Modeling the Sulfate Aerosol Evolution after Recent Moderate Volcanic
Activity, 2008-2012
Abstract
Volcanic activity is a main natural climate forcing and an accurate
representation of volcanic aerosols in global climate models is
essential. This is, however, a complex task involving many uncertainties
related to the magnitude and vertical distribution of volcanic emissions
as well as in observations used for model evaluation. We analyse the
performance of the aerosol-chemistry-climate model SOCOL-AERv2 for three
medium-sized volcanic eruptions. We investigate the impact of
differences in the volcanic plume height and SO2 content on the
stratospheric aerosol burden. The influence of internal model
variability and dynamics are addressed through an ensemble of
free-running and nudged simulations at different vertical resolutions.
Comparing the modeled evolution of the stratospheric aerosol loading to
satellite measurements reveals a good performance of SOCOL-AERv2.
However, the large spread in emission estimates leads to differences in
the simulated aerosol burdens resulting from uncertainties in total
emitted sulfur and the vertical distribution of injections. The
tropopause height varies among the free-running simulations, affecting
model results. Conclusive model validation is complicated by
uncertainties in observations. In nudged mode, changes in convection and
tropospheric clouds affect SO2 oxidation paths and cross-tropopause
transport, leading to increased burdens. This effect can be reduced by
leaving temperatures unconstrained. A higher vertical resolution of 90
levels increases the stratospheric residence time of sulfate aerosol by
reducing the diffusion out of the tropical reservoir. We conclude that
the model set-up (vertical resolution, free-running vs. nudged) as well
as forcing parameters (volcanic emission strength, plume height)
contribute equally to the model uncertainties.