Tyler J. Thorsen
NASA/Langley Research Center
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Uncertainty in observational estimates of the aerosol direct radiative effect and forcing
Aerosols continue to be responsible for the largest uncertainty in determining the anthropogenic radiative forcing of the climate. To both reconcile the large range in satellite-based estimates of the aerosol direct radiative effect (DRE, the direct interaction with solar radiation by all aerosols) and to optimize the design of future observing systems, we build a framework for assessing uncertainty in aerosol DRE and the aerosol direct radiative forcing (DRF, the radiative effect of just anthropogenic aerosols, RF_ari). Shortwave aerosol radiative kernels (Jacobians) were derived using the MERRA-2 reanalysis data. These radiative kernels are used to compute a lower-bound on the systematic uncertainty in observational estimates of the aerosol DRE/DRF by making the optimistic assumption that global aerosol observations can be made with the accuracy found in the Aerosol Robotic Network (AERONET) sun photometer retrievals. The total uncertainty is shown to be dominated by contributions from the aerosol single scattering albedo uncertainty. These uncertainty estimates were compared to a literature survey of mostly satellite-based aerosol DRE/DRF values. Comparisons to previous studies reveal that most have significantly underestimated the aerosol DRE uncertainty. Past estimates of the aerosol DRF uncertainty are smaller (on average) than our optimistic observational estimates, including the aerosol DRF uncertainty given in the Intergovernmental Panel on Climate Change (IPCC) fifth assessment report (AR5). Also quantified is a potential reduction in the current observational uncertainty possible with a future generation of satellite observations that would leverage aerosol typing and more refined vertical information.
Thorsen, T. J., R. A. Ferrare, S. Kato, and D. M. Winker, 2020: Aerosol Direct Radiative Effect Sensitivity Analysis. J. Climate, 33, 6119–6139,
Thorsen, T. J., D. M. Winker, and R. A. Ferrare: Uncertainty in observational estimates of the aerosol direct radiative effect and forcing. J. Climate, in review.
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