Evaluating the Design of an Earth Radiation Budget Instrument with System Simulations. Part III: CERES-I Diurnal Sampling Error

Abstract

Accuracy estimates for the broadband CERES-I (Clouds and Earth's Radiant Energy System Instrument) measurements of daily average radiant exitance are presented. This is a continuation of the authors’ earlier CERES sampling studies published as Part I and II. Daily averaging errors result from not sampling the entire 24-h period with a system of polar satellites. Instantaneous errors, the subject of the previous studies, are also included. Separate estimates for daily average emitted longwave (LW) and reflected shortwave (SW) radiant fluxes are given. The earth SW and LW reference radiation fields are derived from 3-h Geosynchronous Operational Environmental Satellite data, time interpolated between image times, and partitioned into upwelling radiances using scene-dependent angular dependence models (ADMs). Perturbations in these ADMs are introduced to cause instantaneous angular sampling errors (also referred to as ADM errors). These ADM errors, along with spatial sampling errors, are propagated through the time integration process for a more realistic estimate of the daily average error. Three satellite observing configurations are considered. They represent individually, and in combination, a proposed European Polar Orbiting Platform and National Aeronautics and Space Administration Earth Observing System-A sun-synchronous polar-orbiting satellite system. The Earth Radiation Budget Experiment single and multiple satellite time and space averaging algorithms are used for the satellite retrieval. One-satellite spatial root-mean-square (rms) daily averaged SW flux errors of 11–17 W m−2 are obtained for 2.5° latitude-longitude regions over the area studied (15°S–45°N, 50°–120°W). The two-satellite system has errors that are some 40%–60% less, having values between 5 and 9 W m−2. Only the two-satellite system can meet the 10 W m−2 user accuracy requirement for regional daily averaged SW fluxes. Longwave flux errors of 5–6 W m−2 and 3–4 W m−2, respectively, are found for the one- and two-satellite configurations. The largest component of CERES 2.5° daily averaged target area error is due to sparse temporal sampling. The ADM error propagated into the daily average becomes more important as the temporal sampling error is reduced with the two-satellite system. For this system, the ADM error component (of the daily averaged error) for SW radiation reaches a magnitude that can be as large as 8 W m−2 at high solar zenith angles (SZA), where scene anisotropy is usually greatest. Over the study domain, up to 15% of the total rms error is due to ADM errors. Moreover, CERES 2.5° zonal mean daily averaged errors exhibit a latitudinal dependence of some 7 W m−2 for a 60° change in latitude in the presence of 30% systematic errors in the ADMs. This is largely attributable to the SZA dependence of instantaneous ADM error. Without ADM errors, zonal mean daily averaged target area biases range up to 3–4 W m−2 with an irregular latitudinal variation.