Thermodynamic Diagrams

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Thermodynamic Diagrams
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Thermodynamic Diagrams

• What are they and why do we need them?
• Need them to present visualize thermodynamic
  processes
• They are used to keep track of how high in the
  atmosphere the air parcel is (P and z) and what
  temperature it has
• There are many other atmospheric variables on the
  thermo diagram so we can keep track ofwhether
  the parcel of air is dry (or moist) or saturated,
  where clouds form (and how thick they are), how
  much precipitation may fall, how severe
  thunderstorms may get, what type of precipitation
  falls, etc.
• Another advantage we can measure energy
  associated with parcels!

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Lines on a Tephigram

• Dry Adiabats lines of constant lapse rate
  (DALR)
• Pseudo-adiabats or wet adiabats lines of
  constant lapse rate (SALR)
• Temperature in degrees Celsius
• Pressure in millibars (mb)
• Mixing ratio in g/kg
• Area between lifted parcel and environmental
  curve indicates stability

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Closer Look at the Tephi
Isotherms green lines
Deg F
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Closer Look at the Tephi
mixing ratio orange solid lines
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Closer Look at the Tephi
adiabats orange solid lines
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Closer Look at the Tephi
isobars green solid lines (curved)
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Closer Look at the Tephi
Note how SALR slope becomes Similar to DALR as w
decreases!
saturated adiabats orange solid lines (curved)
also called psuedo-adiabats
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Use of Tephigram

• To assess stability
• Stable, unstable, conditionally unstable (or
  conditional instability)
• Slope of the atmospheric profile or environmental
  lapse rate relative to SALR or DALR
• Steeper the slope (leaning more to the left with
  height), more unstable
• Remember lapse rates are negative so bigger
  number then more unstable
• A parcel will rise freely if it is warmer than
  the environment

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The Skew T Diagram

• Dry adiabats are not straight lines, P in the
  vertical is not equally spaced (log profile),
  isotherms are perpendicular to dry adiabats
• Note that all diagrams have ws (related to es)
  and ?s on them as well
• The tephigram skew T-lnP diagrams are used in
  weather offices (tephi in Canada, skew T in
  U.S.)

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Closer Look at the Skew-T
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Closer Look at the Skew-T
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Thermodynamic Diagram

• We can plot the actual atmospheric temp
  humidity vertical profiles (env lapse rates) to
  obtain thermodynamic information about the
  atmosphere (e.g. stability)
• The profiles are sometimes called soundings
• Soundings can be measured by a balloon-borne
  radiosonde or rawinsonde can also use remote
  sensing (satellite ground-based) see figure
  showing N. America UA sites
• Radiosondes measure T, P, Td
• Rawinsondes measure T, P, Td, wind (speed
  direction)

Solid Line env T lapse rate Dashed Line env Td
lapse rate
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(No Transcript)
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What is theta (potential temp)?
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What is theta (potential temp)?
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What is the wet-bulb temp at lowest level? What
is the wet-bulb potential temp?
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wet-bulb temp 8.2 C
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wet-bulb potential temp 9.0 C
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Where is the LCL? What is the equivalent
potential temp?
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LCL at 930 mb
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equivalent potential temp gt 30 C
to 1000 mb level
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Where is the LFC? Where is the CCL and what is
the convective temp?
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LFC at 840 mb (level at which parcel becomes
positively buoyant after being lifted)
Parcel becomeswarmer thanenviro at 840 mb
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2 ways to obtain CCL

• Recall that CCL height at which an air parcel,
  when sufficiently heated from below, rises and
  becomes saturated
• It is where newly forming convective cloud should
  form bases
• CCLP
• Uses sfc dew point to find CCL
• Known as parcel method since it evaluates a
  parcel starting at sfc
• Good for predicting ordinary fair-weather Cu
• CCLML
• Known as moist layer method
• Uses bottom 150 mb moisture to get CCL in a mixed
  PBL
• Good for predicting Cb base heights and
  associated energy

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CCLp just below 910 mb well below LFC follow a
mix ratio line from Td until it intersects the
ELR Follow DALR down to sfc from CCL to get
convective temp 12.0 C
Tcp 12.0 C
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CCLML just below 850 mb just below LFC Draw a
line parallel to ELR but 6 C colder than ELR
between sfc to 850 mb Moist layer is where Td
curve is to the right of your line Bisect the
moist layer (avg mix ratio within moist
layer)follow up the avg mix ratio until it
intersects the enviro temp curve Follow DALR down
to sfc from CCL to get convective temp 13.3 C
Mean wfor moist layer
Line parallelto ELR
TcML 13.3 C
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LFC CCL atsame height whenlayer is well mixed
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Using LFC CCL

• Will free convection occur today??
• Will the daytime high get up to Tc?
• Is there enough mechanical lift to get parcels
  from sfc up to LFC?
• Will the ELR change over the day and why?

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Updraft decel.
EL (CCL)
EL (LFC)
Area proportional to updraft accel
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Example of Thermodynamic Diagrams

• Level of Free Convection (LFC) level at which
  air parcels will rise freely on their own via
  natural buoyancy
• Above the LFC, the air parcels are warmer than
  the env up to some level (i.e. EL)
• Below the LFC, air parcels are mostly colder than
  the env, but not always
• Above the LFC, the buoyancy force does work on
  the parcels (positive work) that is proportional
  to the positive area
• In this positive energy layer, the parcels rise
  freely and accelerate until they reach the
  tropopause (negative area Level of Natural
  Buoyancy (LNB) or equilibrium level (EL))
• Magnitude of positive area is called the
  Convective Available Potential Energy (CAPE)
• Assuming the avg temp difference between the
  parcel and its env is 7 C and that R Rd, we get
  a CAPE 3200 J kg-1

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Example of Thermodynamic Diagrams

• We can estimate the maximum vertical velocity the
  parcel will experience (if all PE is converted to
  KE) as before to get
• Both CINE and CAPE are very useful as they
  provide information on whether or not convection
  will occur (via CINE) and how severe a storm
  might become (via CAPE)
• In the last example, the CAPE is quite high (but
  can have CAPE gt 5000 J kg-1 !)
• Thus, if the parcel makes it to the LFC, deep
  convection will occur
• However, a problem arises trying to predict
  whether severe convection will occur because the
  CINE can also be high
• A forecaster would keep a keen eye on this area
  to see if the CINE (and capping inversion)
  would decline (or break down) over time by
  warming of air at low levels (or cooling in mid
  levels) or whether upward acceleration may be
  aided by frontal/boundary lift, low level jet or
  frontogenesis

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What layers are unstable, stable or neutral?
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What layers are unstable, stable or conditionally
unstable? (slope of SALR DALR compared to ELR
at each layer)
700 - 500 stable
Sfc to 700 Cond unstable