FASTEX IOP 18: A very deep tropopause fold. I: Synoptic description and modelling

The life cycle of a very deep tropopause fold (820 hPa) is documented with aircraft and ship observations during the Intensive Observing Period 18 of the Fronts and Atlantic Storm-Track EXperiment (FASTEX). The initial settin g involves a coherent tropopause disturbance and an associated Arctic tropo pause fold. The confluence episode that results from the phasing up of the tropopause disturbance and a southern. ridge, ends in the formation of an i ntense jet streak, the dynamics of which are associated with the developmen t of a polar tropopause fold. A diagnostic analysis suggests that the final dramatic stratospheric intrusion is the consequence of the vertical superp osition of the Arctic and polar tropopause folds. The Mesoscale Non-Hydrost atic (Meso-NH) model is used to discuss th is hypothesis. Mixing of the pas sive stratospheric tracer within the marine boundary layer is investigated with sensitivity tests which unplug, in turn, the model physical parametriz ations. Finally, upper-level forcings associated with the development of th e tropopause fold are investigated in detail in a companion paper.