Abstract
Abstract. Surface-based measurements of broadband shortwave (solar) and longwave (infrared) radiative fluxes using thermopile radiometers are made regularly around the globe for scientific and operational environmental monitoring. The occurrence of ice on sensor windows in cold environments – whether snow, rime, or frost – is a common problem that is difficult to prevent as well as difficult to correct in post-processing. The Baseline Surface Radiation Network (BSRN) community recognizes radiometer icing as a major outstanding measurement uncertainty. Towards constraining this uncertainty, the De-Icing Comparison Experiment (D-ICE) was carried out at the NOAA Atmospheric Baseline Observatory in Utqiaġvik (formerly Barrow), Alaska, from August 2017 to July 2018. The purpose of D-ICE was to evaluate existing ventilation and heating technologies developed to mitigate radiometer icing. D-ICE consisted of 20 pyranometers and 5 pyrgeometers operating in various ventilator housings alongside operational systems that are part of NOAA's Barrow BSRN station and the US Department of Energy Atmospheric Radiation Measurement (ARM) program North Slope of Alaska and Oliktok Point observatories. To detect icing, radiometers were monitored continuously using cameras, with a total of more than 1 million images of radiometer domes archived. Ventilator and ventilator–heater performance overall was skillful with the average of the systems mitigating ice formation 77 % (many >90 %) of the time during which icing conditions were present. Ventilators without heating elements were also effective and capable of providing heat through roughly equal contributions of waste energy from the ventilator fan and adiabatic heating downstream of the fan. This provided ∼0.6 ∘C of warming, enough to subsaturate the air up to a relative humidity (with respect to ice) of ∼105 %. Because the mitigation technologies performed well, a near complete record of verified ice-free radiometric fluxes was assembled for the duration of the campaign. This well-characterized data set is suitable for model evaluation, in particular for the Year of Polar Prediction (YOPP) first Special Observing Period (SOP1). We used the data set to calculate short- and long-term biases in iced sensors, finding that biases can be up to +60 W m−2 (longwave) and −211 to +188 W m−2 (shortwave). However, because of the frequency of icing, mitigation of ice by ventilators, cloud conditions, and the timing of icing relative to available sunlight, the biases in the monthly means were generally less than the aggregate uncertainty attributed to other conventional sources in both the shortwave and longwave.
Highlights
Radiative fluxes are fundamental environmental observations made regularly from the earth’s surface using thermopile radiometers
The images captured approximately 780 000 views of the De-Icing Comparison Experiment (D-ICE) radiometer domes with an additional 143 000 and 125 000 views captured by Atmospheric Radiation Measurement (ARM) at North Slope of Alaska (NSA) and Oliktok Point (OLI), respectively
While the Baseline Surface Radiation Network (BSRN) instruments that were mounted on the solar tracker were not imaged by the cameras, the tracker instruments provide important information for two reasons: first, the pyrgeometers were shaded, which reduces solar heating of the domes (Alados-Arboledas et al, 1988) and the magnitude of associated corrections that apply to some pyrgeometers (Albrecht and Cox, 1977), and second, because shortwave downwelling (SWD) is more accurately represented by the sum of the DIF and DIR due to increased calibration uncertainty in pyranometers from the direct beam at low sun angles (Michalsky et al, 1995)
Summary
Radiative fluxes are fundamental environmental observations made regularly from the earth’s surface using thermopile radiometers. A consensus in BSRN emerged that heating and ventilation are capable of mitigating ice, but the effectiveness and uncertainties remained poorly quantified, and the range of experiences reported by BSRN users indicated that more work was needed to constrain the attributes of effective designs (BSRN, 2016) To address these objectives, the NOAA Physical Sciences Laboratory (PSL) in partnership with the BSRN-CCIWG and NOAA Global Monitoring Laboratory (GML) carried out the De-Icing Comparison Experiment (D-ICE) to collect data suitable for assessing the influence of icing on the measurements and evaluating the status of ice-mitigation technology. We use these data sets to analyze instantaneous and time-averaged biases caused by ice, to calculate ice-mitigation performance statistics for the participating systems, to discern some of the reasons for successful ice mitigation, and to gather insight for interpretation of ice-contaminated data
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