Search Results

You are looking at 1 - 6 of 6 items for

  • Author or Editor: Herbert Jacobowitz 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
Istvan Laszlo, Arnold Gruber, and Herbert Jacobowitz

Abstract

Observations made with the current and proposed narrowband shortwave channels aboard the NOAA series of satellites were simulated for a number of different surfaces (ocean, vegetative land, desert, cloud and snow) using the ATRAD radiation model to study the relative merit of each channel and, in various combinations to predict the broadband albedo. Solar zenith angles were varied over the range from 0 to 60 degrees. The results indicated that for all of the surfaces considered there would be no significant difference in predicting the broadband albedo with either the current (0.58–0.68 μn) or proposed (0.58–0.68 μm) channel 1 of the AVHRR. The proposed narrower channel 2(0.84–0.87 μm), however, would be a better predictor than the current wider channel 2(0.725–1.0 μm). Channel 1 is better than channel 2 for surfaces of low or moderate reflectivity, while over snow, the error in using channel 2 would be less than half of that for channel 1. Combining channels 1 and 2 would reduce the error by about 50% for vegetation, ocean and snow. Adding the proposed channel 3A (1.58–1.64 μm) to channels 1 and 2 would further improve the prediction of the broadband albedo. Channel 20(0.65–0.73 μm) of the HIRS instrument was similarly studied to ascertain how well the broadband albedo would be predicted if the spectral filter was removed to widen the bandpass. Two different detectors (Si and InGaAs) with the current and a modified beamsplitter were considered. The results indicated that the modified beamsplitter was preferred. The use of this beamsplitter with the Si detector (0.46–1.04 μm) gave the best prediction for ocean, vegetative land, and desert scenes, while the InGaAs detector (0.64–1.74 μm) was best for cloud and snow scenes. Although the use of a widened channel 20 was shown to be less successful than the combination of channels 1, 2 and 3A of the AVHRR, flattening the response curve for the InGaAs detector using a compensating filter was comparable to using the AVHRR channels.

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
Gilbert R. Smith, Robert H. Levin, Peter Abel, and Herbert Jacobowitz

Abstract

A method for calibrating satellite radiometers is investigated. A calibrated spectral radiometer carried aboard a U2 aircraft at an altitude of 60 000 ft was aligned with White Sands. New Mexico along the same view vector as the Advanced Very High Resolution Radiometer (AVHRR) on the NOAA-9 spacecraft at the time of the spacecraft's overpass on 26 August 1985. Both sets of data have been transformed into best estimates of the radiance at satellite altitude inside the footprint of the aircraft radiometer, allowing an estimate of radiance calibration changes in the AVHRR to be made. It is assumed that both instrument systems are linear, that the spectral response function of AVHRR has not changed from its prelaunch value, and that the zero radiance responses of both instruments are accurately known. Extrapolation of the radiances measured from the aircraft to those expected at satellite altitude is achieved by modeling the experimental conditions at White Sands and calculating the ratio of radiances at the two altitudes through the LOWTRAN VI computer program.

Results from data taken within 2 minutes either side of the satellite overpass indicate a 98.9% correlation between the two sets of data, and a change in gain relative to the prelaunch calibration of +2 ± 5% for channel 1 and −2 ± 5% for channel 2 of the NOAA-9 AVHRR. Analysis of other coincident data for the NOAA-9 AVHRR and the aircraft spectral radiometer, including a large dataset from October and November 1986, is now in progress and will establish the day-to-day repeatability of results using this method.

Full access
Larry L. Stowe, Herbert Jacobowitz, George Ohring, Kenneth R. Knapp, and Nicholas R. Nalli

Abstract

As part of the joint National Oceanic and Atmospheric Administration–National Aeronautics and Space Administration (NOAA–NASA) Pathfinder program, the NOAA/National Environmental Satellite, Data and Information Service (NESDIS) has created a research-quality atmospheric, climate-scale dataset through the reprocessing of archived Advanced Very High Resolution Radiometer (AVHRR) observations from four afternoon satellites, in orbit since 1981. The raw observations were recalibrated using a vicarious calibration technique for the AVHRR reflectance channels and an improved treatment of the nonlinearity of the three infrared emittance channels. State-of-the-art algorithms are used in the Pathfinder Atmosphere (PATMOS) project to process global AVHRR datasets into statistics of channel radiances, total cloud amount, components of the earth's radiation budget, and aerosol optical thickness over oceans. The radiances and earth radiation budget components are determined for clear-sky and all-sky conditions. The output products are generated on a quasi-equal-area grid with a spatial resolution of approximately 110 km, with twice-a-day temporal resolution, and averaged over 5-day (pentad) and monthly time periods. The quality of the products is assessed relative to independent surface or satellite observations of these parameters. This analysis shows that the PATMOS data are sufficiently accurate for studies of the interaction of clouds and aerosol with solar and terrestrial radiation, and of climatic phenomena with large signals, for example, the annual cycle, monsoons, and the four ENSOs and two major volcanic eruptions that occurred during the 19-yr PATMOS period. Analysis also indicates that smaller climate signals, such as those associated with longer-term trends in surface temperature, may be difficult to detect due to the presence of artifacts in the time series that result from the drift of each satellite's observation time over its mission. However, a simple statistical method is employed to remove much of the effect caused by orbital drift. The uncorrected PATMOS dataset is accessible electronically.

Full access
Herbert Jacobowitz, Larry L. Stowe, George Ohring, Andrew Heidinger, Kenneth Knapp, and Nicholas R. Nalli

As part of the joint National Oceanic and Atmospheric Administration (NOAA) and National Aeronautics and Space Administration (NASA) Pathfinder program, the NOAA National Environmental Satellite, Data, and Information Service (NESDIS) has created a research-quality global atmospheric dataset through the reprocessing of Advanced Very High Resolution Radiometer (AVHRR) observations since 1981. The AVHRR is an imaging radiometer that flies on NOAA polar-orbiting operational environmental satellites (POES) measuring radiation reflected and emitted by the earth in five spectral channels. Raw AVHRR observations were recalibrated using a vicarious calibration technique for the reflectance channels and an appropriate treatment of the nonlinearity of the infrared channels. The observations are analyzed in the Pathfinder Atmosphere (PATMOS) project to obtain statistics of channel radiances, cloud amount, top of the atmosphere radiation budget, and aerosol optical thickness over ocean. The radiances and radiation budget components are determined for clear-sky and all-sky conditions. The output products are generated on a quasi-equalarea grid with an approximate 110 km × 110 km spatial resolution and twice-a-day temporal resolution, and averaged over 5-day (pentad) and monthly time periods. PATMOS data span the period from September 1981 through June 2001. Analyses show that the PATMOS data in their current archived form are sufficiently accurate for studies of the interaction of clouds and aerosol with solar and terrestrial radiation, and of climatic phenomena with large signals (e.g., the annual cycle, monsoons, ENSOs, or major volcanic eruptions). Global maps of the annual average of selected products are displayed to illustrate the capability of the dataset to depict the climatological fields and the spatial detail and relationships between the fields, further demonstrating how PATMOS is a unique resource for climate studies. Smaller climate signals, such as those associated with global warming, may be more difficult to detect due to the presence of artifacts in the time series of the products. Principally, these are caused by the drift of each satellite's observation time over its mission. A statistical method, which removes most of these artifacts, is briefly discussed. Quality of the products is assessed by comparing the adjusted monthly mean time series for each product with those derived from independent satellite observations. The PATMOS dataset for the monthly means is accessible at www.saa.noaa.gov/.

Full access