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Bruce A. Wielicki, J.T. Suttles, Andrew J. Heymsfield, Ronald M. Welch, James D. Spinhirne, Man-Li C. Wu, David O'C. Starr, Lindsay Parker, and Robert F. Arduini

Abstract

Observations of cirrus and altocumulus clouds during the First International Satellite Cloud Climatology Project Regional Experiment (FIRE) are compared to theoretical models of cloud radiative properties. Three tests are performed. First, Landsat radiances are used to compare the relationship between nadir reflectance at 0.83 μm and beam emittance at 11.5 μm with that predicted by model calculations using spherical and nonspherical phase functions. Good agreement is found between observations and theory when water droplets dominate. Poor agreement is found when ice particles dominate, especially if scattering phase functions for spherical particles am used. Even when compared to a laboratory measured ice particle phase function (Volkovitskiy et al. 1980), the observations show increased side scattered radiation relative to the theoretical calculations. Second, the anisotropy of conservatively scattered radiation is examined using simultaneous multiple-angle views of the cirrus from Landsat and ER-2 aircraft radiometers. Observed anisotropy gives good agreement with theoretical calculations using the laboratory measured ice-particle phase function and poor agreement with a spherical-particle phase function. Third, Landsat radiances at 0.83 μm, 1.65 μm, and 2.21 μm are used to infer particle phase and particle size. For water droplets, good agreement is found with King Air FSSP particle probe measurements in the cloud. For ice particles, the Landsat radiance observations predict an effective radius of 60 μm versus aircraft observations of about 200 μm. It is suggested that this discrepancy may be explained by uncertainty in the imaginary index of ice and by inadequate measurements of small ice particles by microphysical probes.

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