Hurricane Cloud Physics

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Hurricane Cloud Physics

• Robert A. Black
• NOAA/AOML/HRD

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Main Research Topics

• Rainfall and the warm rain process
• Precipitation Chemistry
• Aerosols
• Ice microphysics
• Lightning
• Ice particle nucleation
• Bergeron Process precipitation growth

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How to get rain from vapor

• Hygroscopic particles (CCN)
• Absorb water from vapor to provide droplets
• Problem - This process narrows the size
  distribution

Köhler Curves
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Collision-Coalescence

• Fortunate larger droplets fall faster, collide,
  stick together to form ever larger drops
• Numerical simulations show raindrop formation in
  25-30 min.

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Bergeron Process

• Ice particles grow at the expense of
  (supercooled) cloud
• These get large enough to fall out, melt,
  produce rain

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Ice Crystal Morphology

• Ice crystal shape is governed by the temperature
  and saturation ratio.
• Above the black line, the air is supersaturated
  with respect to water
• Below it is ice saturation only.

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Where does all the ice originate?
Graupel

• Ice nucleation is very inefficient at T -15ºC
• Hallett Mossup, (1974) Provided the clue
• -3ºC T -8ºC, growing graupel ejects numerous
  ice splinters, the ice multiplication mechanism.

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Instruments

• Aircraft-mounted
• Particle Imaging Probes DMT CCP shown.
• Doppler Radar
• Analog measurement devices (T, P, RH, LWC)

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Instruments - Cont.

• Aircraft-mounted
• DMT PIP Probe (top)
• Measures precipitation 0.1 - 6.4 mm diameter
• DMT CAPS Probe (Bottom) includes aerosols and
  cloud drops 0.1 - 50µm

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LWC-100

• A Hot wire
• Cloud water
• Meter, measures
• 0 - 5 g m-3 LWC,
• Sensitive to drops
• 50 µm
• Next, a few images from hurricanes

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PMS 2D-C images 50µm Res.
1.6 mm
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PMS 2D-C images 50µm Res.
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PMS 2D-C images 50µm Res.
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PMS 2D-P images 200µm Res.
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PMS 2DG-P Images 150 µm Res.
9.6 mm
Some very large drops. (9.6 mm between white
lines)
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PMS 2DG-C images 30µm Res.
1.92 mm
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PMS 2DG-P images 150µm Res.
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PMS 2DG-C images 30µm Res.
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PMS 2DG-P images 150µm Res.
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Platform Limitations

• WP-3D maximum altitude too low (-8ºC), or about
  25K ft. (6 hrs into flight)
• Jets wont fly below 35K ft (-40ºC).
• Microphysically important -10ºC to -20ºC is
  unreachable.
• A/C safety precludes flying below 1.5 km in
  eyewall when in precipitation.
• Aircraft and instruments subject to damage from
  ice particle impacts and lightning

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What do hurricanes offer?

• Strong horizontal wind, but not usually damaging
  turbulence
• Long lifetime - often a week or more.
• Moderate updraft
• No hail or tornados
• Usually, little or no lightning

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Hurricane Allen 5 Aug. 1980

• Strong Cat-4
• First mission dedicated to ice microphysics
• First circumnavigation above melting level
• First documented eyewall replacement cycle
• Provided the evidence that killed Project
  Stormfury

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Hurricane Irene 26 Sept. 1981

• First time for a circumnavigation in convection
• Updraft maxima
• Cloud LWC
• Precip. Conc.
• Peak 2D-C 100 l-1

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What purpose for the ice?

• Willoughby et al, 1984 suggested the ice, by
  spreading out radially around the storm and
  inducing downdraft, aided the creation of
  convective rings (the eyewall, in other words)
• Microphysically, the ice saturates and stabilizes
  the storm environment, thereby suppressing other
  convection outside the eyewall.

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LIGHTNING
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Lightning Origins

• Takahashi, 1978, Saunders et al, 1991, 1992, 1996
  all showed that graupel, supercooled cloud water,
  and ice crystals are all necessary for charge
  separation.
• These researchers differ greatly on the relative
  quantities of these particles that are necessary.
  
• These are very difficult measurements to make in
  natural clouds. Hurricanes provide one good place
  to try.

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Hurricane Study

• Black Hallett, 1999
• Measured Ez and Ey
• 2-D particle imagery
• No Cloud LWC
• No cloud droplet measurements of any kind

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Lightning in Hurricanes - Cont.

• Only convective areas were charged
• Stratiform areas had little or no charge
• Transition between areas with much liquid water
  and little ice to all ice and little water were
  abrupt.

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Conceptual model
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Conclusions

• After 30 years, there are still outstanding
  questions.
• The vertical distribution of cloud water is
  unknown
• The partitioning of the condensate mass between
  precipitating and non-precipitating particles,
  graupel, rain, and snow remains hard to quantify.
• The relationship between updraft speed and all
  these particles at various altitudes is still
  uncertain
• The influence of the SSA and other aerosols on
  the convection remains to be measured.
• Lightning is a sign of stronger updraft - what is
  the relationship between flash rate, storm
  structure, and the microphysics that separates
  the charge?