Cloud-top Entrainment Research Articles

東シナ海上の寒気吹き出し雲の雲頂エントレインメント不安定に関する成層について (事例解析)

Evolution of a cloud-topped boundary layer in the East China Sea area in a situation of cold-air outbreak was examined for a case during 12-13 January 1991 based on the operational rawinsonde observation data and the NOAA AVHRR data received by Nagoya University. The analysis was made by comparing with that in the downstream region in the central area of the Japan Sea on 2 February 1991. (1) According to the brief analysis during the period from 11 January to 3 February 1991, the stratification which satisfies the condition for Cloud Top Entrainment Instability (CTEI) was observed at Naze ( about 1000 km downstream from the continent over the East China Sea ) in most cases of cold-air outbreak situations. (2) According to a case study for 12-13 January 1991, it was suggested that the release of CTEI seems to occur and this might affect the change in characteristics of low-level clouds in the downstream of Naze, even in situations of cold-air outbreak over the East China Sea. (3) On the other hand, the unstable stratification satisfying the CTEI condition was formed on passing from Saishuh-toh (about 500 km distant from the continent) to Naze in that case. Besides, the horizontal scales of cloud areas became larger and the cloud tops became somewhat flatter, so that the cloud layer became favorable for release of CTEI in downstream of Naze. (4) The large positive value of the total apparent source just below the top of the mixed layer and its rapid decrease with height above that layer contributed primarily to generating the stratification which satisfies the CTEI condition in the upstream of Naze. The much higher degree of unsaturation (large value of q°-q) above the mixed layer in the East China Sea area than in the Japan Sea area, as well as the huge supply of heat and moisture from the underlying sea, seems to be one of the favorable factors for generating the CTEI condition there, even in situations of cold-air outbreak.

Numerical Simulation of the Stratus-to-Cumulus Transition in the Subtropical Marine Boundary Layer. Part II: Boundary-Layer Circulation

The progression from the stratus-topped boundary layer (STBL) to the trade cumulus boundary layer (TCBL) during a simulated stratus-to-cumulus transition (SCT) involves two intermediate stages: the deep stratus-topped boundary layer (DSTBL) and the “cumulus-under-stratocumulus” boundary layer (CUSBL). The DSTBL, like the STBL, has an active circulation that extends from the surface to the cloud top. The CUSBL, like the TCBL, has an active subcloud-layer circulation that is linked to the cloud layer by narrow cumulus updrafts. It is called a “cumulus-coupled” boundary layer. A generally applicable convective updraft/downdraft partitioning scheme based on trajectory analysis was developed and used to analyze the boundary-layer circulation changes during the simulated SCT. The circulation analysis revealed that as the SST increased and the boundary layer changed from an STBL to a TCBL the updraft fraction in the cloud layer decreased, the convective updrafts strengthened, and the convective downdrafts weakened. The convective mass flux in the cloud layer decreased significantly as SST increased, while in the subcloud layer it changed little. The differences between updraft and downdraft properties and cloud-base levels gradually increased as SST increased. An analysis of the vertical acceleration components of the convective updrafts and downdrafts suggests that there are three steps in the transition from an STBL circulation to a TCBL circulation. First, the STBL deepens due to increased surface buoyancy fluxes as it moves over increasing SST but remains well mixed. Next, the DSTBL gradually changes into the two-layer CUSBL. During this step, negative buoyancy in downdrafts originating near cloud top becomes less important, while positive buoyancy in (cumulus) updrafts becomes more important. This indicates that cloud-top entrainment instability does not play a significant role in the SCT. Finally, the overlying stratocumulus deck gradually dissipates and only the underlying cumulus clouds of a typical TCBL remain. This general sequence of events is supported by recent observational evidence.

