Interactive stratospheric aerosol models' response to different amounts and altitudes of SO₂ injection during the 1991 Pinatubo eruption

Abstract

Abstract. A previous model intercomparison of the Tambora aerosol cloud has highlighted substantial differences among simulated volcanic aerosol properties in the pre-industrial stratosphere and has led to questions about the applicability of global aerosol models for large-magnitude explosive eruptions prior to the observational period. Here, we compare the evolution of the stratospheric aerosol cloud following the well-observed June 1991 Mt. Pinatubo eruption simulated with six interactive stratospheric aerosol microphysics models to a range of observational data sets. Our primary focus is on the uncertainties regarding initial SO2 emission following the Pinatubo eruption, as prescribed in the Historical Eruptions SO2 Emission Assessment experiments (HErSEA), in the framework of the Interactive Stratospheric Aerosol Model Intercomparison Project (ISA-MIP). Six global models with interactive aerosol microphysics took part in this study: ECHAM6-SALSA, EMAC, ECHAM5-HAM, SOCOL-AERv2, ULAQ-CCM, and UM-UKCA. Model simulations are performed by varying the SO2 injection amount (ranging between 5 and 10 Tg S) and the altitude of injection (between 18–25 km). The comparisons show that all models consistently demonstrate faster reduction from the peak in sulfate mass burden in the tropical stratosphere. Most models also show a stronger transport towards the extratropics in the Northern Hemisphere, at the expense of the observed tropical confinement, suggesting a much weaker subtropical barrier in all the models, which results in a shorter e-folding time compared to the observations. Furthermore, simulations in which more than 5 Tg S in the form of SO2 is injected show an initial overestimation of the sulfate burden in the tropics and, in some models, in the Northern Hemisphere and a large surface area density a few months after the eruption compared to the values measured in the tropics and the in situ measurements over Laramie. This draws attention to the importance of including processes such as the ash injection for the removal of the initial SO2 and aerosol lofting through local heating.

Assessing Outcomes in Stratospheric Aerosol Injection Scenarios Shortly After Deployment

“…Previous SAI modeling experiments have provided useful insights into general implications of the intervention, such as the potential for SAI to reduce global mean temperature, the inability of SAI to counteract impacts linked directly to CO 2 concentration, and the risk of rapid climate change if SAI is stopped (“termination shock”; e.g., Bony et al., 2013; Jones et al., 2013; Kwiatkowski et al., 2015; Rasch et al., 2008; Tilmes et al., 2009; Trisos et al., 2018). Many of these experiments (e.g., the Geoengineering Model Intercomparison Project; Kravitz et al., 2011) have relied on models with limited representations of relevant Earth system processes including atmospheric chemistry, stratospheric dynamics, and aerosol microphysics (e.g., McCusker et al., 2015; Quaglia et al., 2023). Many of the SAI scenarios in these experiments are implemented in highly idealized ways, such as by prescribing the aerosol optical depth fields or reducing the model solar constant (Kravitz et al., 2011), which can produce a very distinct climate response from when SAI is more realistically represented with interactive aerosols (Bednarz et al., 2022; Ferraro et al., 2015; Visioni et al., 2021).…”

Assessing Outcomes in Stratospheric Aerosol Injection Scenarios Shortly After Deployment

“…Previous SAI modeling experiments have provided useful insight into general implications of the intervention, such as the risk of rapid climate change if SAI is halted ("termination shock") and the inability of SAI to counteract impacts linked directly to CO2 concentration (Jones et al 2013, Bony et al 2013, Kwiatkowski et al 2015, Trisos et al 2018). Many of these experiments (e.g., the Geoengineering Model Intercomparison Project; Kravitz et al 2011) have relied on models with poor representations of relevant Earth system processes including atmospheric chemistry, stratospheric dynamics, and aerosol microphysics (Richter et al 2017, Quaglia et al 2022). The SAI scenarios in these experiments are implemented in highly idealized ways, such as by prescribing the aerosol optical depth fields or reducing the model solar constant (Kravitz et al 2011), which can produce a very distinct climate response from when SAI is more realistically represented with interactive aerosols (Bednarz et al 2022a).…”

Volcanic Drivers of Stratospheric Sulfur in GFDL ESM4

“…(2018, Table 9) and Quaglia et al. (2023; Figure 1) summarized the wide range of SO 2 emissions used in past modeling studies, with emissions ranging from 5 to 20 Tg and injection height ranging from 15 to 30 km. Lastly, for models that do not interactively simulate aerosol distribution, sulfate dry effective radius used for gravitational settling and optical properties is also varied in past literature, ranging from 0.166 to 1 μm (Aquila et al., 2012; Lacis, 2015; Li & Min, 2002; Li et al., 2001)).…”

“…Lastly, for models that do not interactively simulate aerosol distribution, sulfate dry effective radius used for gravitational settling and optical properties is also varied in past literature, ranging from 0.166 to 1 μm (Aquila et al, 2012;Lacis, 2015;Li & Min, 2002;Li et al, 2001)). For models that simulate aerosol distribution, the effective radius varies depending on location, and they are calculated from Mie theory by integrating the scattering and extinction coefficients or using Mie-theory-based lookup tables (Quaglia et al, 2023). Such a wide range of parameters used to model the same eruption is an indication that models are sensitive to the input for volcanic emissions.…”

“…Aquila et al (2012) performed an ensemble of experiments, with an initial injection height of 16-18 km and found that their model responded quickly to radiatively interacting aerosols, lofting the volcanic aerosol; whereas, Timmreck, Graf, and Feichter (1999) and Zhao et al (1995) based their injection height on SAGE II observations at higher altitudes. The interactive stratospheric aerosol model intercomparison project by Timmreck et al (2018, Table 9) and Quaglia et al (2023; Figure 1) summarized the wide range of SO 2 emissions used in past modeling studies, with emissions ranging from 5 to 20 Tg and injection height ranging from 15 to 30 km. Lastly, for models that do not interactively simulate aerosol distribution, sulfate dry effective radius used for gravitational settling and optical properties is also varied in past literature, ranging from 0.166 to 1 μm (Aquila et al, 2012;Lacis, 2015;Li & Min, 2002;Li et al, 2001)).…”