Search Results

You are looking at 1 - 8 of 8 items for

  • Author or Editor: Richard Hucek x
  • Refine by Access: All Content x
Clear All Modify Search
Richard Hucek and Herbert Jacobowitz

Abstract

Narrowband observations from NOAA's Advanced Very High Resolution Radiometer (AVHRR) are used operationally by NOAA to estimate the earth's broadband planetary albedo. Since May of 1988; these broadband albedo estimates have been derived using the two-channel (visible and near-infrared), scene-independent regression model of Wydick et al. The occurrence of relatively large regional bias errors using this model has led to a study of scene-dependent models that substantially reduce these errors. Three classes of scene stratification are considered: 1) by surface geography type alone (SFC); and 2) and 3) by surface geography type in combination with cloud amount category (SCN) and normalized difference albedo index (NDAI) using AVHRR channels 1 and 2. These and the Wydick model are applied to independent AVHRR global data of 2 July 1985 (hereafter July) and 9 January 1986 (hereafter January). Using ERBE (Earth Radiation Budget Experiment) data as a reference, errors in reflected flux are computed for each day.

The total AVHRR-ERBE shortwave flux difference is separated into two terms. One is due to inaccuracy in the calibration of the AVHRR reflectances (calibration error). The second is the error due to all other sources of the AVHRR-ERBE flux difference. It is referred to as the measurement error. Spatial sampling differences (sampling error) and limitations in the mathematical form and specification of the AVHRR regression model equations (model error) are probably the two primary components of the measurement error.

When calibration error vanishes (due to the implementation of calibration corrections) and sampling differences are small (i.e., for global and zonal averaging), only the model error remains. The Wydick model yields high positive global bias errors of 22 and 37 W m−2 for July and January, respectively. In contrast, errors of ±5 W m−2 are obtained with the scene-dependent models (i.e., SCN). When no calibration adjustments to the AVHRR data are performed, as in operational processing, the Wydick model produces bias errors of −6.8 and 1.5 W m−2 for July and January, respectively. These low bias errors may be misleading though as they result from the near cancellation of large model and calibration error components. The cancellation is not effective at all latitudes, so the Wydick model tends to generate large north-south error gradients. These latitudinal errors are largely removed by all of the scene-dependent models.

Full access
Richard Hucek, Larry Stowe, and Robert Joyce

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.

Full access
Larry Stowe, Richard Hucek, Philip Ardanuy, and Robert Joyce

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.

Full access
Richard R. Hucek, Philip Ardanuy, and H. Lee Kyle

Abstract

The results of a constrained, wide field-of-view (WFOV) radiometer measurement deconvolution are presented and compared against higher resolution results obtained from the Earth Radiation Budget (ERB) Experiment on the Nimbus-7 satellite and from the Earth Radiation Budget Experiment (ERBE). The method is applicable to both longwave and shortwave observations and is specifically designed to treat the problem of anisotropic reflection and emission at the top of the atmosphere (TOA), and low signal-to-noise ratios that arise regionally within the observation field. The latter occur, for example, near the earth's terminator where measured WFOV shortwave signals contain increasing percentages of instrument and modeling errors. Ridge regression and meridional smoothing are used to quell the resulting “local” instability and permit the recovery of a global solution. An optimized retrieval is obtained by tuning the constraints until the recovered solution matches, as well as possible, a known higher resolution product or, lacking that, until unacceptable features in the recovered field no longer appear. The latter approach leads to a set of weight factors that depend on the length of the sampling period and on the desired parameter field, but not on the calendar date. A 1-year study dataset, July 1983 through June 1984, as well as data for the individual months of April 1980 and 1985 have been processed using a preliminary version of these algorithms. Representative deconvolved fields of mean daily longwave flux and albedo are shown for monthly and 8-day inversion periods. When compared to ERB scanner data (April 1980) within 63° of the equator, the WFOV deconvolved solution reduces the RMS error of the WFOV archived results by 31% for longwave flux and 10% for shortwave flux. When compared to the ERBE data of April 1985 over the same domain, error reductions of 25% and 5% are obtained, respectively, for the longwave and shortwave fluxes.

