R. J. Allen, J. R. Norris, M. Wild. Evaluation of multidecadal variability in CMIP5 surface solar radiation and inferred underestimation of aerosol direct effects over Europe, China, Japan and India. Journal of Geophysical Research – Atmospheres. DOI: 10.1002/jgrd.50426
Observations from the Global Energy Balance Archive indicate regional decreases in all sky surface solar radiation from ~1950s-1980s, followed by an increase during the 1990s. These periods are popularly called dimming and brightening, respectively. Removal of the radiative effects of cloud cover variability from all sky surface solar radiation results in a quantity called “clear sky proxy” radiation, in which multidecadal trends can be seen more distinctly, suggesting aerosol radiative forcing as a likely cause. Prior work has shown climate models from the Coupled Model Intercomparison Project 3 (CMIP3) generally underestimate the magnitude of these trends, particularly over China and India. Here, we perform a similar analysis with 173 simulations from 42 climate models participating in the new CMIP5. Results show negligible improvement over CMIP3, as CMIP5 dimming trends over four regions–Europe, China, India and Japan–are all underestimated. This bias is largest for both India and China, where the multi-model mean yields a decrease in clear sky proxy radiation of −1.3 ± 0.3 and −1.2 ± 0.2 W m−2 decade−1, respectively, compared to observed decreases of −6.5 ± 0.9 and −8.2 ± 1.3 W m−2 decade−1. Similar underestimation of the observed dimming over Japan exists, with the CMIP5 mean dimming ~20% as large as observed. Moreover, not a single simulation reproduces the magnitude of the observed dimming trend for these three regions. Relative to dimming, CMIP5 models better simulate the observed brightening, but significant underestimation exists for both China and Japan. Overall, no individual model performs particularly well for all four regions. Model biases do not appear to be related to the use of prescribed versus prognostic aerosols, or to aerosol indirect effects. However, models exhibit significant correlations between clear sky proxy radiation and several aerosol-related fields, most notably aerosol optical depth (AOD), and absorption AOD. This suggests model underestimation of the observed trends is related to underestimation of aerosol direct radiative forcing and/or deficient aerosol emission inventories.
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