Inference of Cirrus Cloud Properties Using Satellite-observed Visible and Infrared Radiances. Part I: Parameterization of Radiance Fields

Patrick Minnis Atmospheric Sciences Division, NASA Langley Research Center, Hampton, Virginia

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Kuo-Nan Liou Department of Meteorology, University of Utah, Salt Lake City, Utah

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Yoshihide Takano Department of Meteorology, University of Utah, Salt Lake City, Utah

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Abstract

Current techniques for deriving cirrus optical depth and altitude from visible (0.65 μm) and infrared (11.5 μm) satellite data use radiative transfer calculations based on scattering phase functions of spherical water droplets. This study examines the impact of using phase functions for spherical droplets and hexagonal ice crystals to analyze radiances from cirrus. Adding-doubling radiative transfer calculations are used to compute radiances for different cloud thicknesses and heights over various backgrounds. These radiances are used to develop parameterizations of top-of-the-atmosphere visible reflectance and infrared emittance utilizing tables of reflectance as a function of cloud optical depth, viewing and illumination angles, and microphysics. This parameterization, which includes Rayleigh scattering, ozone absorption, variable cloud height, and an anisotropic surface reflectance, reproduces the computed top-of-the-atmosphere reflectances with an accuracy of ±6% for four microphysical models: 10–μm water droplet, small symmetric crystal, cirrostratus, and cirrus uncinus. The accuracy is twice that of previous models.

Bidirectional reflectance patterns from theoretical ice-crystal clouds are distinctly different from those of the theoretical water-droplet clouds. In general, the ice-crystal phase functions produce significantly larger reflectances than the water-droplet phase function for a given optical depth. A parameterization relating infrared emittance to visible optical depth is also developed. The effective infrared emittances computed with the adding-doubling method are reproduced with a precision of ±2%. Infrared scattering reduces emittance by an average of 5%. Simulated cloud retrievals using the parameterization indicate that optical depths and cloud temperatures can be determined with an accuracy of ∼25% and ∼6 K for typical cirrus conditions. Retrievals of colder clouds over brighter surfaces are not as accurate, while those of warmer clouds over dark surfaces will be more reliable. Sensitivity analyses show that the use of the water-droplet phase function to interpret radiances from a theoretical cirrostratus cloud will significantly overestimate the optical depth and underestimate cloud height by 1.5–2.0 km for nominal cirrus clouds (temperature of 240 K and visible optical depth of ∼1). The parameterization developed here is economical in terms of computer memory and is useful for both simulation and interpretation of cloud radiance fields.

Abstract

Current techniques for deriving cirrus optical depth and altitude from visible (0.65 μm) and infrared (11.5 μm) satellite data use radiative transfer calculations based on scattering phase functions of spherical water droplets. This study examines the impact of using phase functions for spherical droplets and hexagonal ice crystals to analyze radiances from cirrus. Adding-doubling radiative transfer calculations are used to compute radiances for different cloud thicknesses and heights over various backgrounds. These radiances are used to develop parameterizations of top-of-the-atmosphere visible reflectance and infrared emittance utilizing tables of reflectance as a function of cloud optical depth, viewing and illumination angles, and microphysics. This parameterization, which includes Rayleigh scattering, ozone absorption, variable cloud height, and an anisotropic surface reflectance, reproduces the computed top-of-the-atmosphere reflectances with an accuracy of ±6% for four microphysical models: 10–μm water droplet, small symmetric crystal, cirrostratus, and cirrus uncinus. The accuracy is twice that of previous models.

Bidirectional reflectance patterns from theoretical ice-crystal clouds are distinctly different from those of the theoretical water-droplet clouds. In general, the ice-crystal phase functions produce significantly larger reflectances than the water-droplet phase function for a given optical depth. A parameterization relating infrared emittance to visible optical depth is also developed. The effective infrared emittances computed with the adding-doubling method are reproduced with a precision of ±2%. Infrared scattering reduces emittance by an average of 5%. Simulated cloud retrievals using the parameterization indicate that optical depths and cloud temperatures can be determined with an accuracy of ∼25% and ∼6 K for typical cirrus conditions. Retrievals of colder clouds over brighter surfaces are not as accurate, while those of warmer clouds over dark surfaces will be more reliable. Sensitivity analyses show that the use of the water-droplet phase function to interpret radiances from a theoretical cirrostratus cloud will significantly overestimate the optical depth and underestimate cloud height by 1.5–2.0 km for nominal cirrus clouds (temperature of 240 K and visible optical depth of ∼1). The parameterization developed here is economical in terms of computer memory and is useful for both simulation and interpretation of cloud radiance fields.

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