The Mesoscale and Microscale Structure and Organization of Clouds and Precipitation in Midlatitude Cyclones. Part XIV: Three-Dimensional Airflow and Vorticity Budget of Rainbands in a Warm Occlusion

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.