Ice Initiation in Supercooled Cumulus Clouds

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Ice Initiation in Supercooled Cumulus Clouds

• Bill Cotton
• Colorado State University

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Observations of high ice particle concentrations

• Rangno and Hobbs (1994, 1991) and Hobbs and
  Rangno (1985) report on aircraft observations of
  ice particle initiation in maritime and
  continental cumulus clouds. They find that ice
  particle initiation is a two-stage process.
  During stage 1, initial ice particles seem to
  originate through the freezing of larger cloud
  droplets in concentrations com-parable to that
  estimated with the formula of Meyers et al.
  (1992) derived from IN measurements. During
  stage 2, however, ice particle concentrations of
  tens to 100's per liter form in less than ten
  minutes. Such high concen-trations appear
  coincident with or soon after the formation of
  graupel particles. Supercooled raindrops or
  drizzle drops need not be present. In some of
  their cases the time-scales of high ice particle
  concentration formation is too short for the
  rime-splinter process to account for the
  observations. In other cases it might explain
  the observa-tions. We conclude that there still
  remain mysteries about ice formation in some
  clouds.

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Ice generation during evaporation of ice
particles

• Oraltay and Hallett (1989) and Dong et al. (1994)
  examined the generation of secondary particles
  during the evaporation of ice particles. At
  relative humidities below 70, as many as 30
  pieces per crystal formed. This did not occur
  with columns or solid plates. Evaporation of
  simulated graupel particles of a few millimeters
  produced several hundred ice particles under
  certain conditions. This process has not been
  further quantified in the laboratory or been
  inserted in cloud models to examine its potential
  importance. Circumstantially this process fits
  with numerous observations which suggest
  enhancements of ice crystal concentrations during
  evaporation of clouds or in heavily mixed
  regions. What needs to be determined is will
  these evaporated bits" survive long enough
  during entrainment events to serve as embryos for
  further ice crystal growth?

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Proposed Observations in cumuli

• First attempt to replicate Hobbs and Rangno
  observations
• A single well-instrumented aircraft like the UWYO
  King Air would be perfect.
• It should have LWC, PMS probes, CPI, CCN,
  GCCN(ideal), and IN(CFDC), WCR would be nice too
  but I know that it will not fit with above.
• Soundings(T, RH, winds, CCN, GCCN, IN) before and
  after cloud penetrations
• Obtain multiple penetrations at temperatures
  around -10C and colder to define lifecycle of ice
  evolution
• The study should follow a case study approach to
  enable modelers to determine if current theories
  can account for rapid ice formation

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Locations

• Begin in State of Washington east and west of
  Cascades in locations and time of year similar to
  Hobbs and Rangno to see if their observations can
  be replicated
• Do the same say in NE Colorado
• Do the same in central Texas

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Summary

• The proposed observations should provide a good
  cross section of how ice in supercooled cumuli
  evolves in different airmasses and provide case
  studies that can be used for numerical
  simulations.