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I. The life cycle of extratropical cyclones and extreme events like wind storms.

Interpretation of the cross-jet motion of extratropical cyclones


(Left) Idealized extratropical cyclone crossing the large-scale jet axis (red arrows) in a two-layer quasi-geostrophic model: lower-layer relative vorticity (contours; solid for positive; dashed for negative) and upper-layer relative vorticity (red: positive; blue negative).The solid thick black line shows the trajectory between t=0h and t=15h. (Right) 30h-forecast simulation of the extratropical storm Xynthia by Météo-France ARPEGE-IFS model. The large-scale jet axis is represented by red arrows, Xynthia by its relative vorticity at 850 hPa (black contours) and the upper-level anomalies by the anomalous potential vorticity at 300 hPa (red: positive; blue: negative; units: PVU). The same process is into play to explain the cross-jet motion of the idealized and real storm. The upper-tropospheric dipolar anomaly formed by an upstream trough (red shadings) and a downstream ridge (blue shadings) allows the advection of the surface cyclone across the jet axis (red isolines).


Eddy kinetic-energy (EKE) redistribution within winter storms: the case of Klaus (2009)


A three-dimensional view of the winter storm Klaus at 00 UTC 24 January 2009. The blue semi-transparent shadings represent regions of large baroclinic conversion from eddy potential energy to eddy kinetic energy. The white arrows represent the vertical ageostrophic fluxes in the vertical cross section which tend to redistribute eddy kinetic energy from mid-troposphere to the lower troposphere. In the horizontal plan, at 850 hPa, the horizontal ageostrophic fluxes (yellow arrows) tend to redistribute the eddy kinetic energy (shadings) from the north to the south of the extratropical cyclone. Hence, the ageostrophic geopotential fluxes redistribute eddy kinetic energy from the mid-troposphere to the lower troposphere on the northern side of the low center and then on the southern side in the low troposphere. These fluxes are at the origin of the formation of low-level jet to the south of the low center at the later stage of the extratropical cyclone, in the so-called frontal fracture region.