Introduction to MIMOSA

Background

MIMOSA (Modèle Isentrope du transport Méso-échelle de l’Ozone Stratosphérique par Advection) was developed in the late 1990s at Service d’Aéronomie du CNRS (France) for the European project METRO (MEridional TRansport of Ozone in the lower stratosphere) to capture and quantify the filamentary structures passing over Europe in winter and spring, and to assess the importance of such structures in the irreversible transport of polar air into mid-latitudes.

Principle

In an idealized adiabatic, frictionless atmosphere, isentropic Potential Vorticity (PV) is a conserved tracer with a zonally mean positive latitudinal gradient throughout the stratosphere (i.e., increasing poleward in the Northern hemisphere), and with a sharper gradient in the regions separating the winter polar vortex from the lower latitudes. Therefore, it has tracer characteristics similar to those of ozone in the lower stratosphere and, under conditions of natural ozone balance, we can expect a high degree of correlation between lower stratospheric PV and ozone.
Global analyzed or forecasted isentropic winds and PV (typically coming from the European Centre for Medium range Weather Forecast ECMWF, or from the National Center for Environmental Prediction, NCEP) are input to MIMOSA where they are interpolated onto an azimuthal equidistant projection grid with a very fine resolution, typically several points per degree. The PV is advected isentropically using a 1-hour elementary time-step and re-interpolated onto a regular longitude-latitude grid. A careful validation of the chosen interpolation and re-gridding scheme showed numerical diffusion of the same order as that estimated for the real atmosphere. To account for diabatic processes, the high-resolution PV is relaxed with a ten-day relaxation constant towards the PV extracted from the original model grid. This relaxation technique allows the model to run over several weeks or months with, for example, the diabatic descent inside the winter polar vortex implicitly reproduced. The technique used in MIMOSA (smoothed difference between PV from meteorological analysis and advected PV) assumes that diabatic processes are slow and homogenous on a scale of a few hundred kilometers.

Unlike global analysis such as ECMWF or NCEP interpolated onto a very fine grid, the high-resolution PV advection model MIMOSA is able to reproduce the evolution of fine PV structures such as stretching polar filaments without disconnecting them from the main vortex.

fig 1
Figure 1

The ECMWF T106-analyzed PV fields, and high-resolution MIMOSA-advected PV fields at 435 K are shown for March 16, 2005 at 0000 UT on the Figure 1 (top and bottom respectively). On these two maps (click to enlarge), the vortex edge is contoured with three white lines (two thin lines being the outer and inner edges, and one thick line being the center edge). The large polar filament shown here is largely developed on the MIMOSA advected-PV map, stretching without discontinuity from Alaska to Hawaii, but barely identifiable on the ECMWF T106 PV map, and only a few regions of higher PV can be observed in the vicinity of Hawaii. The improved resolution (T511) of the most recent ECMWF model now allows a better identification of such filaments.

A particularity of a model like MIMOSA is that global winds from various sources can be fed into the model to advect PV.

References

Hauchecorne, A., S. Godin, M. Marchand, B. Heese, and C. Souprayen (2002), Quantification of the transport of chemical constituents from the polar vortex to midlatitudes in the lower stratosphere using the high-resolution advection model MIMOSA and effective diffusivity, J. Geophys. Res., 107 (D20), 8289, doi: 10.1029/2001JD000491.

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