. The two-layer structure of the entrainment zone is found to affect the exponent of the power-law relation between the normalized mean entrainment rate and the convective Richardson number such that the exponent deviates from −1 for typical atmospheric conditions, although it asymptotically approaches −1 for higher Richardson numbers.

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