Improved Instrument Calibration Capabilities Benefit All Sites, Users

 
Published: 30 September 2007

To ensure that ARM precipitation measurements are as accurate as possible, the SGP rain gauges (white cylinder above left) are inspected every two weeks, are statically calibrated in the field every six months, and will now be dynamically calibrated on an annual basis using the system shown above.

The ARM fills a unique position in the scientific community by obtaining long-term continuous measurements of cloud and radiation properties critical to climate modeling. Central to this mission is the ability to maintain and quickly repair instruments, as well as providing integrated instrument calibration processes to ensure ARM provides high quality data products. In the past 15 years of operations, the ARM Southern Great Plains (SGP) site has developed many onsite repair and calibration capabilities that have led to increased efficiencies and cost savings that benefit not just the SGP site and its users, but the other ARM sites as well. In the past year, enhancements to various components of the site’s Radiometer Calibration Facility further increase its ability to provide timely and accurate data to the scientific user community.

A dynamic rain gauge calibration procedure was implemented in 2007. A basic rain gauge calibration is done by dripping a known amount of water into the gauge and reading the amount of water measured. However, this procedure does not take into account errors that might occur during a heavy rainfall event, a common occurrence during spring and summer in Oklahoma. A dynamic rain gauge calibration compares known “rainfall rates” generated by computer-controlled equipment to rates measured by the gauge being calibrated. Differences can then be accounted for in the rain gauge data processing.

The equipment necessary to calibrate multifilter rotating shadowband radiometers and other similar sensors was also recently acquired at the SGP. Because each instrument has seven detectors that require calibration and characterization at all solar viewing angles (sunrise to sunset), a very precise calibration setup is required, as well as a significant amount of time to complete the procedure. Calibration procedures have been formalized, staff training has been completed, and some calibration activities have already begun. This enhancement adds a significant and important calibration capability to the ARM.