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Richard N. Green

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.

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Richard N. Green

Abstract

A parameter estimation technique is presented to estimate the radiative flux density distribution over the earn from a set of radiometer measurements at satellite altitude. The technique analyzes measurements from a wide field of view, horizon to horizon. nadir pointing sensor with a mathematical technique to derive the radiative flux density estimates at the top of the atmosphere for resolution elements smaller than the sensor field of view. A computer simulation of the data analysis technique is presented for both earth-emitted and reflected radiation.

The errors resulting from the assumed directional radiation model, spatial model and random measurement error have little effect an the global mean radiation. Zonal estimates were found to be more sensitive, to the spatial model than to the directional radiation model. Results from analysing medium field of view measurements showed a much greater sensitivity to the directional radiation model, even on a global scale.

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Richard N. Green

Abstract

Three different data analysis techniques—shape factor, parameter estimation, and deconvolution—have been applied to the same set of satellite radiation measurements to determine their effect on the estimated radiation field. The measurements are from a wide-angle, horizon-to-horizon, nadir-pointing sensor. The shape factor technique reduces each measurement to a radiant exitance at the top of the atmosphere by simple division by a constant. The parameter estimation technique processes all measurements together as a batch and defines the radiant exitance as a least-squares fit to the data. The deconvolution technique takes advantage of the fact that spherical harmonies are the eigenfunctions of the measurement operator. All three techniques are derived, and their assumptions, advantages and disadvantages are discussed. Their results are compared globally, zonally, regionally and on a spatial spectrum basis. All three techniques give comparable results for global parameters. However, results on a regional scale were quite different. The standard deviations of the regional differences in radiant exitance varied from 7.4 to 13.5 W m−2. Of the three techniques, the parameter estimation technique produced the best regional results and is the choice of the author.

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Richard N. Green and Lee M. Avis

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 less than 1% in 4 years. There was no statistical evidence that the daytime shortwave or nighttime longwave fluxes had drifted.

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Bruce A. Wielicki and Richard N. Green

Abstract

Derivation of top of atmosphere radiative fluxes requires the use of measured satellite radiances and assumptions about the anisotropy of the Earth's radiation field. The primary modification of the Earth's 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 Earth's radiation field than any previously available models. When used to determine cloud conditions for the inversion of satellite-measured radiances to fluxes, simulations predict that the MLE approach gives substantial improvements over both a Lambertian Earth assumption and the clear/cloud threshold used in the inversion of Nimbus 3 and Nimbus 7 Earth Radiation Budget scanner data. The MLE methodology will be used in the operational processing of the Earth Radiation Budget Experiment (ERBE) scanner data. The present paper serves to document both the philosophy and the form of the MLE methodology. Validation studies using both ERBE and Nimbus 7 radiation budget data will be the subject of future papers by several ERBE Science Team investigators.

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G. Louis Smith and Richard N. Green

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 sensitive to variations in the directional dependence.

The technique is applied to measurements of earth-emitted radiation from the Nimbus 6 ERB (Earth Radiation Budget) experiment WFOV radiometer to produce a resolution enhanced map of emitted radiation for the month of August 1975. For this case, the limit of resolution appeared to be spherical harmonies of degree 15. Comparison with results from the ERB scanning radiometer shows very good agreement.

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Richard N. Green and G. Louis Smith

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.

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T. Dale Bess, Richard N. Green, and G. Louis Smith

Abstract

One year of longwave radiation data from July 1975 through June 1976 from the Nimbus 6 satellite Earth Radiation Budget (ERB) experiment is analyzed by representing the longwave radiation field by a spherical harmonic expansion. The data are from the wide field-of-view (WFOY) instrument. Results show that the limit of the spherical harmonic representation is 12th degree, based on degree valiance plots from 12 months. Degree variance plots also show that most of the power is in the lower degree terms. The axisymmetric (zonal) terms dominate with their coefficients representing approximately 80% of the degree variance. Contour maps of the radiation field show the geographical distribution of earth-emitted radiant exitance (W m−2) and reveal areas of high and low emitted radiation. The analysis also shows differences between the Northern and Southern Hemispheres which is presumably due to land/ocean distribution.

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Norman G. Loeb, Kory J. Priestley, David P. Kratz, Erika B. Geier, Richard N. Green, Bruce A. Wielicki, Patricia O’Rawe Hinton, and Sandra K. Nolan

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 same months over a 5-yr period (1985–89), significant scene-type dependent differences are observed in the SW channel. Full-resolution CERES SW unfiltered radiances are ≈7.5% (≈3 W m−2 equivalent diurnal average flux) lower than ERBS over clear ocean, as compared with ≈1.7% (≈4 W m−2 equivalent diurnal average flux) for deep convective clouds and ≈6% (≈4–6 W m−2 equivalent diurnal average flux) for clear land and desert. This dependence on scene type is shown to be partly caused by differences in spatial resolution between CERES and ERBS and by errors in the unfiltering method used in ERBS. When the CERES measurements are spatially averaged to match the ERBS spatial resolution and the unfiltering scheme proposed in this study is applied to both CERES and ERBS, the ERBS all-sky SW radiances increase by ≈1.7%, and the CERES radiances are now consistently ≈3.5%–5% lower than the modified ERBS values for all scene types. Further study is needed to determine the cause for this remaining difference, and even calibration errors cannot be ruled out. CERES LW radiances are closer to ERBS values for individual scene types—CERES radiances are within ≈0.1% (≈0.3 W m−2) of ERBS over clear ocean and ≈0.5% (≈1.5 W m−2) over clear land and desert.

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Kory J. Priestley, Bruce R. Barkstrom, Robert B. Lee III, Richard N. Green, Susan Thomas, Robert S. Wilson, Peter L. Spence, Jack Paden, D. K. Pandey, and Aiman Al-Hajjah

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 the first 19 months of scientific data collection.

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