Following the Plume: Evaluation of UKESM1 simulation of 2014 Holuhraun eruption in a Lagrangian Framework.

Volcanic eruptions provide invaluable natural experiments to evaluate the transport, evolution, and potential impact of sulphate aerosol on clouds in global climate models (GCMs). The 2014 fissure eruption in Holuhraun, Iceland had an emission rate greater than a third of daily global sulphur dioxide emissions at its peak, resulting in significant perturbations to cloud radiative properties across vast swathes on the North Atlantic. Probing the GCM representation of aerosol lifecycle during transport in the volcanic plume, offers a unique insight to improve aerosol source and sink process understanding, which is essential to reducing one of the largest sources of uncertainty in climate modelling - aerosol-cloud-interactions.

Using rural aerosol measurement sites with climatologically relevant time series, we first perform a Eulerian evaluation of the UK Met Office Earth System Model (UKESM1) simulation of the volcanic eruption. Both in-situ observations and UKESM1 demonstrate a significant increase in aerosol concentration during the volcanic eruption compared to the climatology (2008-2013). However, the ‘standard’ version of the model fails to replicate the significant growth events associated with new particle formation seen in the observations during the volcanic eruption. A second simulation of UKESM1 with the addition of boundary layer nucleation, which is not included in the standard configuration, accurately reproduces the timing of nucleation events seen during the eruption period. Finally, we utilise a Lagrangian framework, in which HYSPLIT trajectories are calculated for both GCMs and reanalysis data, to analyse the relevant aerosol source and sink processes during transport that drive the differences between modelled and observed aerosol at the measurement sites.