Dynamics of Sheared Convective Boundary Layer Entrainment. Part I: Methodological Background and Large-Eddy Simulations

2006 ◽  
Vol 63 (4) ◽  
pp. 1151-1178 ◽  
Author(s):  
Robert J. Conzemius ◽  
Evgeni Fedorovich
Keyword(s):  
Wind Shear ◽  
Geostrophic Wind ◽  
Convective Boundary ◽  
Surface Shear ◽  
Atmospheric Stratification ◽  
Large Eddy ◽  
Entrainment Zone ◽  
Reported Study ◽  
Surface Buoyancy ◽  
Surface Buoyancy Flux

Abstract The reported study examines the dynamics of entrainment and its effects on the evolution of the dry atmospheric convective boundary layer (CBL) when wind shear is present. The sheared CBL can be studied by means of direct measurements in the atmosphere, laboratory studies, and numerical techniques. The advantages and disadvantages of each technique are discussed in the present paper, which also describes the methodological background for studying the dynamics of entrainment in sheared CBLs. For the reported study, large-eddy simulation (LES) was chosen as the primary method of convective entrainment investigation. Twenty-four LES runs were conducted for CBLs growing under varying conditions of surface buoyancy flux, free-atmospheric stratification, and wind shear. The simulations were divided into three categories: CBL with no mean wind (NS), CBL with a height-constant geostrophic wind of 20 m s−1 (GC), and CBL with geostrophic wind shear (GS). In the simulated cases, the sheared CBLs grew fastest, relative to the NS CBLs, when the surface buoyancy flux was weak and the atmospheric stratification was moderate or weak. Three fundamental findings resulted from the investigated CBL cases: (i) the entrainment zone shear is much more important than the surface shear in enhancing CBL entrainment, although entrainment zone shear is indirectly affected by surface shear; (ii) the sheared entrainment zone features a sublayer of nearly constant flux Richardson number, which points to a balance between shear production and buoyancy consumption of turbulence kinetic energy (TKE) that regulates entrainment; and (iii) the fraction of entrainment zone shear-generated TKE spent on the entrainment is lower than suggested by earlier studies.

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.

A Comparison of Roll and Nonroll Convection and the Subsequent Deepening Moist Convection: An LEM Case Study Based on SCMS Data

2009 ◽  
Vol 137 (1) ◽  
pp. 350-365 ◽  
Author(s):  
Qian Huang ◽  
John H. Marsham ◽  
Douglas J. Parker ◽  
Wenshou Tian ◽  
Tammy Weckwerth
Keyword(s):  
Boundary Layer ◽  
Wind Shear ◽  
Moist Convection ◽  
Convective Boundary ◽  
Obukhov Length ◽  
A Value ◽  
Large Eddy ◽  
Wind Shears ◽  
Surface Buoyancy

Abstract Rolls observed during the Small Cumulus Microphysical Study (SCMS) field campaign are simulated using a large eddy model (LEM). The simulated boundary layer properties were in a good agreement with sounding profiles and aircraft observations, and the observed boundary layer rolls were reproduced by the model. Rolls started to decay when −Zi/L exceeded a threshold, with a value between 5 and 45. Here Zi and L refer to the height of the top of convective boundary layer and the Monin–Obukhov length, respectively. This value was found to depend on a nondimensional combination of the low-level wind shear, the height of the CBL, and the eddy velocity scale. Larger surface buoyancy fluxes and smaller shears gave higher thresholds. For the case modeled, rolls persisted for surface buoyancy fluxes less than 110 W m−2, and formed for boundary layer wind shears greater than 5 × 10−3 s−1, which is consistent with previous studies. The simulated roll convection was compared with a nonroll simulation, which was identical except for the wind and the wind shear used. In both the roll and nonroll cases the variability in convective inhibition (CIN) was dominated by the variability in the source air, rather than the lifting of the top of the boundary layer by the convection. Stronger moist updrafts existed in the nonroll convection, whereas roll convection gave a more symmetrical distribution of up and downdrafts, with stronger downdrafts than the nonroll case. The nonroll convection simulations have lower minimum values of CIN and clouds develop 15 min earlier in this case.

