Mesoscale dynamics of cold fronts: Structures described by dropsoundings in FRONTS 87

The F Ronts 87 project was a European experiment to make mesoscale observations to deduce the dynamics of active cold fronts approaching north‐west Europe, with a minimum of orographic influences. In this paper data from dropsoundings made by the Meteorological Office's C‐130 research aircraft are analysed. The aircraft flew a pattern with 4 or 5 runs oriented approximately at right angles to the front, but with each run displaced in the along‐front direction by about 100 km. The dropsondes gave soundings to a height of approximately 7 km with a cross‐frontal spacing of 20 km at best and 60 km on average. Between 30 and 50 soundings were made in each event, over an area of 500 × 500 km 2 . These data are unique in their mesoscale resolution over a synoptic‐scale region; this was made possible by the ability to track up to 5 sondes in the air at any time. Several new aspects of cold‐front dynamics have been identified from these data; the approach here is to treat each run as an instantaneous cross‐frontal section. The discussion is centred on the hypotheses postulated before the experiment; these were intended to allow verification or refutation of aspects of models of frontal dynamics such as those embodied in semi‐geostrophic theory. A minimal and purely objective analysis has been performed on the data presented here, but the high resolution of the dropsonde observations permits evaluation of differentiated quantities, such as potential vorticity (PV), with some confidence. Issues raised here include the degree of thermal‐wind balance, the structure of conserved variables such as equivalent potential temperature and absolute momentum, the cross‐frontal circulation and the role of diabatic processes such as the evaporation, of snow, and the potential vorticity structure on the mesoscale and its interpretation. The intention is to present an integrated view of the dynamical structure of fronts in the light of theoretical rather than conceptual or airflow models.