FASTEX IOP17 cyclone: Introductory synoptic study with field data

This paper introduces the study of the early development phase of the IOP17 frontal cyclone of the FASTEX field experiment (Fronts and Atlantic Storm- Track EXperiment). With the support of observations from FASTEX observing p latforms a synoptic description highlights the development of mesoscale fea tures of interest in the operational Meteo-France ARPEGE (Action de Recherc he Petite Echelle Grand Echelle) analysis cycle, i.e. cloud head, dry wedge and upper-level jet outflow. A discussion attempts to isolate two signific ant differences of the present cyclone life cycle compared with previous we ll-known conceptual models of maritime cyclone life cycles: (i) the indirec t role of an upper-level precursor in triggering the surface low; (ii) the existence of a diabatic regime takings over from the baroclinic development to sustain the deepening rate of the cyclone. These two mechanisms are the subject of two companion papers.