A Method for Determining the Single Scattering Albedo of Clouds Through Observation of the Internal Scattered Radiation Field

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  • 1 Laboratory for Atmospheric Sciences, Goddard Space Flight Center, NASA, Greenbelt; MD 20771
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Abstract

A method is presented for determining the single scattering albedo of clouds at selected wavelengths in the visible and near-infrared wavelength regions. The procedure compares measurements of the ratio of the zenith to nadir propagating intensifies deep within a cloud layer with radiative transfer computations of the same. Analytic formulas are derived which explicitly show the dependence of the internal intensity ratio on ground albedo, optical depth, single scattering albedo and cloud asymmetry factor. The single scattering albedo and cloud asymmetry factor enter the solution in such a way that a similarity relationship exists between these two parameters. As a result, the accuracy with which the single scattering albedo can be determined is dictated by the accuracy with which the asymmetry factor can be estimated. A method of observation is described whereby aircraft measurements of the zenith and nadir propagating intensifies can be used to determine the similarity parameter as a function of wavelength. Since the fractional absorption of a cloud depends on the similarity parameter and not on the single scattering albedo and asymmetry factor separately, this poses no severe limitation to the method. An accurate knowledge of the ground albedo and total optical thickness of a cloud are unnecessary for a solution, provided one associates the wavelength for which the intensity ratio is a maximum with conservative scattering. Under this internal calibration approach, uncertainties in the ground albedo are very nearly compensated by uncertainties in the cloud optical thickness.

Abstract

A method is presented for determining the single scattering albedo of clouds at selected wavelengths in the visible and near-infrared wavelength regions. The procedure compares measurements of the ratio of the zenith to nadir propagating intensifies deep within a cloud layer with radiative transfer computations of the same. Analytic formulas are derived which explicitly show the dependence of the internal intensity ratio on ground albedo, optical depth, single scattering albedo and cloud asymmetry factor. The single scattering albedo and cloud asymmetry factor enter the solution in such a way that a similarity relationship exists between these two parameters. As a result, the accuracy with which the single scattering albedo can be determined is dictated by the accuracy with which the asymmetry factor can be estimated. A method of observation is described whereby aircraft measurements of the zenith and nadir propagating intensifies can be used to determine the similarity parameter as a function of wavelength. Since the fractional absorption of a cloud depends on the similarity parameter and not on the single scattering albedo and asymmetry factor separately, this poses no severe limitation to the method. An accurate knowledge of the ground albedo and total optical thickness of a cloud are unnecessary for a solution, provided one associates the wavelength for which the intensity ratio is a maximum with conservative scattering. Under this internal calibration approach, uncertainties in the ground albedo are very nearly compensated by uncertainties in the cloud optical thickness.

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