Determination of Shortwave Earth Reflectances from Visible Radiance Measurements: Error Estimate Using ScaRaB Data

Abstract

The error resulting from the use of a visible channel to estimate shortwave (SW) (0.2?4 ?m) fluxes reflected by the earth is analyzed. First, the authors compute regression coefficients between visible (0.55?0.65 ?m) and SW radiance measurements made by the ScaRaB (Scanner for Radiation Budget) instrument aboard the Meteor-3/7 satellite between March 1994 and February 1995. These regression coefficients are computed from the 10 months of available ScaRaB measurements in different classes of geotypes and different classes of solar and viewing angles. The regression is applied to the visible radiance measurements to simulate the SW measurements in the operational processing of ScaRaB. For instantaneous fluxes, the visible-to-SW conversion gives a standard deviation of the error smaller than 8%. By comparison, the standard deviation of the instantaneous flux error coming from the angular sampling and the bidirectional reflectance uncertainty is estimated to be about 10%. For monthly mean values the standard deviation of the error is smaller than 4% and is comparable to the expected temporal sampling error made by two polar orbiting satellites. In addition, using ScaRaB visible data with these regression coefficients and with the ScaRaB processing gives a bias smaller than 2% for either instantaneous or monthly mean fluxes. However, calibration and processing problems certainly remain important practical issues for the determination of the earth planetary albedo using only narrowband radiometers.