Numerical Investigations of the Roles of Radiative and Evaporative Feedbacks in Stratocumulus Entrainment and Breakup

Abstract When the surface buoyancy flux is small and the shear is weak, turbulence circulations within a stratus-topped boundary layer are driven by two buoyancy-generating processes at cloud top: radiative cooling and evaporative cooling. These two processes respond very differently to entrainment, however. When the entrainment rate increases, the effectiveness of radiative cooling in driving circulations decreases (a negative feedback) but the effectiveness of evaporative cooling can increase (a positive feedback). The roles of these two competing feedbacks in determining the entrainment rate, and hence in determining cloud breakup, are examined in this paper through large eddy simulations. Three stratus cases (with a small surface buoyancy flux) are simulated: one is stable with respect to the Lilly–Randall–Deardorff cloud-top entrainment instability criterion, and the other two are unstable. Only one of the two cloud decks in the unstable regime dissipates totally; the other remains nearly solid. A me...

Marine boundary layer structure and fractional cloudiness

Radiosonde data collected during the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE), 1987, the Atlantic Stratocumulus Transition Experiment (ASTEX), 1992, and the Tropical Instability and Waves Experiment (TIWE), 1990, were used to develop four composite thermodynamic soundings of the marine boundary layer for cloud conditions ranging from solid stratocumulus to trade cumulus and sea surface temperatures ranging from 16° to 27°C. Average cloud fraction and cloud base height for the composite soundings were estimated using laser ceilometers located at each of the four sites. The average fractional cloudiness varied from 0.83 off the coast of California at San Nicolas Island (33.4°N, 119.6°W) during FIRE to 0.26 over the central equatorial Pacific during TIWE (0°N, 140.0°W). During ASTEX, two sites in the vicinity of the Azores generally experienced cloud conditions characterized by cumulus clouds penetrating into a stratocumulus layer. At the more northerly site, the island of Santa Maria (37.0°N, 25.2°W), the average fractional cloudiness was 0.67 compared with 0.40, 1000 km downstream at the German R/V Valdivia (28.0°N, 24.0°W). The two composite soundings from ASTEX and the composite sounding from TIWE indicate decoupled boundary layer structures. These three soundings have a cloud layer that is conditionally unstable and show a systematic increase in relative humidity with increasing fractional cloudiness. The effect of cloud top entrainment on fractional cloudiness was evaluated. For the four composite soundings the fractional cloudiness decreases as the slope of the normalized w‐θ mixing line increases (greater instability), but this decrease is 4 times less than that from previous studies. Fractional cloudiness diagnosed from parameterizations using cloud layer relative humidity compares well with the observed mean cloudiness.

Precipitation in Stratocumulus Clouds: Observational and Modeling Results

The spatial and temporal variability of precipitating stratocumulus layers is examined using aircraft observations, satellite retrievals of cloud optical depth, and one-dimensional models that include coalescence and a simple representation of layer turbulence. The aircraft observations show large horizontal variations in cloud thickness and precipitation, with local rain rates 4–5 times larger than the replacement moisture flux, and evidence for precipitation scavenging of small cloud droplets. The satellite observations show that, despite this local water loss, the distribution of cloud optical thickness remains nearly constant over the course of a day, indicating that on larger scales precipitation removal and cloud-top entrainment are in approximate balance with the vapor flux. The authors apply analytic and numerical models of steady-state precipitation to the observed microphysical conditions, and find that the models can match the drop size distributions observed during both heavy and light stratocumulus rainfall, but are especially sensitive to the processes governing the growth rate of the smallest drizzle drops.

Sensitivity of a general circulation model to parameterizations of cloud-turbulence interactions in the atmospheric boundary layer

Several approaches to parameterize the turbulent transport of momentum, heat, water vapour and cloud water for use in a general circulation model (GCM) have been tested in 1-dimensional and 3-dimensional model simulations. The schemes differ with respect to their closure assumptions (conventional eddy diffusivity model versus turbulent kinetic energy closure) and also regarding their treatment of cloud–turbulence interactions. The basic properties of these parameterizations are discussed first in relation to column simulations of a stratocumulus–topped atmospheric boundary layer (ABL) under a strong subsidence inversion during the KONTROL experiment in the North Sea. It is found that the K-models tend to decouple the cloud layer from adjacent layers above and below because the turbulent activity is calculated from local variables. The higher-order scheme performs better in this respect because internally generated turbulence can be transported up and down through the action of turbulent diffusion. Thus, the TKE-scheme provides not only a better link between the cloud and the sub-cloud layer but also between the cloud and the inversion as a result of cloud-top entrainment. In the stratocumulus case study, where the cloud is confined by a pronounced subsidence inversion, increased entrainment favours cloud dilution through enhanced evaporation of cloud droplets. In the GCM study, however, additional cloud-top entrainment supports cloud formation because indirect cloud generating processes are promoted through efficient ventilation of the ABL, such as the enhanced moisture supply by surface evaporation and the increased depth of the ABL. As a result, tropical convection is more vigorous, the hydrological cycle is intensified, the whole troposphere becomes warmer and moister in general and the cloudiness in the upper part of the ABL is increased. DOI: 10.1034/j.1600-0870.1995.t01-1-00004.x

