1. Thermodynamics We Can See!
Adapted from ATS 541 notes (Dr. Susan van den Heever)

2. What we have learned so far
~ Moist Adiabatic
~ Dry Adiabatic
Latent Heating ρ
Condensation

3. Cloud Types

4. Skew-T log-P

5. Skew-T log-P Techniques
LCL - Lifting Condensation Level
Level at which a lifted parcel becomes saturated (an estimate of the cloud base height).
To find the LCL, follow a dry adiabat from the temperature of a parcel, and a saturation mixing ratio line from the dew point of the parcel.
The LCL occurs where these two lines intersect (Fig. 1).
LFC - Level of Free Convection
Level at which a lifted parcel begins a free acceleration upward to the equilibrium level.
The LFC is found where the parcel temperature becomes warmer than the environmental temperature (Fig. 1).
For the LFC to be reached, sufficient forcing is required to overcome any CIN.

6. LFC
LCL
Figure 1: Skew-T Ln-P plot from Dodge City, KS at 0000 UTC on 15 September 2004

7. CCL - Convection Condensation Level
EL - Equilibrium Level
In the absence of water loading and entrainment, the parcel freely ascends to the EL.
Level at which a lifted parcel becomes cooler than the environmental temperature and is no longer positively buoyant.
The EL is the point above the LFC where the parcel path crosses the temperature trace
Sometimes referred to as “level of neutral buoyancy” (Fig. 2).
CCL - Convection Condensation Level
The height to which a parcel of air if heated from below will rise adiabatically until just saturated (height of the base of cumuliform clouds) - produced solely from surface heating
The level at which the saturation mixing ratio through the dewpoint temperature of the parcel intersects the temperature curve of the sounding (Fig. 2)

8. CT - Convection Temperature
For forecasting thunderstorms it is best to lift parcels from the forecast high temperature for the day to the CCL.
The convective temperature is the temperature to which the surface must be heated to start the formation of convective clouds through solar heating
It is indicative of the temperature to which we need to raise the surface to initiate convection without dynamical forcing.
The CT is found by following the dry adiabat through the CCL to the surface (Fig. 2).
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9. EL
LFC
CCL CT
Figure 2: Skew-T Ln-P plot from Dodge City, KS at 0000 UTC on 15 September 2004

10. CAPE and CIN Convective Available Potential Energy (CAPE)
The maximum energy available to an ascending parcel, according to parcel theory (Fig. 3)
On a thermodynamic diagram this is called positive area, and can be seen as the region between the pseudoadiabat and the environmental sounding, from the parcel's level of free convection to its level of neutral buoyancy (EL).
CAPE is a measure of instability through the depth of the atmosphere and is related to updraft strength in thunderstorms.
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11. CAPE is given by:
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12. Convective Inhibition (CIN)
The energy needed to lift an air parcel vertically and pseudoadiabatically from its originating level to its LFC (Fig. 3)
The “negative” area on a skew-T ln-P diagram
CIN must be overcome for the initiation of convection
Often arises from the presence of a capping inversion. Even though other factors may be favorable for convection, if CIN is large, deep convection will not develop.
Some CIN can be good for severe storm development because it suppresses growth of a lot of small cells permitting the explosive growth of an energetic few.
Given by:
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13. EL
CAPE
LFC
CIN
Figure 3: Skew-T Ln-P plot from Dodge City, KS at 0000 UTC on 15 September 2004

14. Thermodynamic Stability

15. Semipermanent high-pressure systems
Water world
Real world

16. Subsidence Inversion
Subsidence associated with high-pressure systems produces stable air aloft, which can trap pollutants near the surface
Covers hundreds of thousands of square kms and can persist for days
Los Angeles is at the eastern edge of the semi-permanent North Pacific ant-cyclone – frequently experiences pollutant trapping from subsidence
Short-lived anticyclones can lead to episodic pollution trapping in affected areas

17. Side view
Note how species might get mixed;
mixing relatively slow between hemispheres

18. Isentropes as a path of stratosphere – troposphere exchange
There is an isothermal
layer just above the
tropopause, which
reduces the amount of
stratification in the
lower stratosphere

19. adiabats = isentropes? Starting with the First Law: Now with
and taking
We find
(Write ln θ and differentiate)
Combining
constant Or
Lines of constant
are also lines of constant entropy.
Transformations in which S remains constant, and therefore
is constant,
are called isentropic transformation.
ISENTROPES are lines of constant , and thus constant entropy.

20. Isentropic back-trajectories
“Isentropic trajectories account for adiabatic vertical motions that air parcels may experience en route to their destinations. However, in the near-surface layer an air parcel cannot always be traced isentropically because the isentropic surface on which it is travelling may either intersect the ground or be ill-defined in an unstable boundary layer. ”
HYSPLIT:
Trace motion back in 3D to estimate the likely source region(s) of pollution, etc.
“diabatic” vs. “adiabatic” processes?

21. Courtesy Prof. Wayne Schubert

23. Holton et al., Rev. Geophys., 1995