Revisiting entrainment relationships for shear‐free and sheared convective boundary layers through large‐eddy simulations

Liu, Cheng; Fedorovich, Evgeni; Huang, Jianping

doi:10.1002/qj.3330

Cited by 20 publications

(26 citation statements)

References 53 publications

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“…9c,d). The faster growth of the boundary layer with shear at its top is a well documented feature (Conzemius and Fedorovich, 2006;Liu et al, 2018). The faster-rising cloud layer in SHEAR coincides with an earlier negative buoyancy flux minimum at cloud base and, thus, an earlier decoupling of the cloud layer by almost 2 hours (Table 1).…”

mentioning

confidence: 66%

“…reinforcing the small effect of the mean wind in the transition. The only remarkable difference is a one hour delay in reaching 15 values of r θv near typical CBL of −0.2, due to the fact under convective conditions that mean wind may hamper the turbulent updrafts from the surface to the boundary layer top, thus reducing the related entrainment (Liu et al, 2018). SHEAR shows a qualitatively similar pattern to REF after sunrise with an earlier shift on the sign of r θv of about two hours.…”

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confidence: 80%

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The diurnal stratocumulus-to-cumulus transition over land

Pedruzo-Bagazgoitia

Roode

Adler

et al. 2019

Preprint

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The misrepresentation of the diurnal cycle of boundary-layer clouds by large scale models strongly impacts the modeled regional energy balance in southern West Africa. In particular, recognizing the processes involved in the maintenance and transition of the nighttime stratocumulus to diurnal shallow cumulus over land remains a challenge. This is due to the fact that over vegetation, surface fluxes exhibit a much larger magnitude and variability than on the more researched marine stratocumulus transitions. An improved understanding of the interactions between surface and atmosphere is thus necessary 5 to improve its representation. To this end, the DACCIWA measurement campaign gathered a unique dataset of observations of the frequent stratocumulus to cumulus transition in southern West Africa. Inspired and constrained by these observations, we perform a series of numerical experiments using Large Eddy Simulation. The experiments include interactive radiation and surface schemes where we explicitly resolve, quantify and describe the physical processes driving such transition. Focusing on the local processes, we quantify the transition in terms of dynamics, radiation, cloud properties, surface processes and 10 the evolution of dynamically relevant layers such as subcloud layer, cloud layer and inversion layer. We further quantify the processes driving the stratocumulus thinning and the subsequent transition initiation by using a liquid water path budget.Finally, we study the impact of mean wind and wind shear at cloud top through two additional numerical experiments. We find that the sequence starts with a nighttime well-mixed layer from surface to cloud top, in terms of temperature and humidity, and transitions to a prototypical convective boundary layer by the afternoon. We identify radiative cooling as the largest factor for 15 the maintenance leading to a net thickening of the cloud layer of about 18 g m −2 h −1 before sunrise. Four hours after sunrise, the cloud layer decouples from the surface through a growing negative buoyancy flux at cloud base. After sunrise, the increasing impact of entrainment leads to a progressive thinning of the cloud layer. While the effect of wind on the stratocumulus layer during nighttime is limited, after sunrise we find shear at cloud top to have the largest impact: the local turbulence generated by shear enhances the boundary layer growth and entrainment aided by the increased surface fluxes. As a consequence wind 20 shear at cloud top accelerates the breakup and transition by about 2 hours. The quantification of the transition and its driving factors presented here sets the path for an improved representation by larger scale models.Stratocumulus (Sc) clouds play a critical role on the radiative balance of the planet given their high albedo (Hartmann et al., 1992;Chen et al., 2000) and extensive cover worldwide . These boundarylayer clouds are a common feature in the southern West Africa (SWA), and recur in the night and morning during the Monsoon season between May and Septem...

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confidence: 66%

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confidence: 80%

The diurnal stratocumulus-to-cumulus transition over land

Pedruzo-Bagazgoitia

Roode

Adler

et al. 2019

Preprint

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“…A large number of researchers have used large-eddy simulation (LES) models to establish a better understanding of the mixing processes at work in the IL and its impact on the CBL (e.g., Conzemius & Fedorovich, 2006;Garcia & Mellado, 2014;Gentine et al, 2015;Liu et al, 2018;Mellado et al, 2017). In particular, Sorbjan used his LES model (Sorbjan, 1996) to conclude that the water vapor variance in the IL could be described in terms of the convective velocity scale, the gradient of water vapor mixing ratio, and the Brunt-Vaisala frequency (Sorbjan, 2005(Sorbjan, , 2006.…”

Section: Introductionmentioning

confidence: 99%

Validating the Water Vapor Variance Similarity Relationship in the Interfacial Layer Using Observations and Large‐Eddy Simulations

et al. 2019

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In previous work, the similarity relationship for the water vapor variance in the interfacial layer (IL) at the top of the convective boundary layer (CBL) was proposed to be proportional to the convective velocity scale and the gradients of the water vapor mixing ratio and the Brunt‐Vaisala frequency in the entrainment zone. In the presence of wind shear in the IL, the similarity relationship was hypothesized to also include a dependence on the gradient Richardson number. Simultaneous measurements of the surface buoyancy flux, wind‐shear profiles from a radar wind profiler, water vapor mixing ratio and temperature measurements and their gradients from a Raman lidar provide a unique opportunity to thoroughly examine the function used in defining the variance and validate it. These observations were made over the Atmospheric Radiation Measurement Southern Great Plains site. We identified 19 cases from 2016 during which the CBL was quasi‐stationary and well mixed for at least 2 hr in the afternoon. Furthermore, we simulated the CBL using a large‐eddy simulation (LES) model for these cases and derived the water vapor variance and other profiles to test the similarity function. Utilizing this unique combination of observations and LES, we demonstrate that the water vapor variance in the IL has little‐to‐no dependence on wind shear. Furthermore, we demonstrate that the predicted variance using the original similarity function matches the observed and LES‐modeled variance very well, with linear correlations between the two variances of 0.82 and 0.95, respectively.

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“…4.9c,d). The faster growth of the boundary layer with shear at its top is a well documented feature (Conzemius and Fedorovich, 2006;Liu et al, 2018). The fasterrising cloud layer in SHEAR coincides with an earlier negative buoyancy flux minimum at cloud base and, thus, an earlier decoupling of the cloud layer by almost 2 hours (Table 4.1).…”

Section: Daytime Effectsmentioning

confidence: 77%

“…MEAN shows little variation from REF, reinforcing the small effect of the mean wind in the transition. The only remarkable difference is a one hour delay in reaching values of r θv near typical CBL of −0.2, due to the fact under convective conditions that mean wind may hamper the turbulent updrafts from the surface to the boundary layer top, thus reducing the related entrainment (Liu et al, 2018). SHEAR shows a qualitatively similar pattern to REF after sunrise with an earlier shift on the sign of r θv of about two hours.…”

Section: Representation By Large Scale Metricsmentioning

confidence: 79%

On the interactions between clouds, radiation, turbulence and vegetation in the atmospheric boundary layer

Bagazgoitia

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Revisiting entrainment relationships for shear‐free and sheared convective boundary layers through large‐eddy simulations

Cited by 20 publications

References 53 publications

The diurnal stratocumulus-to-cumulus transition over land

The diurnal stratocumulus-to-cumulus transition over land

Validating the Water Vapor Variance Similarity Relationship in the Interfacial Layer Using Observations and Large‐Eddy Simulations

On the interactions between clouds, radiation, turbulence and vegetation in the atmospheric boundary layer

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