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Results of first outdoor comparison between Absolute Cavity Pyrgeometer (ACP) and Infrared Integrating Sphere (IRIS) Radiometer at PMOD
Atmospheric System Research
Science Team Meeting (March 18‐21, 2013)by
Ibrahim Reda*, Julian Gröbner**, Stefan Wacker**, and Tom Stoffel*
* = NREL , ** = PMOD/WRC
NREL/PR‐3B10‐58121
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Outline
The ACP and IRIS are developed to establish a world reference for calibrating pyrgeometers with traceability to SI units. The two radiometers are unwindowed with negligible spectral dependence, and traceable to SI units through the temperature scale (ITS‐90). The first outdoor comparison between the two designs was held from January 28 to February 8, 2013 at the Physikalisch‐Metorologisches Observatorium Davos (PMOD). The difference between the irradiance measured by ACP and that of IRIS was within 1 W/m2. A difference of 5 W/m2 was observed between the irradiance measured by ACP&IRIS and that of the interim World Infrared Standard Group (WISG).
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Absolute Cavity Pyrgeometer (ACP)‐ ACP Net irradiance:
‐ By cooling the ACP casetemperature, and since Watm
is stable, then,
‐ Then the atmospheric longwave irradiance is,
Where,K1, Vtp, ϵ , K2, Wr , Wc and τ are the reciprocal of ACP’s responsivity, thermopile voltage, gold emittance, detector’semittance, receiver irradiance, CPC irradiance, and throughput (NIST characterization), consecutively.
K V W W K Wtp atm c r1 21 2* * ( ) * ( ) * *
WK V K W W
atmtp r c
1 22 1* ( ) * * ( ) *
KW K W
Vc r
tp1
21 2
( ) * ( ) * *
Reference: Reda, I.; Zeng, J.; Schulch, J.; Hanssen, L.; Wilthen, B.; Myers, D.; Stoffel, T. Dec. 2011. “An absolute cavity pyrgeometer to measure the absolute outdoor longwave irradiance with traceability to International System of Units, SI”. Journal of Atmospheric and Solar-Terrestrial Physics, 77 (2012) 132-143. http://dx.doi.org/10.1016/j.jastp.2011.12.011
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The Infrared Integrating Sphere (IRIS) Radiometer
Key features of the IRIS Radiometer
• Windowless
• Irradiance measurement by using a 60 mm gold‐plated integrating sphere as input optic
• High sensitivity from a windowless pyroelectric detector
• Flat spectral response
• Measurement frequency 0.1 Hz
• Automatic unattended operation
• Nighttime measurements only
Reference surface
DetectorAperture
Shutter
IRIS UncertaintyU95%= 1.8 Wm‐2 summer (+15°C)
2.4 Wm‐2 winter (‐15°C)
Reference: Gröbner, J., A Transfer Standard Radiometer for atmospheric longwave irradiance measurements, Metrologia, 49, S105-S111,2012.
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ACP versus PMOD‐BB on Jan 29 to Feb 2, 2013 ACP Net irradinace vs Vtp during cooling
y = 0.0842x ‐ 255.2R2 = 0.9978
‐285
‐280
‐275
‐270
‐265‐350 ‐300 ‐250 ‐200 ‐150 ‐100 ‐50 0
Vtp (uV)
W/m
2
WACP‐WBB
0
1
2
3
1.00 11.00UTC
W/m
2
Residuals of fitting Net irradiance vs Vtp
‐0.4
‐0.2
0
0.2
0.4
1.00 11.00
UTC
W/m
2
ACP Net irradinace vs Vtp during cooling
y = 0.0834x ‐ 254.62R2 = 1 ‐400
‐350
‐300
‐250‐1400 ‐1200 ‐1000 ‐800 ‐600 ‐400 ‐200 0
Vtp (uV)
W/m
2
WACP‐WBB
0
1
2
3
13.16 13.24 13.33 13.41 13.49UTC
W/m
2
Residuals of fitting Net irradiance vs Vtp
‐0.3
‐0.1
0.1
0.3
13.16 13.24 13.33 13.41 13.49
UTC
W/m
2
ACP Net irradinace vs Vtp during cooling
y = 0.0827x ‐ 254.7R2 = 0.9992
‐360
‐340
‐320
‐300
‐280‐1400 ‐1200 ‐1000 ‐800 ‐600 ‐400 ‐200 0
Vtp (uV)
W/m
2
WACP‐WBB
0
1
2
3
11.62 11.71 11.79 11.87UTC
W/m
2
Residuals of fitting Net irradiance vs Vtp
‐1.5
‐0.5
0.5
1.5
11.62 11.71 11.79 11.87
UTC
W/m
2
ACP Net irradinace vs Vtp during cooling
y = 0.0822x ‐ 273.94R2 = 0.9996
‐380
‐360
‐340
‐320‐1200 ‐1000 ‐800 ‐600 ‐400 ‐200 0
Vtp (uV)
W/m
2
WACP‐WBB
0
0.5
1
1.5
17.66 17.74 17.82UTC
W/m
2Residuals of fitting Net irradiance vs Vtp
‐1.5
‐0.5
0.5
1.5
17.66 17.74 17.82
UTC
W/m
2
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Transient vs steady state in BB, Jan 29‐Feb 2, 2013
WACP‐WBB
0
0.5
1
1.5
2
2.5
3
3.5
12.85 12.94 13.02 13.10 13.19 13.27 13.35 13.44
UTC
W/m
2
Transient during coolingSteady State before cooling
WACP‐WBB
0
0.5
1
1.5
2
2.5
3
3.5
23.38 23.47 23.55 23.63 23.72 23.80
UTC
W/m
2
Transient during coolingSteady State before cooling
WACP‐WBB
0
0.5
1
1.5
2
2.5
3
3.5
11.32 11.41 11.49 11.57 11.66 11.74 11.82 11.91
UTC
W/m
2
Transient during coolingSteady State before cooling
WACP‐WBB
00.20.40.60.81
1.21.41.61.82
17.35 17.43 17.51 17.60 17.68 17.76 17.85
UTC
W/m
2
Transient during coolingSteady State before cooling
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Outdoor ACP&IRIS at night Feb. 5, 2013
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Outdoor ACP&IRIS at night Feb. 5, 2013
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Outdoor ACP, IRIS & WISG at night Feb. 5, 2013
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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‐1IWV
WISG 1 WISG 2 WISG 3
WISG 4
Night averages
IRIS u95
From Julian’s presentation, IRS2012-Germany(Data from 180 nights)
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Irradiance difference (ACP minus WISG) at NREL
Three cooling cycles on November 18 and 21, 2012 with 40% RH at SRRLConsistent with Julian’s observation with high water vapor*
* Algebra is reversed for consistency with NREL’s historical files
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Preliminary Conclusions• Special set‐up of ACP in BB due to unknown gradient in CPC• Outdoor agreement between ACP & IRIS to within 1 W/m2
• Irradiance measured by WISG is ~4 W/m2 lower than that measured by ACP&IRIS. Is Consistent with a Water Vapor Column of 8 mm. This was also observed at NREL/SRRL at RH = 40% (on November 18, 2012 at NREL/SRRL: Water Vapor Column from 7 mm to 9 mm during cooling cycles)
• Future comparison with higher/lower water vapor to resolve observed spectral effect on outdoor pyrgeometer calibrations
• A 3rd design might increase confidence in establishing a consensus reference with traceability to SI units.