Full access
Larry Stowe, Philip Ardanuy, Richard Hucek, Peter Abel, and Herbert Jacobowitz

Abstract

A set of system simulations has been performed to evaluate candidate scanner designs for an Earth Radiation Budget Instrument (ERBI) for the Earth Observing System (EOS) of the late 1990s. Five different instruments are considered: 1) the Active Cavity Array (ACA), 2) the Clouds and Earth's Radiant Energy System-Instrument (CERES-1), 3) the Conically Scanning Radiometer (CSR), (4) the Earth Radiation Budget Experiment Cross-Track Scanner (ERBE), and 5) the Nimbus-7 Biaxial Scanner (N7). Errors in instantaneous, top-of-the-atmosphere (TOA) satellite flux estimates are assumed to arise from two measurement problems: the sampling of space over a given geographic domain, and sampling in angle about a given spatial location. In the limit where angular sampling errors vanish [due to the application of correct angular dependence models (ADMs) during inversion], the accuracy of each scanner design is determined by the instrument's ability to map the TOA radiance field in a uniform manner. In this regard, the instruments containing a cross-track scanning component (CERES-1 and ERBE) do best. As errors in ADMs are encountered, cross-track instruments incur angular sampling errors more rapidly than biaxial instruments (N7, ACA, and CSR) and eventually overtake the biaxial designs in their total error amounts. A latitude bias (north-south error gradient) in the ADM error of cross-track instruments also exists. This would be objectionable when ADM errors are systematic over large areas of the globe. For instantaneous errors, however, cross-track scanners outperform biaxial or conical scanners for 2.5° latitude × 2.5° longitude target areas. providing that the ADM error is less than or equal to 30%.

A key issue is the amount of systematic ADM error (departures from the mean models) that is present at the 2.5° resolution of the ERBE target areas. If this error is less than 30%, then the CERES-I, ERBE, and CSR, in order of increasing error, provide the most accurate instantaneous flux estimates, within 2–3 W m−2 of each other in reflected shortwave flux. The magnitude of this error is near the 10 W m−2 accuracy requirement of the user community. Longwave flux errors have been found to have the same space and time characteristics as errors in shortwave radiation, but only about 25% as large.

Full access
H. Lee Kyle, Richard Hucek, Philip Ardanuy, Lanning Penn, Brian Groveman, John Hickey, and Robert Maschoff

Abstract

This paper describes the production calibration adjustment algorithms used to remove thermal perturbation and stray light noise signals from the Nimbus-7 earth radiation budget (ERB) measurements. Sunlight, both direct and scattered from the sensor baffles, contaminated the ERB measurements at satellite sunrise and sunset. The problem covered subsatellite solar zenith angles from 90° to 120° and reduced the usefulness of the longwave spectral radiation measurements. Scattered light corrections are made from 90° to 99° while orbit-by-orbit interpolation is used frown 99° to 121°. Tests indicate that in the mean the midpoint interpolation error is less than 1 W m−2 with a standard deviation of about 5 W m−2. Thermal perturbations on the total channel 12 (0.2–50 μm) appeared to be always less than 0.3%. However, the Suprasil-W domes on the otherwise similar shortwave channels 13 and 14 in some way helped produce thermal perturbations of up to 6% or more in channel 13 (0.2–3.8 μm and up to 3% or more in channel 14 (0.7–2.8 μm). These perturbations arose from variations in external radiant heating during the day, night, sunrise, and sunset. In addition, the on/off cycles of the ERB and neighboring experiments produced day-to-day variations. The algorithms described here helped produce a stable 9-year-long measurement set. No thermal corrections were made in channel 12 and the obvious thermal perturbations in channels 13 and 14 were corrected. The absolute accuracy of the calibrated measurements is difficult to determine. The remaining uncertainty depends on the perturbing functions that were greater at high latitudes, near satellite sunrise and sunset, than in the Tropics. In June and July, the corrections for the daytime thermal perturbations near the North Pole may be too large by 3–5 W m−2. In general, the Nimbus-7 ERB products show good agreement with the follow-on Earth Radiation Budget Experiment (ERBE) products.

