Recent Updates about the Absolute Cavity 

Pyrgeometer (ACP), InfraRed Integrating Sphere (IRIS), 

and Atmospheric Emitted Radiance Interferometer 

(AERI) Comparisons

Ibrahim Reda, Julian Gröbner, and Dave Turner

BSRN 2018 Workshop

July 19, 2018
NREL | 2

Overview

• Results of five comparisons between ACPs and IRISs.

• Difference between the longwave irradiance 

measured by the ACPs and IRISs versus the irradiance 

measured by the WISG.

• The longwave irradiance measured by the ACPs, IRISs, 

and AERI versus the irradiance measured by the 

WISG.
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Results of First Comparison between ACPs and IRISs-Davos

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0:00:00 0:28:48 0:57:36 1:26:24 1:55:12 2:24:00 2:52:48 3:21:36 3:50:24

Irradiance (W/m2)

Time

Average Irradiance of Two IRISs&ACP95F3 or 96F3 versus the 

WISG Average Irradiance at night on Feb. 5, 2013

(~8 mm H2O vapor column)

Av IRISs&ACP95F3 Av IRISs&ACP96F3 WISG
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Results of Second Comparison between ACPs and IRISs-Davos

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0:00:15 0:25:15 1:14:08 19:00:24 20:22:26 20:47:26 21:27:29 21:52:29 22:46:19 23:11:19 23:51:15

Irradiance (W/m2)

Time

Average Irradiance of Two IRISs&ACP96F3 versus the WISG Average 

Irradiance on Oct. 2&3, 2013

(~15 mm H2O vapor column)

Av IRISs&ACP96F3 WISG
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Results of Third Comparison between ACPs and IRISs-Davos

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18:41:31 20:47:31 22:07:31 22:57:31 23:47:31 0:37:31 1:27:31 2:17:31 3:07:31 19:13:31 21:32:31 22:22:31 23:12:31

Irradiance (W/m2)

Time

Average Irradiance of Four IRISs&ACP95F3 or ACP96F3 versus the WISG 

Average From September 28 to October 16, 2015

(~10 mm H2O vapor column)

AV IRISs&ACP96F3 AV IRISs&ACP95F3 WISG
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Results of Fourth Comparison between ACPs and IRISs-SGP-Phase 1

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20:51:42 23:21:32 2:19:53 22:13:49 0:50:32 3:28:56 18:38:31 23:11:46 1:50:41 4:26:47

Irradiance (W/m2)

CST

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

traceability to WISG on October 16, 17, 18, 24, 25, 26, 2017

AV IRISs&ACPs WISG
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Results of Fifth Comparison between ACPs and IRISs-SGP-Phase 2

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0:00:24 0:29:24 0:54:54 1:20:54 2:13:17 2:49:17 3:52:03 4:30:03 5:16:03 6:05:28

Irradiance (W/m2)

CST

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

traceability to WISG on November 28, 2017

Average ACPs&IRISs WISG
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Summary of the Five comparisons

W/m2 First Second Third Fourth Fifth

Average difference between 

ACPs&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&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
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Results of Comparison between ACPs, IRISs, and AERI at SGP
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Results of Comparison between ACPs, IRISs, and AERI at SGP
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Results of Comparison between ACPs, IRISs, and AERI at SGP
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Results of Comparison between ACPs, IRISs, and AERI at SGP
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Results of Comparison between ACPs, IRISs, and AERI at SGP
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Results of Comparison between ACPs, IRISs, and AERI at SGP
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Water Vapor effect on Irradiance difference (WISG minus 

IRIS) at PMOD

Results

Average Offset (IWV>10) -4.1 ± 1.5 Wm-2

Gradient (IWV<10) -0.45 ± 0.1 Wm-2mm-1

IWV

WISG 1 WISG 2 WISG 3

WISG 4

Night averages

IRIS u95

From J. Gröbner et al., JGR, 2014
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• The 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.

• The irradiance measured by the WISG is lower

than the average irradiance measured by ACPs

and IRISs, magnitude of the difference varied

from 4.4 W/m2 to 6.6 W/m2 depending on the

integrated water vapor.

Conclusion
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WMO CIMO Task Team on Radiation References

• The first session of the World Meteorological Organization (WMO) 

Commission for Instruments and Methods of Observation 

Commission (CIMO) Task Team on Radiation References (TT) was held 

in the National Physical Laboratory (NPL) from November 15 to 17, 

2017 in Teddington, United Kingdom.

• The meeting focused on the traceability of terrestrial radiation 

measurements. It 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 will be submitted to the 

CIMO MG (26-29 March 2018), and most likely to CIMO-17 session 

(12-16 October 2018, Amsterdam) for endorsement. The report of 

the TT meeting will encompass all the recommendations.
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Newton’s Apple Tree in National Physical Laboratory, UK, London-November 2017

Nigel, Reda, Isabelle, Bruce, Julian, Peter, Alison, Nozumo
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency 

and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

www.nrel.gov

NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency 

and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency

and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Thank You

Contact :

Ibrahim Reda 

Ibrahim.Reda@nrel.gov

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 U.S. Department of Energy 

Office of Energy Efficiency and Renewable Energy. 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.

NREL/PR-1900-71819