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 Comparison of Large Eddy Simulations of a convective boundary layer with wind LIDAR measurements

2012 ◽  
Vol 8 (1) ◽  
pp. 83-86 ◽  
Author(s):  
J. G. Pedersen ◽  
M. Kelly ◽  
S.-E. Gryning ◽  
R. Floors ◽  
E. Batchvarova ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Geostrophic Wind ◽  
Large Eddy Simulations ◽  
Horizontal Wind ◽  
Convective Boundary ◽  
Horizontal Wind Speed ◽  
Large Eddy ◽  
Lidar Measurements ◽  
Wind Lidar

Abstract. Vertical profiles of the horizontal wind speed and of the standard deviation of vertical wind speed from Large Eddy Simulations of a convective atmospheric boundary layer are compared to wind LIDAR measurements up to 1400 m. Fair agreement regarding both types of profiles is observed only when the simulated flow is driven by a both time- and height-dependent geostrophic wind and a time-dependent surface heat flux. This underlines the importance of mesoscale effects when the flow above the atmospheric surface layer is simulated with a computational fluid dynamics model.

scholarly journals Scaling Laws for the Heterogeneously Heated Free Convective Boundary Layer

2014 ◽  
Vol 71 (11) ◽  
pp. 3975-4000 ◽  
Author(s):  
Chiel C. van Heerwaarden ◽  
Juan Pedro Mellado ◽  
Alberto De Lozar
Keyword(s):  
Boundary Layer ◽  
Kinetic Energy ◽  
Convective Boundary Layer ◽  
Scaling Laws ◽  
Fixed Ratio ◽  
Buoyancy Flux ◽  
Convective Boundary ◽  
Optimal State ◽  
Surface Buoyancy ◽  
Surface Buoyancy Flux

Abstract The heterogeneously heated free convective boundary layer (CBL) is investigated by means of dimensional analysis and results from large-eddy simulations (LES) and direct numerical simulations (DNS). The investigated physical model is a CBL that forms in a linearly stratified atmosphere heated from the surface by square patches with a high surface buoyancy flux. Each simulation has been run long enough to show the formation of a peak in kinetic energy, corresponding to the “optimal” heterogeneity size with strong secondary circulations, and the subsequent transition into a horizontally homogeneous CBL. Scaling laws for the time of the optimal state and transition and for the vertically integrated kinetic energy (KE) have been developed. The laws show that the optimal state and transition do not occur at a fixed ratio of the heterogeneity size to the CBL height. Instead, these occur at a higher ratio for simulations with increasing heterogeneity sizes because of the development of structures in the downward-moving air that grow faster than the CBL thickness. The moment of occurrence of the optimal state and transition are strongly related to the heterogeneity amplitude: stronger amplitudes result in an earlier optimal state and a later transition. Furthermore, a decrease in patch size combined with a compensating increase in patch surface buoyancy flux to maintain the energy input results in decreasing KE and a later transition. The simulations suggest that a CBL with a heterogeneity size smaller than the initial CBL height has less entrainment than a horizontally homogeneous CBL, whereas one with a larger heterogeneity size has more.

Representing Sheared Convective Boundary Layer by Zeroth- and First-Order-Jump Mixed-Layer Models: Large-Eddy Simulation Verification

2006 ◽  
Vol 45 (9) ◽  
pp. 1224-1243 ◽  
Author(s):  
David Pino ◽  
Jordi Vilà-Guerau de Arellano ◽  
Si-Wan Kim
Keyword(s):  
Large Eddy Simulation ◽  
Boundary Layers ◽  
Mixed Layer ◽  
Convective Boundary ◽  
First Order ◽  
Eddy Simulation ◽  
Jump Model ◽  
Large Eddy ◽  
Entrainment Zone ◽  
Convective Boundary Layers

Abstract Dry convective boundary layers characterized by a significant wind shear on the surface and at the inversion are studied by means of the mixed-layer theory. Two different representations of the entrainment zone, each of which has a different closure of the entrainment heat flux, are considered. The simpler of the two is based on a sharp discontinuity at the inversion (zeroth-order jump), whereas the second one prescribes a finite depth of the inversion zone (first-order jump). Large-eddy simulation data are used to provide the initial conditions for the mixed-layer models, and to verify their results. Two different atmospheric boundary layers with different stratification in the free atmosphere are analyzed. It is shown that, despite the simplicity of the zeroth-order-jump model, it provides similar results to the first-order-jump model and can reproduce the evolution of the mixed-layer variables obtained by the large-eddy simulations in sheared convective boundary layers. The mixed-layer model with both closures compares better with the large-eddy simulation results in the atmospheric boundary layer characterized by a moderate wind shear and a weak temperature inversion. These results can be used to represent the flux of momentum, heat, and other scalars at the entrainment zone in general circulation or chemistry transport models.

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 New Insights into Wind Shear Effects on Entrainment in Convective Boundary Layers Using Conditional Analysis

2020 ◽  
Vol 77 (9) ◽  
pp. 3227-3248
Author(s):  
Katherine Fodor ◽  
Juan Pedro Mellado
Keyword(s):  
Vertical Velocity ◽  
Wind Shear ◽  
External Source ◽  
Buoyancy Flux ◽  
Vortical Flow ◽  
Strong Wind ◽  
Convective Boundary ◽  
Velocity Fluctuations ◽  
Strong Shear ◽  
Entrainment Zone

Abstract Conventional analysis has shown that strong wind shear enhances the entrainment buoyancy flux in the convective boundary layer. By conditioning the entrainment zone into regions of turbulent (i.e., strongly vortical) and nonturbulent (i.e., weakly vortical) flow, some unexpected aspects of this process are revealed. It is found that turbulent regions contribute the most to the entrainment buoyancy flux, but that as wind shear increases, the magnitude of the buoyancy flux in turbulent regions remains approximately constant, or even decreases, despite substantially stronger buoyancy fluctuations. The reason is that the correlation between buoyancy and vertical velocity fluctuations decreases with increasing wind shear, to the extent that it compensates the stronger buoyancy fluctuations. In free convection, this correlation is high because the vertical velocity is mainly determined by the buoyancy force acting in the same direction. Under strong shear conditions, buoyancy is no longer the only external source of vertical velocity fluctuations and their correlation consequently decreases. Hence, shear enhancement of the buoyancy flux in the entrainment zone is primarily due to an increase of the turbulent area fraction, rather than a change of flux inside the turbulent regions.

scholarly journals Characterization of wind-shear effects on entrainment in a convective boundary layer

2018 ◽  
Vol 858 ◽  
pp. 145-183 ◽  
Author(s):  
Armin Haghshenas ◽  
Juan Pedro Mellado
Keyword(s):  
Boundary Layer ◽  
Convective Boundary Layer ◽  
Convective Instability ◽  
Wind Shear ◽  
Scaling Laws ◽  
Strong Wind ◽  
Free Atmosphere ◽  
Convective Boundary ◽  
Entrainment Zone ◽  
Shear Effects

Direct numerical simulations are used to characterize wind-shear effects on entrainment in a barotropic convective boundary layer (CBL) that grows into a linearly stratified atmosphere. We consider weakly to strongly unstable conditions $-z_{enc}/L_{Ob}\gtrsim 4$, where $z_{enc}$ is the encroachment CBL depth and $L_{Ob}$ is the Obukhov length. Dimensional analysis allows us to characterize such a sheared CBL by a normalized CBL depth, a Froude number and a Reynolds number. The first two non-dimensional quantities embed the dependence of the system on time, on the surface buoyancy flux, and on the buoyancy stratification and wind velocity in the free atmosphere. We show that the dependence of entrainment-zone properties on these two non-dimensional quantities can be expressed in terms of just one independent variable, the ratio between a shear scale $(\unicode[STIX]{x0394}z_{i})_{s}\equiv \sqrt{1/3}\unicode[STIX]{x0394}u/N_{0}$ and a convective scale $(\unicode[STIX]{x0394}z_{i})_{c}\equiv 0.25z_{enc}$, where $\unicode[STIX]{x0394}u$ is the velocity increment across the entrainment zone, and $N_{0}$ is the buoyancy frequency of the free atmosphere. Here $(\unicode[STIX]{x0394}z_{i})_{s}$ and $(\unicode[STIX]{x0394}z_{i})_{c}$ represent the entrainment-zone thickness in the limits of weak convective instability (strong wind) and strong convective instability (weak wind), respectively. We derive scaling laws for the CBL depth, the entrainment-zone thickness, the mean entrainment velocity and the entrainment-flux ratio as functions of $(\unicode[STIX]{x0394}z_{i})_{s}/(\unicode[STIX]{x0394}z_{i})_{c}$. These scaling laws can also be expressed as functions of only a Richardson number $(N_{0}z_{enc}/\unicode[STIX]{x0394}u)^{2}$, but not in terms of only the stability parameter $-z_{enc}/L_{Ob}$.

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