Influences of atmospheric reanalysis on the accuracy of clear-sky irradiance estimates: Comparing MERRA-2 and CAMS

This contribution compares the influences of atmospheric reanalysis, in particular, the Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2) produced by NASA's Global Modeling and Assimilation Office and the global reanalysis of atmospheric composition produced by the Copernicus Atmosphere Monitoring Service (CAMS), on the accuracy of clear-sky irradiance estimates. In terms of reanalysis output, comparison shows there exist great differences in aerosol optical depth (AOD) and Angstrom exponent (AE) between the two reanalysis models. Based on 58 sunphotometer stations, it is found that daily MERRA-2 AOD is negatively biased with prominent underestimation in western China. CAMS AOD has considerable improvements in eastern China against MERRA-2, which may be associated with the high-quality assimilation of MODIS AOD and gas emissions, in a timely and consistent fashion. Both reanalyses tend to underestimate the fine-dominated aerosols (AE > 1.6) while overestimating the coarse-dominated ones (AE < 0.8). In terms of irradiance, a high-performance physical clear-sky model called the Reference Evaluation of Solar Transmittance, 2 bands (REST2) is used as the basis of evaluation. The performance of REST2-modeled global horizontal irradiance (GHI) based on the two reanalysis products is subsequently examined using data from 10 ground-based radiometry sites. It is concluded that the clearness index of clear-sky GHI (ktc) computed based on MERRA-2 and that based on CAMS show comparative performance over most parts of China, though there are exceptions. For instance, CAMS-based ktc outperforms MERRA-2 in eastern China, with slopes closer to unity, intercepts and root mean square errors closer to 0, which can be attributed to the improvements in CAMS AOD. The daily accumulated delta GHI (i.e., GHI(MERRA-2) -GHI(CAMS)) changes linearly with delta AOD (i.e., AOD(MERRA-2) -AOD(CAMS)), implying better AOD quality has advantages in accumulated GHI estimation. Therefore, CAMS reanalysis data is recommended for solar resource assessment applications in eastern China, albeit that MERRA-2 has a higher temporal resolution (1 h) and coverage (from 1980 to present).