Download Thermodynamic Diagram

The Thermodynamic
Diagram
Adapted by K. Droegemeier for
METR 1004 from Lectures
Developed by Dr. Frank
Gallagher III
OU School of Meteorology
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What is it?

The thermodynamic diagram, of which
there exist many types, is a chart that
allows meteorologists to easily assess,
via quantitative graphical analysis, the
stability and other properties of the
atmosphere given a vertical profile of
temperature and moisture (i.e., a
sounding).
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Stve Diagram
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Stve Diagram
to be used in
this class
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Skew-T
Log-p
Diagram
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What Can it Be Used to
Estimate?
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Cloud base and cloud top height
Expected intensity of updrafts, downdrafts, and outflow
winds
Likelihood of hail
Storm and cloud type (supercell, multicell, squall line)
Storm motion
Likelihood of turbulence
Likelihood of storm updraft rotation
3D location of clouds
Precipitation amount
High temperature
Destabilization via advection, subsidence
And many others….
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The Stuve Diagram
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Construction:
Altitude in Km or
1,000’s of feet
Pressure levels
in mb.
-400 C
Temperature
+300 C
How high is the
500 mb level?
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Stve Diagram
to be used in
this class
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Thermodynamic Diagram
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Saturation mixing ratio line (yellow):
p
T
It provides the saturation mixing
ratio associated with the dry bulb
temperature, or the mixing ratio
associated with the dew point.
The same line provides both
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Stve Diagram
to be used in
this class
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Thermodynamic Diagram
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Saturation mixing ratio line (yellow):
p
T
It provides the saturation mixing
ratio associated with the dry bulb
temperature, or the mixing ratio
associated with the dew point.
The same line provides both
What is ws at p=1000 mb and T=-100 C?
What is the RH at 1000 mb when T=240 C and Td=130 C?
If T=200 C and RH = 70%, what is Td at 1000 mb?
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Thermodynamic Diagram
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Dry adiabats (green):
p
T
Unsaturated air that rises or sinks
does so parallel to the dry adiabats.
This line simply shows the rate of
temperature decrease with height for
an unsaturated parcel.
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Stve Diagram
to be used in
this class
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Thermodynamic Diagram
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Dry adiabats (green):
p
T
Unsaturated air that rises or sinks
does so parallel to the dry adiabats.
This line simply shows the rate of
temperature decrease with height for
an unsaturated parcel.
What is the temperature of an unsaturated air parcel at 1000 mb and T=200C
if lifted to 900 mb? to 600 mb?
What will be the temperature of an unsaturated air parcel at 600 mb and
T= -200C if it sinks to 1000 mb?
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Temperature of a parcel at 1000 mb
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Tparcel = 20C
Temperature of a parcel at 1000 mb
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Tparcel = 20C
Parcel is unsaturated, so if lifted
to 600 mb, it follows parallel to
a dry adiabat (green line) – note
that the parcel goes parallel to
the NEAREST green line.
Temperature of a parcel at 1000 mb
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Tparcel = 20C
Temperature of a parcel
lifted dry adiabatically to
600 mb. Tparcel = -20C
Temperature of a parcel at 1000 mb
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Tparcel = 20C
Thermodynamic Diagram
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Moist (pseudo) adiabats (red):
p
Saturated air that rises or sinks
does so parallel to the moist adiabats.
This line simply shows the rate of
temperature decrease with height for
a saturated parcel.
T
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Stve Diagram
to be used in
this class
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Thermodynamic Diagram

Moist (pseudo) adiabats (red):
p
Saturated air that rises or sinks
does so parallel to the moist adiabats.
This line simply shows the rate of
temperature decrease with height for
a saturated parcel.
T
Problem:
(a) Moist air rising from the surface (T=12oC) will have a
temperature of _________ at 1 km. (b) If dry, the temperature will be? Why?
(a)
T = 12oC + (-6oC km-1) x (1 km) = 6oC
(b)
T = 12oC + (-10oC km-1) x (1 km) = 2oC
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Using the Thermodynamic Diagram
to Assess
Atmospheric Stability
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The Thermodynamic Diagram

We’ll use two types of thermodynamic
diagrams in this class.
– The simpler of the two is the Stve
diagram, and we’ll use this to familiarize
you with the use of such diagrams
– The more popular (in the U.S.) and more
useful is the Skew-T Log-p diagram, which
we’ll apply later.
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Stve diagram
Green
Dry Adiabats
Red
Moist Adiabats
Yellow
Saturation
Mixing
Ratio
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Thermodynamic Diagram
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Stability: To determine the stability you
must plot a sounding. A sounding is the
temperature at various heights as
measured by a balloon-borne radiosonde.
p
COLD
WARM
The sounding is also
called the environmental
lapse rate (ELR).
T
Note: We also plot dew point on the chart -- we’ll get to that later.
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Types of Stability
Unsat Sat
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Stability May Vary With Height
Stable
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Example: Dry Neutral
Neutral to Dry Processes
Unstable to Moist Processes
ELR
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Example: Moist Neutral
Stable to Dry Processes
Neutral to Moist Processes
ELR
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Example: Absolutely Unstable
Unstable to Dry Processes
Unstable to Moist Processes
ELR
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Example: Conditionally Unstable
Stable to Dry Processes
Unstable to Moist Processes
ELR
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Example: Absolutely Stable
Stable to Dry Processes
Stable to Moist Processes
ELR
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Norman
Sounding
3 February
1999
Temperature
Sounding
Dew Point
Sounding
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Definitions

