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Browsing by Author "Stephens, Graeme L., author"

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    A physical/optical model for atmospheric aerosols with application to visibility problems
    (Colorado State University. Libraries, 1990-01) Tsay, Si-Chee, author; Stephens, Graeme L., author; Cooperative Institute for Research in the Atmosphere (Fort Collins, Colo.), publisher
    The objectives of this report are to describe a conceptually simple but accurate model for efficiently computing the optical properties of atmospheric aerosols. Aerosol characteristics such as size distribution, solubility, mixture and the atmospheric moisture effect are taken into account when computing its optical properties. The dependence of the latter on aerosol microstructure is also discussed. The optical properties of aerosols are computed by employing numerically stable algorithms for obtaining Mie solutions to coated spheres. Resulting bulk quantities such as the extinction/backscatter coefficient, the probability of scattering, and the scattering phase matrix can be incorporated into multiple scattering schemes of radiative transfer for visibility investigations and other types of studies.
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    ItemOpen Access
    CSU radiation budget pilot study for TOGA COARE
    (Colorado State University. Libraries, 1993) Cornwall, Christopher R., author; Combs, Cynthia L., author; Davis, John M., author; Stephens, Graeme L., author; Cox, Stephen K., author; Department of Atmospheric Science, Colorado State University, publisher
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    Nimbus-7 observations of the effect of clouds on the earth's radiation budget
    (Colorado State University. Libraries, 1990-08) Greenwald, Thomas J., author; Stephens, Graeme L., author; Vonder Haar, Thomas H., author; Cooperative Institute for Research in the Atmosphere (Fort Collins, Colo.), publisher
    Outgoing longwave (LW) flux and shortwave (SW) albedo data obtained from narrow field-of-view scanner measurements aboard the Nimbus-7 satellite are used along with coincident cloudiness data to estimate the effect of clouds on the earth's radiation budget (ERB). A simple technique is described to obtain clear sky albedos and LW fluxes using daily Nimbus-7 ERB and total cloud amount data. The analysis is done for the following four seasons: June-August 1979, September-November 1979, December-February 1980, and March-May 1980. When compared with the recent results from the Earth Radiation Budget Experiment (ERBE) the Nimbus-7 derived clear sky LW and SW fluxes are about 5-6 Wm-2 too low and 3-4 Wm-2 too high, respectively, most likely resulting from cloud contamination. The concept of cloud radiative forcing, referred to in this study as cloud effect, can provide a quantitative measure of the impact of clouds on the ERB. It is defined as the difference between the clear sky flux and the cloudy sky flux at the top of the atmosphere. The SW cloud effect is shown to be negative over most of the earth and is greatest in the midlatitudes in areas of stratus clouds and storm tracks. The LW cloud effect, on the other hand, is primarily positive and most significant in the tropics. The net cloud effect is found to be negative over most of the earth, with a near cancellation of the SW and LW effects in the tropics and a significant negative effect in the midlatitudes. Moreover, it is shown that the net cloud effect, when globally averaged, is negative and varies with season. A comparison of the Nimbus-7 derived LW and SW cloud effects to those obtained from ERBE shows, on average, a 5-6 Wm-2 bias resulting from cloud contamination of the Nimbus-7 clear sky fluxes.
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    ItemOpen Access
    Progress in visibility modeling
    (Colorado State University. Libraries, 1992-12) Fuller, Kirk A., author; O'Brien, Dennis M., author; Stephens, Graeme L., author; Cooperative Institute for Research in the Atmosphere (Fort Collins, Colo.), publisher
    The cross section for total scattering by a cluster of spheres is derived from an integration, over a closed spherical surface, of the scattered Poynting flux associated with the different pairs of spheres in the ensemble. With the use of the addition theorem for vector spherical harmonics, the integral can be evaluated analytically. The pair-wise cross sections can be rearranged into an expression for the scattering cross section of sphere aggregates which is analogous to that obtained from Lorenz-Mie theory for a single sphere. This latter formulation, however, is more difficult to treat numerically than is the summation over pair-wise cross sections. The cross section for total scattering by a cluster of spheres thus derived is applied to a study of the effects of scavenging and aggregation on the specific absorption of carbon. Results are presented for polarization- and orientation-dependent absorption cross sections of sulfate haze elements and cloud droplets with small carbon grains (spheres) attached to their surfaces. Soot typically occurs as aggregates of carbon spherules. In order to address the validity of certain assumptions that are made in the analysis of such structures by fractal theory, comparisons between the absorption cross sections of free carbon, linear chains, and tightly clumped carbon spheres are also provided. Monte Carlo integration of the radiative transfer equation is the technique most easily adapted to complex scattering geometries. It is demonstrated that the multidimensional integrals can be evaluated more accurately and more efficiently with quasi Monte Carlo integration and that the convergence of the multiple scattering series can be accelerated by estimating the rate of decay of the tail of the series. Each of these techniques has been found to be robust and applicable to scattering with any geometry.