NATS 101 Lecture 12 Vertical Stability

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NATS 101Lecture 12Vertical Stability

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Tennis Basics
Piece of cake, right?
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Air Molecules Act Similarly
Contracting Boundary
Expanding Boundary
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Rising Air Cools-Sinking Air Warms

• Rising air parcel expands
• Expansion requires work against outside air
• Air molecules rebound from walls at a slower
  speed, resulting in a cooler temperature
• Assuming no transfer of heat across parcel walls
  (adiabatic expansion), cooling rate is 10oC/km

no heat transfer
494 m/s
502 m/s
10oC
502 m/s
494 m/s
8 m/s
1 km
Rising Expanding
Sinking Contracting
20oC
502 m/s
Ahrens, Fig 5.2
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Adiabatic Cooling-Warming
Dew point decreases with height at a rate of
2oC/km b/c DP varies less with Pressure than
Temp. The rate is much less than cooling rate for
air. Thus, unsaturated air can become saturated
IF it rises far enough.
10C 10C
20C 12C
30C 14C
Ahrens, Fig 5.2
RedTemperature BlueDew Point
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Rising, Saturated Air Cools Less

• As a saturated parcel rises and expands, the
  release of latent heat mitigates the adiabatic
  cooling
• Cooling for saturated air varies with mixing
  ratio.
• We will use an average value of 6oC/km for
  moisture lapse rate
• Note sinking clear air always warms at dry lapse
  rate

no heat transfer
14oC
497 m/s
502 m/s
latent heating
502 m/s
497 m/s
5 m/s
1 km
Rising Expanding
Sinking Contracting
20oC
502 m/s
Ahrens, Fig 5.2
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Moist Flow over a Mountain
10?C 2?C DAR
-6?C -6?C MAR
saturated
-6?C -6?C MAR
unsaturated
10?C 2?C DAR
unsaturated
-10?C -2?C DAR
10?C 2?C DAR
Ahrens, Fig 5.12
These concepts can be applied to understand Temp
and DP changes for moist flow over a mountain
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Brain Burners

• Rising and sinking unsaturated (clear) air
• Temp changes at Dry Adiabatic Rate (DAR) of
  10oC/km
• Dew point changes at rate of 2oC/km
• Rising and sinking saturated (cloudy) air
• Temp cools at Moist Adiabatic Rate (MAR) of
  6oC/km
• Dew point decreases at rate of 6oC/km

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Concept of Stability
Stable Rock always returns to starting point
Unstable Rock never returns to starting point
Conditionally Unstable Rock never returns if
rolled past top of initial hill
Ahrens, Fig 5.1
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Archimedes Principle

• Archimedes' principle is the law of buoyancy.
• It states that "any body partially or completely
  submerged in a fluid is buoyed up by a force
  equal to the weight of the fluid displaced by the
  body."
• The weight of an object acts downward, and the
  buoyant force provided by the displaced fluid
  acts upward. If the density of an object is
  greater/less than the density of water, the
  object will sink/float.
• Demo Diet vs. Regular Soda.
• http//www.onr.navy.mil/focus/blowballast/sub/work
  2.htm

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Absolutely Stable Top Rock

• Stable air strongly resists upward motion
• External force must be applied to an air parcel
  before it can rise
• Clouds that form in stable air spread out
  horizontally in layers, with flat bases-tops

Ahrens, Fig 5.3
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Absolutely Unstable Middle Rock

• Unstable air does not resist upward motion
• Clouds in unstable air stretch out vertically
• Absolute instability is limited to very thin
  layer next to ground on hot, sunny days
• Superadiabatic lapse rate

Ahrens, Fig 5.5
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Conditionally Unstable Lower Rock
Ahrens, Fig 5.7
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Environmental Lapse Rate (ELR)
ELR is the Temp change with height that is
recorded by a weather balloon
6.5o C/km
6.0o C/km
ELR is 6.5o C/km, on average, and thus is
conditionally unstable!
10.0o C/km
ELR is absolutely unstable in a thin layer just
above the ground on hot, sunny days
Ahrens, Meteorology Today 5th Ed.
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Lapse Rates and Cumulus Types
Ahrens, Meteorology Today 5th Ed.
The ELR and depth of unstable layer modulates the
type of Cu. As depth increases, the vertical
extent of Cu generally increases. As temp
difference between the air parcel and the
environment increases, the updraft speed and
severity of Cb typically increase.
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Summary Key Concepts I

• Rising unsaturated air, and all sinking air
• Temp changes at DAR of 10oC/km
• DP changes at rate of 2oC/km
• Saturation occurs with sufficient lifting
• Rising saturated air
• Latent Heating Mitigates Adia. Cooling
• Temp and DP cools at MAR of 6oC/km
• Note that MAR is always less than DAR

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Summary Key Concepts II

• Vertical Stability Determined by ELR
• Absolutely Stable and Unstable
• Conditionally Unstable
• Temp Difference between ELR and Air Parcel, and
  Depth of Layer of Conditionally Instability
  Modulates
• Vertical Extent and Severity of Cumulus

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Assignment for Next Lecture

• Topic - Precipitation Processes
• Reading - Ahrens p121-134
• Problems - 5.14, 5.16, 5.17