Thermodynamic Diagrams. What are they and why do we need them?Need them to present
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1. Thermodynamic Diagrams
2. 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 of: whether 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!
3. 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
4. Closer Look at the Tephi
9. 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
10. 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.)
13. 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)
25. 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
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
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
29. 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?
31. 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 Example of Thermodynamic Diagrams
32. 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 > 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 Example of Thermodynamic Diagrams