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Francois Lalaurette, Claude Fischer, and Jean-Pierre Cammas


Both upper-air and surface frontogenesis have often been depicted as processes whose dynamics could he reduced to 2D balanced problems in which “self-sharpening” configurations could be highlighted.

This paper reports on a 3D adiabatic simulation of a baroclinic wave life cycle. Great care has been devoted to the vertical resolution, allowing for a good description of both surface and upper-air frontogenesis. The authors introduce a kinematic diagnostic (Q′ vector) that permits the identification of frontogenetic areas in such complex 3D flows where classical, low-Rossby number balance conditions can be violated. Relations and specificity with respect to frontogenetic forcing diagnostics are discussed. First, Q′ is used for surface frontogenesis, where it describes well the actual frontal activity, including the complex warm-frontal seclusion process. Upper-air frontogenesis is also investigated, both in terms of this kinematic diagnostic or in terms of potential vorticity displacements on isentropic surfaces. Both types of diagnostics clearly distinguish between dynamics of the entrance zone of the northerly jet—where 2D concepts may usefully be applied—and those of the strongly curved zone near the trough axis. Classical cyclogenetic terms (stretching and tilting) as well as the separation of ageostrophic circulations in terms of natural components of the wind also lead to a clear dynamical separation.

The cold front is shown to extend from the surface far into the troposphere. This is shown to be related to a singular property of the 3D flow. Parcels undergoing frontogenegis in the northwesterly upper-air flow are advected on top of those that were forced at the surface cold front in a southwesterly flow. The occurrence of a feedback proem between these upper-air frontogenesis processes and the surface ones is then investigated. Stepwise vertical profiles of horizontal diffusion are used to force local frontolysis. The resulting upper-air frontolysis, despite its local efficiency, does not have any remote effect on the surface front, whose frontolysis in turn has no effect on the upper-air front. The feedback process is thus not occurring in our simulation.

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