The 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 21 and downdrafts ranged from 22 to 26 m s 21 . 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 21 , 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 21 .

This paper presents analyses of the finescale structure of convection in the comma head of two continental winter cyclones and a 16-storm climatology analyzing the distribution of lightning within the comma head. A case study of a deep cyclone is presented illustrating how upper-tropospheric dry air associated with the dry slot can intrude over moist Gulf air, creating two zones of precipitation within the comma head: a northern zone characterized by deep stratiform clouds topped by generating cells and a southern zone marked by elevated convection. Lightning, when it occurred, originated from the elevated convection. A second case study of a cutoff low is presented to examine the relationship between lightning flashes and wintertime convection. Updrafts within convective cells in both storms approached 6–8 m s−1, and convective available potential energy in the cell environment reached approximately 50–250 J kg−1. Radar measurements obtained in convective updraft regions showed enhanced spectral width within the temperature range from −10° to −20°C, while microphysical measurements showed the simultaneous presence of graupel, ice particles, and supercooled water at the same temperatures, together supporting noninductive charging as an important charging mechanism in these storms. A climatology of lightning flashes across the comma head of 16 winter cyclones shows that lightning flashes commonly occur on the southern side of the comma head where dry-slot air is more likely to overrun lower-level moist air. Over 90% of the cloud-to-ground flashes had negative polarity, suggesting the cells were not strongly sheared aloft. About 55% of the flashes were associated with cloud-to-ground flashes while 45% were in-cloud flashes.

Data 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 21 . 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 21 , while wave circulations exhibited maximum updrafts and downdrafts of ;3 m s 21 . 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.

An analysis of the microphysical structure of elevated convection within the comma head region of two winter cyclones over the midwestern United States is presented using data from the Wyoming Cloud Radar (WCR) and microphysical probes on the NSF/NCAR C-130 aircraft during the Profiling of Winter Storms campaign. The aircraft penetrated 36 elevated convective cells at various temperatures T and distances below cloud top zd. The statistical properties of ice water content (IWC), liquid water content (LWC), ice particle concentration with diameter > 500 μm N>500, and median mass diameter Dmm, as well as particle habits within these cells were determined as functions of zd and T for active updrafts and residual stratiform regions originating from convective towers that ascended through unsaturated air. Insufficient data were available for analysis within downdrafts. For updrafts stratified by zd, distributions of IWC, N>500, and Dmm for all zd between 1000 and 4000 m proved to be statistically indistinct. These results imply that turbulence and mixing within the updrafts effectively distributed particles throughout their depths. A decrease in IWC and N>500 in the layer closest to cloud top was likely related to cloud-top entrainment. Within residual stratiform regions, decreases in IWC and N>500 and increases in Dmm were observed with depth below cloud top. These trends are consistent with particles falling and aggregating while entrainment and subsequent sublimation was occurring.

Data from airborne W-band radar, thermodynamic fields from the Weather Research and Forecasting (WRF) Model, and air parcel back trajectories from the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model are used to investigate the finescale reflectivity, vertical motion, and airmass structure of the comma head of a winter cyclone that produced 15–25 cm of snow across the U.S. Midwest on 29–30 January 2010. The comma head consisted of three vertically stacked air masses: from bottom to top, an arctic air mass of Canadian origin, a moist cloud-bearing air mass of Gulf of Mexico origin, and a drier air mass originating mostly at low altitudes over Baja California and the Mexican Plateau. The drier air mass capped the entire comma head and significantly influenced precipitation distribution and type across the storm, limiting cloud depth on the warm side, and creating instability with respect to ice-saturated ascent, cloud-top generating cells, and a seeder–feeder process on the cold side. Convective generating cells with depths of 1.5–3.0 km and vertical air velocities of 1–3 m s−1 were ubiquitous atop the cold side of the comma head. The airmass boundaries within the comma head lacked the thermal contrast commonly observed along fronts in other sectors of extratropical cyclones. The boundary between the Gulf and Canadian air masses, although quite distinct in terms of precipitation distribution, wind, and moisture, was marked by almost no horizontal thermal contrast at the time of observation. The higher-altitude airmass boundary between the Gulf of Mexico and Baja air masses also lacked thermal contrast, with the less-stable Baja air mass overriding the stable Gulf of Mexico air.