The Role of Cloud-Top Generating Cells and Boundary Layer Circulations in the Finescale Radar Structure of a Winter Cyclone over the Great Lakes

Abstract

ata from airborne W-band radar are used in conjunction with thermodynamic fields from the Weather Research and Forecasting Model and air-parcel back trajectories from the HYSPLIT model to investigate the finescale reflectivity, vertical motion, and airmass structure of the comma head of a winter cyclone in the vicinity of the Great Lakes. Cloud-top generating cells formed along an upper-level frontal boundary vertically separating dry air, which 48 h earlier was located in the upper troposphere over south-central Canada, from moist air, which was located in the lower troposphere over the southeast United States. The stronger updrafts within the generating cells had vertical velocities ranging from 1 to 3 m s?1. The generating cells were important to precipitation production within the comma head. Precipitation trails formed within the generating cells could sometimes be followed to the boundary layer before merging.Boundary layer air beneath the cyclone?s comma head exhibited convective circulations and was turbulent. Gravity waves were sometimes observed at the base of the stable layer atop the convective boundary layer. Trajectory analyses showed that boundary layer air sampled by radar beneath the aircraft path had a history of crossing the Great Lakes. The magnitude of updrafts and downdrafts in the boundary layer were 1?2 m s?1, while wave circulations exhibited maximum updrafts and downdrafts of ~3 m s?1. The tops of some boundary layer convective circulations and gravity waves exhibited enhancements in radar reflectivity. The data presented illustrate the impact of the Great Lakes on cyclone mesostructure during the passage of a cyclone through the region.