Richard N. Green

Cloud Identification for ERBE Radiative Flux Retrieval

Abstract Derivation of top of atmosphere radiative fluxes requires the use of measured satellite radiances and assumptions about the anisotropy of the Earths radiation field. The primary modification of the Earths anisotropy is caused by variations in cloud properties. These variations occur rapidly in space and time and provide a challenge for the accurate derivation of radiative flux estimates. The present paper discusses the application of a maximum likelihood estimation (MLE) technique to the problem of cloud determination for coarse resolution broadband satellite data. This methodology is developed in concert with new empirical models of the angular dependence of radiance, and is tested against simulated satellite observations. It is argued that the new angular dependence models are a more complete description of the Earths radiation field than any previously available models. When used to determine cloud conditions for the inversion of satellite-measured radiances to fluxes, simulations predict t...

Determination of Unfiltered Radiances from the Clouds and the Earth's Radiant Energy System Instrument

Abstract A new method for determining unfiltered shortwave (SW), longwave (LW), and window radiances from filtered radiances measured by the Clouds and the Earth’s Radiant Energy System (CERES) satellite instrument is presented. The method uses theoretically derived regression coefficients between filtered and unfiltered radiances that are a function of viewing geometry, geotype, and whether cloud is present. Relative errors in instantaneous unfiltered radiances from this method are generally well below 1% for SW radiances (std dev ≈0.4% or ≈1 W m−2 equivalent flux), less than 0.2% for LW radiances (std dev ≈0.1% or ≈0.3 W m−2 equivalent flux), and less than 0.2% (std dev ≈0.1%) for window channel radiances. When three months (June, July, and August of 1998) of CERES Earth Radiation Budget Experiment (ERBE)-like unfiltered radiances from the Tropical Rainfall Measuring Mission satellite between 20°S and 20°N are compared with archived Earth Radiation Budget Satellite (ERBS) scanner measurements for the sa...

Postlaunch radiometric validation of the Clouds and the Earth's Radiant Energy System (CERES) Proto-Flight Model on the Tropical Rainfall Measuring Mission (TRMM) Spacecraft through 1999

Abstract Each Clouds and the Earth’s Radiant Energy System (CERES) instrument contains three scanning thermistor bolometer radiometric channels. These channels measure broadband radiances in the shortwave (0.3–5.0 μm), total (0.3–>100 μm), and water vapor window regions (8–12 μm). Ground-based radiometric calibrations of the CERES flight models were conducted by TRW Inc.’s Space and Electronics Group of Redondo Beach, California. On-orbit calibration and vicarious validation studies have demonstrated radiometric stability, defined as long-term repeatability when measuring a constant source, at better than 0.2% for the first 18 months of science data collection. This level exceeds by 2.5 to 5 times the prelaunch radiometric performance goals that were set at the 0.5% level for terrestrial energy flows and 1.0% for solar energy flows by the CERES Science Team. The current effort describes the radiometric performance of the CERES Proto-Flight Model on the Tropical Rainfall Measuring Mission spacecraft over t...

Validation of ERBS Scanner Radiances

Abstract The earth radiation budget satellite (ERBS) has made broadband scanner measurements of the earth radiance for over 5 years. The redundancy between the shortwave, longwave, and total scanning radiometers and data averages have been used to validate the long-term consistency among the measurements and to establish how measurement drift has affected the archived top-of-the-atmosphere fluxes. The total channel gain at night was found to be unchanged over a 4-yr test period. Relative to the total channel at night, the longwave channel sensitivity decreased by 0.5% over the same 4 years and the shortwave channel was unchanged. The shortwave part of the total channel, however, gradually increased in gain by 1.3%. Only the daytime longwave flux was affected by these changes. It drifted upward depending on the scene shortwave component. Over 4 years, the clear ocean daytime longwave flux increased by 0.2% and overcast scenes by 2.6%. For all scenes in the Tropics, the daytime longwave flux increased by le...

