Constraining the effects of volcanic eruptions on stratospheric aerosols and radiative forcing

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This project proposal (VolARC) is one of five scientific proposals for the second phase of the DFG research unit VolImpact (FOR 2820), whose first phase has started in spring 2019. The VolARC project deals with key aspects of volcanic effects on stratospheric aerosols and their impact on the radiative balance of the Earth system. Based on the work of phase I, we will address three science objectives: (1) Consolidating our understanding of the evolution of stratospheric aerosol parameters after volcanic eruptions and investigating the reasons for differences between measured and modelled stratospheric aerosol data sets (i.e. aerosol extinction profiles, aerosol optical depth and the aerosol particle size distribution). Here, a special focus will be on understanding the temporal variation of the stratospheric aerosol particle size distribution after volcanic eruptions.(2) Estimating the impact of model resolution and transport on stratospheric aerosol evolution. Significantly enhancing the work in phase I, we will employ a seamless simulation approach involving different nests in order to be able to consistently simulate all processes at their relevant scales, from the initial development of the volcanic plume up to global and long-term scales. (3) Investigating the sensitivity of limb-scatter and solar occultation measurements to stratospheric aerosols after strong volcanic eruptions as well as their capability to monitor possible geoengineering experiments. The first aspect is relevant, because current limb and occultation instruments will become insensitive to stratospheric aerosols already at relatively moderate aerosol optical depths. This is because of the long slant path in limb geometry. These effects are well known, but have never been investigated in a systematic study. The second aspect is also highly important in the light of potential solar radiation management activities based on the injection of sulfur compounds into the stratosphere. The research topics will be addressed by exploiting the synergy of global satellite observations of stratospheric aerosol parameters (mainly in the optical spectral range) and of model simulations with explicit stratospheric aerosol microphysics. We will employ our own algorithms to retrieve aerosol parameters from measurements with past, current and future satellite instruments, including, e.g. OMPS-LP, SAGE III and SCIAMACHY. The model simulations will be mainly based on ICON-ART, but MAECHAM-HAM will also be employed to provide some quick model intercomparison data. The VolARC project is well connected to the other four projects of VolImpact, for example via the overarching science topics that will be addressed by multiple VolImpact projects. These topics are: (1) the aerosol particle size distribution, (2) volcanic H2O injections into the middle atmosphere and (3) radiative forcing. In addition, VolARC will be coordinating the seamless simulation activities across the entire research unit.