Entrainment and mixing in buoyancy-reversing convection with applications to cloud-top entrainment instability

Entrainment and mixing in buoyancy-reversing convection with applications to cloud-top entrainment instability

Observations of Cloud-Top Entrainment in Marine Stratocumulus Clouds

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 occu...

Linear Eddy Modeling of Entrainment and Mixing in Stratus Clouds

Mixing of entrained air in stratus clouds is an important but poorly understood process. It is a crucial ingredient of cloud-top entrainment instability (CEI). CEI has been proposed as a breakup mechanism for stratus clouds. A recently developed model called the linear eddy model was used to simulate mixing of air entrained into stratus clouds. The linear eddy approach involves stochastic simulation on a one-dimensional domain with sufficient resolution to include all physically relevant length scales. In each realization, molecular diffusion is implemented explicitly, while a sequence of statistically independent “rearrangement events” represents the effect of turbulent eddies. Inertial range scaling is incorporated. The linear eddy model was used to simulate the mixing of one or more wisps of entrained air with a specified volume of cloud-topped boundary layer (CTBL) air. The volume was idealized to be a horizontal slab of fluid that travels from the top of the CTBL down to the surface in the descending branch of a large convective eddy. The probability density function of the mixing fraction of entrained air was determined from linear eddy model simulations as a function of time for a mean mixing fraction of 0.05 and three wisp sizes. The effect of the mixing on the mean buoyancy of the downdraft could then be calculated given a specification of the buoyancy as a function of mixing fraction. In the simulations, the entrained air did not completely mix with cloudy air just below the CTBL top, nor was uniform saturation maintained. Furthermore, when buoyancy functions typical of observed CTBLs were used, the mean downdraft buoyancy due to entrainment and mixing integrated over the cloud layer remained positive. This suggests that CEI is unlikely in stratocumulus. An additional conclusion is that using reduced spatial resolutions typical of published large-eddy simulations (LES) of CTBLs in mixing simulations significantly underestimates the buoyancy in the cloud layer near cloud top. This may explain why low-resolution LFS simulations have exhibited CEI under conditions for which CEI is not observed in the atmosphere.

A Numerical Investigation of the Criterion for Cloud-Top Entrainment Instability

Abstract An investigation of cloud-top entrainment instability (CTEI) has been carried out using a fine-resolution two-dimensional numerical model. Initial conditions having specified values of R = cpΔθe/LΔqt were used. Here, Δθe, and Δqt, are the jumps in equivalent potential temperature and total water mixing ratio across cloud top. In order to isolate the effects of entrainment across cloud top, cloud microphysics and surface fluxes were excluded from all the integrations. Radiative processes were generally also excluded, although a number of runs with longwave radiative cooling were performed. Integrations were carried out for specified values of R, using various subgrid models, including several constant values of eddy viscosity. Because the crucial process underlying CTEI is small-scale mixing, which must be parameterized in this model, only those results that are not critically dependent on the precise form of the subgrid model are likely to have any general validity. Fortunately, significant concl...

