Simulation of ash clouds after a Laacher See-type eruption

Abstract

Abstract. Dated to approximately 13 000 years ago, the Laacher See (East Eifel volcanic zone) eruption was one of the largest midlatitude Northern Hemisphere volcanic events of the Late Pleistocene. This eruptive event not only impacted local environments and human communities but probably also affected Northern Hemispheric climate. To better understand the impact of a Laacher See-type eruption on NH circulation and climate, we have simulated the evolution of its fine ash and sulfur cloud with an interactive stratospheric aerosol model. Our experiments are based around a central estimate for the Laacher See aerosol cloud of 15 Tg of sulfur dioxide (SO2) and 150 Tg of fine ash, across the main eruptive phases in May and a smaller one in June with 5 Tg SO2 and 50 Tg of fine ash. Additional sensitivity experiments reflect the estimated range of uncertainty of the injection rate and altitude and assess how the solar-absorptive heating from the fine ash emitted in the first eruptive phase changed the volcanic clouds' dispersion. The chosen eruption dates were determined by the stratospheric wind fields to reflect the empirically observed ash lobes as derived from geological, paleoecological and archeological evidence linked directly to the prehistoric Laacher See eruption. Whilst our simulations are based on present-day conditions, and we do not seek to replicate the climate conditions that prevailed 13 000 years ago, we consider our experimental design to be a reasonable approximation of the transport pathways in the midlatitude stratosphere at this time of year. Our simulations suggest that the heating of the ash plays an important role for the transport of ash and sulfate. Depending on the altitude of the injection, the simulated volcanic cloud begins to rotate 1 to 3 d after the eruption. This mesocyclone, as well as the additional radiative heating of the fine ash, then changes the dispersion of the cloud itself to be more southward compared to dispersal estimated without fine ash heating. This ash-cloud-generated southerly migration process may at least partially explain why, as yet, no Laacher See tephra has been found in Greenland ice cores. Sulfate transport is similarly impacted by the heating of the ash, resulting in stronger transport to low latitudes, later arrival of the volcanic cloud in the Arctic regions and a longer lifetime compared to cases without injection of fine ash. Our study offers new insights into the dispersion of volcanic clouds in midlatitudes and addresses a likely behavior of the ash cloud of the Laacher See eruption that darkened European skies at the end of the Pleistocene. In turn, this study can also serve as significant input for scenarios that consider the risks associated with re-awakened volcanism in the Eifel.

The effect of ash, water vapor, and heterogeneous chemistry on the evolution of a Pinatubo-size volcanic cloud

“…Brühl et al (2015) obtained equatorial average SAOD=0.38 compared to SAOD=0.11 reported by LeGrande et al (2016) for 17 Mt of injected SO 2 . Niemeier et al (2021) and Stenchikov et al (2021) obtained similar SAOD which is consistent with observations for 17 Mt of injected SO 2 . Dhomse et al (2014), using a detailed aerosol microphysics model, found that in simulations of a Pinatubo-like eruption with a 10 Mt of SO 2 injection, SAOD matches observations better than that with larger SO 2 emission.…”

“…Ash particles have a wide range of sizes from sub-microns to millimeters (Rose and Durant, 2009) and highly irregular shapes. Large ash particles with radii r>1 µm sediment relatively quickly (Niemeier et al, 2021(Niemeier et al, , 2009Stenchikov et al, 2021), and are believed to contribute little in the long-term evolution of a volcanic cloud. Fine ash particles with r<1 µm disperse over vast distances and can survive in the stratosphere for several months (Pueschel et al, 1994;Zhu et al, 2020;Russell et al, 1996;Vernier et al, 2016), but their radiative effect is small because of their relatively smaller mass.…”

Dependency of the impacts of geoengineering on the stratospheric sulfur injection strategy – Part 1: Intercomparison of modal and sectional aerosol modules

“…As in Niemeier and Timmreck (2015), we changed this threshold radius between the accumulation and coarse modes (the two largest modes) from 0.5 to 0.2 µm. Our model set-up does not include additional stratospheric chemistry, the limitation of available OH for oxidation of SO 2 under extreme high-SO 2 conditions (> 1000 Tg (S)) or the forced evaporation of sulfate over 30 km, as in Niemeier and Timmreck (2015) and Niemeier et al (2021). Even though the mode set-up of the model was modified to satisfactorily represent the stratospheric aerosol at the expense of the representation of the tropospheric aerosols (especially sea salt and dust), we also include all anthropogenic emissions and natural surface emission.…”

Interactive Stratospheric Aerosol models response to different amount and altitude of SO2 injections during the 1991 Pinatubo eruption

“…The sulfate was radiatively active for both SW and LW radiation and coupled to the radiation scheme of ECHAM. Further details are described in Niemeier et al (2021).…”

“…S4 and S5): in the first months after the eruption models and observations show large differences, especially for SAD and extinction, which are overestimated in both the latitudes considered. This may be related both to the sensitivity to the actual meteorological conditions that climate models are unable to accurately replicate, and to the absence in HErSEA simulations of volcanic ash injection that could remove some of the initial SO 2 gas or affect the local winds and the SO 2 dispersion (Ayris et al, 2013;Zhu et al, 2020;Dhomse et al, 2020;Kloss et al, 2021;Niemeier et al, 2021). This sensitivity to the initial conditions decreases the more time passes after the eruption.…”