A Cold Frontal Rainband Observed during the LANDES-FRONTS 84 Experiment: Mesoscale and Small-Scale Structure Inferred from Dual-Doppler Radar Analysis.

Abstract

This paper presents the kinematic structure of a cold frontal rainband associated with a secondary cold front which moved over southwest of France on 4 June 1984. Observations were performed during the French cooperative field experiment LANDES-FRONTS 84 and data used in the present study come primarily from dual Doppler radar, upper air soundings, surface meteorological mesonetwork and satellite imagery. The mesoscale reflectivity field is characterized by a 80 km-wide rainband oriented along the surface front and normal to the vertical shear of the horizontal wind with precipitation cores (spaced 30?40 km apart) elongated in the direction of this shear. It appears that both synoptic ascent (Warm Conveyor Belt), slantwise convection along the tilted frontal zone and upright convection in the first 10 km at the leading edge of the system are present. The characteristics of the rainband are analyzed in terms of the relevant dynamical mechanisms, including (i) the frontogenetical forcing of the prefrontal boundary layer air by the low level cold flow acting at small scale as a gravity current (ii) the mesoscale organization of precipitation by the conditional symmetric instability which develops inside the Warm Conveyor Belt, (iii) the release of the convective instability in altitude within the rearward inflow along the frontal surface.