Air Parcel

1
Air Parcel

• Poisson equation

2
Air Parcel

• For an air parcel, going though dry adiabatic
  process,

• Three lapse rates

• (Environmental) lapse rate
• Dry adibatic lapse rate
• Moist adiabatic lapse rate

3
Dry Static Stability

• Due to restoring force after displacement in a
  stratified stable atmosphere.
• Starting from vertical momentum equation

• Displacement

Environment
environment
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Dry Static Stability

• In the air parcel

P adjusted to the environment immediately

• Assume environment is hydrostatic balance

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Dry Static Stability

• Back to the RHS of the momentum equation.
• In the air parcel,

• From ideal gas law and Poisson equation, one can
  get

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Dry Static Stability

• Therefore

• In troposphere, the average of N is about 1.2 X
  10-2 s-1. So, the period
• implying that the parcel oscillating one
  cycle needs about 10 min.

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Dry Static Stability

• So

the atmosphere is statically stable
the atmosphere is statically neutral
the atmosphere is statically unstable

• This is for dry atmosphere. The moist atmosphere
  will be more sophisticated!

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Sounding
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Instability
unstable
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Potential Instability

• (Also called convective instability, thermal
  instability.) The state of an unsaturated layer
  or column of air in the atmosphere with a
  wet-bulb potential temperature (or equivalent
  potential temperature) that decreases with
  elevation.
• If such a column is lifted bodily until
  completely saturated, it will become unstable
  (i.e., its temperature lapse rate will exceed the
  saturation-adiabatic lapse rate) regardless of
  its initial stratification.
• From AMS

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Potential Instability
Unstable
Absolutely Stable
Air Layer dp
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Instability
13
Inversion

• Subsidence inversion
• Radiative inversion
• Mixed layer inversion
• Frontal inversion

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Subsidence Inversion
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Subsidence Inversion
Highs
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Subsidence Inversion
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Mixed Layer Inversion

• Shear stability

When there is no wind shear,
When there is shear
Ri lt 0.25 Shear instability (unstble)
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Mixed Layer Inversion
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Mixed Layer Inversion
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Radiative Inversion
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Frontal Inversion
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Frontal Inversion
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Frontal Inversion
OUN
LCH
GGG
DOC
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Frontal Inversion
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Frontal Inversion
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Frontal Inversion
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Frontal Inversion