PNNL

Abstract

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. Fourteen simultaneous measurements were obtained over a two-week period under mostly clear skies with low to moderate aerosol loading. Overcast data were obtained during the morning of one day. The purpose of the comparison was to assess the level of agreement in 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 very good during the IOP with no outages associated with tracker problems. Five of the measurements are reproducible to about 2 W/m2 at the 95% confidence level. Four more agree with the most consistent group to about 4 W/m2 at the 95% confidence level after correction for thermal offsets. The prototypes agree less well with the most consistent group.