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M. B. Baker

Abstract

The statistical properties of airborne tracer concentrations, and the time-lagged correlation between concentration and wind velocity, are calculated from a simple model for the fluctuations in wind velocity. The mean concentration distribution thus calculated is compared with that obtained using the usual diffusivity formulation for transport, and the limitations on the diffusivity approximation discussed for various sources and sinks. It is shown that the model provides a method for calculation of the concentration statistics in the presence of velocity-dependent sources.

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T. Ackerman and M. B. Baker

Abstract

Clouds in some polluted areas may contain high concentrations of anthropogenic aerosol particles. The possible role of these particles in perturbing the optical and dynamical properties of the clouds is an important question for climate studies. The direct radiative effects of unactivated aerosol particles in stable stratus clouds have been calculated at λ = 0.5 μm. Several simplifying assumptions have been made relating the behavior of such particles in the high humidity environment within the cloud to their physicochemical make-up. It is shown that the energy absorbed by particles within the clouds may be, for realistic concentrations, comparable to the latent heat released and thus may play a significant role in cloud dynamics in some areas. These results are shown to be relatively insensitive to the assumptions about the particle properties within the cloud.

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M. B. Baker and J. Latham

Abstract

Calculations have been made of the evolution of droplet spectra within small cumulus clouds which are entraining undersaturated environmental air. The mixing process is assumed to be highly inhomogeneous. In the extreme situation considered, environmental air is entrained in discrete blobs or parcels, causing some droplets of all sizes to be completely removed from the condensate spectrum, while others do not change in size. This model, which is based on laboratory experiments, corresponds to a situation in which the time constant for droplet evaporation is small relative to that for turbulent mixing; in the classical (homogeneous) model, which has been used by other workers, the reverse applies. The calculations produce spectral shapes which agree well with those observed in cumulus by Warner (1969), and they indicate that favored droplets may grow very much faster through the condensate spectrum than is predicted classically.

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Keith Ronnholm, M. B. Baker, and Halstead Harrison

Abstract

While it is now possible to compute the optical transmission, reflection and absorption of a homogeneous horizontal layer of known parameters to great accuracy, the input parameters (optical depth, scattering phase function and single-scattering albedo) are in general neither precisely known nor exactly constant over a layer. In this paper several simple representations of the distributions of input parameters are used to compute the mean values and standard deviations of layer transmission, reflection and absorption. The effects of variability in input parameters depend on the mean-layer properties; under certain conditions this variability induces errors on the order of 5% in the derived optical properties. This magnitude is comparable to the differences between those obtained by the Eddington, delta-Eddington, and discrete ordinate four-steam approximations, and a more precise 20-stream doubling method.

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W. P. Giddings and M. B. Baker

Abstract

Haze particles coated by organic substances have been found in some maritime regions. It is shown that the most likely origin of surfactant material on atmospheric droplets is the ocean surface, which contains fatty acids and other organics previously identified in atmospheric aerosol particles as well as calcium soaps and proteinaceous substances which could act as surfactants. Persistent hazes, but not fogs, may result from stabilization of unactivated atmospheric droplets by organic surfactants in maritime regions. Calculations show that under time-varying atmospheric conditions the primary effect of surfactants is to decrease the steady-state portion of. the growth rate. A kinetic mechanism is proposed for the monolayer inhibitory action, implying limitations on the utility of the accommodation coefficient formulation.

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J. B. Jensen, P. H. Austin, M. B. Baker, and A. M. Blyth

Abstract

The analysis of Paluch suggests that some cumuli contain cloudy air from only two sources: cloud base and cloud top. A framework is presented for the investigation of droplet spectral evolution in clouds composed of air from only these two sources. The key is the investigation of the dependence of droplet concentration N on the fraction of cloud base air F in a sample of cloudy air. This N-vs-F analysis is coupled with an investigation of droplet spectral parameters to infer the types and scales of entrainment and mixing events.

