Interactive stratospheric aerosol simulations of the Hunga-Tonga aerosol cloud re: stronger than expected observed mid-visible stratospheric AOD

In this presentation we present findings from a series of interactive stratospheric aerosol simulations of the Hunga-Tonga volcanic aerosol cloud with the UM-UKCA composition-climate model.   The model experiments apply the same version of the UM-UKCA model published for the Agung, El Chichon and Pinatubo aerosol clouds (Dhomse et al., 2020), those runs aligned with the Historical Eruption SO2 emissions Assessment experiment within ISA-MIP (Timmreck et al., 2018).

A consistent feature of interactive stratospheric aerosol simulations of Pinatubo (e.g. Dhomse et al., 2014; Sheng et al., 2015; Mills et al., 2016) is an over-prediction of stratospheric AOD, for a given emission of SO2, requiring a downward-adjustment of emitted SO2 (e.g. Timmreck et al., 2018). Dhomse et al. (2020) indicated models may be missing an important removal process such as heterogeneous uptake of SO2 onto fine ash particles, the process recently demonstrated to have removed ~43% more sulphate than SO2-only simulations for the Kelut aerosol cloud (Zhu et al., 2020).

For Hunga-Tonga, any volcanic ash was likely removed within the initial days (Sellitto et al., 2022) and the co-emission of ~100Tg of water vapour (e.g. Carn et al., 2022) has been shown by Zhu et al. (2022) to increase the scattering efficiency of the Hunga-Tonga cloud, both via hygroscopic growth and changes in coagulation, leading to a potential systematic underestimation of stratospheric AOD among interactive stratospheric aerosol models.

In a series of UM-UKCA model experiments, we explore the unexpectedly strong optical depth from Hunga-Tonga aerosol via SO2-only simulations, increasing by a factor 2 and 3 the observed 0.4-0.5Tg of SO2 (Carn et al., 2022), and aligning with the protocol from a multi-model Hunga-Tonga aerosol intercomparison co-ordinated by the University of Colorado (Clyne et al., 2021).  The Tonga-MIP protocol emits 0.5Tg of SO2 at 25-30km, within a 6-hour period, with models also enacting a meridional emission spread between 22S and 14S, matching approaches used to account for unresolved early-phase plume transport for Pinatubo (see e.g. Quaglia et al., 2022).

We evaluate the strength of the simulated Hunga-Tonga aerosol cloud in these SO2-only emission runs, comparing to the magnitude and timing of maximum strat-AOD observed from OMPS (Taha et al., 2022), and the altitude of the progressing aerosol cloud compared to ground-based lidar measurements from Reunion Island (Baron et al., 2022) and Sao Paulo (Landulfo et al., 2022).