New strategies for vertical transport in chemistry transport models: application to the case of the Mount Etna eruption on 18 March 2012 with CHIMERE v2017r4

Excessive numerical diffusion is one of the major
limitations in the representation of long-range transport
by chemistry transport models. In the present study, we focus
on excessive diffusion in the vertical direction, which has
been shown to be a major issue, and we explore three possible
ways of addressing this problem: increasing the vertical resolution,
using an advection scheme with anti-diffusive properties
and more accurately representing the vertical wind. This
study was carried out using the CHIMERE chemistry transport
model for the 18 March 2012 eruption of Mount Etna,
which released about 3 kt of sulfur dioxide into the atmosphere
in a plume that was observed by satellite instruments
(the Infrared Atmospheric Sounding Interferometer instrument,
IASI, and the Ozone Monitoring Instrument, OMI) for
several days. The change from the classical Van Leer (1977)
scheme to the Després and Lagoutière (1999) anti-diffusive
scheme in the vertical direction was shown to provide the
largest improvement to model outputs in terms of preserving
the thin plume emitted by the volcano. To a lesser extent, the
improved representation of the vertical wind field was also
shown to reduce plume dispersion. Both of these changes
helped to reduce vertical diffusion in the model as much as a
brute-force approach (increasing vertical resolution).