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Shouping Wang

Abstract

A two-layer model of the marine boundary layer is described. The model is used to simulate both stratocumulus and shallow cumulus clouds in downstream simulations. Over cold sea surfaces, the model predicts a relatively uniform structure in the boundary layer with 90%–100% cloud fraction. Over warm sea surfaces, the model predicts a relatively strong decoupled and conditionally unstable structure with a cloud fraction between 30% and 60%. A strong large-scale divergence considerably limits the height of the boundary layer and decreases relative humidity in the upper part of the cloud layer; thus, a low cloud fraction results. The effects of drizzle on the boundary-layer structure and cloud fraction are also studied with downstream simulations. It is found that drizzle dries and stabilizes the cloud layer and tends to decouple the cloud from the subcloud layer. Consequently, solid stratocumulus clouds may break up and the cloud fraction may decrease because of drizzle.

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Shouping Wang

Abstract

A prognostic cloud scheme is described based on Tiedtke's cloud parameterization for large-scale meteorological models, which uses prognostic equations for both mean liquid water content and cloud fraction. The present work parameterizes the relevant physical processes in the framework of convective circulation in the cloud-topped marine boundary layer. Both the steady-state and downstream solutions of the scheme are studied under the conditions of marine boundary layers. The results show that the predicted cloud fraction and mean liquid water content are strongly regulated by the strength of mass-flux detrainment and relative humidity in the environmental downdrafts in the upper part of the cloud layer. Thus, the intensity of the turbulent mixing is important in defining marine boundary layer clouds in the scheme. When coupled with a boundary-layer model that has a mass-flux representation of convective mixing, the cloud scheme gives reasonable downstream variations of marine boundary layer clouds. The sensitivities of the cloud scheme to some parameters are discussed in the context of the boundary layer model.

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Shouping Wang and Qing Wang

Abstract

An analysis of the condensation and evaporation processes involved in the classic Sommeria–Deardoff–Mellor turbulence cloud models is presented. A liquid water budget is derived from the diagnostic Gaussian cloud relations. It is found that the adiabatic condensation generated by turbulent eddies at the cloud base and the mean radiative cooling at the cloud top are the two major processes responsible for the condensation in the parameterizations for stratocumulus-topped boundary layers. The evaporation is directly related to the stratification of the boundary layers and the turbulence variability (variance and covariance) of the conserved thermodynamic variables. The evaporation caused by the turbulence variability plays a dominant role at the cloud top. The analysis also shows that the profile of the parameterized liquid water flux is primarily determined by the turbulence-generated condensation and evaporation in the cloud model. This model is also compared with other prognostic cloud schemes.

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Shouping Wang and Qing Wang

Abstract

This study focuses on the effects of drizzle in a one-dimensional third-order turbulence closure model of the nocturnal stratus-topped marine boundary layer. When the simulated drizzle rate is relatively small (maximum ∼0.6 mm day−1), steady-state solutions are obtained. The boundary layer stabilizes essentially because drizzle causes evaporative cooling of the subcloud layer. This stabilization considerably reduces the buoyancy flux and turbulence kinetic energy below the stratus cloud. Thus, drizzle tends to decouple the cloud from the subcloud layer in the model, as suggested by many observational studies. In addition, the evaporation of drizzle in the subcloud layer creates small scattered clouds, which are likely to represent cumulus clouds, below the solid stratus cloud in the model. The sensitivity experiments show that these scattered clouds help maintain a coupled boundary layer.

When the drizzle rate is relatively large (maximum ∼0.9 mm day−1), the response of the model becomes transient with bursts in turbulent fluxes. This phenomenon is related to the formation of the scattered cloud layer below the solid stratus cloud. It appears that the model is inadequate to represent the heat and moisture transport by strong updrafts covering a small fractional area in cumulus convection.

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Qingfang Jiang and Shouping Wang

Abstract

The impact of gravity waves on marine stratocumulus is investigated using a large-eddy simulation model initialized with sounding profiles composited from the Variability of American Monsoon Systems (VAMOS) Ocean–Cloud–Atmosphere–Land Study Regional Experiment (VOCALS-Rex) aircraft measurements and forced by convergence or divergence that mimics mesoscale diurnal, semidiurnal, and quarter-diurnal waves. These simulations suggest that wave-induced vertical motion can dramatically modify the cloud albedo and morphology through nonlinear cloud–aerosol–precipitation–circulation–turbulence feedback.

In general, wave-induced ascent tends to increase the liquid water path (LWP) and the cloud albedo. With a proper aerosol number concentration, the increase in the LWP leads to enhanced precipitation, which triggers or strengthens mesoscale circulations in the boundary layer and accelerates cloud cellularization. Precipitation also tends to create a decoupling structure by weakening the turbulence in the subcloud layer. Wave-induced descent decreases the cloud albedo by dissipating clouds and forcing a transition from overcast to scattered clouds or from closed to open cells. The overall effect of gravity waves on the cloud variability and morphology depends on the cloud property, aerosol concentration, and wave characteristics. In several simulations, a transition from closed to open cells occurs under the influence of gravity waves, implying that some of the pockets of clouds (POCs) observed over open oceans may be related to gravity wave activities.

