Abstract
Abstract. Aerosol–cloud interactions (ACI) are the consequence of perturbed aerosols affecting cloud drop and crystal number, with corresponding microphysical and radiative effects. ACI are sensitive to both cloud microphysical processes (the "C" in ACI) and aerosol emissions and processes (the "A" in ACI). This work highlights the importance of cloud microphysical processes, using idealized and global tests of a cloud microphysics scheme used for global climate prediction. Uncertainties in key cloud microphysical processes examined with sensitivity tests cause uncertainties of nearly −30 to +60 % in ACI, similar to or stronger than uncertainties identified due to natural aerosol emissions (−30 to +30 %). The different dimensions and sensitivities of ACI to microphysical processes identified in previous work are analyzed in detail, showing that precipitation processes are critical for understanding ACI and that uncertain cloud lifetime effects are nearly one-third of simulated ACI. Buffering of different processes is important, as is the mixed phase and coupling of the microphysics to the condensation and turbulence schemes in the model.
Highlights
Aerosols represent the largest uncertainty in our estimates of current anthropogenic forcing of climate (Boucher et al, 2013), limiting our ability to constrain the sensitivity of the current climate to radiative forcing
Cloud microphysics determines how much water precipitates, the amount of water remaining in the cloud, and the resulting population of cloud drops
Gettelman: Aerosol–cloud interactions clouds interact with aerosols, assuming aerosols translated into cloud drop numbers based on fixed cloud dynamics and water content (Carslaw et al, 2013), largely ignoring the “C” in aerosol–cloud interactions (ACI)
Summary
Aerosols represent the largest uncertainty in our estimates of current anthropogenic forcing of climate (Boucher et al, 2013), limiting our ability to constrain the sensitivity of the current climate to radiative forcing. Aerosols affect climate through direct effects of absorption or scattering, and indirect effects (Twomey, 1977) by changing the number of cloud drops and resulting complex microphysical interactions. Recent work (Carslaw et al, 2013; Ghan, 2013; Kiehl et al, 2000) found large sensitivities of ACI to uncertainty in natural emissions and pre-industrial aerosols: the “A” in ACI.
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