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Model physics and chemistry causing intermodel disagreement within the VolMIP-Tambora Interactive Stratospheric Aerosol ensemble

Margot Clyne 1, 2 Jean-Francois Lamarque 3 Michael Mills 3 Myriam Khodri 4 William Ball 5, 6, 7 Slimane Bekki 8 Sandip Dhomse 9 Nicolas Lebas 4 Graham Mann 9, 10 Lauren Marshall 9, 11 Ulrike Niemeier 12 Virginie Poulain 13 Alan Robock 14 Eugene Rozanov 5, 6 Anja Schmidt 11, 15 Andrea Stenke 6 Timofei Sukhodolov 5 Claudia Timmreck 12 Matthew Toohey 16, 17 Fiona Tummon 6, 18 Davide Zanchettin 19 Yunqian Zhu 2 Owen Toon 1, 2
Abstract : As part of the Model Intercomparison Project on the climatic response to Volcanic forcing (VolMIP), several climate 30 modeling centers performed a coordinated pre-study experiment with interactive stratospheric aerosol models simulating the volcanic aerosol cloud from an eruption resembling the 1815 Mt Tambora eruption (VolMIP-Tambora ISA ensemble). The pre-study provided the ancillary ability to assess intermodel diversity in the radiative forcing for a large stratospheric-injecting equatorial eruption when the volcanic aerosol cloud is simulated interactively. An initial analysis of the VolMIP-Tambora ISA ensemble showed large disparities between models in the stratospheric global mean aerosol optical depth (AOD). In this study, we 35 now show that stratospheric global mean AOD differences among the participating models are primarily due to differences in aerosol size, which we track here by effective radius. We identify specific physical and chemical processes that are missing in some models and/or parameterized differently between models, which are together causing the differences in effective radius. In particular, our analysis indicates that interactively tracking hydroxyl radical (OH) chemistry following a large volcanic injection of sulfur dioxide (SO 2) is an important factor in allowing for the timescale for sulfate formation to be properly simulated. In 40 addition, depending on the timescale of sulfate formation, there can be a large difference in effective radius and subsequently AOD that results from whether the SO 2 is injected in a single model gridcell near the location of the volcanic eruption, or whether it is injected as a longitudinally averaged band around the Earth.
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Margot Clyne, Jean-Francois Lamarque, Michael Mills, Myriam Khodri, William Ball, et al.. Model physics and chemistry causing intermodel disagreement within the VolMIP-Tambora Interactive Stratospheric Aerosol ensemble. Atmospheric Chemistry and Physics Discussions, European Geosciences Union, In press, pp.(Under Review). ⟨10.5194/acp-2020-883⟩. ⟨insu-02937474⟩

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