Shortwave Shape Factor Inversion of Earth Radiation Budget Observations

Abstract

The shape factor technique is routinely used to invert wide-angle radiometric measurements at satellite altitude to flux at the top of the atmosphere. The derivation of a shortwave shape factor requires assumptions on both the viewed radiation field and the angular distribution of the radiance. This paper describes the effect on the shape factor of assuming a constant flux field, a constant albedo field, and a variable albedo field. In addition, three assumptions on the angular distributions are investigated: Lambertian, an analytic model, and the Earth Radiation Budget Experiment (ERBE) bidirectional models. The accuracies and resolutions of the shape factor flux estimates arising from these assumptions are determined by simulating the shape factor inversion technique with ERBE scanner data. First, the scanner data are summed at satellite altitude to simulate the wide-angle radiometric measurements. Radiant flux at the top of the atmosphere is then estimated from these simulated wide-angle measurements with the various shape factors and compared to the original scanner flux field. The resulting biases and variances are given for both the ERBE medium-field-of-view and wide-field-of-view radiometers.