Evaluating the Design of an Earth Radiation Budget Instrument with System Simulations. Part II: Minimization of Instantaneous Sampling Errors for CERES-I

Abstract

Much of the new record of broadband earth radiation budget satellite measurements to be obtained during the late 1990s and early twenty-first century will come from the dual-radiometer Clouds and Earth's Radiant Energy System Instrument (CERES-1) flown aboard sun-synchronous polar orbiters. Simulation studies conducted in this work for an early afternoon satellite orbit indicate that spatial rms sampling errors of instantaneous CERES-I shortwave flux estimates will range from about 8.5 to 14.0 W m?2 on a 2.5° latitude and longitude grid resolution. Root-mean-square errors in longwave flux estimates are only about 20% as large and range from 1.5 to 3.5 W m?2. These results are based on an optimal cross-track scanner design that includes 50% footprint overlap to eliminate gaps in the top-of-the-atmosphere coverage, and a ?smallest? footprint size to increase the ratio in the number of observations lying within to the number of observations lying on grid area boundaries. Total instantaneous measurement error depends additionally on the variability of anisotropic reflectance and emission patterns and on the retrieval methods used to generate target area fluxes. Three retrieval procedures are investigated, all relying on a maximum-likelihood estimation technique for scene identification. Observations from both CERES-1 scanners (cross-track and rotating azimuth plane) are used. One method is the baseline Earth Radiation Budget Experiment (ERBE) procedure, which assumes that errors due to the use of mean angular dependence models (ADMs) in the radiance-to-flux inversion process nearly cancel when averaged over grid areas. In a second (estimation of N) method, instantaneous ADMs are estimated from the multiangular, collocated observations of the two scanners. These observed models replace the mean models in the computation of the satellite flux estimates. In the third (scene flux) approach, separate target-area retrievals are conducted for each ERBE scene category and their results are combined using area weighting by scene type. The ERBE retrieval performs best when the simulated radiance field departs from the ERBE mean models by less than 10%. For larger perturbations, both the scene flux and collocation methods produce less error than the ERBE retrieval. The scene flux technique is preferable, however, because it involves fewer restrictive assumptions.