Stability and Charging Characteristics of the Comma Head Region of Continental Winter Cyclones

Abstract

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.

Structure and Statistical Analysis of the Microphysical Properties of Generating Cells in the Comma Head Region of Continental Winter Cyclones

“…Additionally, the PLOWS measurements included observations of SLW occurring in GC regions over a larger range of temperatures than in the past studies. The measured LWCs were similar to those observed by Houze et al (1981), Rauber and Grant (1986), and Ikeda et al (2007) but larger than those observed by Evans et al (2005) in newly formed convective cells. The difference is attributed to the fact that a much larger range of cell lifetimes was sampled during PLOWS.…”

“…The presence of SLW in the GCs is important because of its major impacts on both primary ice nucleation and particle growth processes (e.g., Rauber and Tokay 1991;Westbrook and Illingworth 2011). Supercooled water's production enhances primary ice nucleation by allowing multiple nucleation processes (deposition, condensation, and contact) to take place, and its consistent presence in the PLOWS dataset implies the production of considerable ice supersaturations over a large temperature range.…”

“…Liquid water has been measured at cloud top in a variety of environments in past studies [e.g., Rauber and Grant (1986) in orographic clouds; McFarquhar et al (2011) in Arctic stratocumulus] and has major impacts on both ice nucleation and growth. Rauber and Tokay (1991) provide a theoretical basis for why SLW can exist in the cloud-top region. In the PLOWS dataset, the GCs occurring at higher temperatures atop shallow precipitation commonly had SLW present.…”

A Polarimetric and Microphysical Investigation of the Northeast Blizzard of 8–9 February 2013

“…Depolarization streaks occurred in the southern comma-head region of the synoptic-scale low pressure system from 2100 through 0300 UTC 9 February 2013, and were most frequent and pronounced during the 2300 UTC hour, when Z H exceeded 55 dBZ near the surface. The location of these streaks within the system supports the work by Rauber et al (2014), who documented a climatology of electrification in the comma heads of 16 continental winter cyclones. They found that lightning originated in elevated convective cells within the southern region of the comma head, due to advancement of upper-tropospheric dry air within the dry slot over the low-level moist air.…”

Influence of Terrain Resolution on Banded Convection in the Lee of the Rocky Mountains

“…These banded precipitation structures occur in a variety of different synoptic environments. For example, various types of precipitation bands have been observed within extratropical cyclones (e.g., Hobbs 1978;Houze and Hobbs 1982;Novak et al 2004Novak et al , 2006Novak et al , 2008Novak et al , 2009Novak et al , 2010Rauber et al 2014). Snowbands have been observed in association with mesoscale gravity waves (e.g., Bosart et al 1998;Koch and Siedlarz 1999).…”