Season-dependent uncertainties in Baseline Surface Radiation Network (BSRN) solar radiation data

Abstract

A novel radiation calibration method (Qiu et al., 2014) is improved using two new calibration selection criteria for quality control and is then used to analyze the uncertainties in global, direct and scattering solar irradiance (GSI, DSI and SSI, respectively) measurements at Izana, a Baseline Surface Radiation Network (BSRN) site. A total of 476 days of accepted calibration results from 2011 to 2014 are employed. The results reveal season-dependent uncertainties in the GSI, DSI and SSI measurements. The GSI, DSI and SSI measurements are probably lower by 3% ~ 6% in the summer and higher by 1% ~ 4% in the winter. The temperature dependency of solar radiometers is the most likely cause for this result. The calibration results are validated through two types of comparisons. The first is a comparison between aerosol optical thickness (AOT) retrievals from the vertical and horizontal DSI and AOT retrievals from the Aerosol Robotic Network (AERONET). The second is a comparison between in situ radiation measurements and model simulations. Higher GSI and DSI measurements in the summer result in systematically larger AOT retrievals, and they are also responsible for evidently lower measurements relative to model simulations. The winter case shows evidence to the contrary. Differences between in situ radiation measurements and model simulations also indicate the seasonal dependence of the measurement uncertainties. These results are highly significant for understanding the uncertainties in historical solar radiation data.