High resolution transport model MIMOSA

Alain Hauchecorne

Service d’Aéronomie, CNRS, 91371 Verrières le Buisson Cedex

Alain.hauchecorne@aerov.jussieu.fr

http://www.aero.jussieu.fr/equipe/DCAM

 

MIMOSA (Modélisation Isentrope du transport Méso-échelle de l'Ozone Stratosphérique par Advection) high resolution advection model of potential vorticity (PV) has been developed at Service d’Aéronomie in the frame of the European Union project METRO (MEridional TRansport of Ozone in the lower stratosphere) which was part of THESEO (Third European Stratospheric Experiment on Ozone) – THESEO 2000 campaigns. The model was used to interpret the observations of ozone laminae in lidar profiles, especially at Observatoire de Haute-Provence (OHP, 44°N, 5.7°E), and to support the planning of an air-borne ozone lidar [Heese et al., 2001] on board of a French Falcon (Mystère 20). The basic assumption is that PV and ozone mixing ratio are very well correlated on an isentropic surface and the location of ozone filaments can be visualized using PV as a quasi passive tracer. A full description of MIMOSA is given in Hauchecorne et al. [2001].

The model runs on an isentropic surface, starting on an orthogonal grid in an azimuthal equidistant projection centered at North Pole (parallels are represented as concentric equidistant circles) It covers latitudes between 30°N and 90°N in the present version. The size of an elementary grid cell 37*37 km (3 grid points/degree of latitude). Data are taken from meteorological analysis and forecast fields of zonal wind, meridional wind and temperature on a 3D latitude-longitude-pressure grid. Data are first interpolated at isentropic levels and on the fine MIMOSA grid. PV fields are computed on isentropic surfaces using horizontal wind and temperature data on pressure surfaces.

Advection and regridding

The model starts from the meteorological PV field interpolated on the MIMOSA orthogonal grid. The PV of each grid point is advected using meteorological winds interpolated on the MIMOSA grid at the specified isentropic level. We have to keep in mind that the quantity advected by the model is not the true dynamical PV but a “quasi-passive PV” which correlates well with the concentration of ozone or others long-lived trace species in the lower stratosphere. In particular, the true PV is probably poorly conserved in small-scale filaments due to radiative and dynamical processes. In order to avoid confusion, we will call the quantity advected by MIMOSA “advected potential vorticity” (APV) in the continuation of the paper.

If we consider a square formed by 4 adjacent grid points at the initial time, this square is stretched and deformed by horizontal gradients in the wind field. After a given time, it is necessary to re-interpolate the APV field to the original grid in order to keep the distance between two adjacent points approximatively constant. A time interval of 6 hours has been chosen between two successive regriddings. For this time interval, the average change of the distance between 2 adjacent grid points ranges from 10% to 15% in the region between 400 and 675 K potential temperature. The regridding process produces a numerical diffusion as explained in Annex 1. If a bilinear interpolation is used, the equivalent numerical diffusivity is equal to 5280 m²s^-1 for the nominal horizontal resolution Dx = 37 km. In order to minimize the numerical diffusion, an interpolation scheme, based on the preservation of the second order momentum of a PV perturbation, has been implemented. It limits the effect of numerical diffusivity to 1350 m²s^-1 [Hauchecorne et al., 2001]. This value is close to the value of 1000 m²s^-1 estimated by Waugh et al. [1997] by tracer-tracer scatter plots from aircraft data or to the upper limit deduced from the vertical diffusion 0.01 m²s^-1 obtained by Balluch and Haynes [1997] using aircraft data and an aspect ratio 300 between horizontal and vertical structures (k_xx = 0.01 * 300² = 900 m²s^-1).

Relaxation

In the lower stratosphere PV is assumed to be conserved on isentropic surfaces for periods of 1 to 2 weeks [Orsolini, 1995]. For periods longer than 2 weeks, the diabatic transport across isentropic surfaces due to radiative cooling and warming of air as well as the diabatic advection term in the PV equation have to be taken into account. See Haynes and McIntyre [1990] for further discussion on PV conservation. The information on diabatic changes in the PV field, at least for larger scales, can be extracted from ECMWF fields. In MIMOSA this is made by applying to the APV field a relaxation toward the ECMWF PV field with a time constant of 10 days. In order to preserve the filamentation structure, the relaxation term is only applied to scales larger than 300 km. To do that, both MIMOSA and ECMWF fields are smoothed to the same resolution and the difference between the two fields is used to compute the relaxation term. Using this procedure, it is possible to run continuously the model and to follow the evolution of filaments during several months, for instance from November to April for the study of the filamentation and the final break up of the Arctic vortex.

Accuracy of filament location

At the beginning of December 1997, a large filament developed above Western Europe and the ozone lidar profile at OHP detected a layer of high ozone concentration during the night from 4^th to 5^th December peaking around 445 K [Godin et al., 2001], which indicates the presence of air of polar origin above the lidar station.. The comparison between the altitude of filaments shown in lidar profiles and predicted by MIMOSA during this period was less than 500m and, taking into account the slope of filament with altitude, corresponded to an error of less than 100 km in the predicted horizontal location. This is in good agreement with the estimation of the accuracy on the filament location simulated with contour advection codes by comparison with airborne ozone lidar observations [Flentje et al., 2000; Heese et al., 2001] . The strongest error in the location of filament was found to be due to the inaccuracy in meteorological wind field (about 1 ms^-1) used for the advection of PV filaments. 

 

References

Balluch M. G. and P. H. Haynes, Quantification of lower stratospheric mixing processes using aircraft data, J. Geophys. Res, 102, 23,487-23,504, 1997.

Flentje, H., W. Renger, and W. Wirth, comparison of airborne lidar measurements with contour advection simulations, J. Geophys. Res, 105, 15,417-15-437, 2000.

Godin S., M. Marchand, and A. Hauchecorne, Influence of the Arctic polar vortex erosion on the lower stratospheric ozone amounts at Haute-Provence Observatory (44°N, 6°E), J. Geophys. Res, 2001, in press.

Haynes, P.H., and M.E. McIntyre, On the conservation and impermeability theorems for potential vorticity, J. Atmos. Sci. , 47, 2021-2031, 1990.

Heese, B., S. Godin and A. Hauchecorne, Heese, B., S. Godin and A. Hauchecorne, Forecast and simulation of stratospheric ozone filaments: A validation of a high resolution PV advection model by airborne ozone lidar measurements in winter 1998-1999, J. Geophys. Res., 20,011-20,024, 2001.

Orsolini, Y. J., On the formation ozone laminae at the edge of the Arctic polar vortex, Q. J. R. Meteorol. Soc., 121, 1923-1941, 1995.

Waugh, D. W., R. A. Plumb, J. W. Elkins, D. W. Fahey, K. A. Boering, G. S. Dutton, C. M. Volk, E. Keim, R.-S. Gao, B. C. Daube, M. Loewenstein, J. R. Podolske, K. R. Chan, M