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

Abstract

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 248 and 2458C 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.

Dynamics of Cloud-Top Generating Cells in Winter Cyclones. Part I: Idealized Simulations in the Context of Field Observations

“…Given that Alberta clippers are the most common cyclone type in the Great Lakes region (Bowie and Weightman 1914;Isard et al 2000) and that the central pressure of the 14-15 February 2010 cyclone of 1008 hPa was near the peak in central pressure frequency in that region (Isard et al 2000), this cyclone was representative of typical continental extratropical winter cyclones. The microphysical structure of precipitation within the cyclone is discussed in Plummer et al (2014Plummer et al ( , 2015. Supercooled liquid water was not observed in the GCs between 6-and 8-km altitude in this cyclone since the temperature in their vicinity ranged from approximately 2458 to 2558C ).…”

“…A detailed discussion of the quality control and processing procedures for the WCR reflectivity and vertical radial velocity data is included in Rosenow et al (2014). Processing procedures for data from the microphysical probes are presented in Plummer et al (2014Plummer et al ( , 2015. The very high resolution of the WCR dataset (;15 m), paired with a recent upgrade of the WCR that increased sensitivity to a minimum detectable equivalent reflectivity of 225 dBZ e (Wang et al 2012), enabled very-high-resolution observations of GC structure (Rosenow et al 2014;Plummer et al 2014;Rauber et al 2014aRauber et al , 2015.…”

“…The simulated bulk cloud microphysical properties are compared with in situ observations from 14 to 15 February 2010 derived by Plummer et al (2014) in Table 3. The ice water content (IWC) derived by Plummer et al at 6.7 and 7.3 km has been converted to ice precipitation mixing ratio q i using the air density at the appropriate level in nighttime idealized simulation.…”

“…While updrafts of that magnitude are considered weak for summertime convection, in the ice-supersaturated environments common at the top of winter cyclones they can promote rapid depositional growth of ice crystals (Plummer et al , 2015. Trails of hydrometeors originating from GCs have been observed merging into bands of heavy precipitation or have been traced individually to the surface in winter cyclones (Evans et al 2005;Cunningham and Yuter 2014;Plummer et al 2014;Rauber et al 2014aRauber et al , 2015Rosenow et al 2014), demonstrating that GCs are a critical component of the precipitation process in extratropical cyclones. Accordingly, an understanding of the dynamics and thermodynamics that control their development and maintenance is needed.…”

“…In this paper, cloud-top diabatic forcing and the corresponding evolution of the vertical velocity field within generating cells are quantified using simulations representative of the comma-head region of an extratropical winter cyclone that was sampled on 14-15 February 2010, during the Profiling of Winter Storms (PLOWS) field campaign (Rosenow et al 2014;Plummer et al 2014Plummer et al , 2015. Specifically, the goal of this paper is to 1) quantify the temperature tendency at the GC level due to radiative and latent heating, 2) relate diabatic processes to upper-tropospheric stability and presence or absence of GCs, and 3) compare the structure and bulk microphysical characteristics of simulated GCs to data collected within the GCs during PLOWS.…”