For the opposite case of irradiance observations over ocean, the effect of a distant snow surface is not significant at ranges greater than 2 or 3 km. Since the radius of influence depends on atmospheric transmission and surface albedo, the effective albedo varies spectrally. Neglect of this nonlocal albedo effect may significantly degrade the accuracy of radiation diagnostics that depend on spectral intensity ratios.
IntroductionThe environment in high-latitude regions presents an unusual challenge to remote sensing and surface radiation measurements interpretation. In addition to the well-known problems discriminating between clouds and high-albedo surfaces, the low Sun angles make satellite observations of the surface difficult to interpret. Even areas of modest topographic relief are subject to large variations in the surface insolation due to shadowing and shallow angles of illumination. These geometrical effects must be properly considered in order to make quantitative use of the satellite observations. The analysis and interpretation of surface radiation measurements in high-latitude regions represents an important challenge. The extensive surface coverage of snow and ice increases multiple reflection and causes a significant sensitivity of downwelling irradiance on the surface reflectivity. Under cloudy conditions the contrast between a snow surface and a dark ground cover can make a factor of 2 difference in the downwelling shortwave ( produce an effective surface albedo that could be used in plane-parallel radiative transfer (RT) models. In overcast conditions, physical considerations of cloud and surface reflectance suggest that the radius of influence of a given surface element is of the order of the cloud base height. Presumably, surface regions that remain uniform over a horizontal scale much larger than the cloud base height should be well characterized by the local surface albedo. However, in areas where the snow or sea ice coverage may vary on the kilometer scale, it is difficult to know how to weigh the importance of contributions at different radii around the sensor.In an effort to quantify these effects we have developed the SAMCRT (Surface-Atmosphere Monte Carlo Radiative Transfer) code, which is designed to model radiative interactions of atmospheric radiation with complex surface features. During the austral spring, Palmer Station is subjected to episodes of significant UVB enhancements as the outer edge of the ozone hole passes over it. We have undertaken a thorough study of the radiation environment at this site