Microphysical Properties of Convectively Generated Fall Streaks within the Stratiform Comma Head Region of Continental Winter Cyclones

This paper presents analyses of the microphysical structure of comma head stratiform precipitation in 14 continental cyclones, focusing on fall streaks of hydrometeors produced by cloud-top convective generating cells. Data were obtained at temperatures between −4° and −45°C using in situ instrumentation and the W-band University of Wyoming Cloud Radar, all operated aboard the National Science Foundation/National Center for Atmospheric Research C-130. Analyses are presented first for a case study of one cyclone, followed by statistical analyses of the full dataset. Using radar-based objective classifications, the statistical percentile number concentrations averaged 1.9 times larger within the fall streaks compared to the regions between them, and the corresponding ice water content and median mass diameter values averaged 2.2 and 1.1 times larger. Ice-phase conditions were predominant within the stratiform precipitation, with deposition and aggregation the primary ice growth mechanisms. No distinct vertical velocity signatures were associated with the fall streaks, and similar ice growth mechanisms were common within and between them. Combined with observations of cloud-top generating cells in many of the same cyclones, these analyses provide a more complete description of the comma head microphysical structure and the physical processes producing precipitation. Whereas the generating cells are critical to nucleation and initial ice growth, the majority of ice growth (exceeding 90% of the median ice water contents in the case study) typically occurred below the generating-cell level, where enhanced moisture associated with synoptic-scale ascent was present.