Comparison of long-term solar radiation trends from CM SAF satellite products with ground-based data at the Iberian Peninsula for the period 1985-2015

The aim of this work is to analyse the quality of long-term trends of surface incoming shortwave solar radiation (SIS) derived from two satellite datasets from the EUMETSAT Satellite Application on Climate Monitoring (CM SAF): the SIS Data Set from the Advanced Very High-Resolution Radiometer (AVHRR) data, Edition 2 (CLARA-A2), and the SIS Data Set-Heliosat, Edition 2 (SARAH-2). In order to achieve this goal, reference ground-based SIS measurements recorded at 12 stations over the Iberian Peninsula for the period 1985-2015 are used in this study. Firstly, the two satellite datasets have been compared against ground-based SIS measurements at 12 surface sites, showing a good agreement (i.e., R = 0.83 in SARAH-2 and R = 0.80 in CLARA-A2 on an annual basis). However, the two satellite datasets substantially underestimate the SIS trends found for the ground-based measurements. Thus, while the ground-based SIS data reported trends between -0.5 and + 6.5 Wm(-2)decade(-1) (with statistical significance at 95% level at most stations), the satellite datasets gave trends lower for all locations (without statistical significance); between -0.4 and + 3.8 Wm(-2)decade(-1) for CLARA-A2, and between + 0.2 and + 2.8 Wm(-2)decade(-1) for SARAH-2. It is worth to mention that the seasonal analysis of the SIS trends for both ground-based and satellite data displays a reasonably good agreement in spring (i.e., high positive trends), in accordance with the notable decline in the cloudiness for this season in the study region. By contrast, satellite products exhibit smaller SIS anomalies than ground-based data in summer, particularly from the beginning 2000s, which could be related to well-known decrease in the aerosol load over the study region.