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Ping Zhu and Bruce Albrecht

Abstract

The formation of fair-weather cumuli (FWC) has been analyzed in this study based on both a simple mixed layer model and a subset of the data collected from the Atmospheric Radiation Measurement (ARM) Program at the southern Great Plains (SGP) site. By analyzing conditions for the formation of FWC, the authors illustrate how different processes—such as the surface heat fluxes, the surface thermodynamic conditions, the entrainment processes at the boundary layer top, the vertical thermodynamic structure above the boundary layer, and large-scale subsidence—control the formation of clouds. The results of the analysis show that it is the highly nonlinear interaction among these factors that gives rise to the formation of FWC. For this reason, the occurrence of FWC may not simply follow changes in the surface conditions. The analysis indicates that the entrainment of moisture and surface processes play important roles in the formation of FWC, and the net effects of these processes can be evaluated by a parameter (l − β 2)/B, where β 2 is the ratio between the entrainment moisture flux and the surface moisture flux, and B is the extended Bowen ratio defined as the ratio of the surface buoyancy flux to the surface latent heat flux. The stratification above the inversion is another key parameter that influences cloud formation. The weaker the stability, the greater the potential for cloud formation. In most situations the net effect of subsidence is to reduce the relative humidity at the top of the mixed layer and thus is unfavorable for cloud formation, but the intensity of this reduction may vary depending on conditions of the boundary layer. In some specific conditions such as a moist boundary layer over an area with relatively small surface Bowen ratio, the net effect of subsidence on the relative humidity budget at the top of the mixed layer can be weak even though subsidence reduces the mixed layer depth substantially. In this study, some issues related to cloud onset and fractional cloudiness are also discussed based on the ARM SGP observational data.

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Pavlos Kollias and Bruce Albrecht

Abstract

Fair-weather cumuli are fundamental in regulating the vertical structure of water vapor and entropy in the lowest 2–3 km of the earth’s atmosphere over vast areas of the oceans. In this study, a long record of profiling cloud radar observations at the Atmospheric Radiation Measurement Program (ARM) Climate Research Facility (ACRF) at Nauru Island is used to investigate cloud vertical air motion statistics over an 8-yr observing period. Appropriate processing of the observed low radar reflectivities provides radar volume samples that contain only small cloud droplets; thus, the Doppler velocities are used as air motion tracers. The technique is applied to shallow boundary layer clouds (less than 1000 m thick) during the 1999–2007 period when radar data are available. Using the boundary layer winds from the soundings obtained at the Nauru ACRF, the fair-weather cumuli fields are classified in easterly and westerly boundary layer wind regimes. This distinction is necessary to separate marine-forced (westerlies) from land-forced (easterlies) shallow clouds because of a well-studied island effect at the Nauru ACRF. The two regimes exhibit large diurnal differences in cloud fraction and cloud dynamics as manifested by the analysis of the hourly averaged vertical air motion statistics. The fair-weather cumuli fields associated with easterlies exhibit a strong diurnal cycle in cloud fraction and updraft strength and fraction, indicating a strong influence of land-forced clouds. In contrast over the fair-weather cumuli with oceanic origin, land-forced clouds are characterized by uniform diurnal cloudiness and persistent updrafts at the cloud-base level. This study provides a unique observational dataset appropriate for testing fair-weather cumulus mass flux and turbulence parameterizations in numerical models.

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Bruce A. Albrecht

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.

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Keith Brill and Bruce Albrecht

Abstract

Cloud fraction observations and trade-wind inversion base heights from both GATE and the undisturbed period of ATEX are examined for diurnal variation. The cloud fraction tends to be larger in the morning than in the afternoon. The inversion base height has a morning maximum and an afternoon minimum. A time series of horizontal divergence is calculated using ATEX wind data. The maximum divergence occurs in the morning at about 0700 LST.

A one-dimensional time-dependent trade-wind boundary layer model is used to study diurnal variations. A scheme is used that includes solar heating below as well as within the inversion in the cloud-covered portion of the model boundary layer. Using this parameterized heating distribution, the model reasonably simulates the observed diurnal variations. Model experiments are carried out to ascertain the effects of the solar heating distribution and horizontal divergence on diurnal variation. The model-simulated diurnal variation of the cloud cover, inversion height, and the thermodynamic and moisture structure of the cloud layer is sensitive to the partitioning of the solar heating between the top half of the cloud layer and the inversion. The variations due to divergence and solar radiation appear to contribute additively to the total diurnal variation of the model inversion height.

The observations and model results support the conclusion that significant diurnal variation of the trade-wind boundary layer occurs and is primarily due to the diurnal variation of radiative forcing and secondarily to diurnally varying large-scale divergence.

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Bruce A. Albrecht

Abstract

Several applications of a simple layered model of the temperature and moisture structure of the trade-wind boundary layer are considered. The formation of the trade inversion in the wake of disturbed conditions is simulated. This simulation indicates that the observed thermodynamic structure can easily evolve from a shallow mixed layer in less than 24 h. The growth rate of the depth of the boundary layer is shown to be most sensitive to parameters which directly influence the surface fluxes of heat and moisture. Steady-state model results are compared to the boundary-layer structure observed during the 1969 Atlantic Trade-Wind Experiment (ATEX). The predicted structure is slightly warmer and more moist than the observed structure. These differences are largely eliminated if the variation of the model structure following a surface air trajectory is predicted. The effect of a diurnally varying radiative heating and divergence on the height of the inversion is evaluated. These model results are compared to diurnal variations in the height of the inversion observed from the ship Meteor during ATEX. The model is also used to simulate the thermodynamic structure when the cloud layer becomes saturated. The steady-state and time-dependent results of this simulation are in very good agreement with the results obtained with a cloud-topped mixed-layer model.

