Local Adaptive Calibration of the Satellite-Derived Surface Incident Shortwave Radiation Product Using Smoothing Spline

Incident solar radiation (Rs) over the Earth's surface plays an important role in determining the Earth's climate and environment. Generally, Rs can be obtained from direct measurements, remotely sensed data, or reanalysis and general circulation model (GCM) data. Each type of product has advantages and limitations: the surface direct measurements provide accurate but sparse spatial coverage, whereas other global products may have large uncertainties. Ground measurements have been normally used for validation and occasionally calibration, but transforming their “true values” spatially to improve the satellite products is still a new and challenging topic. In this paper, an improved thin-plate smoothing spline approach is presented to locally “calibrate” the Global LAnd Surface Satellite (GLASS) Rs product using the reconstructed Rs data from surface meteorological measurements. The influence of surface elevation on Rs estimation was also considered in the proposed method. The point-based surface reconstructed Rs was used as the response variable, and the GLASS Rs product and the surface elevation data at the corresponding locations as explanatory variables to train the thin-plate spline model. We evaluated the performance of the approach using the cross-validation method at both daily and monthly time scales over China. We also validated the estimated Rs based on the thin-plate spline method using independent ground measurements and independent satellite estimates of Rs. These validation results indicated that the thin-plate smoothing spline method can be effectively used for calibrating satellite-derived Rs products using ground measurements to achieve better accuracy.