

Kato, S, Rose, FG, Charlock, TP (2005). Computation of domainaveraged irradiance using satellitederived cloud properties. JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY, 22(2), 146164. Abstract The respective errors caused by the gammaweighted twostream approximation and the effective thickness approximation for computing the domainaveraged broadband shortwave irradiance are evaluated using cloud optical thicknesses derived from 1 h of radiance measurements by the Moderate Resolution Imaging Spectrometer (MODIS) over footprints of Clouds and the Earth's Radiant Energy System (CERES) instruments. Domains are CERES footprints of which dimension varies approximately from 20 to 70 km, depending on the viewing zenith angle of the instruments. The average error in the topofatmosphere irradiance at a 30degrees solar zenith angle caused by the gammaweighted twostream approximation is 6.1 W m(2) (0.005 albedo bias) with a onelayer overcast cloud where a positive value indicates an overestimate by the approximation compared with the irradiance computed using the independent column approximation. Approximately onehalf of the error is due to deviations of optical thickness distributions from a gamma distribution and the other half of the error is due to other approximations in the model. The error increases to 14.7 W m(2) (0.012 albedo bias) when the computational layer dividing the cloud layer is increased to four. The increase is because of difficulties in treating the correlation of cloud properties in the vertical direction. Because the optical thickness under partly cloudy conditions, which contribute twothirds of cloudy footprints, is smaller, the error is smaller than under overcast conditions;, the average error for partly cloudy condition is 2.4 W m(2) (0.002 albedo bias) at a 30degrees solar zenith angle. The corresponding average error caused by the effective thickness approximation is 0.5 W m(2) for overcast conditions and 21.5 W m(2) (0.018 albedo bias) for partly cloudy conditions. Although the error caused by the effective thickness approximation depends strongly on the optical thickness, its average error under overcast conditions is smaller than the error caused by the gammaweighted twostream approximation because the errors at small and large optical thicknesses cancel each other. Based on these error analyses, the daily average error caused by the gammaweighted twostream and effective thickness approximations is less than 2 W m(2). DOI:
ISSN: 07390572 