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Doug M. Smith
,
Nick J. Dunstone
,
Rosie Eade
,
David Fereday
,
Leon Hermanson
,
James M. Murphy
,
Holger Pohlmann
,
Niall Robinson
, and
Adam A. Scaife
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David M. Plummer
,
Jeffrey R. French
,
David C. Leon
,
Alan M. Blyth
,
Sonia Lasher-Trapp
,
Lindsay J. Bennett
,
David R. L. Dufton
,
Robert C. Jackson
, and
Ryan R. Neely

Abstract

Analyses of the radar-observed structure and derived rainfall statistics of warm-season convection developing columns of enhanced positive differential reflectivity Z DR over England’s southwest peninsula are presented here. Previous observations of Z DR columns in developing cumulonimbus clouds over England were rare. The observations presented herein suggest otherwise, at least in the southwesterly winds over the peninsula. The results are the most extensive of their kind in the United Kingdom; the data were collected using the National Centre for Atmospheric Science dual-polarization X-band radar (NXPol) during the Convective Precipitation Experiment (COPE). In contrast to recent studies of Z DR columns focused on deep clouds that developed in high-instability environments, the COPE measurements show relatively frequent Z DR columns in shallower clouds, many only 4–5 km deep. The presence of Z DR columns is used to infer that an active warm rain process has contributed to precipitation evolution in convection deep enough for liquid and ice growth to take place. Clouds with Z DR columns were identified objectively in three COPE deployments, with both discrete convection and clouds embedded in larger convective complexes developing columns. Positive Z DR values typically extended to 1–1.25 km above 0°C in the columns, with Z DR ≥ 1 dB sometimes extending nearly 4 km above 0°C. Values above 3 dB typically occurred in the lowest 500 m above 0°C, with coincident airborne measurements confirming the presence of supercooled raindrops. Statistical analyses indicated that the convection that produced Z DR columns was consistently associated with the larger derived rainfall rates when compared with the overall convective population sampled by the NXPol during COPE.

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Robert M. Rauber
,
David M. Plummer
,
Matthew K. Macomber
,
Andrew A. Rosenow
,
Greg M. McFarquhar
,
Brian F. Jewett
,
David Leon
,
Nathan Owens
, and
Jason M. Keeler

Abstract

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−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 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−1, while wave circulations exhibited maximum updrafts and downdrafts of ~3 m s−1. 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.

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Gabor Vali
,
Robert D. Kelly
,
Jeffrey French
,
Samuel Haimov
,
David Leon
,
Robert E. McIntosh
, and
Andrew Pazmany

Abstract

Observations were made of unbroken marine stratus off the coast of Oregon using the combined capabilities of in situ probes and a 95-GHz radar mounted on an aircraft. Reflectivity and Doppler velocity measurements were obtained in vertical and horizontal planes that extend from the flight lines. Data from three consecutive days were used to examine echo structure and microphysics characteristics. The clouds appeared horizontally homogeneous and light drizzle reached the surface in all three cases.

Radar reflectivity is dominated by drizzle drops over the lower two-thirds to four-fifths of the clouds and by cloud droplets above that. Cells with above-average drizzle concentrations exist in all cases and exhibit a large range of sizes. The cells have irregular horizontal cross sections but occur with a dominant spacing that is roughly 1.2–1.5 times the depth of the cloud layer. Doppler velocities in the vertical are downward in all but a very small fraction of the cloud volumes. The cross correlation between reflectivity and vertical Doppler velocity changes sign at or below the midpoint of the cloud, indicating that in the upper parts of the clouds above-average reflectivities are associated with smaller downward velocities. This correlation and related observations are interpreted as the combined results of upward transport of drizzle drops and of downward motion of regions diluted by entrainment. The in situ measurements support these conclusions.

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Sonia Lasher-Trapp
,
David C. Leon
,
Paul J. DeMott
,
Cecille M. Villanueva-Birriel
,
Alexandria V. Johnson
,
Daniel H. Moser
,
Colin S. Tully
, and
Wei Wu

Abstract

Three flights from the Ice in Clouds Experiment–Tropical (ICE-T) field campaign examined the onset of ice near the ascending cloud tops of tropical maritime cumuli as they cooled from 0° to −14°C. Careful quantitative analysis of ice number concentrations included manual scrutiny of particle images and corrections for possible particle-shattering artifacts. The novel use of the Wyoming Cloud Radar documented the stage of cloud development and tops relative to the aircraft sampling, complemented the manual estimates of graupel concentrations, and provided new clear evidence of graupel movement through the rime-splintering zone. Measurements of ice-nucleating particles (INPs) provided an estimate of primary initiated ice.

