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Chin-Hoh Moeng

Abstract

Two sets of large-eddy simulation data were used to study some of the assumptions about the cloud-topped boundary layer (CTBL) structure which are used in mixed-layer models. The roles of buoyant production and cloud-top radiative cooling in turbulent kinetic energy generation were examined.

The buoyant production in the turbulent kinetic energy (TKE) budgets was partitioned into production and consumption components by grouping the warm-rising and cold-sinking parcels and the warm-sinking and cold-rising parcels, separately. The results indicate that the ratio of the consumption part of the buoyant production to the sum of the shear production and the production part of the buoyant production is 0.15 for the clear convective mixed layer, and 0.22 for the CTBL.

Almost all mixed-layer models use the experimentally (from either direct measurement or tank experiment) obtained ratio of the buoyancy flux at the top of the clear convective boundary layer to the flux at the surface for their entrainment constant. Those direct measurement or tank experiment studies adopt the horizontal average in the usual Eulerian coordinates to define their ensemble average. A mixed-layer model, therefore, must also use the same type of averaging process to define the radiative cooling distribution near the cloud top. In that case, the 1arge-eddy simulation results indicate that about 85% of the cooling must occur within the entrainment zone and 15% within the well-mixed layer, for relatively dense clouds.

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Chin-Hoh Moeng

Abstract

The structure of a stratus-topped boundary layer is observed through large-eddy simulation which includes the interaction of longwave radiation and turbulence processes. This simulated boundary layer has a relatively warm and dry overlying inversion, a weak surface buoyancy flux, no solar heating, and an insignificant wind shear across the cloud top. The cloud top height and the layer-averaged buoyancy flux inside the cloud layer define a velocity scale appropriate for this of boundary layer.

In the cloud layer, buoyancy generates the vertical component of the turbulent kinetic energy, while pressure effect transfer some of this energy into the horizontal components. In the subcloud layer, the only source of the vertical energy other than the surface buoyancy is import from above and the only source of the horizontal energy other than the mean shear is the vertical energy transferred through pressure effects.

The profiles of the vertical velocity variance and kinetic energy flux in the stratus-topped boundary layer depend on the relative contributions of the surface beating and cloud-top cooling to turbulence. Therefore, the vertical velocity variance is decomposed into two components: one entirely due to surface heating and the other entirely due to cloud-top cooling; the dimensionless profile of the latter is presented.

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Chin-Hoh Moeng

Abstract

A closure relationship between subgrid-scale (SGS) updraft–downdraft differences and resolvable-scale (RS) variables is proposed and tested for cloud-resolving models (CRMs), based on a data analysis of a large-eddy simulation (LES) of deep convection. The LES flow field is partitioned into CRM-RS and CRM-SGS using a cutoff scale that corresponds to a typical CRM grid resolution. This study first demonstrates the capability of an updraft–downdraft model framework in representing the SGS fluxes of heat, moisture, and momentum over the entire deep convection layer. It then formulates a closure scheme to relate SGS updraft–downdraft differences to horizontal gradients of RS variables. The closure is based on the idea that largest SGS and smallest RS motions are spectrally linked and hence their horizontal fluctuations must be strongly communicated. This relation leads to an SGS scheme that expresses vertical SGS fluxes in terms of horizontal gradients of RS variables, which differs from conventional downgradient eddy diffusivity models. The new scheme is shown to better represent the forward and backscatter energy transfer between CRM-RS and CRM-SGS components than conventional eddy-viscosity models.

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Chin-Hoh Moeng

Abstract

The large eddy simulation technique is used to search for key factors in determining the entrainment rate, cloud fraction, and liquid water path in the stratocumulus-topped boundary layer (STBL), with the goal of developing simple schemes of calculating these important quantities in climate models. In this study an entrainment rate formula is proposed where the entrainment rate is determined by four variables—total jump of the liquid water potential temperature across the entrainment zone, surface heat flux, net radiative flux away from the top of the STBL, and liquid water path. This study also shows that buoyancy reversal, measured here as the ratio between the equivalent potential temperature jump and the total moisture jump across the cloud top, plays a major role in reducing the simulated cloud amount, both cloud fraction and liquid water path. For cases where no buoyancy reversal occurs, the simulated cloud fraction remains 100% and the liquid water path depends solely on the cloud height.

