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

Abstract

<jats:p>Abstract. 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 to address this problem: increase vertical resolution, use an advection scheme with antidiffusive properties, and represent more accurately the vertical wind. This study is done with the CHIMERE chemistry-transport model, for the March 18, 2012 eruption of Mount Etna, which has released about 3 kt of sulphur dioxide in the atmosphere into a plume that has been observed by satellite instruments (IASI and OMI) for several days. The change from the classical Van Leer et al., (1977) scheme to the Després and Lagoutière (1999) antidiffusive scheme in the vertical direction has been shown to bring the largest improvement to model outputs in terms of preserving the thin plume emitted by the volcano. To a lesser extent, improved representation of the vertical wind field has also been shown to reduce plume dispersion. Both these changes help reducing vertical diffusion in the model as much as a brute-force approach (increasing vertical resolution). </jats:p>

Using an antidiffusive transport scheme in the vertical direction: a promising novelty for chemistry-transport models

“…The numerical experiments that have been presented confirm the interest of using the Després and Lagoutière (1999) scheme for vertical transport in chemistry-transport models, as already claimed by Lachatre et al (2020). The idealized framework set up here permits to examine some of the questions that were out of reach in the real case of Lachatre et al (2020) due to uncertainties on the forcing meteorological fields, the volcanic emissions and to the lack of accurate measurement of plume structure.…”

“…While reduction of diffusion is in line with the results of Lachatre et al (2020) and could be expected by design of the Després and Lagoutière (1999) schemes, improved accuracy in presence of sharp gradients is a strong argument in favor of using the Després and Lagoutière (1999) scheme for vertical transport in chemistry-transport models.…”

“…We think that these results are important because they explain under which conditions the Després and Lagoutière (1999) is able to reduce excessive vertical diffusion in CTMs, as was observed in Lachatre et al (2020), show that this reduction in numerical diffusion is not obtained at the expanse of accuracy, which is improved as well. Since the vertical resolution of chemistry-transport model is usually coarse in the free troposphere, while the advected plumes tend to be extremely thin in this part of the atmosphere due to the action of wind shear, the present study along with Lachatre et al (2020) advocates for using the Després and Lagoutière (1999) transport scheme for chemistry-transport modelling in the free troposphere, and probably even more in the stratosphere where vertical diffusion needs to be extremely reduced. More investigation is needed in real and/or idealized cases to adress several questions:…”

“…Regarding chemistry-transport models, in practice, approximated values of F i− 1 2 are inferred from the wind and density fields provided by the forcing meteorological dataset. Here we will avoid the typical problem of mass-wind inconsistenciesdiscussed in, e.g., Jöckel et al (2001); Emery et al (2011); Lachatre et al (2020), by working with analytically-defined non-divergent mass fluxes, and constant and uniform air density, so that Eq. 12 is verified exactly by construction.…”

“…Therefore, the present study aims at answering the questions on the use of Després and Lagoutière (1999) that could not be adressed in the realistic framework of Lachatre et al (2020). For that purpose, we have designed three idealized test cases that permit to compare the performance of the Després and Lagoutière (1999) scheme with the classical schemes of Van Leer (1977) and Colella and Woodward (1984), not only in terms of diffusion but also in terms of accuracy compared to the exact solution, which could not be done in Lachatre et al (2020). We also examine how the performance of Després and Lagoutière (1999) compares to the two above-cited schemes and to the order-1 upwind Godunov scheme when resolution increases.…”