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  • Author or Editor: Ellsworth G. Dutton x
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Ellsworth G. Dutton

Abstract

Differences between observed and LOWTRAN7-computed downward longwave irradiances were examined at each of four globally diverse locations for an entire year at each site. The final results are restricted to times determined to be completely or nearly cloud-free. The irradiances from 367 such times range from 60 to 435 W m−2, and results indicate that the modeled irradiances and those measured directly using a pyrgeometer agree to within 5 W m−2 at individual sites and to within lm than 0.2 W m−2 when averaged over all four sites, neglecting any site-specific biases. The standard deviations and standard errors associated with these results are roughly 10 and 1 W m−2, respectively. An unbiased estimate of the agreement between the model and observations results in a mean difference of 0.62 W m−2 with standard deviation of 5 W m−2 but an even larger 95% confidence interval because of the small sample size. The comparison variance can be logically ascribed to a number of different sources, including atmospheric variability and inhomogeneity, as well as to short-term instrument and LOWTRAN7 input variations. LOWTRAN7 and the observations agree better, in the mean, than the commonly accepted uncertainties for either would suggest. Maximum cloud radiative forcing at the surface for each site is quantified as a by-product of the comparison process.

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Barry A. Bodhaine
,
Norman B. Wood
,
Ellsworth G. Dutton
, and
James R. Slusser

Abstract

Many different techniques are used for the calculation of Rayleigh optical depth in the atmosphere. In some cases differences among these techniques can be important, especially in the UV region of the spectrum and under clean atmospheric conditions. The authors recommend that the calculation of Rayleigh optical depth be approached by going back to the first principles of Rayleigh scattering theory rather than the variety of curve-fitting techniques currently in use. A survey of the literature was conducted in order to determine the latest values of the physical constants necessary and to review the methods available for the calculation of Rayleigh optical depth. The recommended approach requires the accurate calculation of the refractive index of air based on the latest published measurements. Calculations estimating Rayleigh optical depth should be done as accurately as possible because the inaccuracies that arise can equal or even exceed other quantities being estimated, such as aerosol optical depth, particularly in the UV region of the spectrum. All of the calculations are simple enough to be done easily in a spreadsheet.

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Barry A. Bodhaine
,
Ellsworth G. Dutton
,
Richard L. McKenzie
, and
Paul V. Johnston

Abstract

A UV spectroradiometer was installed at Mauna Loa Observatory (MLO), Hawaii, in July 1995. This instrument has been employed to characterize several broadband UV instruments of a type commonly used to estimate erythemal irradiance at many sites around the globe. One year of clear-sky data from MLO has been analyzed for solar zenith angles (SZAs) of 5°–85°, in steps of 5°, and for total ozone values in the range 220–310 DU measured with a Dobson spectrophotometer. Because the spectral responses of various broadband instruments can be quite different, and particularly because the erythemal response defined for human skin is significantly different than that of many broadband instruments, the calibration of a broadband instrument reporting in erythemal units is strongly dependent on total ozone and SZA. When a broadband instrument is placed in the field it is necessary to know the calibration as a function of ozone and SZA to determine accurate erythemal irradiance. However, the manufacturers of broadband instruments do not generally provide information on the ozone dependence of the calibration. A procedure is described here for determining the calibration of a broadband UV instrument by comparison with a calibrated spectroradiometer. This procedure does not require precise knowledge of the spectral response of the broadband instrument. This analysis shows that if, for example, total ozone concentration decreased from 300 to 200 DU, the calibration constant of a broadband instrument should be increased by almost 20%. Therefore, the broadband instrument would significantly underestimate the increase of erythema.

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Ellsworth G. Dutton
,
Joseph J. Michalsky
,
Thomas Stoffel
,
Bruce W. Forgan
,
John Hickey
,
Donald W. Nelson
,
Timothy L. Alberta
, and
Ibrahim Reda

Abstract

Diffuse-sky solar irradiance is an important quantity for radiation budget research, particularly as it relates to climate. Diffuse irradiance is one component of the total downwelling solar irradiance and contains information on the amount of downward-scattered, as opposed to directly transmitted, solar radiation. Additionally, the diffuse component is often required when calibrating total irradiance radiometers. A variety of pyranometers are commonly used to measure solar diffuse irradiance. An examination of some instruments for measuring diffuse irradiance using solar tracking shade disks is presented, along with an evaluation of the achieved accuracy. A data correction procedure that is intended to account for the offset caused by thermal IR exchange between the detector and filter domes in certain common diffuse pyranometers is developed and validated. The correction factor is derived from outputs of a collocated pyrgeometer that measures atmospheric infrared irradiance.

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Joseph Michalsky
,
Ellsworth G. Dutton
,
Donald Nelson
,
James Wendell
,
Stephen Wilcox
,
Afshin Andreas
,
Peter Gotseff
,
Daryl Myers
,
Ibrahim Reda
,
Thomas Stoffel
,
Klaus Behrens
,
Thomas Carlund
,
Wolfgang Finsterle
, and
David Halliwell

Abstract

In the most comprehensive pyrheliometer comparison known to date, 33 instruments were deployed to measure direct normal solar radiation over a 10-month period in Golden, Colorado. The goal was to determine their performance relative to four electrical-substitution cavity radiometers that were calibrated against the World Radiometric Reference (WRR) that is maintained at the World Radiation Center in Davos, Switzerland. Because of intermittent cabling problems with one of the cavity radiometers, the average of three windowed, electrical-substitution cavity radiometers served as the reference irradiance for 29 test instruments during the 10-month study. To keep the size of this work manageable, comparisons are limited to stable sunny conditions, passing clouds, calm and windy conditions, and hot and cold temperatures. Other variables could have been analyzed, or the conditions analyzed could have employed higher resolution. A more complete study should be possible now that the instruments are identified; note that this analysis was performed without any knowledge on the part of the analyst of the instruments’ manufacturers or models. Apart from the windowed cavities that provided the best measurements, two categories of performance emerged during the comparison. All instruments exceeded expectations in that they measured with lower uncertainties than the manufacturers’ own specifications. Operational 95% uncertainties for the three classes of instruments, which include the uncertainties of the open cavities used for calibration, were about 0.5%, 0.8%, and 1.4%. The open cavities that were used for calibration of all pyrheliometers have an estimated 95% uncertainty of 0.4%–0.45%, which includes the conservative estimate of 0.3% uncertainty for the WRR.

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