CH5 Stability, clouds and rain

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CH5- Stability, clouds and rain
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• Importance of stability
• 1- crucial for forecasting cloud types, heavy vs.
  light rain, severe weather (tornadoes, hail,
  strong straight line winds)
• 2- the greater the instability the stronger the
  updrafts in a Cband the taller the cloud
• 3- large Cbs are the cylinders of the engine
  for a hurricane
• 4- deep clouds make a major contribution to the
  energy balance of the atmosphere-ocean-earth
• system

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• To assess the stability of a sample of air, or a
  parcel, we need two items
• 1- how does T vary with height (z)? - sounding
• a. use a balloon, aircraft, rocket or even
  
• radiometric measurements to obtain
• b. the sounding can and does change from day
  
• to day
• 2- how does the parcel T change as it ascends or
  descends?
• a. physical relationships govern this ideal
  gas law, hydrostatic assumption, 1st law of
  thermo

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Stepped Art
Fig. 5-2, p. 113
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• To assess the stability of an unsaturated parcel
• 1- select a parcel on the sounding
• 2- move this parcel parallel to the dry adiabatic
  guide line to a new level
• 3- compare T of parcel with T of sounding, if
• TP lt TE parcel is denser and sinks back to
  original position.stable
• TP TE parcel is same density as environment,
  stays at new levelneutral
• TP gt TE parcel is less dense than environment
  and continues upwardunstable

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• A parcel that does not have water vapor changing
  vapor to liquid is considered to be dry adiabatic
• Adiabatic no energy exchanged between parcel and
  environment, no mixing of the parcel as it rises
• dry adiabatic lapse rate
• Rate of cooling (upward) or warming (downward) is
  1 C per 100 m or
• -10 C on ascent or 10 C on descent per km
• 
  (1000 m)

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The slope of the sounding determines the
stability S, U, N
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• what happens if the parcel is saturated ?
• e es, RH 100, T Tdew
• as parcel ascends it cools like before, but now
  more and more water vapor changes to liquid, this
  releases energy and warms the parcel, the result
  is that the rate of cooling is less, roughly half
  that of the dry adiabatic process
• saturated or moist adiabatic lapse rate is about
• 5 C per km

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Saturated or moist adiabatic guide line
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Fig. 5-10, p. 118
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Fig. 5-14, p. 119
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Figure 1, p. 129
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Fig. 5-17, p. 121
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• Precipitation formation
• 1- condensation vapor collects on nuclei,
  changes
• to liquid
• a. rate of growth slows greatly by .02 mm,
• would take days to grow to raindrop size
• 2- collision and coalescence droplets of varying
  size collide and some stick together, the larger
  drops fall out of the updraft
• 3-ice crystal (Bergeron) process if cloud grows
  high enough to have snowflakes then these flakes
  grow rapidly at the expense of the liquid droplets

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Large cloud droplet
Small cloud droplets
Small droplets captured in wake
(a)
(b)
Editable Text
Fig. 5-18, p. 122
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Water droplet
Ice crystal
1.
2.
3.
Temperature - 15oC
Editable Text
Fig. 5-22, p. 124
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Stepped Art
Fig. 5-22, p. 124
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Fig. 4-5, p. 83
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Ice only (glaciated) (-40oC)
7600 m (26,000 ft)
5500 m (18,000 ft)
Mixed ice and water (-20oC)
Freezing level (0oC)
Liquid water only
1000 m (3000 ft)
Fig. 5-20, p. 123
Editable Text
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• Precipitation types
• 1- drizzle drops with diameters lt 0.5 mm
• 2- rain diameter gt 0.5 mm
• 3- virga droplets evaporate before they hit the
  ground
• 4- sleet frozen raindrops
• 5- freezing rain rain that freezes after landing
• 6- graupel soft hail
• 7- hail graupel collects supercooled water
• a. hailstorms cause gt 700 mil per yr
• b. max frequency in WY and CO
• c. max size 1.7 lbs in Coffeyville, KA

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Fig. 5-30, p. 131
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• Key issue is the fall velocity of precip. types
• Type fall velocity (m/s)
• snow 1
• drizzle 1-2
• raindrops 6-9
• hail 25-50
• Once a particle gets large it needs a strong
  updraft to keep it aloft

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Fig. 5-34, p. 132
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Fig. 5-36, p. 134
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Fig. 5-35, p. 134
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Fig. 5-32, p. 132
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Figure 2, p. 133
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Fig. 5-31, p. 131
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• Cloud seeding make rain and inhibit hail
• Both schemes attempt to alter the ice process in
  precipitation formation
• To form ice in the atmosphere at T gt -40 one
  needs ice nuclei (IN), these can be rare unlike
  the plentiful CCN needed for condensation
• To increase rain add IN to get ice process
  started
• To inhibit hail add excess of IN, spread liquid
  over many small stones rather than fewer, large
  damaging ones
• USA (AgI) vs Soviet (PbI) experience

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• Summary CH5
• If a parcel
• 1-ascends, it moves to lower P, expands and cools
• 2-descends, it moves to higher P, is compressed
  and warms
• parcels, if they do not mix may follow 2 paths
• 1- dry adiabatic 10 C per km
• 2- saturated or moist adiabatic 5 C per km
• adiabatic no energy or mass mixed into or out
  of
• parcel

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• Parcels can be stable, unstable or neutral
  (S,U,N)
• to assess stability look at the slope of the
  sounding
• Cloud formation mechanisms include
• 1- orographic lifting
• 2- heating of surface air
• 3- convergence
• 4- frontal lifting
• mountain ranges have a wet windward and drier,
  warmer leeward side

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• Precipitation processes
• 1- condensation liquid forms on nuclei (CCN)
• 2- collision and coalescence (warm rain method)
• 3- ice crystal process (need supercooled water)
• supercooled water still liquid despite T lt 0 C
• Fall velocities of precip. help explain what
  conditions are needed for their formation size
  (mass) and fall speed are related

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Fig. 5-23, p. 125
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Fig. 5-19, p. 122