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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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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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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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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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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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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 and Bruce A. Albrecht

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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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Bingfan Yin and Bruce A. Albrecht

Abstract

Variations in the atmospheric boundary layer structure over the eastern equatorial Pacific are analyzed using 916 soundings collected during the First Global Atmospheric Research Program Global Experiment. Unstable boundary layer structures are observed much more frequently in soundings north of the ocean front located near 2.5°N in the eastern equatorial Pacific than in soundings south of the front. An objective criterion is applied to identify the presence of the transition layer, a weak stable layer near cloud base, in the soundings. The transition is observed in about 45% of the soundings in both the unstable and the inversion categories. A comparison of soundings over the cold tongue with those over the ITCZ indicates that differences in static stability between these regions are limited to the layer from the surface to about 850 mb, which is the mean height of the inversions capping the cloud layer over the cold tongue. The cold tongue soundings on average are found to be drier from the surface to 300 mb than the ITCZ soundings with the largest average difference (∼5 g kg−1) between these two groups of soundings observed just above the inversion layer. Compensating subsidence from the ITCZ may account for some of the drying observed just above the cold tongue inversions, although horizontal advection may also be a factor. North–south cross sections (10°S–15°N) of potential temperature, mixing ratio, equivalent potential temperature, and meridional wind across the cold tongue–ITCZ complex (CTIC) were constructed for two longitudinal bands: 95°–105°W and 105°–115°W. There is little latitudinal variation of the average height of the trade inversion and the height of the transition layer across the CTIC. Although the average lifting condensation level (LCL) at 980 mb is located near the average top of the transition layers observed over the cold tongue, the average 980-mb LCL is close to the average height of the base of the transition layers observed over the ITCZ. These differences, while subtle, may have a substantial impact on the coupling between the subcloud and the cloud layer in these two regions. Strong boundary layer meridional winds are observed near the surface at about 7.5°N over the higher SSTs north of the cold tongue. The average meridional winds over the cold tongue show little vertical shear over the lowest 100 mb of the boundary layer.

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Gerald A. Meehl and Bruce A. Albrecht

Abstract

A tropical Pacific sea surface temperature (SST) anomaly is included in a global general circulation model with a hybrid convection scheme. Results are compared with a similar version of the same model that includes a simple moist convective adjustment scheme and with observed data analyses from the European Centre for Medium-Range Weather Forecasts. The model with the hybrid scheme transports proportionately more heat and moisture higher into the troposphere in the vicinity of the SST anomaly, and the extratropical sensitivity to the SST anomaly is greater (larger-amplitude anomalies). Improvements in the simulated mean climate with the hybrid scheme result in an altered and more realistic extratropical response to the tropical SST anomaly. This is especially true for the Southern Hemisphere where the improvements to the mean climate simulation are most dramatic. This result is consistent with linear models that show a great dependence on the mean extratropical climate simulation for the type of response to a tropical heating anomaly. The mechanisms in the model that produce warming in the tropical troposphere at all longitudes (as observed) are influenced by the type of convection scheme used. Results suggest that climate sensitivity to an external forcing (such as a tropical SST anomaly) depends on the convective scheme in two ways. First, the convective scheme is important to the simulation of the basic climate and thus influences the nature of the extratropical response to the forcing. Second, due to the altered properties of the vertical transports of heat and moisture (particularly in the tropics associated with deep convection), the convective scheme can influence the size of the heating anomaly and the magnitude of the resulting extratropical circulation anomalies.

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Gerald A. Meehl and Bruce A. Albrecht

Abstract

This note documents improvements in the Southern Hemisphere midlatitude circulation associated with a revised convective scheme that was implemented in a version of the NCAR Community Climate Model. Perpetual July and January runs are performed for a standard control and an experiment with a revised convective parameterization. No other boundary conditions in the model are changed (e.g., sea-surface temperatures or sea ice, which are prescribed from observations). The most notable result is a warmer troposphere at all latitudes that is closer to the observed than to the control. Since the warming is greatest in the tropics, the equator-to-pole temperature gradient is increased in both July and January. As a consequence, the midlatitude u wind maximum is stronger at both times of the year in the lower and mid-troposphere. Sea-level pressure is also lower at high southern latitudes in the circumpolar trough surrounding Antarctica. Greatest improvements of the Southern Hemisphere (SH) circulation in the model experiment with the warmer tropical troposphere occur in southern summer, the time of year when the climate of the control simulation was most poorly simulated. Midlatitude eddy heal transport increases somewhat with the revised convective scheme, but greatest changes are seen in large increases of poleward eddy momentum transport. Results show that notable improvement in the SH mid- and high-latitude circulation in this case can be realized by changing a model parameterization whose greatest temperature effects are in the tropical troposphere.

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