Anisotropic reflection of UV radiation at the top of the atmosphere: Characteristics and models obtained from Meteor 3TOMS

Reflection of solar adiation usually exhibits a strong dependence on viewing geometry relative to the Sun's position. Such a dependence needs taking into account in remote sensing studies employing satellite measurements made with scanning radiometers. While many investigations have been conducted concerning the angular variation of radiation in the total solar, visible, and IR wavelengths, no study was devoted to the UV radiation whose dependence on viewing geometry may differ from radiation in other spectral regions. On the basis of two and half years of Meteor-3/TOMS data (from January 1992 to May 1994) the angular characteristics of UV radiation reflected at the top of the atmosphere (TOA) were studied extensively. A set of angular dependence models (ADMs) was developed for UV radiation at 340, 360, and 380 nm over 10 scene types. The results show that the angular dependence of UV-reflected radiation is strong and rather different from that for total shortwave (SW) radiation as derived for the Earth Radiation Budget Experiment (ERBE). The largest discrepancy occurs under clear conditions over oceans for overhead Sun and large viewing zenith angles. The discrepancy in TOA mean albedos derived using the two sets of ADM can reach up to 48%. The discrepancy diminishes to about 10% for clear land scenes. Comparisons were a!s0 made for regional monthly mean TOA a!bedos over the whole globe derived from the ERBE SW ADM, TOMS UV ADM, and the assumption of isotropic reflection from an independent data set acquired by Nimbus 7/TOMS in January and July 1991. The differences exhibit an apparent zonal trend, large at high latitudes, moderate at midlatitudes, and small in the tropics.