The Swiss government offered the WMO to operate the WRC as a contribution to the World Weather Watch Program (WWW )
A further service was added in 1996 with the operation of the World Optical depth Research and Calibration Center (WORCC), a Swiss contribution to the WMO Global Atmosphere Watch program with the following terms of reference:
- Develop accurate radiometric references for spectral solar radiometry used to determine optical depth (as in the case of the World Radiometric Reference for total solar irradiance radiometry);
- Develop procedures to ensure world-wide homogeneity of optical depth measurements by e.g. providing transfer standards for precision filter radiometry (formerly called sunphotometry);
- Develop and test new instrumentation and methods for the determination of optical depth;
- Implement a trial phase of Precision Filter Radiometers (PFR) at GAW GLOBAL OBSERVATORIES to test methods for optical depth determination and calibration transfer;
- Develop relevant quality control procedures in cooperation with GAW Quality Assurance/Science Activity Centers;
- Training of staff to operate Precision Filter Radiometers.
The World Radiation Center (WRC) has the following terms of reference (Fröhlich et al., 1995):
- Guarantee world-wide homogeneity of meteorological radiation measurements by maintaining the World Standard Group (WSG) which is used to establish the World Radiometric Reference (WRR, Fröhlich, 1978).
- Calibration of meteorological radiation instruments.
- Research and development in radiometry and methods of observation of atmospheric radiation parameters.
- Training of radiation specialists.
Every five years, an International Pyrheliometer Comparison (IPC) is held at PMOD/WRC to transfer the WRR to the participating pyrheliometers in order to ensure world-wide homogeneity of solar radiation measurements. It is chiefly intended for the calibration of absolute radiometers from the Regional Radiation Centers of the six WMO regions. The next IPC(X) will be held in 2015.
The World Radiometric Reference is the measurement standard representing the SI unit of irradiance. It was introduced in order to ensure world-wide homogeneity of solar radiation measurements and is in use since 1980.
The WRR was determined from the weighted mean of the measurements of a group of 15 absolute cavity radiometers which have been fully characterized. It has an estimated accuracy of 0.3%.
The WMO introduced its mandatory use in its statutes in 1979.
Definition of the WRR using the results of 15 absolute cavity radiometers. The shaded area represents the ± 0.2% deviation from the WRR (Fröhlich, 1978).
The World Standard Group (WSG)
The WRR is determined by a group of absolute cavity radiometers named the World Standard Group (WSG). At the moment, the WSG is composed of 6 instruments: PMO-2, PMO-5, CROM-2L, PACRAD-3, TMI-67814 and HF-18748.
GAW-PFR AOD Network
* Classic extinction measurements at the recommended 4 WMO wavelengths 368, 415, 500 and 862 nm using Precision Filter Radiometers (PFRs).
* PFRs are manufactured by PMOD/WRC, and have optical filters from the same batch.
* Continuous sampling at a 1- minute frequency by automated systems.
* Data are transferred on a monthly basis to WORCC which acts as a central processing hub for data evaluation.
* Quality control flags cover solar pointing, housing temperature, and cloud screening.
* Continuous in-situ and/or (annual) centralized calibration based on high altitude references, and linear interpolation in time.
* Data products: AOD and the Angström coefficients alpha and beta (no inversions).
* Hourly mean AOD archived at the World Data Center for Aerosols (WDCA) at NILU. Data with a 1-minute resolution are available from WORCC upon request.
Calibration of Radiation Instruments at PMOD/WRC
Radiation instruments can be separated into three different categories: Shortwave Radiometers, Longwave Radiometers, and Spectral Radiometers. The calibration of radiometers is routinely performed at PMOD/WRC and is described in the sections (Index):
- Calibration of Shortwave Radiometers
- Calibration of Longwave Radiometers
- Calibration of Spectral Radiometers
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The “Physikalisch-Meteorologisches Observatorium Davos” (PMOD) was founded in 1907 by Carl Dorno as a privately operated institute. Dorno began operational measurements of the direct solar irradiance in 1909, initiating the world’s longest and still continuing time series of this kind.
In addition, he began investigating the biological influences of ultra-violet radiation – then known as Dorno Radiation, and today as UV-B.
With his world famous solar and UV radiation measurements he set the corner-stone of the PMOD which has continued his work as an internationally recognized center for research in radiation measurements and instrumentation.
Since 1926, PMOD has been part of the SFI private foundation (“Schweizerisches Forschungsinstitut für Hochgebirgsklima und Medizin”) .
CH-7260 Davos Dorf