Parameterization of sheared entrainment in a well-developed CBL. Part I: Evaluation of the scheme through large-eddy simulations

Liu, Peng; Sun, Junqiang; Shen, Lidu

doi:10.1007/s00376-016-5208-x

Cited by 8 publications

(16 citation statements)

References 45 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…We note that, although the change of these heights is small relative to the CBL depth, those changes are approximately 100 % of the entrainment-zone thickness, and are relevant for the local analysis of the entrainment zone in § 4. Figure 5(a,b) also shows that wind-shear effects diminish and eventually vanish as the CBL grows and thermals ascending from the mixed layer become more vigorous and dominate mixing in the entrainment zone (Mahrt & Lenschow 1976;Pino & Vilà-Guerau De Arellano 2008;Liu, Sun & Shen 2016). Hence, shear effects depend not only on the Froude number Fr 0 , or, equivalently, the wind velocity in the free atmosphere, but also on the normalized CBL depth z enc /L 0 .…”

Section: Wind-shear Effects On Buoyancy and Velocitymentioning

confidence: 88%

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

Haghshenas

Mellado

2018

J. Fluid Mech.

View full text Add to dashboard Cite

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 $-z_{enc}/L_{Ob}$.

show abstract

Section: Wind-shear Effects On Buoyancy and Velocitymentioning

confidence: 88%

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

Haghshenas

Mellado

2018

J. Fluid Mech.

View full text Add to dashboard Cite

show abstract

“…Because of this advantage, they argued that the FOM is superior to the ZOM. Following this line of argumentation and also the necessity of predicting the finite thickness of the transition layer for some applications, most recent work made the effort to further develop a FOM (Sun and Xu 2009;Huang et al 2011;Gentine et al 2015;Liu et al 2016). In this paper, we show that the infinitesimal transition-layer representation of the ZOM is sufficient to precisely reproduce bulk properties in the cloud-free sheared CBL, as long as the entrainment closure appropriately represents the local effects of wind shear on entrainment.…”

Section: Introductionmentioning

confidence: 80%

“…We note that Sun and Xu (2009) and Liu et al (2016) have derived the entrainment parameterization in the FOM framework, but we obtain their corresponding parameterization in the ZOM framework by setting to zero the variable that represents the thickness of the transition layer between the mixed layer and the free atmosphere. This approach has also been applied in Pino et al (2006) and Conzemius and Fedorovich (2007).…”

Section: Entrainment Closure Equationmentioning

confidence: 99%

“…Nonetheless, uncertainties still remain in some key aspects associated with the surface and entrainment closures. The work presented here focuses on the entrainment closure and is motivated by challenges identified in previous work, namely, the lack of agreement on the minimum complexity of the bulk model that is necessary to accurately represent sheared CBLs (Pino et al 2006;Liu et al 2016) and a singularity in the entrainment closure that can appear at finite wind strength (Driedonks 1982;Conzemius and Fedorovich 2004;Liu et al 2018). In this work, we address these two issues.…”

Section: Introductionmentioning

confidence: 99%

“…In this paper, we show that the infinitesimal transition-layer representation of the ZOM is sufficient to precisely reproduce bulk properties in the cloud-free sheared CBL, as long as the entrainment closure appropriately represents the local effects of wind shear on entrainment. If required, the actual finite thickness of the transition layer can be constructed a posteriori at the top of the predicted zero-order CBL depth using either the relationships between the zero-order CBL depth and various actual CBL depths provided in Haghshenas and Mellado (2019) or the transition-layer parameterizations proposed in previous work (Pino et al 2006;Kim et al 2006;Liu et al 2016). (By the term ''actual CBLs,'' we explicitly mean atmospheric CBLs or threedimensional simulations of them.…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Nonsingular Zero-Order Bulk Models of Sheared Convective Boundary Layers

Haghshenas

Mellado

Hartmann

2019

Journal of the Atmospheric Sciences

View full text Add to dashboard Cite

Two zero-order bulk models (ZOMs) are developed for the velocity, buoyancy, and moisture of a cloud-free barotropic convective boundary layer (CBL) that grows into a linearly stratified atmosphere. The models differ in the entrainment closure assumption: in the first one, termed the “energetics-based model,” the negative and positive areas of the buoyancy flux are assumed to match between the model and the actual CBL; in the second one, termed the “geometric-based model,” the modeled CBL depth is assumed to match different definitions of the actual CBL depth. Parameterizations for these properties derived from direct numerical simulation (DNS) are employed as entrainment closure equations. These parameterizations, and hence the resulting models, are free from the potential singularity at finite wind strength that has been a major limitation in previous bulk models. The proposed ZOMs are verified using the DNS data. Model results show that the CBL depths obtained from the energetics-based model and previous ZOMs correspond to the height that marks the transition from the lower to the upper entrainment-zone sublayer; this reference height is few hundred meters above the height of the minimum buoyancy flux. It is also argued that ZOMs, despite their simplicity compared to higher-order models, can accurately represent CBL bulk properties when the relevant features of the actual entrainment zone are considered in the entrainment closures. The vertical structure of the actual entrainment zone, if required, can be constructed a posteriori using the available relationships between the predicted zero-order CBL depth and various definitions of the actual CBL depth.

show abstract

Shallow Convection Dataset Simulated by Three Different Large Eddy Models

Zhao,

Wang,

Liu

et al. 2024

Adv. Atmos. Sci.

View full text Add to dashboard Cite

Shallow convection plays an important role in transporting heat and moisture from the near-surface to higher altitudes, yet its parameterization in numerical models remains a great challenge, partly due to the lack of high-resolution observations. This study describes a large eddy simulation (LES) dataset for four shallow convection cases that differ primarily in inversion strength, which can be used as a surrogate for real data. To reduce the uncertainty in LES modeling, three different large eddy models were used, including SAM (System for Atmospheric Modeling), WRF (Weather Research and Forecasting model), and UCLA-LES.Results show that the different models generally exhibit similar behavior for each shallow convection case, despite some differences in the details of the convective structure. In addition to grid-averaged fields, conditionally sampled variables, such as in-cloud moisture and vertical velocity, are also provided, which are indispensable for calculation of the entrainment/detrainment rate. Considering the essentiality of the entraining/detraining process in the parameterization of cumulus convection, the dataset presented in this study is potentially useful for validation and improvement of the parameterization of shallow convection.

show abstract

Parameterization of sheared entrainment in a well-developed CBL. Part I: Evaluation of the scheme through large-eddy simulations

Cited by 8 publications

References 45 publications

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

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

Nonsingular Zero-Order Bulk Models of Sheared Convective Boundary Layers

Shallow Convection Dataset Simulated by Three Different Large Eddy Models

Contact Info

Product

Resources

About