Binormal Model of Ensemble Partial Cloudiness

Abstract A new formulation of partial cloudiness parameterization has been introduced that agrees with that for a random model in one limit and approaches the simple updraft/downdraft model of larger-scale models in the limit of very highly skewed flow. Each of the conserved variables, liquid potential temperature and total humidity, along with the vertical velocity, are assumed to have probability distributions that may be parameterized as combinations of two multivariate normal distributions. This allows the skewness of the variables to be controlled by the bias between the means of the two normals and their relative fractions. It also provides a smooth transition between the normal distribution and the two limiting delta function distribution of the updraft/downdraft model. Comparisons with large-eddy-simulation data show this new model to be valid over a much wider range of conditions than the single normal distribution. When a simple cloud-top entrainment instability (CTEI) analysis is made using the...

Vertical Mixing in Clear Air and Clouds

The realization that clouds are diluted by the entrainment of dry air from outside the cloud boundaries began more than 40 years ago. Cloud-top entrainment was first clearly recognized by Squires in 1958. Warner and Squires showed that the dilution increased towards cloud top and was uniform across the cloud perpendicular to the shear. Not until the mid 1970s, however, did more evidence begin to show that cumulus clouds have a structure from upshear to downshear where rapid dilution by the mixing of dry air from above cloud top eventually totally evaporates the cloud parcels. This slow development was partially caused by the lack of appropriate thermodynamic diagrams for cloud microphysics analysis. In this paper, an easy approach to thermodynamic analysis is introduced for studying cloud mixing and entrainment structure. The advantages of using the temperature and wet-bulb temperature of air parcels, after compression to 1000 mb, as the coordinates of a general thermodynamic diagram are discussed and illustrated with examples. These variables are easily calculated on a small computer and then directly plotted. No further graphical calculations are needed for parcel mixing or for comparing densities. Total water mixing ratios and condensation pressures are directly readable for each composition.more » An important point is that these displays can be automatically produced without including any significant approximations. Thus, the details of how to process the data, or plot points and draw lines, is no longer a consideration in the use of such analysis, and no analog calculations are needed to produce the graphs. Observational examples are discussed, and the use of the computer program raises new questions about radiative cooling in and above cloud top and about the penetration by turbules carrying heat and moisture of marine stratus clouds and subcloud air. 16 refs., 10 figs.« less

The Stability of Cloud Top with Regard to Entrainment: Amendment of the Theory of Cloud-Top Entrainment Instability

The stability of a uniformly saturated cloud layer separated from an overlying nonturbulent unsaturated layer by a thin inversion is considered. The stability of the interface can be described by entraining a parcel of air from above the inversion into the cloud layer below and by subsequently studying the effect of the mixing on the buoyancy of the parcel. From the relevant momentum equation for the parcel it is shown that the important quantity to consider is the total buoyancy (total mass of the parcel times the virtual potential temperature difference between parcel and environment) of the parcel per unit mass of entrained air. The total buoyancy is a more general and useful concept than all other parameters discussed in the literature.

A study of a two-dimensional cloudiness transition during a cold air outbreak event

A two-dimensional version of a hydrostatic mesoscale model with a partial cloudiness scheme is used to study a cold air outbreak event during MASEX (Mesoscale Air-Sea Exchange experiment). The model produces a weak mesoscale circulation which is slightly advected offshore and simulates well an observed cloudiness transition zone of 80 km where the cloud cover ranges between 0 and 100%. It is shown that the cloud top entrainment instability criterion can explain the observed cloudiness phenomenon i.e., the transition from a cumulus field to a solid stratocumulus cloud layer. The usefulness of the partial cloudiness scheme is demonstrated by comparing the results to those obtained with a simple “all or nothing” condensation scheme. A model sensitivity study shows that the sea surface temperature and the horizontal advection speed control the surface heat and moisture fluxes and so explain the structure and horizontal extent of the cloudiness transition zone.

Observations of Cloud-Top Entrainment in Cumuli

Sulfur hexafluoride tracer gas was released during single aircraft passes just above growing convective turrets to study its entrainment into the clouds as they grew through the release altitude. The tracer was sampled in situ from a second research aircraft that carried a real-time sulfur hexafluoride analyzer. The results from three experiments are presented. They were done with clouds ranging in size from a vigorous convective turret to a small cumulus.The observations suggest that during the early stages of entrainment, the tracer remained mostly out of the cloud and was carried alongside the upper cloud regions by the circulation present there. In each experiment, concentrated tracer was first found on the edges of the turrets. Later, the tracer mixed into the central portions of the turrets where it had diluted considerably and mixed through most of the turret. The observations are consistent with the hypothesis that cloud-top entrainment occurs through a vortex-like circulation that brings air from above the cloud into the central region of the cloud. The results are compared to some recent conceptual and numerical models of entrainment.

