The three-dimensional (3-D) kinematics of four precipitation features in a warm occlusion and a warm front are investigated using data from scanning dual-Doppler radars. The high density of measurements resolves the structure of the weak vertical motions associated with the precipitation. Kinematic fields are described with respect to the small-scale precipitation cores that made up two rainbands. The 3-D motions in this core-relative frame provide insights into relationships between the kinematic and precipitation fields. Two rainbands were situated near the centers of cyclonic vorticity and upward motion in the young, warm 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), non-divergent, 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 vertical shear of the wind acorss the rainband. The vorticity budget in the other rainband was not computed because the updrafts and the vertical shear were too weak to allow accurate computation of the budget components. The 3-D structure of the warm front and its precipitation features are also examined. Evidence is presented to support a staircase-like structure of the warm-frontal surface and significant flow of air through the front from the warm side. The cyclonic vertical vorticities within a banded and a non-banded precipitation feature were very weak. The primary source of the vertical vorticity appears to have been advected horizontally from behind the frontal zone by a strong, low-level inflow. Vortex-stretching was generally weak. Tilting terms in the vorticity budget were primarily sinks. Kinematic factors that played a role in the formation of the banded and non-banded precipitation features associated with the warm front are discussed and some generalizations made to other precipitation systems.
- Pub Date:
- Physics: Atmospheric Science