This Historical Eruption SO₂ Emission Assessment (HErSEA) experiment will involve each participating model running a limited ensemble of simulations for each of the three largest volcanic perturbations to the stratosphere in the last 100 years: 1963 Mt. Agung, 1982 El Chichón and 1991 Mt. Pinatubo.
The main aim is to use a wide range of stratospheric aerosol observations to constrain uncertainties in the SO2 emitted for each eruption (amount, injection height). Several different aerosol metrics will be intercompared to assess how effectively the emitted SO₂ translates into perturbations to stratospheric aerosol properties and simulated radiative forcings across interactive stratospheric aerosol CCMs with a range of different complexities. Whereas the TAR simulations (see section 3.2) use specified dynamics, and are suitable for chemistry transport models, for this experiment, simulations must be free-running with radiative coupling to the volcanically-enhanced stratospheric aerosol, thereby ensuring the composition-radiation-dynamics interactions associated with the injection are resolved. We are aware that this specification inherently excludes chemistry transport models, which must impose atmospheric dynamics. However, since the aim is to apply stratospheric aerosol observations in concert with the models to re-evaluate current best-estimates of the SO₂ input, and in light of the first order impact the stratospheric heating has on hemispheric dispersion from these major eruptions (e.g. Young, R. E. et al., 1994), we assert that this apparent exclusivity is entirely justified in this case.
As well as analysing and evaluating the individual model skill and identifying model consensus and disagreement for these three specific eruptions, we also seek to learn more about major eruptions which occurred before the era of satellite and in-situ stratospheric measurements. Our understanding of the effects from these earlier eruptions relies on deriving volcanic forcings from proxies such as sulphate deposition to ice sheets (Gao et al., 2007; Sigl et al., 2015; Toohey et al., 2013), from photometric measurements from astronomical observatories (Stothers, 1996, 2001) or from documentary evidence (Stothers, 2002; Stothers and Rampino, 1983; Toohey et al., 2016a).