Observational and Theoretical Studies of Solar Radiation in Arctic Stratus Clouds

Abstract

A series of clouds-radiation experiments was carried out in June 1980 in Arctic stratus clouds occurring over the Beaufort Sea using the NCAR Electra aircraft. This paper is an analysis of the hemispheric radiation fields obtained with Eppley pyranometers and silicon flux detectors, and the cloud microphysical data obtained with the Knollenberg FSSP and 200X probes. These data were collected in a series of 12 vertical profiles in a variety of boundary layer stratus and altostratus situations. The results are interpreted with the aid of a theoretical radiative transfer model developed by Slingo and Schrecker. The Knollenberg measurements indicated a wide range of cloud liquid water contents which generally increased from cloud base to cloud top. Values of the equivalent radius of the droplet size distribution are presented. Based on the agreement between the radiation measurements and the model calculations, it is concluded that the Knollenberg FSSP probe produced realistic liquid water concentrations in the present study. Cloud reflectivity, transmissivity, and absorptivity were obtained with the Eppley pyranometers over the total solar spectrum, and also in the visible and near infrared regions, while the silicon detector produced values over the total spectrum, and in the near-infrared. The reflectivity and transmissivity obtained with the two systems agreed well with each other, and also with the model calculations. The measured values of absorptivity were systematically greater than those predicted, particularly in the visible region. The theoretical model was used to investigate in each cloud profile the roles of aerosols, gaseous and droplet absorptions, and surface reflectivity. In particular, it was found that Arctic summer aerosol did not significantly affect the bulk radiative properties of the clouds, but did have a large effect on the ratio of diffuse to total radiation above the cloud.