Improvements in the Shortwave Cloud-free Radiation Budget Accuracy. Part II: Experimental Study Including Mixed Surface Albedos

Abstract

A method is presented to obtain area mean radiant flux densities at the surface in the solar spectral range in cloud-free conditions. This is accomplished by fitting a realistic radiation model with actual atmospheric data and an anisotropic mixed surface reflection function. The latter is assumed to model anisotropy correctly but not the angular average value, i.e., the surface albedo. The surface albedo is tuned until computed and measured radiances incident on a satellite radiometer agree. Then the radiant flux densities, and the albedos and net fluxes, produced by the model are considered to be correct. In a case study, the method is applied in La Mancha, Spain. Uniqueness and accuracy of the tuned radiation model is assessed by means of airborne radiance measurements. The overall agreement of measured and computed radiances is found to be 5%. Flux densities and net fluxes derived from the tuned radiation model have accuracies of about ±15 W m?2, limited mainly by the present accuracy of the METEOSAT VIS channel calibration; surface albedo can be determined to approximately ±0.01 to ±0.015. These accuracies are obtained for instantaneous flux densities allowing their diurnal variability without time averaging to be derived.