Vertical Velocity and Physical Structure of Generating Cells and Convection in the Comma Head Region of Continental Winter Cyclones

Abstract

he vertical motion and physical structure of elevated convection and generating cells within the comma heads of three continental winter cyclones are investigated using the Wyoming W-band cloud radar mounted on the National Science Foundation/National Center for Atmospheric Research (NSF/NCAR) C-130, supplemented by analyses from the Rapid Update Cycle model and Weather Surveillance Radar-1988 Doppler (WSR-88D) data. The cyclones followed three distinct archetypical tracks and were typical of those producing winter weather in the midwestern United States. In two of the cyclones, dry air in the middle and upper troposphere behind the Pacific cold front intruded over moist Gulf of Mexico air at lower altitudes within the comma head, separating the comma head into two zones. Elevated convection in the southern zone extended from the cold-frontal surface to the tropopause. The stronger convective updrafts ranged from 2 to 7 m s?1 and downdrafts ranged from ?2 to ?6 m s?1. The horizontal scale of the convective cells was approximately 5 km. The poleward zone of the comma head was characterized by deep stratiform clouds topped by cloud-top generating cells that reached the tropopause. Updrafts and downdrafts within the generating cells ranged from 1 to 2 m s?1, with the horizontal scale of the cells from about 1 to 2 km. Precipitation on the poleward side of the comma head conformed to a seeder?feeder process?the generating cells seeding the stratiform cloud?which was forced by synoptic-scale ascent. In one case, shallow clouds behind the cyclone?s cold front were also topped by cloud-top generating cells, with vertical motions ranging from 1 to 2 m s?1.