thermo_diagrams

8/7/2019 thermo_diagrams

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Thermodynamic Diagrams What are they and why do we need them?

Need them to present & visualize thermodynamicprocesses

They are used to keep track of how high in theatmosphere the air parcel is (P and z) and whattemperature it has

There are many other atmospheric variables on thethermo 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 muchprecipitation may fall, how severe thunderstorms may get,

what type of precipitation falls, etc.

Another advantage we can measure energy associatedwith parcels!


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Lines on a Tephigram

Dry Adiabats lines of constant lapse rate(DALR)

Pseudo-adiabats or wet adiabats lines ofconstant 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

Deg F


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also called

psuedo-adiabats

Note how SALR slope becomes

Similar to DALR as w decreases!


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Use of Tephigram

To assess stability Stable, unstable, conditionally unstableStable, unstable, conditionally unstable (or conditional(or conditional

instability)instability)

Slope of the atmospheric profile or

environmental lapse rate relative to SALR orDALR

Steeper the slope (leaning more to the leftwith height), more unstable

Remember lapse rates are negative so bigger numberRemember lapse rates are negative so bigger numberthen more unstablethen more unstable

A parcel will rise freely if it is warmer than theenvironment


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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 areused 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 DiagramThermodynamic Diagram

We can plot the actualatmospheric temp & humidity vertical

profiles (env lapse rates) to obtain thermodynamic informationabout the atmosphere (e.g. stability)

The profiles are sometimes called soundings

Soundings can be measured by a balloon-borne radiosonde orrawinsonde can also use remote sensing (satellite & ground-

based) see figure showing N. Ame

rica

UA sites Radiosondes: measure T, P, Td Rawinsondes: measure T, P, Td, wind (speed & direction)

WARMCOLD

T

p

Solid Line: env T lapse rate

Dashed Line: env Td lapse rate


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T = 10 ; Td = 7T = 8 ; Td = 6

T = 5 ; Td = 1

T = 2 ; Td = 0

T = - 2 ; Td = - 9

T = - 8 ; Td = - 15

T = - 15 ; Td = - 20

T = - 25 ; Td = - 32


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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 > 30 C


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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 becomes

warmer than

enviro at 840 mb


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2 ways to obtain CCL

Recall that CCL : height at which an air parcel, whensufficiently heated from below, rises and becomes

saturated

It is where newly forming convective cloud should form basesIt is where newly forming convective cloud should form bases

CCLP Uses sfc dew point to find CCLUses sfc dew point to find CCL

Known as parcel method since it evaluates a parcel starting at sfcKnown as parcel method since it evaluates a parcel starting at sfc

Good for predicting ordinary fairGood for predicting ordinary fair--weather Cuweather Cu

CCLML

Known as moist layer methodKnown as moist layer method

Uses bottom 150 mb moisture to get CCL in a mixed PBLUses bottom 150 mb moisture to get CCL in a mixed PBL

Good for predicting Cb base heights and associated energyGood for predicting Cb base heights and associated energy


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CCLpjust below 910 mb well below LFC

follow a mix ratio line from Td until it intersects the ELR

Follow DALRdown to sfc from CCL to get convective temp = 12.0 C

Tcp= 12.0 C


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CCLMLjust 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 w

for moist layer

TcML= 13.3 C

Line parallelto ELR


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LFC & CCL at

same height when

layer is well mixed


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Using LFC & CCL

Will free convection occur today??

Will the daytime high get up to Tc?Will the daytime high get up to Tc?

Is there enough mechanical lift to get parcelsIs there enough mechanical lift to get parcelsfrom sfc up to LFC?from sfc up to LFC?

Will the ELR change over the day and why?Will the ELR change over the day and why?


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EL (LFC)

EL (CCL)

Area proportional

to updraft accel

Updraft decel.


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Level of Free Convection (LFC): level at which air parcels will risefreely on their own via natural buoyancy

Above the LFC, the air parcels are warmer than the env up to somelevel (i.e. EL)

Below the LFC, air parcels are mostly colder than the env, but notalways

Above the LFC, the buoyancy force does work on the parcels (positivework) that is proportional to the positive area

In this positive energy layer, the parcels rise freely and accelerate untilthey 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 7C and that R = Rd, we get a CAPE = 3200 J kg

-1

Example of Thermodynamic Diagrams

CAPE R T T d P P

P

LFC

LNB

! ( ) ln( )/


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We can estimate the maximum vertical velocity the parcel willexperience (if all PE is converted to KE) as before to get:

Both CINE and CAPE are very useful as they provide information onwhether or not convection will occur (via CINE) and how severe astorm 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 convectionwill occur because the CINE can also be high

A forecaster would keep a keen eye on this area to see if the CINE (andcapping inversion) would decline (or break down) over time bywarming of air at low levels (or cooling in mid levels) or whetherupward acceleration may be aided by frontal/boundary lift, low level

jet or frontogenesis

Example of Thermodynamic Diagrams

v CAPE msmax ! }

2 80 1


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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)

Sfc to 700

Cond unstable

700 - 500

stable

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