A Field Intercomparison Technique to Improve the Relative Accuracy of Longwave Radiation Measurements and an Evaluation of CASES-99 Pyrgeometer Data Quality

Techniques for improving the relative accuracy of longwave radiation measurements by a set of pyrgeometers [the Eppley Laboratory Precision Infrared Radiometer (Model PIR)] are presented using 10 PIRs from the 1999 Cooperative Atmosphere-Surface Exchange Study (CASES-99). The least squares-based optimization technique uses a field intercomparison (i.e., a time period during which all the PIRs were upward looking and set up side by side) to determine a set of optimization coefficients for each PIR. For the 10 CASES-99 PIRs, the optimization technique improved the standard deviation of the difference of downwelling irradiance between the PIRs from + or -0.75 to + or -0.4 W m^sup -2^ (for nighttime data). In addition to presenting the optimixation method, various PIR data quality checks are outlined and applied to the PIR data. Based on these quality checks, the measured case and dome temperatures of the CASES-99 PIRs were all reasonable. Using the 10 CASES-99 PIRs, simple estimates of the average nighttime net radiative flux divergence within the layer between 2 and 48 m were determined and resulted in cooling rates over a range from 0 to -1.3[degrees]C h^sup -1^, depending on the assumptions made for the upwelling irradiance at 2 m. The effect of the coefficient optimization on the calculated net radiative flux divergence is explored.