Results from the first ARM diffuse horizontal shortwave irradiance comparison

[1] The first intensive observation period (IOP) dedicated exclusively to the measurement of diffuse horizontal shortwave irradiance was held in the Fall 2001 at the central facility of the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site with the cooperation of the Baseline Surface Radiation Network (BSRN) community. The purpose of the study was to compare diffuse irradiance measurements among most commercial pyranometers and a few prototypes calibrated independently using current practices. The hope was to achieve a consensus for this measurement with the goal of improving the uncertainty of shortwave diffuse irradiance measurements. All diffuse broadband measurements were made using the same type of two-axis tracker with the direct beam blocked by shading balls. Tracking was excellent during the IOP with no lost data associated with tracker problems. Fourteen simultaneous measurements were obtained over a two-week period under mostly clear skies with low to moderate aerosol loading. Totally overcast data were obtained during the morning of one day. Five of the measurements are reproducible to about 2 W/m(2) at the 95% confidence level. Three more agree with the mean of these five to about 4 W/m(2) at the 95% confidence level after correction for thermal offsets.