Hail: theory and observations

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Hail theory and observations

• 30 November 2006

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Supercell schematic
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Why is it important to study hail?

• Relatively rare occurrence
• Not well understood
• Consequently difficult to forecast
• Potential for destructive property damage
• Cars (personal and dealerships)
• Livestock
• Structures (windows/roofs)
• Agriculture
• Airline industry

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Hail days per year
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Large (gt 5 cm diameter) hail days per year
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Norman, OK
Houston, TX
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Hagelgefährdung in Österreich
Mäßig gefährdete Gebiete sind gelb umrandet
(praktisch gesamt Österreich exklusive dem
Hochgebirge), stärker bedrohte Regionen rot.
Besonders gefährdet sind Teile der SO-Stmk (nicht
selten mehrere Hageltage an einem Punkt), was
schwarz gekennzeichnet ist.
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What is hail?

• Frozen water which accumulates in a thunderstorm
  and eventually precipitates out
• DIFFERENT FROM WINTER PRECIP
• Hail is NOT sleet or freezing rain!
• Forms in the convective process of a storm
• Can you get hail without a thunderstorm? NO
• Size oblong to spherical
• Largest hail often takes irregular sizes
• Can be aggregates of other hail stones
  (collisions)

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How does hail form?

• Reconsider the collision-coalesence process
• Tiny ice nuclei (dust/aerosols) meet supercooled
  water droplets
• a hail embryo forms (this occurs at -15C)
• Most of these ice particles are swept up into the
  anvil part of the storm
• Embryos on the edge of the main updraft fall back
  into the supercooled cloud droplets
• Collision-coalesence process grows them into
  graupel

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How does hail form?

• Most graupel particles end up melting and falling
  as raindrops
• Some, however, go on to become hailstones
• Small hail (5mm to 2cm)
• graupel particle swept into the updraft and up
  through mesocyclone
• hailstone falls out due to its weight
• Large hail (2cm to 10cm)
• graupel particle (likely originating near the
  hook) swept into the turbulent part of the
  updraft
• spirals up through a region very rich with
  supercooled water
• grows tremendously large in this utopia of
  sorts!
• finally becomes heavy enough to fall out

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Hail location and growth process

• Largest hail is typically found adjacent to the
  main updraft
• It is heaviest and thus falls out first
• Important note hailstone makes only ONE pass
  through the updraft!
• Two growth processes wet and dry
• Wet growth supercooled water does NOT freeze on
  contact coats the hailstone
• Leads to more spherical shapes
• Dry growth supercooled water freezes on
  contact air bubbles trapped

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Hail growth in a supercell
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Typical hail formation region
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Hail detection

• Newest RADAR technology can detect presence of
  hail
• Uses polarization of different beams
• Hail reflects differently than raindrops
• Raindrops resemble hamburger buns (NOT
  classical teardrop shape!!)
• Hail is more spherical than rain
• Thus can detect presence of hail by lack of
  differential reflectivities

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Hail recap.

• Hail measurements are standardized by objects
• Dime/penny/nickel/quarter/half-dollar
• Baseball/tennis ball
• Softball
• Pictures from hail events
• 5 April 2003 Woodson, TX (video from Mon.)
• 22 June 2003 largest hail ever recorded

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An unfortunate casualty.
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How can radar detect hail?
Using conventional (horizontal polarization)
radar, hail can sometimes be detected with the
presence of a hail spike. Radar beam reaches
hail stone, is reflected to the surface, then
reflected back to the hail stone, and finally
back to the radar site. The computer algorithm
interprets this false signal as precipitation
occurring several tens of km beyond the
thunderstorm.
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How can radar detect hail?
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Examples of hail spikes, or three-body scatter
(the hail, the ground, and the hail three
bodies) on radar
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• http//blaze.ocs.ou.edu/dcheresn/Video/HailinWood
  son_4_5_03.wmv