Observations Suggestive of Ice Production through Secondary Processes in Convective Clouds in Southwest England Jeffrey French, David Leon, Robert Jackson, David Plummer, and Jason Sulskis, University of Wyoming Sonia Lasher-Trapp, University of Illinois -UC Alan Blyth, University of Leeds, UK funded through NSF AGS & AGS

AGU-2015; San Francisco 14 December, 2015 Overarching objective: to improve understanding and representation of convective processes that result in heavy precipitation events and could result flash-flooding COnvective Precipitation Experiment (COPE-13) Study focus area in SW England with convective clouds: Low, cool cloud bases (~ ℃ ) Relatively shallow (cloud tops to -10 to -20 ℃ ) Low CAPE (typically ~150 J kg -1 ) Maritime-coastal environment with low CDNC (few hundred cm -3 ) Convergence line influencing growth and persistence of storms For project overview – see Leon et al., 2016 (available in pre-release from Bull. Am. Soc.)

AGU-2015; San Francisco 14 December, 2015 Motivation and Objectives: Schematic illustration of microphysical pathways active in a mixed phase convective cloud Microphysics And Entrainment Dependencies Investigate competing microphysical processes to predict conditions under which processes dominate In particular, drop development through warm process and how that impacts Primary and Secondary Ice production Overall precipitation production

Data Set and Sampling Strategy AGU-2015; San Francisco 14 December, 2015 Ground-based X-band radar from 3 August Univ. Wyoming King Air Penetration at -9 C on 2 August, 500 m below cloud top Cloud measurements focus on new growth along main convective line In Situ measurements near cloud tops as they grow through 0 to -12 C level Characterize cloud microphysical and dynamical structure – bulk measures, cloud particle spectra, and vertically pointing cloud radar

AGU-2015; San Francisco 14 December, 2015 Overview of four days in COPE Four days with cloud top growth to ~ -15 C level (-25 C on Aug02) Total of ~150 penetrations between 0 and -13 C Define updraft cores: ~250 m wide with continuous updraft > 3 m s -1 (1.5 m s -1 ) and investigate measurements from within cores DayCloud BaseCloud Droplets#cores (1.5/3.0)Updrafts (peak)N >100  m range Jul 2811 – 12 C350 – 400 cm -3 10/4 – 40 pens8 – 10 m s – 300 L -1 Jul 2910 – 11 C250 – 350 cm -3 18/10 – 35 pens11 – 13 m s – 70 L -1 Aug 0210 – 11 C550 – 650 cm -3 31/18 – 45 pens m s -1 1 – 5 L -1 Aug0311 – 12 C300 – 350 cm -3 9/7 – 45 pens9 – 12 m s – 70 L -1 Statistical analyses of these data are presented in Poster A41H (Jackson et al.) on Thursday AM (Mixed Phase Clouds) In the following slides, we segregate measurements within updraft cores into 3 different levels: (a) near 0 C, (b) -4 – -9 C, (c) -9 – -13 C and examine a few cases from each.

AGU-2015; San Francisco 14 December, 2015 Aug 02 Case Largest CAPE, Highest droplet number, Strongest updrafts High droplet number impeded growth from collision- coalescence resulting in few drizzle drops at mid-levels The few drops that did form, grew rapidly in high LWC, but were also carried quickly to higher regions; or grew to sizes that fell through updraft (ZDR columns) Little evidence of ice precipitation for any clouds with tops > -14 C 1.6 mm Particle -8 C Cloud droplets with few larger hydrometeors -- mostly liquid Narrow droplet spectra (High N) Low concentration of precip Radar echo weak – indicating only some precipitation (~5 dBZ)

AGU-2015; San Francisco 14 December, 2015 July 29 Case -12 C -8 C -1 C Moderate precipitation production by 0 C level evidenced by hydrometeor spectra & radar – large liquid drops (~2.5 mm at 0 C) Stepping up through clouds OAP images indicate a gradual change of liquid to graupel of larger particles, pockets of needles, and increasing concentration of small ice at higher altitudes (colder temperatures)

AGU-2015; San Francisco 14 December, 2015 Aug 03 Case -1 C all liquid D < 1 mm -8 C only rare liquid (d<500  m) heavily rimed graupel to 2 mm irregular and columnar ice Increasing concen. of precipitation particles (w/ altitude) at all size ranges Depletion of cloud liquid water with altitude due to scavenging by precip. -12 C copious amounts of small irregular ice graupel to 3 mm NO liquid precip Much weaker updrafts than previous days Production of precip. through Collision- Coalescence produced drops < 1mm at reasonably high concentrations Rapid increase in precipitation concentration Little or no liquid precipitation at -8 C and above

AGU-2015; San Francisco 14 December, 2015 July 28 Case -2 C -6 C-10 C Drops with D<1 mm produced through warm processes at 0 C Increase in small precipitation by -6 C, mostly ice – mix of small graupel and linear ice Completely glaciated at -10 C, nearly all cloud water depleted Much greater production of ice than on any of the other days whilst having the lowest (warmest) cloud tops of the 4 days in this study. Although no evidence of ingestion of ice from higher clouds – it cannot be ruled out.

AGU-2015; San Francisco 14 December, 2015 Summary All days show development of ~1 mm or larger drops by the 0 C level within updraft cores, yet only 3 of the 4 days indicate significant ice production by the -12 C level: Aug02 had lowest concentration of raindrops and virtually no ice was produced for clouds with tops warmer than -15 C. Interestingly, however, this day also produced the largest raindrops evidenced by airborne microphysical measurements and radar ZDR analysis and these drops were lofted up to -10 C level by strong updrafts. July28 produced the greatest ice concentrations whilst having warmer cloud tops and a similar ‘input’ of precipitation and droplet distribution as other study days. Weaker updrafts allowed more time for interactions between particles within the expected Hallet-Mossop zone ultimately producing more ice by the -10 C level. Aug03 and July29 produced similar concentrations of ice precipitation, but the precipitation glaciated at different rates. Some liquid precipitation still existed at -10 C on July 29 (stronger updrafts). On Aug03 glaciation proceeded more rapidly with height and growth of larger ice particles (through accretion) produced higher concentrations of graupel with D>1 mm at -12 C, with no liquid precipitation at this level.

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