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Pavlos Kollias and Bruce Albrecht

Abstract

The turbulent-scale vertical velocity structure in a continental stratocumulus cloud is studied using a 3-mm wavelength Doppler radar operating in a vertically pointing mode. The radar observations provided 30-m sampling in the vertical with 2-s averages of 10 000 samples. Vertical velocity measurements were made continuously for an 8-h period and were further supported by measurements of cloud-base height from a laser ceilometer and liquid water path from a microwave radiometer. During the beginning of the observational period, the cloud layer extended between 200 and 800 m. The vertical velocity variance profiles evolved systematically over the period from a well-defined peak in the upper part of the cloud layer of ∼0.7 m2 s−2 to a peak in the lower part of the cloud layer of 0.2 m2 s−2 as the layer became decoupled later in the observing period. The vertical velocity skewness during the well-coupled conditions was negative through most of the cloud, consistent with the presence of relatively narrow downdrafts. A positive skewness in the top 100 m of the cloud is consistent with relatively narrow penetrating updrafts at this level.

The radar vertical velocities are used to compare the directly observed updraft fractional coverage and mass flux with those obtained from the bulk statistics. These comparisons are consistent with similar comparisons made using a large eddy simulation model. The fractional coverage and the mass flux associated with coherent updraft structures are obtained for a range of criteria used to define the updrafts. A more detailed analysis of the vertical velocities in the cloud confirms the existence of well-defined downdrafts extending through the entire cloud depth. These downdrafts are estimated to have horizontal dimensions of about 200 m and appear to originate on the downshear side of updrafts. The reduction of radar reflectivity at cloud top in the downdrafts is consistent with the entrainment of drier air. This study further illustrates the utility of millimeter-wavelength radars for studying turbulence in boundary layer clouds and particularly in defining the vertical structure of coherent eddies.

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Bruce A. Albrecht

Abstract

A scheme for parameterizing the fractional cloud coverage of trade cumuli is developed which gives the cloud cover in terms of the cloud-layer relative humidity and the liquid water content of the convective elements. This parameterization gives good agreement with cloud observations obtained during the Atlantic Trade-Wind Experiment, 1969. The scheme was incorporated into a simple one-dimensional model of the thermodynamic structure of the trade-wind boundary layer in order to evaluate the dependence of cloud cover on sea surface temperature, surface wind speed, and radiative processes. For steady-state conditions over relatively cold water (∼16°C), the cloud cover decreases as sea surface temperature increases while over warmer water (∼24°C) there is a slight increase in cloud cover as the sea surface temperature increases. When used in a downstream mode, the model structure obtained by moving from cold to warm water can be used to evaluate the effect of advection. Over cold water, the advective case results in a cloud cover which is greater than the corresponding steady-state cloud cover, while over warm water the cloud cover is less than the corresponding steady-state cloud cover. The model predicted cloud cover is positively correlated with wind speed and the net radiative cooling of the boundary layer.

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Eunsil Jung and Bruce Albrecht

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Circulations in and around cumulus clouds are inferred by using a passive tracer (radar chaff) and an airborne cloud radar during the Barbados Aerosol Cloud Experiment (BACEX). The radar chaff elements used for this experiment are fibers that are cut to a length of about ½ of the radar wavelength to maximize radar returns by serving as dipole antennas. The fibers are packed in fiber tubes and are mounted in a dispenser beneath the wing of the aircraft. The chaff was released near the tops and edges of a growing small cumulus cloud. The aircraft then made penetrations of the cloud at lower levels to observe the chaff signals above the aircraft with the zenith-pointing cloud radar. This study shows that the environmental air above the cloud top descends along the downshear side of the cloud edge and is subsequently entrained back into the same cloud near the observation level. The in-cloud flow follows an inverted letter P pattern. The merits and limitations of the chaff method for tracking circulations in and around small cumuli are discussed.

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Bruce A. Albrecht

Abstract

A scheme for parameterizing the effects of cumulus convection for the maintenance of the thermodynamic structure of the tropical atmosphere is described. This parameterization is used in a one-dimensional model that represents the vertical structure of the atmosphere as a function of Chebyshev polynominals.

The convective fluxes in this model are represented as a product of an “effective” convective mass flux and cloud-environment differences in thermodynamic properties for a reference cloud. The conversion of cloud water to precipitation is assumed to be proportional to the cloud water content. The effects of shallow clouds on the thermodynamic structure new cloud base are represented by constraining the derivative of the mass flux near cloud base. An equilibrium assumption is used to obtain the mass flux associated with deep clouds.

The sensitivity of the vertical distributions of the parameterized heating and moistening rates to cloud model assumptions is determined. The inefficient conversion of cloud water to precipitation significantly cools and moistens the upper portion of the cloud layer. The evaporation of rain significantly cools the lower layers, although in the moisture budget the evaporation of rain is less important than other effects. The importance of shallow clouds for maintaining the heat and moisture budgets near cloud base is clearly demonstrated.

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Qing Wang and Bruce A. Albrecht

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.

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