Asymmetry in the diurnal variation of surface albedo

Remote sensing of surface properties and estimation of clear-sky and surface albedo generally assume that the albedo depends only on the solar zenith angle. The effects of dew, frost, and precipitation as well as evaporation and wind can lead to some systematic diurnal variability resulting in an asymmetric diurnal cycle of albedo. This paper examines the symmetry of both surface-observed and top-of-the-atmosphere (TOA) albedos derived from satellite data. Broadband surface albedos were measured at the Department of Energy Atmospheric Radiation Measurement (ARM) Program Southern Great Plains Central Facility near Lamont, Oklahoma and several extended facilities. GOES satellite radiance data are converted to broadband albedo using bidirectional reflectance functions and an empirical narrowband-to-broadband relationship. The surface and top-of-atmosphere albedos vary in a consistent fashion during both the morning and afternoon. The initial results indicate that surface moisture, probably in the form of dew, has a significant effect and can change the albedo by 10% at a given solar zenith angle between the morning and afternoon. Wind speed is well correlated with the diurnal albedo asymmetry. Light winds and small dew point depressions are associated with the greatest morning/afternoon albedo differences. Aerosols tend to moderate those differences. Changes in the surface properties from dew may alter the bidirectional reflectance characteristics of the scene, affecting the interpretation of remote sensing data. Errors in the diurnally averaged albedos derived from Sun-synchronous satellite measurements that arise from albedo asymmetry are generally less than 3%. Further examination of surface albedo asymmetry is needed to assess its influence on satellite measurements and the surface energy budget over a range of land surface types.