Lifting and precipitation

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Lifting and precipitation

• How do you get air to rise?
• What happens to air when it rises?
• Will condensation occur?
• How does precipitation occur?

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Why we get clouds and precipitation

• Air is cooled to saturation

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1. How do you get air to rise?

• Three mechanism
• Orographic
• Frontal
• Convective

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Orographic lifting
Air is forced up over a mountain range
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Frontal lifting
A cold front produces rapid, violent lifting
A warm front slides gently over the colder air.
Cloud formation is slow.
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Convective lifting

• Solar heating of ground surfaces followed by
  heating of the lower atmosphere
• Causes decrease in density and rising

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2. What happens when air rises?

• It gets colder

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Reasons for vertical temperature profile

• Troposphere temperature declines due to
  decreases in pressure causing decreases in the
  average kinetic energy
• Conceptually the molecules of air are moving
  around more slowly, causing temperature to
  decline
• This can be predicted mathematically

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Where q is the heat flow in or out of the
parcel cpis the specific heat of air (J kg-1
K-1) T is temperature (K) P is pressure (Pa) ? is
density (kg m-3)
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We can generally assume that there is no heat in
or out of the parcel
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Remembering the ideal gas law (P?RT or ?P/RT) ,
we can then substitute for density
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Then divide both sides by T
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The hydrostatic law says dP -? g dz or that
upward forces are balanced by gravitational forces
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remember that density is
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After substitution and elimination of units
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And finally rearrange
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Change some units and you get
This is the dry adiabatic lapse rate 10K/km.
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So when will rising and cooling really occur?

• Once a parcel is lifted (orographic, frontal,
  convective) the continued rising is determined by
  stability

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When rising wont happen

• If the parcel is very stable
• As during morning inversions
• Or for the entire Cache Valley winter
• The air at the ground is cold and dense
• When a parcel of this air is raised adiabatically
  it quickly becomes colder than the surrounding
  air and descends

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When rising will happen

• Instability
• Warm air at the bottom, cold higher up
• If a parcel is less dense than the surrounding
  air it is unstable and will rise
• The parcel is warmer than the surrounding air
• The parcel rises adiabatically
• Stays warmer than the surrounding air for a long
  time

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3. Will condensation occur?

• Possibly!
• If the parcel keeps rising and cooling long
  enough
• It will reach saturation (dew point) and
  condensation will dominate evaporation. Latent
  heat release reduces the lapse rate.
• This height is called the lifting condensation
  level or LCL

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4. How does precipitation occur?

• Water droplets continue to condense and expand
  with time.
• When condensation nuclei are present (almost
  always)
• Needs to be about 0.2 mm in diameter to form
  precipitation
• Condensation alone is very slow - usually causes
  only some forms of snow.

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Rapid precipitation formation

• Collision and coalescence
• Occurs mostly in warm clouds composed of liquid
  water

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The Bergeron Process

• Water droplets become supercooled
• Release water vapor
• Condenses onto ice crystals
• Ice crystals grow through
• The most common way for precipitation to occur

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