Optimal Measurement of Surface Shortwave Irradiance Using Current Instrumentation

Abstract

Although most measurements of total downwelling shortwave irradiance are made with pyranometers, the World Climate Research Program?s Baseline Surface Radiation Network has recommended the use of the summation of shortwave components in which the direct normal irradiance is measured and multiplied by the cosine of the solar zenith angle and then added to the diffuse horizontal irradiance measured by a pyranometer that is shaded from direct solar radiation by a disk. The nonideal angular response of most pyranometers limits their accuracy to about 3%, or 20?30 W m?2, for instantaneous clear-sky measurements. An intensive study of 21 separate measurements of total horizontal irradiance was conducted during extreme winter conditions of low sun and cold temperatures over 12 days at the National Oceanic and Atmospheric Administration?s Climate Monitoring and Diagnostics Laboratory. The experiment showed that the component sum methodology could lower the uncertainty by a factor of 2 or 3. A clear demonstration of this improvement was realized in a separate experiment conducted at the Atmospheric Radiation Measurement Southern Great Plains Cloud and Radiation Testbed site during April 1996. Four independent measurements of downwelling shortwave irradiance using the component sum technique showed typical differences at solar noon of about 10 W m?2. The mean of these summed measurements at solar noon was lower than the mean of the most-well-calibrated pyranometer measurements, acquired simultaneously, by about 30 W m?2, which is consistent with the typical angular response of many pyranometers.