A numerical investigation of cloud-top entrainment instability and related experiments

Buoyancy reversal occurs between two homogeneous fluids when as a result of non-linear mixing the mixture becomes denser than either of the fluids. In 1980 both Randall and Deardorff suggested that such nonlinear mixing, which may occur across a stratocumulus cloud-top as a consequence of evaporative cooling. could create an entrainment instability; the ensuing rapid entrainment could lead to stratocumulus break-up. This break-up mechanism is known as cloud-top entrainment instability or CTEI. In 1990 Shy and Breidenthal investigated buoyancy reversal within a laboratory environment and proposed a model for turbulent entrainment, and conditions under which buoyancy reversal may lead to an entrainment instability. They found that an instability could occur only if D (the ratio of the density difference between the most cooled mixed parcel and the lower fluid to the density difference across the unmixed layers) was approximately one or larger. This restriction of a strong buoyancy reversal for an instability (hence SBRI) is more stringent than Randall's condition, D ≥ 0, and is almost never satisfied at the inversion topping a marine stratocumulus layer. We have used numerical simulations of a single eddy at an interface between two fluids to explore the entrainment model of Shy and Breidenthal. Consistent with SBRI. we find that in all cases a critical value of D, Dc, between 0.3 and 2.0 exists. For D Dc, kinetic energy is found to increase on the scale of the entraining eddy. We expand the domain to model the interaction of the multiple eddies with an entrainment interface. We find a similar Dc where unstable entrainment is exhibited, and a tendency for larger eddies to develop if Dc. > D. We explore the influence of the fluid and flow characteristics on Dc to bridge the gap between the laboratory and the atmosphere. We find that Dc is lower at Schmidt numbers of order unity, characteristic of the atmosphere, than for those equal to 103, typical of the laboratory fluid. In all cases Dc remains too large to be a factor in stratocumulus break-up.

Saturation Point Representation of Cloud-Top Entrainment Instability

Cloud-top entrainment instability was investigated using a mixing line analysis. Mixing time scales are closely related to the actual size of the parcel, so that local instabilities are largely dependent on the scales of mixing near the cloud top. Given a fixed transport velocity, variation over a small range of parcel length scales (parcel mixing velocities) turns an energy-producing mixing process into an energy-consuming mixing process. It is suggested that a single criterion for cloud-top entrainment instability will not be found due to the role of at least three factors operating more or less independently: the stability of the mixing line, the entrainment speed, and the strength of the internal boundary-layer circulation.

Fractional Cloudiness and Cloud-Top Entrainment Instability

Abstract Temperature and moisture profiles from soundings obtained over the central Atlantic during the Atlantic Trade-Wind Experiment were used to determine if cloud-top entrainment instability can explain observed variations in cloudiness. Differences in temperature and moisture across the trade inversion associated with fractional cloudiness near 100% were nearly the same as those associated with less than 25% cloud cover. NO correlation between fractional cloudiness and boundary layer mixing-line slopes was found.

Cloud albedo control by cloud-top entrainment

Cloud albedo control by cloud-top entrainment

Cloud albedo control by cloud-top entrainment

Marine stratus and stratocumulus clouds exert a considerable influence on the Earth's heat budget, mainly due to their high albedos relative to the ocean surface. It is therefore important to understand the processes that control the radiative properties of these extensive cloud systems, particularly during daylight hours. Aircraft measurements of a stratocumulus cloud deck taken around local noon during the 1987 field phase of the First International Satellite Cloud Climatology Project Regional Experiment are the topic of this paper. A mixing line analysis of data from a series of flight tracks across a strong gradient in cloud albedo provides evidence that variations in the water vapour content of the air above the marine inversion can be responsible for the albedo change. The implications of this unexpected result for climate modelling are discussed. DOI: 10.1034/j.1600-0870.1991.00004.x