This study raises an interesting feature about what controls the entrainment rate of the STBL. The two cases with a larger surface heat flux studied here show that the net impact of surface heating on the entrainment rate could be negligible if surface heating also leads to enhanced cloud-top evaporation; enhanced evaporation then results in smaller cloud amount and hence smaller radiative forcing for entrainment. Since larger surface heat flux always significantly increases the layer-averaged buoyancy flux and the turbulence intensity, the entrainment rate of the STBL for a given inversion strength is therefore not always directly proportional to the layer-averaged buoyancy flux or to the turbulence intensity.

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Chin-Hoh Moeng

Abstract

A large-eddy-simulation (LFS) model explicitly calculates the large-eddy field and parameterizes the small eddies. The large eddies in the atmospheric boundary layer are believed to be much more important and more flow-dependent than the small eddies. The LES model results are therefore believed to be relatively insensitive to the parameterization scheme for the small eddies.

Deardorff first applied this type of numerical model to boundary-layer turbulence. In order to continue his important work, and to take advantage of the fast Fourier transformation algorithm, a new LES model code which uses a mixed pseudospectral finite-difference method was developed. This LES model is described here and tested with a simple vortex flow and with the Wangara day-33 data.

This model will be used to systematically investigate fundamental problems in the area of boundary-layer turbulence. It is hoped that three-dimensional simulations will give useful statistical information about turbulence structural and improve the closure assumptions in ensemble-mean turbulence modeling.

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Ulrich Schumann
and
Chin-Hoh Moeng

Abstract

From results Of large-eddy simulations of the clear convective boundary layer and of a stratus-topped boundary layer, mean properties of “plumes” that consist of “updrafts” and “downdrafts” are determined. The plumes are defined locally by the sign of the vertical velocity or of moisture fluctuation or by a combination of both. As a further alternative, updraft and downdrafts in which the vertical velocity magnitude exceeds certain threshold values are considered. The first two variants divide the motion field into two streams, whereas in the other variants “environmental” air forms a separate stream. The computed mean properties are in general agreement with existing measurements. From the results we compute mean vertical fluxes assuming “top-hat profiles” and compare these with the actual fluxes. It is shown that the most uniform flux approximation is obtained if the plume structure is classified in terms of vertical velocity w. For such “w plumes,” the top-hat profiles approximate about 60% of the actual fluxes if updrafts and downdrafts are distinguished with zero threshold values just according to the sign of the vertical velocity. A higher percentage is obtained with nonzero threshold values.

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Chin-Hoh Moeng
and
Richard Rotunno

Abstract

A number of puzzling features of the skewness of the vertical velocity field, Sw (z), are found in observations and large-eddy simulations (LES) of the buoyancy-driven planetary boundary layer (PBL). For example, observations of Sw (z) in cases where the air is heated from below indicate that Sw (z) > 0 and remains relatively constant for z ≳ 0.3zi , whereas all large-eddy simulations of these cases show a continuing increase of Sw (z) with height. In cases where the air is both heated from below and cooled from above, as in some of the stratus-topped PBL cases, large-eddy simulations show a rather curious feature: Sw is positive in the upper layer and negative in the lower layer. In considering these features, it occurred to us that a theoretical model of what one should expect of the skewness distribution, even in simple situations, did not exist. Hence in the present paper we examine the skewness distributions from direct numerical simulations of several simple archetypes of buoyancy-driven turbulent flow. While these simulations do not resolve the discrepancy between LES and observations, they help in understanding the LES results, and suggest avenues for future research.

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Chin-Hoh Moeng
and
Akio Arakawa

Abstract

A model for numerical simulation of stratus cloud layers is constructed by combining a second-order closure, turbulent transfer model with a thermal radiative transfer model. The turbulent transfer model allows water vapor saturation. The combined turbulence-radiation model is applied to both a horizontally uniform one-dimensional case and a horizontally nonuniform two-dimensional case. In the latter, the dynamics of mesoscale circulations are also incorporated.

Results of the two-dimensional simulation show that the layer cloud instability occurs where the sea surface temperature is high and the large-scale subsidence is weak. The simulated instability is analyzed in view of an instability criterion, the eddy kinetic energy budget, and evaporative cooling near the cloud top.

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Chin-Hoh Moeng
and
John Wyngaard

Abstract

Not available

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Chin-Hoh Moeng
and
Akio Arakawa

Abstract

No abstract.

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