Last Lab: Hail formation

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Last Lab (Hail formation)

• Anthony R. Lupo
• Atms 4310 / 7310
• Lab 13

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Last Lab (Hail formation)

• ? Hail is of course fronzen precipitation that
  forms within the updraft region of a severe
  thunderstorm.
• ? A large droplet may be carried aloft into the
  colder regions of the cloud and freeze, the
  droplet may fall again and accumulate more
  unfrozen droplets (may go up or continue down
  again). This next coating of water may freeze and
  the stone grows.
• ? The process repeats itself until the stones are
  too heavy to be supported by the updrafts.

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Last Lab (Hail formation)

• ? Thus hail size is proportional to updraft
  strength and instability
• ? Severe thunderstorm forecast . these
  thunderstorms could be accompanied by wind gusts
  of up to 80 mph and hail up to 2 in diameter!
• ? Weve already examined an algorithm for wind
  gusts.

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Last Lab (Hail formation)

• Hail formation
• Miller-Fawbush technique
• developed by Air Force Col. Miller. (of sounding
  fame).
• ? Again, like some indicies and wind gusts, this
  is an empirical relationship based on on solid
  thermodynamic principles and using thermodynamic
  quantities weve derived.

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Last Lab (Hail formation)

• Recall finding wet bulb temperature?
• ? To determine hail-size must first determine
  wet-bulb zero. (WBZ)
• -or-
• determine height above the surface of Tw 0
  C
• ? This height is assumed to be a indicator of
  the height of the 0 C level in a precipitating
  convective storm.

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Last Lab (Hail formation)

• Relationship to Hail formation
• ? Empirical evidence suggests that 90 of hail
  accurrences are associated with WBZ between 1.5
  km (850 hPa) to 4 km (roughly 600 hPa) with a
  maximum occurrence of roughly 3 km (700 hPa).
• ? If WBZ is below 850 hPa, the air is too cold or
  dry for deep convection.
• ?If above 600 hPa, air tends to be too warm and
  hail will melt.

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Last Lab (Hail formation)

• Use of CCL and Fawbush Miller charts (hail
  size)
• ? Determine the CCL as the intersection of the
  mean mixing ratio of lowest 150 hPa with the
  sounding.
• ? Then, trace the moist adiabat from the CCL up
  to the pressure level at which T sounding -5 C

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Last Lab (Hail formation)

• ? This forms a triangle bounded by the sounding,
  the moist adiabat and the pressure level.
• If the area is negative (-) stop!!
• No hail formation!
• ? If we have a () area
• the 5 C level temp difference between the
  sounding and the moist adiabat is identified as
  the triangle base (B B) Base

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Last Lab (Hail formation)

• A second dimension, the altitude
• ? is the temperature difference along the dry
  adiabat extending from the CCL to the level of
  the triangle base and passing through the
  triangle.( H H) Height
• ? Enter the Fawbush Miller chart values chart 1
  using the base and height!

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Last Lab (Hail formation)

• Determine WBZ
• ? Pick any point above zero on the sounding,
  determine the LCL and follow moist adiabat down
  to that level.
• ? This is Tw, you may have to find wet-bulb-zero
  iteratively.
• If WBZ gt 10,500 ft (aaprox. 3.2 km or 680 hPa)
  adjust size for melting using the second chart!

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Last Lab (Hail formation)

• The End!

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Last Lab (Hail formation)

• Questions?
• comments?
• Critique?