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David J. Yanuk
and
Robert O. Ellingson

Abstract

The procedure used by NOAA to estimate the outgoing longwave irradiance at the top of the atmosphere from an array of satellite window region measurements includes the averaging of the flux values inferred from the columns encompassing the satellite scan spots. The flux associated with a particular averaging region of interest is assumed to have contributions from only those points contained within the region. However, the area immediately surrounding a particular averaging region contributes to the flux estimate associated with that large averaging area. A new technique, based on the geometrical description of the flux associated with a single scan spot, is presented to estimate the uncertainty of the current averaging technique and to provide an alternative method for performing the averaging. Comparisons between the operational technique and the new technique show flux differences as large as 7% in the presence of high clouds when there are differences of 60% or more in cloud amounts between the averaging region and the area immediately surrounding it.

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Robert G. Ellingson
,
David J. Yanuk
, and
Arnold Gruber

Abstract

The technique used by NOAA to estimate the outgoing longwave flux from 10 μm window radiance observations has been reexamined because the data that result from the application of the empirically determined regression equation are systematically lower than those obtained from regression models based on earlier radiative transfer calculations. A new set of radiation calculations was made from a set of 1600 atmospheric soundings and the resulting regression equation gives flux differences from the empirical model that are of the order of ±5 W m−2 over the range of 150 to 300 W m−2, as compared to the +10 W m−2 systematic differences from previous studies. The differences are attributed to the size and representativeness of the sample of soundings used in the radiation calculations.

The results also show that although the explained variance of the regression is of the order of 95%, this type of estimation technique may make errors of ±20 W m−2 or larger for a given radiance observation. This will lead to biased average flux estimates in geographical regions where the temperature and moisture profiles change little over extended time periods.

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Robert G. Ellingson
,
Hai-Tien Lee
,
David Yanuk
, and
Arnold Gruber

Abstract

Simultaneous observations by the Earth Radiation Budget Experiment (ERBE) scanning radiometer and the High-Resolution Infrared Sounder (HIRS) on board the NOAA-9 spacecraft have been used to validate a multispectral technique for estimating the outgoing longwave radiation (OLR) from the earth-atmosphere system. Results farm approximately 100 000 collocated observations show that the HIRS technique provides instantaneous OLR estimates that agree with the ERBE observations just as well as different ERBE scanners agree with each other—about 5 W m−2 rms. Although there are differences between the HIRS and ERBE estimates that depend upon the scene type and time of day, the HIRS technique explained more than 99% of the variance of the ERBE observations for both day and night observations. The results suggest that the HIRS OLR technique is a suitable replacement for the Advanced Very High Resolution Radiometer technique now used by the National Oceanic and Atmospheric Administration for operational estimates of the OLR.

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Robert G. Ellingson
,
David J. Yanuk
,
Hai-Tien Lee
, and
Arnold Gruber

Abstract

A new technique for estimating outgoing longwave radiation from observations on the NOAA operational satellites has been developed from a regression analysis of radiation model calculations. The technique consists of a weighted sum of radiance in but four intervals sensed by the High-resolution InfraRed Sounder (HIRS). The analysis shows that model outgoing flux may be reproduced to within ±2 W · m−2 rms, which is about a factor of 4 smaller than the rms error of the method used by NOAA to estimate flux from the AVHRR. The small errors suggest that the new technique holds the promise of eliminating the large systematic errors possible with the current NOAA technique. Additionally, the new technique often the possibility of directly relating flux changes to changes in atmospheric parameters.

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