The data portray a dynamically complex picture of hydrometeor transport contributing to, and likely resulting from, the rime-splintering process. Hundreds per liter of supercooled raindrops ascended within the updrafts as the cloud tops reached 0°C and contributed in part to the 0.1 L−1 graupel detected soon after the cloud tops cooled to −5°C. Rime splintering could thus be initiated upon first ascent of the cloud top through that zone and arguably contributed to the 1 L−1 or more graupel observed above it. Graupel ascending/descending into, or balanced within, the rime-splintering zone were found. In wider, less isolated clouds with dying updrafts and tops near −14°C, ice particle concentrations sometimes reached 100 L−1. Future 3D numerical modeling will be required to evaluate if rime splintering alone can explain the difference of three to four orders of magnitude in the observed INPs and the graupel observed at −5°C and colder.

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Robert M. Rauber
,
Joseph Wegman
,
David M. Plummer
,
Andrew A. Rosenow
,
Melissa Peterson
,
Greg M. McFarquhar
,
Brian F. Jewett
,
David Leon
,
Patrick S. Market
,
Kevin R. Knupp
,
Jason M. Keeler
, and
Steven M. Battaglia

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.

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David C. Leon
,
Jeffrey R. French
,
Sonia Lasher-Trapp
,
Alan M. Blyth
,
Steven J. Abel
,
Susan Ballard
,
Andrew Barrett
,
Lindsay J. Bennett
,
Keith Bower
,
Barbara Brooks
,
Phil Brown
,
Cristina Charlton-Perez
,
Thomas Choularton
,
Peter Clark
,
Chris Collier
,
Jonathan Crosier
,
Zhiqiang Cui
,
Seonaid Dey
,
David Dufton
,
Chloe Eagle
,
Michael J. Flynn
,
Martin Gallagher
,
Carol Halliwell
,
Kirsty Hanley
,
Lee Hawkness-Smith
,
Yahui Huang
,
Graeme Kelly
,
Malcolm Kitchen
,
Alexei Korolev
,
Humphrey Lean
,
Zixia Liu
,
John Marsham
,
Daniel Moser
,
John Nicol
,
Emily G. Norton
,
David Plummer
,
Jeremy Price
,
Hugo Ricketts
,
Nigel Roberts
,
Phil D. Rosenberg
,
David Simonin
,
Jonathan W. Taylor
,
Robert Warren
,
Paul I. Williams
, and
Gillian Young

Abstract

The Convective Precipitation Experiment (COPE) was a joint U.K.–U.S. field campaign held during the summer of 2013 in the southwest peninsula of England, designed to study convective clouds that produce heavy rain leading to flash floods. The clouds form along convergence lines that develop regularly as a result of the topography. Major flash floods have occurred in the past, most famously at Boscastle in 2004. It has been suggested that much of the rain was produced by warm rain processes, similar to some flash floods that have occurred in the United States. The overarching goal of COPE is to improve quantitative convective precipitation forecasting by understanding the interactions of the cloud microphysics and dynamics and thereby to improve numerical weather prediction (NWP) model skill for forecasts of flash floods. Two research aircraft, the University of Wyoming King Air and the U.K. BAe 146, obtained detailed in situ and remote sensing measurements in, around, and below storms on several days. A new fast-scanning X-band dual-polarization Doppler radar made 360° volume scans over 10 elevation angles approximately every 5 min and was augmented by two Met Office C-band radars and the Chilbolton S-band radar. Detailed aerosol measurements were made on the aircraft and on the ground. This paper i) provides an overview of the COPE field campaign and the resulting dataset, ii) presents examples of heavy convective rainfall in clouds containing ice and also in relatively shallow clouds through the warm rain process alone, and iii) explains how COPE data will be used to improve high-resolution NWP models for operational use.

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