Spectral Reflectance and Atmospheric Energetics in Cirrus-like Clouds. Part II: Applications of a Fourier-Riccati Approach to Radiative Transfer

Si-Chee Tsay Laboratory for Atmospheres, NASA/Goddard Space Flight Center, Greenbelt, Maryland

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Philip M. Gabriel Departments of Atmospheric Science, Colorado State University, Fort Collins, Colorado

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Michael D. King Earth Sciences Directorate, NASA/Goddard Space Flight Center, Greenbelt, Maryland

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Graeme L. Stephens Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado

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Abstract

One of the major sources of uncertainty in climate studies is the detection of cirrus clouds and characterization of their radiative properties. Combinations of water vapor absorption channels (e.g., 1.38 µm), ice-water absorption channels (e.g., 1.64 µm), and atmospheric window channels (e.g., 11 µm) in the imager, together with a lidar profiler on future EOS platforms, will contribute to enhancing our understanding of cirrus clouds.

The aforementioned spectral channels are used in this study to explore the effects exerted by uncertainties in cloud microphysical properties (e.g., particle size distribution) and cloud morphology on the apparent radiative properties, such as spectral reflectance and heating and cooling rate profiles. As in Part I of our previous study, which establishes the foundations of the Fourier-Riccati method of radiative transfer in inhomogeneous media, cloud extinction and scattering functions are characterized by simple spatial variations with measured and hypothesized microphysics to facilitate our understanding of their radiative properties.

Results of this study suggest that (i) while microphysical variations in the scattering and extinction functions of clouds affect the magnitudes of their spectral reflectances, cloud morphology significantly alters the shape of their angular distribution; (ii) spectral reflectances viewed near nadir are least affected by cloud variability; and (iii) cloud morphology can load to spectral heating and cooling rate profiles that differ substantially from their plane-parallel averaged equivalents. Since there are no horizontal thermal gradients in plane-parallel clouds, it may be difficult to correct for this deficiency.

Abstract

One of the major sources of uncertainty in climate studies is the detection of cirrus clouds and characterization of their radiative properties. Combinations of water vapor absorption channels (e.g., 1.38 µm), ice-water absorption channels (e.g., 1.64 µm), and atmospheric window channels (e.g., 11 µm) in the imager, together with a lidar profiler on future EOS platforms, will contribute to enhancing our understanding of cirrus clouds.

The aforementioned spectral channels are used in this study to explore the effects exerted by uncertainties in cloud microphysical properties (e.g., particle size distribution) and cloud morphology on the apparent radiative properties, such as spectral reflectance and heating and cooling rate profiles. As in Part I of our previous study, which establishes the foundations of the Fourier-Riccati method of radiative transfer in inhomogeneous media, cloud extinction and scattering functions are characterized by simple spatial variations with measured and hypothesized microphysics to facilitate our understanding of their radiative properties.

Results of this study suggest that (i) while microphysical variations in the scattering and extinction functions of clouds affect the magnitudes of their spectral reflectances, cloud morphology significantly alters the shape of their angular distribution; (ii) spectral reflectances viewed near nadir are least affected by cloud variability; and (iii) cloud morphology can load to spectral heating and cooling rate profiles that differ substantially from their plane-parallel averaged equivalents. Since there are no horizontal thermal gradients in plane-parallel clouds, it may be difficult to correct for this deficiency.

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