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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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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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Mark A. Miller
and
Bruce A. Albrecht

Abstract

Data obtained from the island of Santa Maria in the Azores, during the Atlantic Stratocumulus Transition Experiment (ASTEX) are used to describe cloud and boundary-layer structure for a 24-h period on 15 June 1992 over the east-central Atlantic (37°N, 25°10′W). The evolution of the mesoscale cloud structure during a 24-h period in the vertical column above the surface site was characterized using a 94-GHz radar, a laser ceilometer, 3-h radiosonde ascents, and surface micrometeorological instrumentation. Mesoscale circulations and drizzle were found to be key elements of the boundary-layer clouds observed in this region.

During the late-night and predawn hours of the study period, a single layer of stratocumulus that averaged ∼200 m in thickness topped a well-mixed marine boundary layer. Mesoscale cellular convection (MCC), which had ascending regions with horizontal dimensions of ∼7 km, was observed during this period. At sunrise, decoupling was imposed on this MCC, and extreme mesoscale variations in the cloud thickness and surface precipitation rate were observed. These variations included mesoscale patches of cumulus that rose from the surface-lifting condensation level into the overlying stratocumulus (cumulus–stratocumulus interaction), co-existing with patches of decoupled stratocumulus that occasionally had small, shallow cumulus beneath. The average horizontal scale of cumulus–stratocumulus interaction regions was found to be on the order of ∼12 km, and mesoscale variations in the cloud thickness of as much as 400 m were indicated in the remote sensor data during the daytime.

Major drizzle events observed at the surface were shown to correspond with the deepening of the cumulus layer. Evidence was presented that mesoscale cumulus-stratocumulus interaction regions were affecting the surrounding decoupled regions through (i) the vertical transport of properties of the surface moist layer to decoupled stratocumulus by nearby cumulus and (ii) the extended influence of subcloud-layer wakes induced by major precipitation events to nearby decoupled regions. It was suggested that the enhanced precipitation due to cumulus convection may be an important feedback mechanism between the cloud and subcloud layers in the transition region.

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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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Kevin A. Kloesel
and
Bruce A. Albrecht

Abstract

The structure of the boundary layer over a broad region of the equatorial Pacific is studied using dropwinsonde measurements made in January, February, May and June of 1979. Low-level inversions of sufficient strength to inhibit deep convection are found to be present in more than 50% of the soundings. These inversions appear to play a critical role in regulating convective activity over the central and eastern Pacific. The tops of the inversions have an average pressure level of approximately 300 mb and show little latitudinal or longitudinal variation. The majority of the inversion soundings (approximately 70%) have a reversal in the mixing ratio profile (qreversal) above the inversion that appears as a dry layer at the top of the inversion layer capped by a relatively moist layer. This moist layer is on the average 2 g kg−1 more moist than the corresponding soundings that have no qreversal. No systematic regional or temporal variations in the frequency of occurrence of the qreversal or the structure of the boundary layer associated with this feature were observed. In previous studies it was suggested that the qreversal could be formed by nearby convection that is penetrating to higher levels, moistening thou levels, and producing downdrafts that spread out at the top of the inversion as a dry layer. Differences between the thermodynamic structure of soundings with and that without the qreversal support this idea. It is suggested that relatively dry layers may form above inversions of all types and heights in areas where there is nearby convection and associated downdrafts.

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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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Alan K. Betts
and
Bruce A. Albrecht

Abstract

An analysis of FGGE dropwindsonde data using conserved thermodynamic variables shows mixing line structures for the convective boundary layer over the equatorial Pacific. Deeper boundary layers show a double structure. Reversals of the gradients of mixing ratio and equivalent potential temperature above the boundary- layer top are present in all the averages and suggest that the origin of the air sinking into the boundary layer needs further study.

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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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