David M. Plummer

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

AbstractThis 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 ...

Vertical Velocity and Physical Structure of Generating Cells and Convection in the Comma Head Region of Continental Winter Cyclones

AbstractThe 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−1 and downdrafts ranged from −2 to −6 m s−1. The horizontal scale of the ...

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

AbstractThis paper presents analyses of the microphysical structure of cloud-top convective generating cells at temperatures between −10° and −55°C across the comma head of 11 continental cyclones, using data collected by the W-band Wyoming Cloud Radar and in situ instrumentation aboard the National Science Foundation (NSF)/NCAR C-130. A case study of one cyclone is presented, followed by statistical analyses of the entire dataset.Ice particle number concentrations averaged 1.9 times larger inside generating cells compared to outside, and derived ice water contents and median mass diameters averaged 2.2 and 1.1 times larger in cells, respectively. Supercooled water was directly measured at temperatures between −31.4° and −11.1°C, with the median and 95th-percentile liquid water content increasing from ~0.09 to 0.12 g m−3 and 0.14 to 0.28 g m−3 over this temperature range, respectively. Liquid water was present in 26% of observations within cells and 18% of observations between cells over the same temperat...

Discrimination of Mixed- versus Ice-Phase Clouds Using Dual-Polarization Radar with Application to Detection of Aircraft Icing Regions*

Abstract Dual-polarization radar measurements and in situ measurements of supercooled liquid water and ice particles within orographic cloud systems are used to develop probabilistic criteria for identifying mixed-phase versus ice-phase regions of sub-0°C clouds. The motivation for this study is the development of quantitative criteria for identification of potential aircraft icing conditions in clouds using polarization radar. The measurements were obtained during the Mesoscale Alpine Programme (MAP) with the National Center for Atmospheric Research S-band dual-polarization Doppler radar (S-Pol) and Electra aircraft. The comparison of the radar and aircraft measurements required the development of an automated algorithm to match radar and aircraft observations in time and space. This algorithm is described, and evaluations are presented to verify its accuracy. Three polarization radar parameters, the radar reflectivity factor at horizontal polarization (ZH), the differential reflectivity (ZDR), and the s...

Finescale Radar and Airmass Structure of the Comma Head of a Continental Winter Cyclone: The Role of Three Airstreams

AbstractData 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 col...

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

AbstractThis 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 distinc...

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

AbstractData 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 circulati...

A Microphysical Analysis of Elevated Convection in the Comma Head Region of Continental Winter Cyclones

AbstractAn 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 indisti...