High-Resolution Analysis of Frontal Fracture

Abstract

A mesoscale array of dropwindsondes, released in a small rapidly deepening frontal wave cyclone in the eastern North Atlantic during the FRONTS 92 experiment, has been assimilated into a 17-km-grid mesoscale model nested within the Meteorological Office?s operational Limited Area Model. The mesoscale model reproduced the evolving cloud pattern, with ?cloud head? and ?dry slot,? seen in satellite imagery. It also revealed a well-defined evolution in the three-dimensional thermodynamic structure associated with the process known as frontal fracture. The frontal fracture was revealed most clearly in the pattern of wet-bulb potential temperature ?w, which was distorted by an effective differential rotation, the rotation increasing with height. This led to backward-tilted ?w surfaces (and ana-cold-frontal characteristics) in the cloud head to the north of the center of rotation, and to forward-tilted ?w surfaces (and kata-cold-frontal characteristics) in the dry slot to the south of the center of rotation. The effective differential rotation was associated with a local maximum of potential vorticity aloft within a developing tropopause fold.