2019 ARM/ASR PI Meeting

Bethesda, MD

June 10-13, 2019

NREL/PO-1900-73811

Establishing the World Reference for Measuring the Atmospheric Longwave

Irradiance with Traceability to the International System of Units

Ibrahim Reda and Manajit Sengupta | National Renewable Energy Laboratory

This work was authored by the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding provided by the U.S. Department of Energy 

Office of Energy Efficiency and Renewable Energy Solar Energy Technologies Office. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government retains and the publisher, by accepting 

the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.

ABSTRACT

OVERVIEW

• Advancing climate change research requires accurate and traceable measurement of atmospheric longwave irradiance. 

Measurement capabilities are limited to an estimated uncertainty of larger than ±6 W/m2 using the interim World Infrared 

Standard Group (WISG).

• Two independently designed and calibrated absolute radiometers measuring downwelling longwave irradiance were 

compared during five outdoor comparisons in 2013, 2015, and 2017 at the Physikalisch Meteorologisches Observatorium 

Davos‒World Radiation Center (PMOD/WRC) and the U.S. Department of Energy (DOE) Atmospheric Radiation 

Measurement program (ARM) in the Southern Great Planes (SGP). Two Absolute Cavity Pyrgeometers (ACPs) developed by 

the National Renewable Energy Laboratory (NREL) and four Integrating Sphere Infrared Radiometers (IRISs) developed by 

PMOD/WRC took part in these intercomparisons.

• Results presented on five comparisons between ACPs and IRISs.

• Difference between longwave irradiance measured by ACPs and IRISs versus irradiance measured by WISG.

ACKNOWLEDGMENTS

We sincerely appreciate the support of the U.S. DOE Atmospheric System Research Program and the Atmospheric Radiation 

Measurement User Facility, staff at NREL’s Metrology Laboratory within the Solar Radiation Research Laboratory (SRRL) for help 

in setting up instruments and maintaining the quality of SRRL data, and Martina Newman for her continuous support. We thank 

the U.S. DOE Office of Energy Efficiency and Renewable Energy Solar Energy Technologies Program, and NREL’s Environment, 

Safety, Health, and Quality Office for providing the funds.

CONCLUSION

• Difference between the irradiance measured by the ACPs, IRISs, and AERI varied from 0.2 W/m2 to 2.5 W/m2 based on the 

atmospheric conditions, which is within the stated uncertainties of ±3 W/m2.

• Irradiance measured by the WISG is lower than the average irradiance measured by ACPs and IRISs. The magnitude of the 

difference varied from 4.4 W/m2 to 6.6 W/m2 depending on the integrated water vapor.

• Longwave irradiance measured by ACPs, IRISs, and AERI versus irradiance measured by WISG.

• Recommend establishing world reference for measuring atmospheric longwave irradiance with traceability to SI.

W/m2 First Second Third Fourth Fifth

Average Difference between ACPs and IRISs 0.10 0.31 -1.17 -1.58 -1.77

StDev of Difference 0.08 0.65 0.70 1.15 0.88

Difference within 95% 0.19 1.34 1.82 2.79 2.50

Average of ACPs and IRISs‒WISG 3.93 6.14 3.82 3.50 6.50

StDev of Difference 0.97 0.76 0.67 0.81 0.66

Difference within 95% 4.38 6.33 4.05 3.86 6.63

Summary of the Five Comparisons Meeting of WMO CIMO Task Team on Radiation References

• Focused on traceability of terrestrial radiation measurements. Reviewed and evaluated recent developments of reference instruments for 

terrestrial radiation, and developed recommendations to the attention of CIMO on the appropriateness, requirements, and timeliness for a 

possible future modification of the current reference.

• Relevant recommendations from the TT was approved by CIMO MG on April 18, 2019; Link to the report: 

http://www.wmo.int/pages/prog/www/IMOP/reports.html

• From the five comparisons, the difference between the irradiance measured by ACPs and IRISs varied from 0.2 W/m2 to 2.5 W/m2 based on 

atmospheric conditions, which is within the combined stated uncertainties of ±3 W/m2.

• The irradiance measured by the WISG is lower than the average irradiance measured by ACPs and IRISs. The magnitude of difference varied 

from 0.2 W/m2 to 6.6 W/m2 depending on the integrated water vapor.

• A concerted effort to establish a world reference for measuring the atmospheric longwave irradiance with lower uncertainty and with traceability 

to the International System of Units (SI) by using the ACPs and IRISs as the reference started at the World Meteorological Organization 

(WMO). The Commission for Instruments and Methods of Observation Commission (CIMO) Task Team on Radiation References (TT) meeting 

was held in 2017 at the National Physical Laboratory (NPL), in Teddington, United Kingdom to discuss specific recommendation on the 

traceability of the atmospheric longwave irradiance to the International System of units (SI).

Average Irradiance of Two IRISs and ACP95F3 or 96F3 Versus the WISG 

Average Irradiance at Night on Feb. 5, 2013 (~8 mm H2O vapor column)

Results of First Comparison between ACPs and IRISs–Davos Results of Second Comparison between ACPs and IRISs–Davos Results of Fifth Comparison between ACPs and IRISs-SGP–Phase 2 Results of Third Comparison between ACPs and IRISs–Davos Results of Fourth Comparison between ACPs and IRISs-SGP–Phase 1

Average Irradiance of Two IRISs and ACP96F3 versus the WISG 

Average Irradiance on Oct. 2 and 3, 2013 (~15 mm H2O vapor column)

Average Irradiance of Four IRISs and ACP95F3 or ACP96F3 Versus WISG 

Average from Sept. 28, 2015 to Oct. 16, 2015 (~10 mm H2O vapor column)

Average of Three IRISs and Two ACPs Versus PIR-31197F3 with 

Traceability to WISG on Oct. 16, 17, 18, 24, 25, 26, 2017

Average of Three IRISs and Two ACPs Versus PIR-30475F3 with 

Traceability to WISG on Nov. 28, 2017

October 18, 2017

Results of Comparison between ACPs, IRISs, and AERI at SGP

October 24, 2017 October 26, 2017 November 28, 2017 December 6, 2017