Diffuse fraction distributions 1901-2017 in support of carbon cycle modelling: Constant aerosols.

This diffuse fraction dataset offers 6-hourly distributions of the diffuse fraction of surface shortwave fluxes over the period 1901-2017. This is a companion dataset of doi:10.6084/m9.figshare.14423690, where time variations in tropospheric aerosols are ignored, and stratospheric aerosols are not accounted for. Radiative transfer calculations are based on monthly-averaged distributions of tropospheric aerosol optical depth, and 6-hourly distributions of cloud fraction. Methods follow those described in the Methods section of Mercado et al. (doi:10.1038/nature07949, 2009), but with updated input datasets. The time series of speciated tropospheric aerosol optical depth is taken from the historical and RCP8.5 simulations by the HadGEM2-ES climate model (Bellouin et al., doi:10.1029/2011JD016074, 2011). To correct for biases in HadGEM2-ES, tropospheric aerosol optical depths are scaled over the whole period to match the global and monthly averages obtained over the period 2003-2017 by the CAMS Reanalysis of atmospheric composition (Inness et al., doi:10.5194/acp-19-3515-2019, 2019), which assimilates satellite retrievals of aerosol optical depth. Monthly distributions of sulfate and carbonaceous optical depth are then averaged over the 1901-1920 period, and those multi-annual monthly averages are used for the whole 1901-2017 period. The only interannual changes in tropospheric aerosols therefore come from mineral dust and seasalt. Stratospheric aerosol optical depth is not accounted for in the radiative transfer calculations. The time series of cloud fraction is obtained by scaling the 6-hourly distributions simulated in the Japanese Reanalysis (JRA; Kobayashi et al., doi:10.2151/jmsj.2015-001, 2015) to match the monthly-averaged cloud cover in the CRU TS v4.03 dataset (Harris et al. doi:10.1038/s41597-020-0453-3, 2020). Surface radiative fluxes account for aerosol-radiation and aerosol-cloud interactions by tropospheric aerosols, except mineral dust which only exerts aerosol-radiation interactions. The radiative effects of aerosol-cloud interactions are assumed to scale with the radiative effects of aerosol-radiation interactions of tropospheric aerosols, using regional scaling factors derived from HadGEM2-ES. Diffuse fraction is assumed to be 1 in cloudy sky. Atmospheric constituents other than aerosols and clouds are set to a constant standard mid-latitude summer atmosphere, but their variations do not affect the diffuse fraction of surface shortwave fluxes.