HIGH-RESOLUTION ANALYSIS OF FRONTAL FRACTURE

A mesoscale array of dropwindsondes, released in a small rapidly deepe ning frontal wave cyclone in the eastern North Atlantic during the FRO NTS 92 experiment, has been assimilated into a 17-km-grid mesoscale mo del nested within the Meteorological Office's operational Limited Area Model. The mesoscale model reproduced the evolving cloud pattern, wit h ''cloud head'' and ''dry slot,'' seen in satellite imagery. It also revealed a well-defined evolution in the three-dimensional thermodynam ic structure associated with the process known as frontal fracture. Th e frontal fracture was revealed most clearly in the pattern of wet-bul b potential temperature theta(w), which was distorted by an effective differential rotation, the rotation increasing with height. This led t o backward-tilted theta(w) surfaces (and ana-cold-frontal characterist ics) in the cloud head to the north of the center of rotation, and to forward-tilted theta(w), surfaces (and kata-cold-frontal characteristi cs) in the dry slot to the south of the center of rotation. The effect ive differential rotation was associated with a local maximum of poten tial vorticity aloft within a developing tropopause fold.