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Towards a better understanding of the sudden growth in cyclogensis during the jet crossing

The dynamics of some frequently-observed sudden growths of East-Atlantic extratropical cyclones are still an open issue. Using highly idealized models, some dynamical aspects that need to be better understood are explored. The study aims at shedding some light on the interaction between high and low level cyclonic vortices embedded in different zonal jets that have both a vertical and a horizontal shear. In an anticyclonic shear which is observed on the south side of jets where a large number of future storms appear, the tropopause and surface anomalies are strongly horizontally stretched but maintain their vertical tilt so that they can strongly extract energy from their environment through baroclinic processes. On the contrary, in a cyclonic shear, the rotation induced by the environmental jet contributes to the structures’ rotation. This leads to the relative isotropy of both structures which wrap around the other. In this case, the initial westward vertical tilt of the perturbations which is the optimal configuration to extract energy from the environment is rapidly lost. It has otherwise been shown that the meridional gradient of environmental potential vorticity was a key factor controlling the poleward shift of cyclones, through similar processes as those in the dynamics of tropical cyclones and oceanic lenses. These mechanisms are two basic components of a synthetic explanation of extratropical cyclone growth when they cross the jet axis.

Evolution in a simple baroclinic two-layer model at t=0h, 12h, 24h and 36h from top to bottom of vorticity anomalies (interval : 2 10-5 s-1) at the upper (solid black) and lower (dashed red) level embedded in an environment characterized by a uniform vertical shear (2,4.10-5s-1) and a uniform horizontal shear (2,4. 10-5 s-1 in the upper layer, 1,2.10-5s-1 in the lower layer). Left panels : anticyclonic shear ; right panels : cyclonic shear.