FASTEX IOP 18: A very deep tropopause fold. II: Quasi-geostrophic omega diagnoses

A set of two companion papers is dedicated to the documentation of the life cycle of a very deep tropopause fold (820 hPa) during the Intensive Observ ing Period 18 of the Fronts and Atlantic Storm-Track EXperiment (FASTEX). I n this second part, diagnoses of vertical motion using a Q-vector partition ing in the natural coordinate system that follows the geostrophic wind are analysed. The partitioning allows the evaluation of vertical motions associ ated with forcing mechanisms such as confluence and diffluence, thermal adv ection by the horizontal geostrophic shear (shear advection) and curvature of the flow. The synoptic situation involves the formation of an intense up per-level jet streak when an Arctic trough and a southern ridge move into p hase with each other. Results assessed in the course of the tropopause-fold life cycle show that subsiding vertical motions associated with each of th e forcing mechanisms (confluence, shear advection and curvature) overlap on the cyclonic-shear side of the entrance region of the jet streak. It is sh own that an additional effect of shear advection over the confluence, a nec essary ingredient for the development of deep tropopause folds in two-dimen sional contexts, takes place in the present case-study. However, the forcin g mechanisms that contribute mostly to subsiding vertical motions over the warm side of the upper-level frontal zone (i.e. with a maximum frontogeneti c effect) are, in order of importance, the shear advection and the curvatur e.