TITLE

Study of the orographic influences upon the ageostrophic circulations in the jet-front systems: Application to IOP N^o 8 of the "PYREX 90" experiment. Thèse de l'Université Blaise Pascal; spécialité : Physique de l'Atmosphère. Clermont-Ferrand, septembre 1995, 246 p. (in french).

SUMMARY

The aim of this study is to analyse the orographic influence upon the large scale atmospheric circulation, and upon the vertical transport, in jet-front systems. This work is based on the ECMWF (T106/L19 and T213/L31 versions) and on the PERIDOT model analysis, on the TOMS/Nimbus 7 measurements and on the ST radar data.

The study of the quasi geostrophic vorticity advection terms allows to localize the influences of the large scale motion (acceleration and curvature effects) on the vertical velocity field. Then, the study of the total ageostrophic wind and of the potential vorticity shows that the orographic impact can cancel the local ageostrophic circulations, and may involve a smaller development over orography of the induced tropopause folding. A kinetic trajectory calculation software, the accuracy of which is evaluated, puts in evidence that the local orographic effect strongly alters, in this case, the curvature of synoptic particle trajectories, the vertical transports and the stratosphere-troposphere exchanges.

The PERIDOT data analyses allow to better localize the jet-streak in relation to the relief as a function of time, and to deduce the expected vertical motion over the mountains. The intensity (0,1 m/s upstream of the orography and up to 2,5 m/s over and downstream of it) and the intensity variations (waves) of the vertical velocities recorded by ST radars show their ability to probe the vertical motions of synoptic origin (upstream of the mountain) and of orographic origin (over and downstream of it).

Key words

Jet-front system, jet-streak, ageostrophic circulations, tropopause folding, quasi geostrophic potential vorticity, strato-tropopheric exchanges, ST radars, vertical velocity.

Y.Pointin

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