Lifting Condensation Level (LCL)
– The level to which a parcel must be raised dry
adiabatically, and at constant mixing ratio, in
order to achieve saturation
– It is determined by lifting the surface dew point
upward along a mixing ratio line, and the
surface temperature upward along a dry
adiabat, until they intersect.
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Notes:
Example: LCL
Dry adiabatic
ascent from
surface
Constant
mixing ratio
RH increases
as parcel
ascends (T and
Td approach
one another;
RH is
100% at LCL
Surface Data
T = 10oC
Td = 3oC
Mixing Ratio = 5 g kg-1
Data at LCL
TLCL = 2oC
Mixing Ratio = 5 g kg-1
LCL = 900 mb
Td
T
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Definitions
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Lifting Condensation Level (LCL)
– The LCL is CLOUD BASE HEIGHT for a
parcel lifted mechanically, e.g., by a front.
Remember, it is the LIFTED OR LIFTING
condensation level.
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Notes:
Example: LCL
Dry adiabatic
ascent from
surface
Constant
mixing ratio
RH increases
as parcel
ascends (T and
Td approach
one another;
RH is
100% at LCL
Surface Data
T = 10oC
Td = 3oC
Mixing Ratio = 5 g kg-1
LCL = 900 mb
Td
T
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Definitions
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Level of Free Convection (LFC)
– The level to which a parcel must be lifted in
order for its temperature to become equal to
that of the environment.
– It is found by lifting a parcel vertically until it
becomes saturated, and then lifting it further
until the temperature of the parcel crosses the
ELR
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Example: LFC
LFC = 840 mb
Surface Data
T = 10oC
Td = 3oC
Mixing Ratio = 5 g kg-1
LCL = 900 mb
Td
T
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Definitions
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Level of Free Convection (LFC)
– Any subsequent lifting will result in the parcel
being warmer than the environment, i.e.,
instability.
– This is what “free convection” means – the
parcel will convect freely after reaching the
LFC
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Example: LFC
LFC = 840 mb
Surface Data
T = 10oC
Td = 3oC
Mixing Ratio = 5 g kg-1
LCL = 900 mb
Td
T
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Definitions
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Equilibrium Level
– A level higher than the LFC above which
the temperature of a rising parcel
becomes equal to that of the environment,
i.,e. the parcel has zero buoyancy or is in
equilibrium with the environment
– It is found by lifting a parcel until its
temperature becomes equal to the ELR
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Example: LFC and EL
EL = 580 mb
LFC = 840 mb
Surface Data
T = 10oC
Td = 3oC
Mixing Ratio = 5 g kg-1
LCL = 900 mb
Td
T
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Definitions
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Equilibrium Level
– Any subsequent lifting above the EL
leads to stability
– The EL marks the “top” of
thunderstorms, though in reality the
upward momentum of updraft air makes
thunderstorms overshoot the EL
(overshooting top)
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Example: LFC and EL
EL = 580 mb
LFC = 840 mb
Surface Data
T = 10oC
Td = 3oC
Mixing Ratio = 5 g kg-1
LCL = 900 mb
Td
T
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Definitions
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Convective Condensation Level
– The level at which convective clouds will form due to
surface heating alone.
– It is found by taking the surface dew point upward
along a mixing ratio line until it intersects the ELR.
Convective Temperature (Tc)
– The temperature required at the ground for convective
clouds to form.
– It is found by taking a parcel at the CCL downward
along a dry adiabat to the surface.
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Example: LCL, CCL, and Tc
CCL = 750 mb
LCL = 900 mb
Surface Data
T = 10oC
Td = 3oC
Mixing Ratio = 5 g kg-1
Td
T
Tc = 23oC
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Example: Positive and Negative Areas
EL = 510 mb
Positive Area
Negative
Area
LFC = 800 mb
Surface Data
T = 10oC
Td = 3oC
Mixing Ratio = 5 g kg-1
Parcel warmer
than environment!
Need to push
parcel up!!!!
LCL = 900 mb
Td
T
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CAPE
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Convective Available Potential Energy
– The “positive area” on a thermodynamic
diagram, or the area between the MALR and
ELR curves in the layer where the parcel is
warmer than the environment, is proportional to
the energy available in the atmosphere to do the
work of lifting/accelerating a parcel vertically.
– The theoretical maximum updraft of a
thunderstorm is equal to the square root of
2xCAPE
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How Can CAPE Increase?
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How Can CAPE Increase?
Hotter surface temperature
 More low-level moisture
 Cool the mid-levels
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Td
T
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W(surface) = 11 g/kg
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W(surface) = 14 g/kg
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W(surface) = 16 g/kg
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What Changes with Height as a Parcel Rises?
Below LCL
(cloud base)
T
Td
w
ws
RH
Td
T
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What Changes with Height as a Parcel Rises?
Below LCL
(cloud base)
T decreases
Td decreases
w is constant
ws decreases
RH increases
Td
T
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What Changes with Height as a Parcel Rises?
At LCL
T
w
RH
LCL = 900 mb
Td
T
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What Changes with Height as a Parcel Rises?
At LCL
T = Td
w = ws
RH = 100%
LCL = 900 mb
Td
T
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What Changes with Height as a Parcel Rises?
Above LCL
T
Td
w
ws
RH
LCL = 900 mb
Td
T
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What Changes with Height as a Parcel Rises?
Above LCL
T decreases
Td decreases
w decreases
ws decreases
RH = 100%
LCL = 900 mb
Td
T
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