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.

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.

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.

Abstract

Isolated cumuli penetrating through marine stratocumulus clouds were documented during the Atlantic Stratocumulus Transition Experiment. This paper aims at understanding the role of the penetrating cumulus in regulating stratocumulus and boundary-layer structure through analysis of data from the NCAR Electra aircraft. When penetrating cumulus clouds are present, the boundary layer is generally decoupled from the near-surface air except in the cumulus region. Therefore, air in the cumulus region includes air entrained at the cloud top, as well as air modified by surface processes. In the stratocumulus region, however, entrained inversion air and moist surface air are confined to separate layers. As a result, large horizontal variations are found in scalars, such as ozone and water vapor. Turbulence statistics and conditional sampling of entrainment events in the cumulus and stratocumulus regions indicate that stronger entrainment may occur at the cumulus top compared to the surrounding stratocumulus. This analysis is, however, complicated by insufficient sampling of cloud-top jump conditions in both regions.

Convergent flow in the lower boundary layer and compensating diverging flow in the upper boundary layer were identified along the flight trark. This flow field, together with the vertical coupling of surface air with the cloud layer in the cumulus region, helps to transport moisture upwards from the sea surface and disperse it to the surrounding stratocumulus sheet, thus helping to maintain the stratocumulus cover.

Abstract

Measurements of the thermodynamic and dynamic properties of entrainment events in marine stratocumulus are used to explain why cloud-top entrainment instability may not lead to the breakup of the clouds and to define the role of cloud-top entrainment on the turbulent mixing processes when buoyancy reversal due to mixing is released. The measurements were made off the coast of California during the First ISCCP Regional Experiment (FIRE 1987) by the NCAR Electra research aircraft. The data used in this study were collected on a day when the cloud-top jump conditions indicate possible buoyancy reversal for the entrained parcels that mix with cloudy air. The entrainment events are identified using a conditional sampling method. Ozone concentration is used as a tracer of inversion air to define the entrainment mixing fraction.

It is found that cloud-top entrainment ceases to be a mere interfacial phenomenon when buoyancy reversal of the entrainment parcel occurs. Strong entrainment preferentially occurs in the downdraft branch of the boundary-layer circulation, and its effect is not confined to a region near the cloud top. In the case studied here, the contribution to the negative buoyancy in the entrainment downdrafts through evaporative cooling is comparable with that from radiative cooling. The buoyancy deficit as the result of evaporation of cloud droplets is found to be insufficient to promote enhanced entrainment that leads to the breakup of the cloud deck, as suggested by the simple application of cloud-top entrainment instability (CTEI). A conceptual model for cloud-top entrainment that results in buoyancy reversal is proposed. This model emphasizes the interaction between entrainment and the boundary-layer circulation. According to this conceptual model, while buoyancy reversal tends to maintain a well-mixed boundary layer by providing deficit negative buoyancy to drive turbulent mixing, it may also accelerate the thinning and dissipation of a cloud deck once the boundary layer is decoupled by other processes such as solar absorption or drizzle. It is suggested here that a simple criterion for CTEI based solely on the cloud-top discontinuities is unlikely to exist since the dynamics of the entire boundary layer are involved in the entrainment process.

Abstract

The characteristics of a convective internal boundary layer (CIBL) documented offshore during the East Coast phase of the Coupled Air–Sea Processes and Electromagnetic Ducting Research (CASPER-EAST) field campaign has been examined using field observations, a coupled mesoscale model (i.e., Navy’s COAMPS) simulation, and a couple of surface-layer-resolving large-eddy simulations (LESs). The Lagrangian modeling approach has been adopted with the LES domain being advected from a cool and rough land surface to a warmer and smoother sea surface by the mean offshore winds in the CIBL. The surface fluxes from the LES control run are in reasonable agreement with field observations, and the general CIBL characteristics are consistent with previous studies. According to the LESs, in the nearshore adjustment zone (i.e., fetch < 8 km), the low-level winds and surface friction velocity increase rapidly, and the mean wind profile and vertical velocity skewness in the surface layer deviate substantially from the Monin–Obukhov similarity theory (MOST) scaling. Farther offshore, the nondimensional vertical wind shear and scalar gradients and higher-order moments are consistent with the MOST scaling. An elevated turbulent layer is present immediately below the CIBL top, associated with the vertical wind shear across the CIBL top inversion. Episodic shear instability events occur with a time scale between 10 and 30 min, leading to the formation of elevated maxima in turbulence kinetic energy and momentum fluxes. During these events, the turbulence kinetic energy production exceeds the dissipation, suggesting that the CIBL remains in nonequilibrium.