Temperature, Pressure, and Composition

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Skew-T Stability Analysis

• Notes for Lab 5

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Skew-T
Temperature
Dew Point

• Present a vertical view of the atmosphere
• Measurements taken from a balloon
• Can help assess stability

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Stability

• Stability a condition of equilibrium
• Rock pushed will return to its original position
• Instability a condition where a perturbation
  tends to grow
• Rock pushed keeps going
• Our atmosphere tends to be stable
• Need instability for storms, clouds

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Stability

• Recall. . .air that warmer is also less dense by
  the ideal gas law
• P rRT
• Warmer air is buoyant, and tends to rise
• Convection
• Unstable, because perturbed parcel keeps going
• Colder air would tend to sink
• Stable condition, because a perturbed parcel
  returns to its original position

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Rising Parcels

• Consider rising parcels to be closed from the
  environment
• Unsaturated air will change temperature at DALR
  (9.8oC/km)
• Saturated air will change temp at MALR (6.5oC/km)
• Why are these different?

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Rising Parcels
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Rising Parcels

• Consider rising parcels to be closed from the
  environment
• Unsaturated air will change temperature at DALR
  (9.8oC/km)
• Saturated air will change temp at MALR (6.5oC/km)
• Why are these different?

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Stability

• If after vertical displacement, the parcels is
  colder than the environment, it will tend to sink
  (stable)
• Parcels warmer than the environment will tend to
  rise (unstable)
• As long as the parcel remains warmer than the
  surrounding air, its rate of ascent will increase
• If a parcel will neither rise or sink after
  perturbation, it is considered neutral

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What do the lines mean?

• Isotherm line of constant temperature
• Isobar line of constant pressure
• Saturation mixing ratios
• Dry adiabats
• Moist adiabats

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Isobars

• Run horizontally from left to right, and are
  labeled on the left side of the diagram
• 10 mb spacing
• Spacing increases as we go up

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Isotherms

• Run diagonally from lower left to upper right
• 1oC
• Labeled in Fahrenheit on bottom of diagram and
  Celsius throughout
• Skewed

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Saturation Mixing Ratio Lines

• Also run diagonally from lower left to upper
  right (steeper than isotherms)
• Dashed
• Labeled at bottom of diagram

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Dry Adiabats

• Nearly straight curves running from lower right
  to upper left
• Unsaturated air parcels assumed to change
  temperature at the dry adiabatic lapse rate

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Moist Adiabats

• psuedo-adiabats
• Run from bottom of diagram and gradually curve to
  up eventually parallel to dry adiabats
• saturated air parcels assumed to change
  temperature at the moist adiabatic lapse rate

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Plotting a Sounding

• Pressure (mb) Temperature (C)
  Dewpoint (C)
• 1000 24
  18
• 900 19
  13
• 850 15
  10
• 800 11
  8
• 700 3
  -1
• 600 -6
  -10
• 500 -12
  -18
• 400 -18
  -30

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Levels

• Lifting Condensation Level (LCL)
• Level at which the air parcel becomes saturated
• Consider dry adiabatic ascent from the surface,
  but since parcel does not interact, moisture
  content (mixing ratio) remains same
• On Skew-T, follow mixing ratio from dewpoint, and
  dry adiabatic line from surface temp, LCL is
  where they intersect

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Levels

• Level of Free Convection (LFC)
• Level at which rising parcel becomes buoyant
  (warmer than its environment)
• From the LCL, consider moist adiabatic ascent
  until the parcel becomes warmer than the
  environment

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Levels

• Equilibrium Level (EL)
• First level above LFC in which parcel temperature
  colder than air temperature
• This is the level where the parcel will stop
  rising

EL
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Indices

• Lifted Index
• LI T500 observed T500 parcel
• Indicator of stability
• Values below 0 mean the air is unstable at 500
  mb. The more negative, the better
• LI over 0 stable but weak convection possible
  for LI 1-3 if strong lifting is present
• LI 0 to 3 marginally unstable
• LI 3 to 6 moderately unstable
• LI 6 to 9 very unstable
• LI below 9 extremely unstable

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Indices

• K index
• K T850 T500 Td850 (T700 Td700)
• Combines the effects of stability and moisture
• First two terms stability (ELR)
• Last three terms moisture
• Note the K-index does not deal with parcel
  temp/dewpt, only the environment conditions
• In general probability for thunderstorms gt 75
  for K gt 35
• Watch out for dry air at 700 mb. K will be
  lower, but this is a favorable condition for
  thunderstorms
• K better measure for heavy rain

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Indices

• Convective Available Potential Energy
• Energy avaliable as the air parcel rises from LFC
• Larger CAPE more buoyancy and stronger updrafts
  could result

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Indices

• Convective Inhibition
• Amount of energy needed to lift a parcel to the
  LFC
• Larger CIN more difficult to lift, and
  convection may be suppressed

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Tropical Sounding

• Would tend to be moist throughout the low to mid
  atmosphere
• Surface Temperatures higher (time of year)
• Higher tropopause
• Less nocturnal inversion

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Continental Sounding

• Typically drier, unless heavy rains
• Stronger nocturnal inversion at night (under
  light winds)
• Tropopause is higher

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Example Soundings

• Finding the tropopause
• Temperature inversions
• Dry layers
• Tropical vs. Midlatitude