News & Events > Field Notes

Calibration Season Begins

 
Published: 5 October 2015

Editor’s Note: Craig Webb, calibration technician at the Southern Great Plains site, sent this update.

A view of a collection of radiometers taken from the now closed Tropical Western Pacific facility.
A view of a collection of radiometers taken from the now closed Tropical Western Pacific facility.

The ARM Climate Research Facility requires accurate measurements of solar radiation from radiometers used in ground-based networks and airborne instrument platforms. These measurements are needed to evaluate and improve the mathematical description of radiative transfer processes simulated in global climate models used to predict climate change. The Radiometric Calibration Facility (RCF), located at the Southern Great Plains (SGP) ARM site operated by Argonne National Laboratory, now provides shortwave radiometer calibrations traceable to the World Radiometric Reference. The RCF is a collaboration between the National Renewable Energy Laboratory (NREL) and the ARM Facility.

Team members, Craig Webb and Matt Gibson, work to begin radiometer calibrations at the Southern Great Plains site.
Team members, Craig Webb and Matt Gibson, work to begin radiometer calibrations at the Southern Great Plains site.

Beginning in July, the team began the precision infrared radiometer (PIR), also known as a pyrogeometer, calibrations. The RCF is designed to simultaneously calibrate up to 11 PIR radiometers in outdoor conditions similar to those the field instruments would experience in routine monitoring.Instruments calibrated at the site include:

  1. pyrheliometers, also known as NIPs, which are used to measure direct (beam) solar irradiance;
  2. pyranometers, also known as PSPs, which have a hemispheric (fish-eye) field of view and are designed to measure the total direct and diffuse (sky) solar irradiance on a flat surface; and
  3. pyrgeometers, also known as PIRs, which are similar to pyranometers, but have an outer blocking filter to eliminate the high energy solar irradiance and measure only the longwave, thermal (or infrared) radiation from the atmosphere.
Data quality of the measurements from radiometers requires accurate and regular recalibration traceable to the World Radiometric Reference, the international standard of solar radiation measurement.
Data quality of the measurements from radiometers requires accurate and regular recalibration traceable to the World Radiometric Reference, the international standard of solar radiation measurement.

The process to calibrate the PIRs takes about 3 to 6 weeks to complete, and  can only occur at night between dark and dawn. The nighttime weather provides a good mix of cloudy nights and unpredictable conditions. Downtime is less than ever before due to this new capability being added to the SGP site.

The team is working on a total of 15 instruments, one of which will be a measurement assurance standard, one control standard, and two reference standards.  After the calibration period is complete, only 11 precision infrared radiometers will be ready for the field—if they all pass testing data. Instrument calibrations at SGP are now planned on an annual basis.