A Parameterization for the Shortwave Transmissivity of Stratiform Water Clouds Based on Empirical Data and Radiative Transfer Theory

V. E. Derr NOAA, Environmental.Research Laboratories, Boulder, Colorado

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R. S. Stone Cooperative Institute for Research in Environmental Sciences, University of Colorado/NOAA, Boulder, Colorado

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H. P. Hanson Cooperative Institute for Research in Environmental Sciences, University of Colorado/NOAA, Boulder, Colorado

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L. S. Fedor NOAA/ERL, Wave Propagation Laboratory, Boulder, Colorado

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Abstract

Surface measurements of solar flux and total integrated liquid-water content, radiosonde data, and infrared satellite images are analyzed in conjunction with radiative transfer calculations to derive an empirical parameterization for the shortwave transmissivity of continental stratiform water clouds. The data were collected near Denver, Colorado, over a period of six years. Seventeen days on which uniform stratiform clouds persisted over the observing site were selected for detailed analysis, and form the basis for deriving the parameterization. A mulitiple reflection radiative transfer model is employed to estimate stratus cloud transmissivity in terms of the measurable liquid-water path (LWP). A nonlinear fit of estimated transmissivities to the corresponding observations of LWP yields close agreement with a previous, more complicated parameterization. The derived expression for cloud transmissivity is used to predict mean daily surface fluxes for 61 days during which periods of stratiform clouds were observed over the Denver area. A comparison between predicted and measured fluxes shows agreement to within ±4%, with best agreement for clouds of moderate optical thickness. Potential sources of error are identified with sensitivity studies.

Abstract

Surface measurements of solar flux and total integrated liquid-water content, radiosonde data, and infrared satellite images are analyzed in conjunction with radiative transfer calculations to derive an empirical parameterization for the shortwave transmissivity of continental stratiform water clouds. The data were collected near Denver, Colorado, over a period of six years. Seventeen days on which uniform stratiform clouds persisted over the observing site were selected for detailed analysis, and form the basis for deriving the parameterization. A mulitiple reflection radiative transfer model is employed to estimate stratus cloud transmissivity in terms of the measurable liquid-water path (LWP). A nonlinear fit of estimated transmissivities to the corresponding observations of LWP yields close agreement with a previous, more complicated parameterization. The derived expression for cloud transmissivity is used to predict mean daily surface fluxes for 61 days during which periods of stratiform clouds were observed over the Denver area. A comparison between predicted and measured fluxes shows agreement to within ±4%, with best agreement for clouds of moderate optical thickness. Potential sources of error are identified with sensitivity studies.

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