A diagnostic analysis and a modeling study of the life cycle of a very deep tropopause fold (820 hPa) associated with the triggering of an explosive cold air cyclogenesis is presented. The case study if from the Fronts and Atlantic Storm Tracks EXperiment (FASTEX). The observations come from dropsondes and radiosondes launched by FASTEX aircraft and ships. Diagnoses are based on 4D-Var analyses of the french model ARPEGE. The modeling part is from the mesoscale non-hydrostatic model (Meso-NH).

The observations suggest that the final dramatic stratospheric intrusion is the consequence of the vertical superposition of the arctic and polar tropopause folds. The Meso-NH model is used to discuss this hypothesis. Upper level forcings associated with the development of the tropopause fold are investigated with diagnoses of vertical motion by using a Q-Vector partitioning in the natural coordinate system that follows the geostrophic wind. The partitioning allows to evaluate vertical motions associated with forcing mechanisms such as confluence and diffluence, thermal advection by the horizontal geostrophic shear (shear advection) and curvature of the flow. It is shown that the forcing mechanisms that contribute mostly to subsiding vertical motions over the warm side of the upper level frontal zone (i.e. with a maximum frontogenetic effect) are the shear advection and the curvature in order of growing importance. Finally, the role of the tropopause fold in the triggering of the cold air cyclogenesis is investigated. It is shown that the very first low level precursor of the cyclogenesis is a comma cloud forced by the dynamics of the tropopause fold.