Deconvolution of Wide Field-of-View Radiometer Measurements of Earth-Emitted Radiation. Part I: Theory

Abstract The problem of relating satellite measurements from wide field-of-view (WFOV) radiometers to the radiant exitance emitted from the top of the atmosphere is treated. The problem is formulated as an integral equation to be solved for the radiant exitance distribution in terms of the measurements. An analytical solution to this integral equation in terms of spherical harmonies is presented for the case in which the directional dependence of the outgoing radiation is a function of zenith angle only. It is shown that the resolution which can be obtained under real conditions is limited. The sensitivity of the derived radiant exitance distribution to the directional dependence of the out-going radiation is studied, and results are presented for WFOV flat-plate and spherical radiometers and for restricted field-of-view flat-plate radiometers. It is demonstrated that this sensitivity is a function of the scale of spatial resolution; thus higher resolutions in the radiant exitance distribution are more se...

Estimation of angular distribution models from radiance pairs

A new approach to constructing angular distribution models (ADMs) from satellite data has been developed. The ADMs model the radiance anisotropy and are used to convert satellite measured radiance to flux at the top of the atmosphere. The radiance pairs method (RPM) processes radiance pairs that view approximately the same area at the same time. By ratioing the paired radiances, the flux or field strength is eliminated, producing ratios of anisotropies which are taken as the data source for the ADMs. The ADMs are modeled as random functions, and the RPM estimates the mean of the ADM. The RPM is compared to the standard method of sorting by angular bins (SAB) and is shown to remove questionable assumptions, converge faster, and give better accuracy than the SAB method. Both methods were applied to the same Nimbus 7 ERB data and resulted in statistically different longwave ADMs.

Top‐of‐atmosphere albedo estimation from angular distribution models: A comparison between two approaches

Empirical angular distribution models (ADMs) are commonly used to convert satellite-measured radiances to top-of-atmosphere (TOA) radiative fluxes. This study compares two methods of developing ADMs: (1) the radiance pairs method (RPM), which composits ratios of near-simultaneous radiance measurements over the same scene to construct the ADMs; (2) the sorting-into-angular-bins (SAB) method, which estimates ADM anisotropic factors from the ratio of the mean radiance in each angular bin to the mean flux determined by direct integration of the mean radiances. Theoretical simulations and analyses of measurements from the CERES (Clouds and Earths Radiant Energy System) satellite instrument show that the RPM method provides a better estimate of the true mean ADM for a population of scenes, while the SAB method is better suited for top-of-atmosphere flux estimation. The CERES results also show that a variable field of view size with viewing zenith angle can cause an ≈10% (relative) change in estimated all-sky mean albedo with viewing zenith angle.

Early radiometric validation results of the CERES Flight Model 1 and 2 instruments onboard NASA's Terra Spacecraft

Abstract The CERES Flight Model 1 and 2 instruments were launched aboard NASAs Earth Observing System (EOS) Terra Spacecraft on December 18, 1999 into a 705 Km sun-synchronous orbit with a 10:30 a.m. equatorial crossing time. These instruments supplement measurements made by the CERES Proto Flight Model (PFM) instrument launched aboard NASAs Tropical Rainfall Measuring Mission (TRMM) spacecraft on November 27, 1997 into a 350 Km, 38-degree mid-inclined orbit. An important aspect of the EOS program is the rapid archival and dissemination of datasets measured by EOS instruments to the scientific community. On September 22, 2000 the CERES Science Team voted to archive the Edition 1 CERES/Terra Level 1b and Level 2 and 3 ERBE-Like data products. These products consist of instantaneous filtered and unfiltered radiances through temporally and spatially averaged TOA fluxes. CERES filtered radiance measurements cover three spectral bands including shortwave (0.3 to 5 μm), total (0.3 to μ m) and an atmospheric window channel (8 to 12 μm). The current work summarizes both the philosophy and results of validation efforts undertaken to quantify the quality of the Terra data products as well as the level of agreement between the Terra and TRMM datasets.

Accuracy and Resolution of Earth Radiation Budget Estimates

Abstract A numerical filter inversion technique that reduces wide-angle satellite measurements to top-of-the-atmosphere radiant exitances has been proposed for the Earth Radiation Budget Experiment (ERBE). The matrix formulation of this technique is presented, and the design of the numerical filter is discussed. The filter is smoothed with a singular value decomposition. The inversion process is simulated by generating synthetic measurements from a 24 degree spherical harmonic radiation field derived from Nimbus 6 ERB data. The numerical filter is applied to these measurements after they are corrupted with instrument error. The results are curves of expected error versus resolution area.

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 measu...