Abstract
Abstract. As part of the Model Intercomparison Project on the climatic response to Volcanic forcing (VolMIP), several climate 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 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 (SO2) is an important factor in allowing for the timescale for sulfate formation to be properly simulated. In 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 SO2 is injected in a single model grid cell near the location of the volcanic eruption, or whether it is injected as a longitudinally averaged band around the Earth.
Highlights
Volcanic eruptions impact climate by cooling temperatures (Robock, 2000)
For global mean stratospheric aerosol optical depth (AOD), the peak values of the models vary by 65 % above to 19 % below the multi-model mean maximum value for the original VolMIP-Tambora Interactive stratospheric aerosol (ISA) ensemble models that were included in Marshall et al (2018), and the peak values vary by 63 % above and 34 % below the multimodel mean maximum when LMDZ-S3A is included
We sought to answer the question: why do the VolMIPTambora ISA models drastically disagree on global stratospheric AOD under a coordinated injection experiment protocol designed to eliminate confounding variables? We have identified physical and chemical processes that some models handled differently, made simplifying assumptions about, or even left out entirely, which contributed to the intermodel disagreement on the Reff and stratospheric sulfate burden and led to a wide range of simulated magnitude and duration of the volcanic perturbation to AOD
Summary
Volcanic eruptions impact climate by cooling temperatures (Robock, 2000). They inject sulfur dioxide gas (SO2) into the atmosphere. The sulfate aerosol scatters sunlight and causes an increase in aerosol optical depth, which is a key volcanic forcing parameter. There is uncertainty about the true values of these basic volcanic injection parameters due to limited availability of observational data for each eruption. The VolMIP-Tambora ISA experiment was created to assess intermodel differences by using a consistent set of volcanic injection parameters across models. The Tambora eruption was chosen as an example because it was large enough to have significantly altered the climate but had no observations of the volcanic cloud so that modelers did not know the answer in advance
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