Solar Radiation in Summertime Arctic Stratus Clouds

Abstract

The results are reported of pyranometric measurements of solar radiation in Arctic stratus decks made from aircraft flights over the Beaufort Sea during late July 1975. The reflectance of these cloud layers was nearly constant over the range of cloud thicknesses investigated, indicating the importance of the high surface reflectivity. The following ranges of reflectance are obtained: over the total solar spectrum, 60?750%; visible spectrum, 70?85%; near?infrared, 50?65%. Transmittances for the cloud layers are presented as a function of cloud geometrical depth, and the bulk absorptance averaged over all cloud decks was 7% in the total solar spectrum, and 5% and 9% in the visible and near?infrared, respectively. Additional cloud parameters, namely, single scatter albedo ??vand absorption optical depth τv, are derived by fitting the upward and downward flux profiles from each flight to a two?stream approximation to estimate the absorption optical depth. By assuming a linear relation between absorption optical depth and cloud thickness, the scattering parameter ?v, which defines the increased path length caused by multiple scattering, is determined from a best fit to the complete set of observed reflectances and transmittances. The following ranges of ?v, are estimated: total solar, 8.75?10.7; visible, 9.01?14.3; near?infrared, 6.86?8.12. By assuming an asymmetry factor of 0.85 these values of ?v yield estimates of the single scattering albedo (?v) of 0.994?0.996 over the total solar spectrum, 0.994?0.998 in the visible, and 0.990.993 in the near?infrared. Examples are presented of cloud absorption calculated with these derived values of ??v.