Downward longwave irradiance uncertainty under arctic atmospheres: Measurements and modeling

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Abstract
Measurement and modeling of downward longwave irradiance are a special challenge in arctic winter due to its low water vapor content and the extreme meteorological conditions. There are questions about the representativeness of the instrument calibration, the consistency and unfcertainty of measurements and models in these environments. The Second International Pyrgeometer and Absolute Sky‐scanning Radiometer Comparison (IPASRC‐II), which was conducted at Atmospheric Radiation Measurement (ARM) program's North Slope of Alaska (NSA) site in Barrow provided a unique opportunity to compare high accuracy downward longwave irradiance measurements and radiative transfer model computations during arctic winter. Participants from 11 international institutions deployed 14 pyrgeometers, which were field‐calibrated against the Absolute Sky‐scanning Radiometer (ASR). Continuous measurements over a 10‐day period in early March 2001 with frequent clear‐sky conditions yielded downward longwave irradiances between 120 and 240 W m−2. The small average difference between ASR irradiances, pyrgeometer measurements, MODTRAN and LBLRTM radiative transfer computations indicates that the absolute uncertainty of measured downward longwave irradiance under arctic winter conditions is within ±2 W m−2.