Critical assessment of surface incident solar radiation observations collected by SURFRAD, USCRN and AmeriFlux networks from 1995 to 2011

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 .