Performance of Longwave Radiation Instruments: Comparisons Between Measured and Modeled Irradiances During Arctic Winter

The longwave downward irradiance depends directly on the magnitude of the greenhouse effect. A recent comparison of longwave downward irradiances between global climate models and the best available surface measurements showed an especially large disagreement at observation sites with cold and dry climates. Accurate measurements and modeling of this irradiance is therefore required to detect and monitor possible changes in the greenhouse effect. The low water vapor content and the frigid meteorological conditions in the Arctic make measurements and modeling of the longwave downward irradiance a special challenge. The International Pyrgeometer and Absolute Sky-scanning Radiometer Comparison (IPARSC-II), which was conducted at DOE's Atmospheric Radiation Measurement (ARM) program site in Barrow, Alaska provided a unique opportunity to compare high accuracy longwave downward irradiance measurements and radiative transfer model computations during arctic winter. Continuous measurements over a 10 days period in early March 2001 with frequent clear-sky conditions yielded longwave downward irradiances between 120 and 160 W m^-2. Participants from 12 international institutions deployed 15 pyrgeometers, which were field-calibrated against the Absolute Sky-scanner. High accuracy radiosondes equipped with chilled mirror hygrometers and a Dobson spectrometer provided data that were used as input for the radiative transfer model computations.