Introduction and motivation
The Infrared Radiometry Section of the World Radiation Centre (WRC-IRS) was established in January 2004 at the PMOD/WRC following
the recommendation of the Commission for instruments and methods of observation (CIMO) at its thirteenth session in Bratislava,
25 September to 3 October 2002 (Recommendation 1, CIMO-XIII). The WRC-IRS establishes an interim WMO Pyrgeometer Infrared Reference
using the procedures and instrumentation that make up the World Infrared Standard Group of Pyrgeometers (WISG). The WRC-IRS holds the
global infrared radiation reference and as such defines the longwave infrared scale to which all longwave infrared radiation measurements
should be traced. The role of the WRC-IRS is to disseminate this scale to the worldwide community either by individual instrument
calibrations at the PMOD/WRC, or preferably through the creation of regional calibration centers which are themselves traceable to the WRC-IRS.
The absolute sky-scanning radiometer consists essentially of a pyroelectric detector mounted on a tracking platform which observes the l
ongwave infrared radiance with a five degree field of view entrance optic (Philipona, 2001). The absolute calibration is obtained from measurements
of a temperature stabilised black-body before, during, and after the sky radiance measurements. One measurement cycle lasts for about 20 minutes and
consists of the measurement of the sky radiance at a total of 32 zenith angles in 4 planes. These 32 measurements are integrated using a gauss-quadrature
integration to produce one value for the downwelling longwave infrared irradiance. Since the pyroelectric detector is used without a sun-blocking filter,
measurements are only performed during nighttime. Furthermore, since the gauss-quadrature method requires a smooth function of sky radiance versus zenith
angle, measurements are only done under stable and clear nighttime conditions without clouds.
The WISG is currently composed of four pyrgeometers, two precision infrared radiometers (PIR) from Eppley, and two CG4 from Kipp&Zonen.
Three instruments were installed in September 2003 on the measurement platform on the roof of PMOD/WRC, while the fourth pyrgeometer was
installed in the summer of 2004. All instruments are mounted on a tracker (BRUSAG) which provides shading from the direct solar irradiance
using shading disks. Measurements are performed every second and two minute averages and their standard deviation are stored in a daily data file.
All pyrgeometers are mounted in ventilation units which also contain a heating ring to heat the air flowing around the dome. The longwave downward
irradiance is determined from the thermopile voltage and the dome and body thermistors using the Philipona et al. equation:
Figure 1. World infrared standard group of Pyrgeometers (WISG) at PMOD/WRC
- Black- Body
A black-body is used in the laboratory of WRC-IRS to characterise the pyrgeometers, i.e. to determine the calibration constants k1, k2, and k3
used in the equation above. The black-body is described in Philipona et al., 1995. Measurements are done at various equilibrium temperatures of the
black-body and of the pyrgeometer to simulate outdoor conditions (see Figure 2). To determine k3 on Eppley pyrgeometers, the dome is heated by a
heating ring installed around the rim of the dome so as to reach a temperature difference between dome and body of about 1 degree. The coefficients
k1, k2, and k3 can be usually retrieved with an uncertainty of 0.01, 0.0006, and 0.1 respectively.
Figure 2. Pyrgeometer characterisation in the black-body. The upper figure shows the temperature of the black-body (red), and the pyrgeometer body (green).
The lower figure displays the thermopile voltage in mV.
The absolute calibration of the WISG is traceable to the International Pyrgeometer and ASR comparison (IPASRC-I) campaign held at
the Atmospheric Radiation Measurement program's Southern Great Plains site in Oklahoma in September 1999 (Philipona et al., 2001).
Stability of the WISG
The stability of the WISG has been monitored since its inception in September 2003. Figure 3 shows a 30 day running average of nighttime longwave
irradiance measured by the WISG pyrgeometers relative to their mean. As can be seen in the figure, inter instrument variabilities are below ±1 W/m2,
which represents a relative variation of typically less than 0.5%.
Figure 3. World Infrared Standard Group pyrgeometers. Shown is the 30 day average deviation in nighttime downwelling longwave irradiance relative
to the average of the WISG. The gray background represents nightly averages of the WISG pyrgeometers.
Maintenance and quality control
The pyrgeometers on the measurement platform are inspected daily, and domes cleaned if necessary. Desiccant is checked every three weeks.
Measured data is visually inspected every day to check for instrumental problems.
Time keeping of the data acquisition system is referenced to an internet time server several times a day.
The measured data is stored on the computer controlling the data acquisition, and daily data files are archived every day on the PMOD/WRC server.
The server itself has a backup schedule to prevent data loss.
Philipona et al., Atmospheric longwave irradiance uncertainty: Pyrgeometers compared to an absolute
sky-scanning radiometer, atmospheric emitted radiance interferometer, and radiative transfer model calculations, J. Geophys. Res., 106, 28129-28141, 2001.
Philipona, R., Sky-scanning radiometer for absolute measurements of atmospheric long-wave radiation, Appl. Opt., 40, 2376-2383, 2001.
Philipona, R., . Fröhlich, and Ch. Betz, Characterisation of pyrgeometers and the accuracy of atmospheric long-wave radiation measurements,
Appl. Opt., 34, 1598-1605, 1995.
| For further information please contact: