Thermodynamic Diagrams

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Thermodynamic Diagrams

• EAS 211
• Spring 2005
• 03/13/05

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Purpose

• Isobaricpressure is constant.
• Isothermaltemperature is constant.
• Dry adiabaticno heat is gained or lost in
  displacement of unsaturated parcel.
• Moist adiabatic/pseudo-adiabaticno heat is lost
  or gained in displacement of saturated parcel.

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Skew T-log P

• Isobarsstraight, horizontal brown lines drawn at
  10 mb intervals.
• Isothermsstraight brown lines sloping from
  bottom left to top right, drawn at 1 degree
  Celsius intervals.
• Dry adiabatsnearly straight brown lines sloping
  from bottom right to top left, draw at 1 degree
  Celsius intervals represents the rate at which
  an unsaturated parcel would cool during ascentor
  warm during descent.
• Saturation/Moist adiabatscurved green lines from
  bottom right to top left, drawn at 2 degree
  Celsius intervals represents the rate at which a
  saturated parcel would cool during ascent or warm
  descent. They tend to diverge near the top and
  become parallel to dry adiabats.
• Saturation mixing ratio linesdashed green lines
  sloping from bottom left to top right.
• Windsplot on open circles at the mandatory
  levels on vertical black line to the right.

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Plotting

• Plot temperature and dew point (using isotherms)
  for each level reported.
• Connect the temps (solid red) and dew points
  (solid green or blue)
• Plot winds at each level on the right-hand-side

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Why use a Skew T-log P?

• Isopleths are mostly straight
• Entire sounding (including lower stratosphere)
  can be displayed
• Easier to compute stability parameters because
  adiabats are perpendicular to the isotherms.

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Things we can find on a Skew T

• Mixing ratio (w)mass of water vapor in g/kg of
  dry air
• Read the saturation mixing ratio line that
  intersects the dew point plot at the level of
  interest.
• Saturation mixing ratio (ws)amount of water
  vapor in g/kg of saturated air
• Read the saturation mixing ratio line that
  intersects the temperature plot at the level of
  interest.
• Relative Humidity (RH)identify w and ws
• RH w/ ws 100

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• Vapor Pressure (e)contribution of water vapor to
  total atmospheric pressure (in mb)
• From the dew point plot at a given level, follow
  isotherms to 622 mb read the saturation mixing
  ratio line that intersects this point.
• Saturation Vapor Pressure (es)the pressure which
  water vapor would contribute to total atmospheric
  pressure if the air were saturated.
• From the Temp plot at a given level, follow
  isotherm to 622 mb and read the saturation mixing
  ratio line through this point.

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• Lifting condensation level (LCL)the height at
  which a parcel of air becomes saturated when
  lifted adiabatically (base of stratiform clouds)
• From the dew point, follow the saturation mixing
  ratio line up from the temperature follow the
  dry adiabat up the intersection of the two is
  the LCL.
• Convective condensation level (CCL)height at
  which a parcel of air, if heated sufficiently
  from below, will rise dry adiabatically until
  saturation (base of cumuliform clouds)
• From the dew point plot, follow saturation mixing
  ratio line up until it intersects the temperature
  curve this intersection point is the CCL.
• CCL is always equal to or higher than the LCL
  (CCLLCL if the ELR DALR)

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• Convective Temperature (Tc)surface temperature
  that must be reached to start the formation of
  convective clouds due to only solar heating
• Find CCL, from CCL, follow the dry adiabat back
  down to the surface and read the intersecting
  isotherm.
• Level of free convection (LFC)height at which a
  rising parcel of air becomes warmer than the
  surrounding environment and will rise without any
  forcing (it becomes positively buoyant)
• From the LCL, follow saturation adiabat up until
  it intersects the plotted temperature, this is
  the LFC.
• Equilibrium Level (EL)height at which a rising
  parcel of air becomes colder than the environment
  (above the LFC).
• From the LFC, continue to follow the saturation
  adiabat up until it intersects the plotted
  temperature curve again. This intersection is
  the EL.
• Represents the height of the cloud tops, since
  the parcel is now negatively buoyant.

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• Negative AreaArea in which Temp of the
  environment is greater than temp of a parcel
  (parcel is negatively buoyant)
• CINConvective inhibitionnegative area between
  the surface and LFC.
• Positive AreaArea in which temp of the
  environment is less than that of a parcel (parcel
  is positively buoyant)
• CAPEConvective Available Potential
  Energypositive area b/w LFC and EL (in Midwest
  CAPE approximately 2500 J/kg is a pretty fair bet
  for severe weather)

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• Potential Temperature (?)temperature a parcel
  would have if expanded or compressed dry
  adiabatically from a given temperature and
  pressure to 1000 mb.
• Starting at a given pressure level, follow the
  dry adiabat from the temperature plot down (or
  up) to 1000 mb. Read the intersecting isotherm.
  This is ? in Celsius, add 273 to convert to
  Kelvin.
• Where
• Rd 287 J/(kgK)
• Cp 1004 J/(kgK)
• Rd/Cp 0.286

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• Wet bulb temperature (Tw)the lowest temperature
  to which a parcel can be cooled by evaporating
  water into it.
• From the dew point at a given level, draw line up
  along the saturation mixing ratio line. From
  Temp at same level, draw a line up the dry
  adiabat until it intersects the line drawn in
  step one. From this intersection, follow the
  saturation adiabat down to the original pressure
  and read the isotherm value.
• Wet bulb potential temp (?w)Tw of a parcel if
  brought moist-adiabatically to 1000 mb.
• From Tw follow saturation adiabat to 1000 mb.
  Read the isotherm value.

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• Equivalent Temperature (Te)Temperature a parcel
  would have if all its moisture were condensed out
  by a pseudo-adiabatic process and then it was
  brought dry-adiabatically back to its original
  level
• From the dewpoint at a given pressure level, draw
  a line up the saturation mixing ratio line. From
  the temp at the same level, draw a line up the
  dry-adiabat until it intersects the line drawn in
  the first step (LCL)
• From the LCL, follow the saturation adiabat up to
  a level where the saturation and dry-adiabats are
  parallel (essentially lifting a parcel until all
  its moisture is condensed out)
• From this level, follow the dry adiabat back to
  the original level and read the isotherm
  intersecting this point.

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• Equivalent potential temperature (?e)temperature
  a parcel would have if all its moisture were
  condensed out by a pseudo-adiabatic process and
  then it was brought dry adiabatically down to
  1000 mb
• Find Te, then keep following the dry adiabat down
  to 1000 mb. Read the intersecting isotherm
• Virtual Temperature(Tv)the virtual temperature
  at which a dry sample would have the same density
  as the moist sample (holding Pressure constant.
• Where Tv T(10.6w)