PYIRLOSS

Pyranometer IR Loss Study

7 August 2006 - 31 July 2007

Lead Scientist: Scott Richardson

Observatory: NSA (North Slope of Alaska)

Unlike the ARM SGP and TWP sites, which have periods of darkness and daylight each day of the year, the NSA site experiences extended periods of both darkness and daylight. Additionally, due to the harsh conditions present there, the broadband radiometers are equipped with electric heaters inside the ventilators to keep hoarfrost and snow from undue interference with the measurements. Both of these issues appeared to have an effect on our ability to correct the broadband shortwave measurements for known IR loss problems. Anomalous results were discovered during testing of the ARM Diffuse Correction VAP when applied to NSA data. The correction methodology (Younkin and Long, 2004, ARM-TR-009) used night time offset data to establish a correction relationship that was applied during all hours. However, at NSA the majority of the "night time" data occurs during the winter months, while most of the daylight occurs during the summer months under far different ambient conditions. In addition, our IR loss correction methodology was developed without the use of heaters in the ventilators, which have been shown to have an effect on the instrument case and dome temperatures (Michalsky et al., 2005, JGR; Michalsky et al., 2003, JGR; Philipona, 2002, JGR). We had no comparative data with which to investigate and quantify these effects, both for correcting for IR loss, and on the ARM NSA surface broadband measurements themselves. This IOP provided a long term (1-year) set of measurements needed to investigate and refine the IR loss correction methodology for the ARM NSA data. In addition, an extensive effort was directed toward gaining an understanding of what effects the ventilator heaters were having on the basic measurements. We also investigated various permutations of the amount of ventilation and heating to perhaps optimize the ARM broadband measurement strategy for NSA. These latter studies naturally somewhat evolved as the early results were garnered. Overall, the IOP targeted improving both our understanding, and the quality and accuracy, of our ARM NSA measurements.