Asymmetry in the Diurnal Variation of Surface Albedo

This paper examines the symmetry of both surface-observed albedos and top-of-theatmosphere (TOA) albedos derived from satellite data. Broadband and visible surface albedos were measured at the Department of Energy Atmospheric Radiation Measurement (ARM) Program Southern Great Plains Central Facility, at some fields near the ARM site, and over a coniferous forest in eastern Virginia. Surface and wind conditions are available for most cases. GOES-8 satellite radiance data are converted to broadband albedo using bidirectional reflectance functions and an empirical narrowband-to-broadband relationship. The initial results indicate that surface moisture has a significant effect and can change the albedo in the afternoon by 20% relative to its morning counterpart. Such effects may need to be incorporated in mesoscale and even large-scale models of atmospheric processes. INTRODUCTION 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 (SZA). This assumption results in an albedo variation symmetrical about local noon. Both regular and irregular changes in the surface state can negate this assumption. 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. A combination of satellite and surface albedo measurements are used here to examine asymmetry in the diurnal cycle of clear-sky albedo. DATA Geostationary Operational Environmental Satellite (GOES) visible radiance data taken over a 0.3° grid centered on the Atmospheric Radiation Measurement (ARM) Program Southern Great Plains Central Facility (SCF) were converted to visible narrowband and shortwave broadb..