The technique is used in a case study of a small, nonprecipitating continental cumulus cloud which was sampled during the 1981 CCOPE project in eastern Montana. The mixing between cloudy and entrained air in this cloud often appears to occur without total removal of droplets, although there is evidence that total evaporation occurs in some regions with low liquid water content. The observed droplet spectra are compared with those calculated from an adiabatic parcel model. The spectral comparison and the results of the N-vs-F analysis support the hypothesis that cloudy and environmental air interact on fairly large scales with subsequent homogenization of the large-scale regions. This description is consistent with recent models of mixing in turbulent flows.

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P. H. Austin, M. B. Baker, A. M. Blyth, and J. B. Jensen

Abstract

We have analyzed small-scale fluctuations in microphysical, dynamical and thermodynamical parameters measured in two warm cumulus clouds during the Cooperative Convective Precipitation Experiment (CCOPE) project (1981) in light of predictions of several recent models. The measurements show the existence at all levels throughout the sampling period of two statistically distinct kinds of cloudy regions, termed “variable” and “steady,” often separated by transition zones of less than ten meters. There is some evidence for microphysical variability induced by local fluctuations in thermodynamic and dynamic parameters; however, the predominant variations are of a nature consistent with laboratory evidence suggesting that mixing is dominated by large structures. Entrainment appears to occur largely near cloud top but the data presented here do not permit identification of a mechanism for transport of the entrained air throughout the cloud.

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L. R. Vargas Zeppetello, Étienne Tétreault-Pinard, D. S. Battisti, and M. B. Baker

Abstract

Climate models show that soil moisture and its subseasonal fluctuations have important impacts on the surface latent heat flux, thus regulating surface temperature variations. Using correlations between monthly anomalies in net absorbed radiative fluxes, precipitation, 2-m air temperature, and soil moisture in the ERA-Interim reanalysis and the HadCM3 climate model, we develop a linear diagnostic model to quantify the major effects of land–atmosphere interactions on summertime surface temperature variability. The spatial patterns in 2-m air temperature and soil moisture variance from the diagnostic model are consistent with those from the products from which it was derived, although the diagnostic model generally underpredicts soil moisture variance. We use the diagnostic model to quantify the impact of soil moisture, shortwave radiation, and precipitation anomalies on temperature variance in wet and dry regions. Consistent with other studies, we find that fluctuations in soil moisture amplify temperature variance in dry regions through their impact on latent heat flux, whereas in wet regions temperature variability is muted because of high mean evapotranspiration rates afforded by plentiful surface soil moisture. We demonstrate how the diagnostic model can be used to identify sources of temperature variance bias in climate models.

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M. B. Baker, R. E. Breidenthal, T. W. Choularton, and J. Latham

Abstract

Turbulent mixing of cloudy and cloud-free air may play an important role in determining the overall dynamic and microphysical behavior of warm clouds. We present a model of turbulent mixing based on laboratory and theoretical studies of chemically reacting shear layers, extended to include the effects of buoyancy instabilities and droplet sedimentation. It is found to be consistent with recent observations of microphysical variability in natural clouds.

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K. N. Bower, T. W. Choularton, J. Latham, J. Nelson, M. B. Baker, and J. Jensen

Abstract

Simple parameterizations of droplet effective radius in stratiform and convective clouds are presented for use in global climate models. Datasets from subtropical marine stratocumulus, continental and maritime convective clouds, and hill cap clouds in middle latitudes and a small amount of data from stratocumulus clouds in middle latitudes have been examined. The results suggest strongly that a simple relationship exists between droplet effective radius and liquid water content in layer clouds with the droplet effective radius proportional to the cube root of the liquid water content. The constant of proportionality is different over oceans and continents. In current global climate models liquid water content is not a predicted variable in convective clouds, and the data strongly suggest that a fixed value of droplet effective radius between 9 and 10 μm should be used for continental clouds more than 500 m deep and 16 μm for maritime cumulus more than 1.5 km deep.

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