LOCATION AND INTERACTION OF UPPER-TROPOSPHERE AND LOWER-TROPOSPHERE ADIABATIC FRONTOGENESIS

Both upper-air and surface frontogenesis have often been depicted as p rocesses whose dynamics could be reduced to 2D balanced problems in wh ich ''self-sharpening'' configurations could be highlighted. This pape r 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 a uthors introduce a kinematic diagnostic (Q' vector) that permits the i dentification of frontogenetic areas in such complex 3D flows where cl assical, low-Rossby number balance conditions can be violated. Relatio ns and specificity with respect to frontogenetic forcing diagnostics a re discussed. First, Q' is used for surface frontogenesis, where it de scribes well the actual frontal activity, including the complex warm-f rontal 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 diagnost ics clearly distinguish between dynamics of the entrance zone of the n ortherly jet-where 2D concepts may usefully be applied-and those of th e strongly curved zone near the trough axis. Classical cyclogenetic te rms (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 t he surface far into the troposphere. This is shown to be related to a singular property of the 3D flow. Parcels undergoing frontogenesis in the northwesterly upper-air Bow are advected on top of those that were forced at the surface cold front in a southwesterly flow. The occurre nce of a feedback process between these upper-air frontogenesis proces ses and the surface ones is then investigated. Stepwise vertical profi les of horizontal diffusion are used to force local frontolysis. The r esulting upper-air frontolysis, despite its local efficiency, does not have any remote effect on the surface front, whose frontolysis in tur n has no effect on the upper-air front. The feedback process is thus n ot occurring in our simulation.