Surface Albedo Estimates from Nimbus-7 ERB Data and a Two-Stream Approximation of the Radiative Transfer Equation

Abstract

Solar zenith angle-dependent surface albedo is determined by equating top of the atmosphere (TOA) albedo evaluated from Nimbus-7 data with TOA albedo predicted by a two-layer, two-stream radiative transfer model of the atmosphere. Results are presented for solar zenith angles ten than 82° for two locations in Western Australia, the Sahara/Arabian desert and the Ar Rab Al Khali basin in Saudi Arabia. They are compared with previous estimates, theoretical model results and values used in some GCMS. Uncertainties in surface albedo are determined for precipitable water amounts between 1 and 3 g cm⁻² and tropospheric aerosol broadband optical depths up to 0.1. Seasonal mean values of precipitable water vapor and ozone are adequate for estimating albedo by this method. For solar zenith angles less than 60°, aerosols of optical depth less than 0.1 can probably be neglected. Otherwise, neglect of aerosol backscatter can lead to significant errors in albedo estimates. Surface albedos obtained by averaging over the entire solar spectrum are usually within 5% of those obtained by splitting the solar spectrum into visible and near infrared components. Although a value for the ratio of near infrared to visible region albedo must be assumed, error is small for ratios larger than 1, which are generally the case. Neglect of multiple reflections between surface and atmosphere introduces systematic error into albedo estimates up to ≈4%. Our estimates agree quite well in both magnitude and form with estimates from the δ-Eddington method applied to sand and with estimates from a geometric vegetation albedo model. Differences between our estimates and those used in some GCMs may be significant to simulations of regional climate.