Kaicun Wang,¹ John Augustine,² and Robert E. Dickinson³

¹ State Key Laboratory of Earth Surface Processes and Resource Ecology, College of Global Change and Earth System Science, Beijing Normal University, Beijing, China;

² NOAA Earth System Research Laboratory, Boulder, Colorado, USA;

³ Department of Geological Sciences, University of Texas at Austin, Austin, Texas, USA.

 

Abstract: Surface incident solar radiation (R_s) drives weather and climate changes. Observations of R_s have been widely used as reference data to evaluate climate model simulations and satellite retrievals. However, few have studied uncertainties of R_s observations, especially long term. This paper compares R_s from 1995 to 2011 at collocated sites collected by the Surface Radiation Budget Network (SURFRAD), the U.S. Climate Reference Network (USCRN) and the AmeriFlux network. SURFRAD stations have measured separately the diffuse and direct components of R_s as well as R_s by a pyranometer, while R_s was measured by a pyranometer or a net radiometer at the USCRN and AmeriFlux sites. R_s can be calculated by summing the diffuse and direction radiation measurements. R_s measured by the summation technique was compared those measured by a pyranometer or a net radiometer at collocated sites. Agreement among these four independent R_s measurements is good with correlation coefficients higher than 0.98 and an average error (one standard deviation) of about 4% at both hourly and monthly time scales. R_s has a large spatial variability at the hourly time scale, even exceeding 100 W m^-2 in ~6 km. This spatial variability is substantially reduced at the monthly time scale. The two independent measurement systems at the SURFRAD sites agree rather well in annual variability of R_s with an average relative standard deviation error of 34%. The errors are 71%and 85% for the USCRN and AmeriFlux sites. Evidently, caution should be taken when using the R_s data collected at the USCRN and AmeriFlux sites to study annual variability of R_s.