Large Differences in Diffuse Solar Radiation among Current-Generation Reanalysis and Satellite-Derived Products

Abstract Although the partitioning of shortwave radiation K↓ at the surface into its diffuse (K↓,d) and direct beam (K↓,b) components is relevant for, among other things, the terrestrial energy and carbon budgets, there is a dearth of large-scale comparisons of this partitioning across reanalysis and satellite-derived products. Here we evaluate K↓, K↓,d, and K↓,b, as well as the diffuse fraction kd of solar radiation in four current-generation reanalysis datasets (NOAA–CIRES–DOE, NCEP–NCAR, MERRA-2, and ERA5) and one satellite-derived product (CERES) using ≈1400 site-years of observations. Although the systematic positive biases in K↓ are consistent with previous studies, the biases in gridded K↓,d and K↓,b vary in direction and magnitude, both annually and across seasons. The intermodel variability in cloud cover strongly explains the biases in both K↓,d and K↓,b. Over Europe and China, the long-term (10 yr and longer) trends in K↓,d in the gridded products differ noticeably from corresponding observations and the grid-averaged 35-yr trends show an order of magnitude variability. In the MERRA-2 reanalysis, which includes both clouds and assimilated aerosols, the reductions in both clouds and aerosols reinforce each other to establish brightening trends over Europe, whereas the effect of increasing aerosols overwhelms the effect of decreasing cloud cover over China. The intermodel variability in kd seen here (from 0.27 to 0.50 from CERES to MERRA-2) suggests substantial differences in shortwave parameterization schemes and their inputs in climate models and can contribute to intermodel variability in coupled simulations. From these results, we call for systematic evaluations of K↓,d and K↓,b in CMIP6 models.