Atmospheric Stability

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Atmospheric Stability
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Atmospheric Stability

• Refers to a condition of equilibrium
• Stable equilibrium if marble pushed up side of
  bowl and let go, it will return to original
  position
• Unstable equilibrium if marble on top of bowl
  receives a push, it will roll off bowl and not
  return to original position

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Atmospheric Stability

• Apply concepts to the atmosphere
• When the atmosphere is in stable equilibrium, it
  will want to return to its original position
  after being raised or lowered
• Vertical motions inhibited
• When the atmosphere is in unstable equilibrium,
  it will want to move farther away from its
  original position when raised or lowered
• Vertical motions can occur

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Atmospheric Stability

• To explore the behavior of rising/sinking air, we
  use an air parcel
• Imaginary small body of air a few meters wide
• Can expand and contract freely
• Does not break apart
• External air and heat cannot mix with the air
  inside the parcel
• Space occupied by air molecules inside parcel
  defines the air density
• Average speed of molecules directly related to
  air temperature
• Molecules colliding against parcel walls define
  the air pressure inside

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Atmospheric Stability

• A rising parcel of air expands and cools, while a
  sinking parcel of air compresses and warms
• Pressure decreases with height
• If parcel is raised to a lower pressure,
  molecules inside it will want to push walls
  outward
• Molecules use up energy, slowing down their
  average speeds and thus decreasing the
  temperature
• Opposite is true if parcel is lowered to a higher
  pressure

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Atmospheric Stability
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Atmospheric Stability

• How do we assess stability?
• Bottom line we must compare parcels
  temperature, T(p), with the temperature of the
  surrounding environment, T(e), at some altitude
• T(p) gt T(e) parcel rises (positively buoyant)
• T(p) lt T(e) parcel sinks (negatively buoyant)
• T(p) T(e) parcel does not rise or sink
  (neutrally buoyant)

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Vertical Profile of Atmospheric Temperature
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Adiabatic Processes

• Adiabatic process a process in which no heat is
  exchanged between the parcel and its surroundings
  as it expands and cools or compresses and warms
• Lapse rate the rate at which the temperature
  changes with height
• Two adiabatic lapse rates
• Dry adiabatic lapse rate (DALR)
• Moist adiabatic lapse rate (MALR)

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Absolute Stability

• The atmosphere is absolutely stable when the
  environmental lapse rate (ELR) is less than the
  MLR
• ELR lt MALR lt DALR
• A saturated OR unsaturated parcel will be cooler
  than the surrounding environment and will sink,
  if raised

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Absolute Stability

• Inversion layers are always absolutely stable
• Temperature increases with height
• Warm air above cold air very stable

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Absolute Instability

• The atmosphere is absolutely unstable when the
  ELR is greater than the DALR
• ELR gt DALR gt MALR
• An unsaturated OR saturated parcel will always be
  warmer than the surrounding environment and will
  continue to ascend, if raised

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Conditional Instability

• The atmosphere is conditionally unstable when the
  ELR is greater than the MALR but less than the
  DALR
• MALR lt ELR lt DALR
• An unsaturated parcel will be cooler and will
  sink, if raised
• A saturated parcel will be warmer and will
  continue to ascend, if raised

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Conditional Instability

• Example parcel at surface
• T(p) 30C, Td(p) 10C (unsaturated)
• ELR 8C/km for first 8km
• Parcel is forced upward, following DALR
• Parcel saturated at 2km, begins to rise at MALR
• At 4km, T(p) T(e)this is the level of free
  convection (LFC)

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Conditional Instability

• Example continued
• Now, parcel will rise on its own because T(p) gt
  T(e) after 4km
• The parcel will freely rise until T(p) T(e),
  again
• This is the equilibrium level (EL)
• In this case, this point is reached at 9km
• Thus, parcel is stable from 0 4km and unstable
  from 4 9km

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Lifting by Convection

• As the earth is heated by the sun, thermals
  (bubbles of hot air) rise upward from the surface
• The thermal cools as it rises, losing some of its
  buoyancy (its ability to rise)
• The vertical extent of the cloud is largely
  determined by the stability of the environment

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Lifting by Convection

• A deep stable layer restricts continued vertical
  growth
• A deep unstable layer will likely lead to
  development of rain-producing clouds
• These clouds are more vertically developed than
  clouds developed by convergence lifting

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Lifting by Convergence

• Convergence exists when there is a horizontal net
  inflow into a region
• When air converges along the surface, it is
  forced to rise

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Lifting by Convergence

• Large scale convergence can lift air hundreds of
  kilometers across
• Vertical motions associated with convergence are
  generally much weaker than ones due to convection
• Generally, clouds developed by convergence are
  less vertically developed

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Lifting due to Topography

• This type of lifting occurs when air is
  confronted by a sudden increase in the vertical
  topography of the Earth
• When air comes across a mountain, it is lifted up
  and over, cooling as it is rising
• The type of cloud formed is dependent upon the
  moisture content and stability of the air

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Lifting Along Frontal Boundaries

• Front The transition zone between two air
  masses of different densities.
• Lifting occurs along two different types of
  fronts.
• Cold Front
• Warm Front

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Lifting Along Cold Fronts

• A colder,denser air mass lifts the warm, moist
  air ahead of it
• As the air rises, it cools and condenses,
  producing clouds and precipitation
• The steep slope of the cold front leads to more
  vigorous rising motion
• Hence, cold fronts are often associated with
  thunderstorms

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Lifting Along Cold Fronts
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Lifting Along Warm Fronts

• A warmer, less dense air mass rises up and over
  the cold air ahead of the warm front
• Air rises, cools and condenses
• Warm fronts have gentler slopes and move slower
  than cold fronts
• Generally, precipitation is more steady and
  widespread

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Lifting Along Warm Fronts