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High-Resolution Analysis of Frontal Fracture

K. A. BrowningJoint Centre for Mesoscale Meteorology, University of Reading, Reading, United Kingdom

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S. P. BallardJoint Centre for Mesoscale Meteorology, University of Reading, Reading, United Kingdom

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C. S. A. DavittJoint Centre for Mesoscale Meteorology, University of Reading, Reading, United Kingdom

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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 θw, which was distorted by an effective differential rotation, the rotation increasing with height. This led to backward-tilted θw surfaces (and ana-cold-frontal characteristics) in the cloud head to the north of the center of rotation, and to forward-tilted θw 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.

* Current affiliation: Meteorological Office, Bracknell, United Kingdom.

Corresponding author address: K. A. Browning, Joint Centre for Mesoscale Meteorology, Dept. of Meteorology, University of Reading, P.O. Box 243, Reading RG6 6BB, United Kingdom.

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 θw, which was distorted by an effective differential rotation, the rotation increasing with height. This led to backward-tilted θw surfaces (and ana-cold-frontal characteristics) in the cloud head to the north of the center of rotation, and to forward-tilted θw 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.

* Current affiliation: Meteorological Office, Bracknell, United Kingdom.

Corresponding author address: K. A. Browning, Joint Centre for Mesoscale Meteorology, Dept. of Meteorology, University of Reading, P.O. Box 243, Reading RG6 6BB, United Kingdom.

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