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.

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.

Aspects of the convective boundary layer structure over complex terrain

1998 ◽  
Vol 32 (7) ◽  
pp. 1323-1348 ◽  
Author(s):  
M. Kossmann ◽  
R. Vögtlin ◽  
U. Corsmeier ◽  
B. Vogel ◽  
F. Fiedler ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Convective Boundary Layer ◽  
Complex Terrain ◽  
Layer Structure ◽  
Convective Boundary ◽  
Boundary Layer Structure

scholarly journals The performance of RAMS in representing the convective boundary layer structure in a very steep valley

2005 ◽  
Vol 5 (1-2) ◽  
pp. 35-62 ◽  
Author(s):  
Stephan F. J. De Wekker ◽  
D. G. Steyn ◽  
J. D. Fast ◽  
M. W. Rotach ◽  
S. Zhong
Keyword(s):  
Boundary Layer ◽  
Convective Boundary Layer ◽  
Layer Structure ◽  
Convective Boundary ◽  
Boundary Layer Structure

scholarly journals Measurement report: characteristics of clear-day convective boundary layer and associated entrainment zone as observed by a ground-based polarization lidar over Wuhan (30.5° N, 114.4° E)

2021 ◽  
Vol 21 (4) ◽  
pp. 2981-2998
Author(s):  
Fuchao Liu ◽  
Fan Yi ◽  
Zhenping Yin ◽  
Yunpeng Zhang ◽  
Yun He ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Convective Boundary Layer ◽  
Growth Stage ◽  
Late Spring ◽  
Atmospheric Measurements ◽  
Convective Boundary ◽  
Late Autumn ◽  
Early Autumn ◽  
Entrainment Zone ◽  
Aerosol Backscatter

Abstract. Knowledge of the convective boundary layer (CBL) and associated entrainment zone (EZ) is important for understanding land–atmosphere interactions and assessing the living conditions in the biosphere. A tilted 532 nm polarization lidar (30∘ off zenith) has been used for the routine atmospheric measurements with 10 s time and 6.5 m height resolution over Wuhan (30.5∘ N, 114.4∘ E). From lidar-retrieved aerosol backscatter, instantaneous atmospheric boundary layer (ABL) depths are obtained using the logarithm gradient method and Harr wavelet transform method, while hourly mean ABL depths are obtained using the variance method. A new approach utilizing the full width at half maximum of the variance profile of aerosol backscatter ratio fluctuations is proposed to determine the entrainment zone thickness (EZT). Four typical clear-day observational cases in different seasons are presented. The CBL evolution is described and studied in four developing stages (formation, growth, quasi-stationary and decay); the instantaneous CBL depths exhibited different fluctuation magnitudes in the four stages and fluctuations at the growth stage were generally larger. The EZT is investigated for the same statistical time interval of 09:00–19:00 LT. It is found that the winter and late autumn cases had an overall smaller mean (mean) and standard deviation (SD) of EZT data compared to those of the late spring and early autumn cases. This statistical conclusion was also true for each of the four developing stages. In addition, compared to those of the late spring and early autumn cases, the winter and late autumn cases had larger percentages of EZT falling into the subranges of 0–50 m but smaller percentages of EZT falling into the subranges of > 150 m. It seems that both the EZT statistics (mean and SD) and percentage of larger EZT values provide measures of entrainment intensity. Common statistical characteristics also existed. All four cases showed moderate variations of the mean of the EZT from stage to stage. The growth stage always had the largest mean and SD of the EZT and the quasi-stationary stage usually the smallest SD of the EZT. For all four stages, most EZT values fell into the 50–150 m subrange; the overall percentage of the EZT falling into the 50–150 m subrange between 09:00 and 19:00 LT was > 67 % for all four cases. We believe that the lidar-derived characteristics of the clear-day CBL and associated EZ can contribute to improving our understanding of the structures and variations of the CBL as well as providing a quantitatively observational basis for EZ parameterization in numerical models.

scholarly journals On the Relationship between the TKE Dissipation Rate and the Temperature Structure Function Parameter in the Convective Boundary Layer

2020 ◽  
Vol 77 (7) ◽  
pp. 2311-2326
Author(s):  
Hubert Luce ◽  
Lakshmi Kantha ◽  
Hiroyuki Hashiguchi ◽  
Abhiram Doddi ◽  
Dale Lawrence ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Structure Function ◽  
Convective Boundary Layer ◽  
Dissipation Rate ◽  
Function Parameter ◽  
Mixing Efficiency ◽  
Buoyancy Frequency ◽  
Temperature Structure ◽  
Convective Boundary

AbstractUnder stably stratified conditions, the dissipation rate ε of turbulence kinetic energy (TKE) is related to the structure function parameter for temperature , through the buoyancy frequency and the so-called mixing efficiency. A similar relationship does not exist for convective turbulence. In this paper, we propose an analytical expression relating ε and in the convective boundary layer (CBL), by taking into account the effects of nonlocal heat transport under convective conditions using the Deardorff countergradient model. Measurements using unmanned aerial vehicles (UAVs) equipped with high-frequency response sensors to measure velocity and temperature fluctuations obtained during the two field campaigns conducted at Shigaraki MU observatory in June 2016 and 2017 are used to test this relationship between ε and in the CBL. The selection of CBL cases for analysis was aided by auxiliary measurements from additional sensors (mainly radars), and these are described. Comparison with earlier results in the literature suggests that the proposed relationship works, if the countergradient term γD in the Deardorff model, which is proportional to the ratio of the variances of potential temperature θ and vertical velocity w, is evaluated from in situ (airplane and UAV) observational data, but fails if evaluated from large-eddy simulation (LES) results. This appears to be caused by the tendency of the variance of θ in the upper part of the CBL and at the bottom of the entrainment zone to be underestimated by LES relative to in situ measurements from UAVs and aircraft. We discuss this anomaly and explore reasons for it.

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.

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