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Shouping Wang and Bjorn Stevens

Abstract

Large eddy simulation is used to study top-hat parameterizations of second- and third-order scalar statistics in cumulus and stratocumulus cloud-topped boundary layers (CTBLs). Although the top-hat parameterizations based on commonly used conditional sampling methods are a useful approach to modeling the vertical fluxes in the simulated CTBLs, they fail to realistically represent the scalar variances. The reason is that the common sampling methods are based at least in part on the sign of vertical velocity, but not on the sign of the scalars whose variances are represented and that scalars and velocity are not perfectly correlated. Furthermore, the self-correlation nature for a variance means that all the fluctuations contribute to its value, while the top-hat models completely ignore the deviations from the top-hat means and thus considerably degrade the representation of the variance. For the fluxes, however, only the coherent convective elements make the most contribution. Analysis of analytic models and “toy” time series indicates in a more generic setting that the effect of poor correlations between the signal upon which the sampling is based and the signal whose variance is to be represented tends to degrade the ability of top-hat parameterizations to capture the variance. The analysis of toy time series also indicates that variability among individual events within a composite degrades the top-hat representation of the variance more than variability within events. For the vertical velocity–scalar-related third-order moments, the top-hat model gives reasonable estimates for the cumulus CTBL but not for the stratocumulus CTBL. These differences are explained by structural differences (tied to circulation differences in the two CTBLs) in their respective joint probability density functions of vertical velocity and various scalars.

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Qingfang Jiang and Shouping Wang

Abstract

The impact of aerosol replenishment in a stratocumulus-topped boundary layer (STBL) on cloud morphology and albedo is examined using a large-eddy simulation (LES) model in conjunction with a prey–predator-type dynamical model following Koren and Feingold. In both the LES and the prey–predator models, the aerosol replenishment is represented as a relaxation term toward an ambient aerosol concentration with a time scale . The LESs suggest the existence of three distinct cloud regimes corresponding to different aerosol relaxation times. Specifically, for a small , the simulations are characterized by a large aerosol concentration, weak precipitation, relatively thick cloud depth, and closed cells. For a moderate , the simulated clouds exhibit open cellular patterns, in accordance with low aerosol concentration and moderate precipitation that oscillates in time. For a large , the aerosol may be depleted by intense drizzling and accordingly the cloud disappears. The critical aerosol relaxation times that separate these regimes vary with the ambient aerosol number concentration and cloud depth. Solutions from the low-order dynamical model with parameters relevant to the LESs are in general consistent with the LES results and provide further insight into the interplay among clouds, aerosol, and precipitation.

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Shouping Wang and Bruce A. Albrecht

Abstract

A mean-gradient model of the dry convective boundary layer is developed using a convective mass flux representation of the turbulent fluxes. A top-hat model of thermals is used to represent the average characteristics of updrafts and downdrafts in the buoyantly driven circulation. The convective mass flux is related to the convective velocity scale. Although the model is computationally simple, it describes the basic structure of the CBL. In this parameterization the vertical gradients of conserved variables are controlled by internal mixing through vertical mass transports inside the convective boundary layer, bottom mixing due to the surface processes, top mixing due to the entrainment, and lateral mixing between updrafts and downdrafts.

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Shouping Wang and Bruce A. Albrecht

Abstract

An updraft and downdraft circulation model based on a mass flux representation of convective fluxes is incorporated into a classic cloud-topped, mixed layer model. The convective mass flux is assumed to be proportional to the convective velocity scale. The closure is obtained by assuming that the turbulent kinetic energy consumption due to entrainment is a small portion of its production due to buoyancy. The model is used to study how cloud-top entrainment instability, partitioning or radiative cooling, and drizzle affect the boundary layer, and the stratocumulus circulation.

The results obtained with the model indicate that the intensity of the internal circulation may be critical in regulating the structure of stratocumulus clouds so that the cloud-top entrainment instability as it is presently used is not sufficient to define the breakup of stratocumulus. The vertical distribution of radiative cooling in the mixed layer is found to decrease the amplitude of the diurnal cycle of the cloud thickness. Precipitation processes are incorporated into the model to study their effects on the bulk properties of the boundary layer.

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Shouping Wang, Qing Wang, and Graham Feingold

Abstract

Condensation and turbulent liquid water transport in stratocumulus clouds involve complicated interactions between turbulence dynamics and cloud microphysical processes, and play essential roles in defining the cloud structure. This work aims at understanding this dynamical–microphysical interaction and providing information necessary for parameterizations of the ensemble mean condensation rate and turbulent fluxes of liquid water variables in a coupled turbulence–microphysics model. The approach is to simulate nonprecipitating stratocumulus clouds with a coupled large eddy simulation and an explicit bin-microphysical model, and then perform a budget analysis for four liquid water variables: mean liquid water content, turbulent liquid water flux, mean cloud droplet number concentration, and the number density flux. The results show that the turbulence contribution to the mean condensation rate comes from covariance of the integral cloud droplet radius and supersaturation, which enhances condensation in turbulent updrafts and reduces evaporation in the downdrafts. Turbulent liquid water flux results from a close balance between turbulence dynamics and microphysical processes. Consequently, the flux can be parameterized in terms of the common diffusive downgradient formulation, fluxes of conservative thermodynamic variables, the turbulence mixing timescale, and the condensation timescale, which is determined by the droplet spectrum. The results also suggest that the condensation timescale regulates the turbulence fields, as does the number concentration, because it affects the condensation fluctuation, which is highly correlated with the turbulence vertical motion. A saturation adjustment cloud model, which diagnoses liquid water content at its equilibrium level, instantly condenses (evaporates) all available water vapor (liquid water) surplus. Consequently, there is likely to be a systematic difference between the turbulence field resolved with this type of model and that with a supersaturation-based cloud scheme for which a finite condensation timescale applies.

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