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Owen Hertzman and Peter V. Hobbs

Abstract

Two rainbands in a young, warm occlusion are examined using dual-Doppler radar data and supporting mesoscale measurements. The high density of measurements permits the structure of the weak vertical motions associated with the precipitation to be resolved. Kinematic fields are described with respect to the small-scale precipitation cores that made up two rainbands. The three-dimensional motions in this core-relative frame provide insights into the relationship between the kinematic and precipitation fields.

The rainbands were situated near the centers of cyclonic vorticity and upward motion in the occlusion. They were oriented perpendicular to the wind shear in the vertical and they appeared to be maintained through the interaction of small-scale (∼5 km) updrafts and band-scale (∼40 km), nondivergent, cyclonic circulations. Updrafts, forced by convergence in the boundary layer ahead of the surface occlusion and ahead of the cold air aloft, enhanced the growth of precipitation at and above the level of the band-scale cyclonic circulations. These circulations played an important role in determining the widths of the rainbands.

The primary source of vertical vorticity in one of the rainbands was the tilting of the horizontal vorticity associated with the shear in the vertical of the wind across the rainband.

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Owen Hertzman, Peter V. Hobbs, and John D. Locatelli

Abstract

The three-dimensional structure of a warm front and its precipitation features are caused using due-Doppler radar data and supporting mesoscale measurements. Evidence is presented to support a staircase-like structure of the warm-frontal surface and significant flow of air through front from the warm side.

The cyclonic vertical vorticities within both the principal banded and nonbanded precipitation features were very weak. The primary source of the vertical vorticity appears to have been advected horzontally from behind the frontal zone by a strong low-level inflow. Vortex streteching was generally weak. Tilting terms is in the vorticity budget were primarily sinks.

Kinematic factors that must have played a role in the formation of the banded and irregular precipitation feature associated with this front are discussed and some generalizations are made to other preciptations systems.

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