Full access
H. Lee Kyle, Richard Hucek, Philip Ardanuy, Lanning Penn, John Hickey, and Brian Groveman

Abstract

Much of the early record of spectrally broadband earth radiation budget (ERB) measurements was taken by the ERB instrument launched on the Nimbus-7 spacecraft in October 1978. The wide-field-of-view (WFOV) sensors measured the emitted and reflected radiation from November 1978 through January 1993, and the first nine years have been processed into a stable, long-term dataset. However, heating and cooling of the ERB experiment introduced thermal perturbations in the original measurements that were only significant in the shortwave (SW) channels. These sensors were covered by spherical filter domes to absorb incident longwave (LW) radiation. In this paper, a thermal regression model—the thermal calibration adjustment table (CAT)—is developed to track and remove these thermal signals from the SW data. The model relies on instrument temperatures within and near the surface of the ERB instrument, and the observed nonzero nighttime sensor readings represent the thermal signals. Confidence that the model is stable for daytime applications was gained by smoothing the solution using ridge regression and noting the effect on the solution coefficient vector. The bias signal produced by the thermal CAT portrays the balance of instrument heating and cooling within the Nimbus-7 variable external radiation environment. Cooling occurs over about two-thirds of an orbit including satellite night. During the nighttime, the sensor bias change is about 17 W m−2 (compare with mean daytime SW flux of about 200 W m−2) with little seasonal or annual fluctuation. Strong warming takes place during morning and evening twilight when direct solar radiation illuminates the WFOV sensors. This warming effectively compensates for nighttime cooling when the opposite thermal signature is found. Additional daytime warming occurs for satellite positions near the solar declination when the effects of combined LW and SW terrestrial fluxes exceed thermal cooling to space. However, this heating is influenced by the terrestrial scene and so it varies seasonally.

The thermal CAT was one of two semi-independent procedures, each of equal mean accuracy, developed to validate and correct for thermally induced sensor signals. The other, called the global CAT, is described in the second paper in this series. Although the thermal CAT was considered heuristically superior, the global CAT was chosen for the basic calibration work since it was thought to be potentially more stable for the production of a consistent long-term ERB dataset.

Full access
H. Lee Kyle, Richard Hucek, Philip Ardanuy, Lanning Penn, and Brian Groveman

Abstract

Sensitivity changes in the four wide-field-of-view (WFOV) Nimbus-7 earth radiation budget (ERB) sensors were monitored over a 9-yr period (November 1978–October 1987) by use of a number of reference sources. The sun was the primary reference and was used to check the shortwave (SW; about 0.2–4 μm) sensitivities on the twin total channels 11 and 12. The longwave (LW; greater than 4 μm) sensitivity in channel 12 was checked by a time series analysis of the nighttime mean global terrestrial signal, but the method could not be usefully applied to channel 11 because it was shuttered too much of the time. The accuracy of this type of analysis was verified by comparing a similar shortwave time series analysis with the solar calibration results. It was also checked by comparing channel 12 nighttime measurements with those from the companion scanner. The scanner had a built-in blackbody for calibration, but the scanner failed after 20 months. As a result of this comparison, a bias adjustment of 12.6 W m−2 was made in the channel 12 measurements. In addition, channels 11 and 12 were compared to each other. The shortwave channels 13 (0.2–4 μm) and 14 (0.7–2.8 μm) were covered by Suprasil-W domes that blocked radiation greater than 4 μm. A piece of red glass in channel 14 further restricted its spectral range to the near infrared. After launch, these domes fogged asymmetrically. For this reason, the effective sensitivity changes in these channels were monitored by comparison with channel 12 using the whole earth as a transfer target. The shortwave range mentioned above for channels 11 and 12 really refers to channel 13 and not to channels 11 and 12. By October 1987, the following sensitivity decreases had occurred: channel 11 (no observable change), channel 12 (LW 2.5%, SW 1.5%), channel 13 (13.3%), and channel 14 (6%). Corrections for these changes kept the calibrated signals stable to better than 0.5% over the 9-yr period. Year-to-year annual global mean longwave shifts of 0.1%–0.4% have been related to climate perturbations and appear real.

Full access