Downward Longwave Irradiance Uncertainty Under Arctic Atmospheres: Measurements and Modeling

Marty, C., Swiss Federal Institute of Snow and Avalanche Research

Radiation Processes

Radiative Processes

Marty, C., R. Philipona, J. Delamere, E.G. Dutton, J. Michalsky, K. Stamnes, R. Storvold, T. Stoffel, S.A. Clough, and E.J. Mlawr, Downward longwave irradiance uncertainty under arctic atmospheres: Measurements and modeling, J. Geophys. Res., 108(D12), 4358, doi:10.1029/2002JD002937, 2003.

IPASRC-II instruments deployed at ARM's Barrow Station.

IPASRC-II instruments deployed at ARM's Barrow Station.

Members of 11 international institutions converged at the Atmospheric Radiation Measurement (ARM) Program's North Slope of Alaska (NSA) site in Barrow, Alaska, to conduct the Second International Pyrgeometer and Absolute Sky-scanning Radiometer Comparison (IPASRC-II) in March 2001. Organized by OBER's ARM Program, this event provided a unique opportunity to compare high-accuracy downward longwave irradiance measurements and radiative transfer model computations during arctic winter. The extreme cold results in low water vapor concentrations and the associated low values of downward longwave irradiance stresses the measurement equipment. These conditions present special challenges for measurement and modeling of downward longwave irradiance, including questions about the representativeness of the instrument calibration, and consistency and uncertainty of measurements and models in these environments. Accurate measurements of longwave irradiance are important to understanding the total energy balance of the earth-ocean-atmosphere system.

In all, 14 pyrgeometers—which measure radiation from the earth's atmosphere down to the surface—were field-calibrated against the Absolute Sky-scanning Radiometer (ASR), the reference instrument for this experiment. Continuous measurements over a 10-day period 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, and state-of-the-art radiative transfer computations indicates the absolute uncertainty of measured downward longwave irradiance under arctic winter conditions is within ±2 W m-2. Another key finding was the demonstrated importance of field calibrations (versus laboratory blackbody calibration) for the relevant environmental conditions. Final experimental results represent the combined efforts of National Oceanic and Atmospheric Administration, State University of New York at Albany, Physickalish-Meteorologisches Observatorium Davos and World Radiation Center (Switzerland), and the National Renewable Energy Laboratory.