08-Tephigrams

8/12/2019 08-Tephigrams

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ENVI 1400 : Meteorology and Forecasting 2

Tephigrams are thermodynamic

diagrams one of a range of such

diagrams developed to help in the visual

analysis of atmospheric profiles.

They have the property that equal areas

on the diagram represent equal amounts

of energy.


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Tephigram

Thermodynamic

diagram showing

the vertical structure

of the atmosphere.

Temperature (C)

Dewpoint

temperature (C)


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Temperature (C)

Pressure (mb)

Potential

Temperature (C)

or dry adiabat


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Saturated Adiabat

Saturation

mixing ratio (g kg-1)


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Potential Temperature

Much of the change in air

temperature with altitude is

due purely to the reduction in

pressure.

It is often easier to work with ameasure of temperature that

accounts for this pressure-

related change in T, allowing

us to focus on real differences

in the energy content of the

gas. The PotentialTemperature is one such

measure.

Potential temperature, (K) is

defined as the temperature a

parcel of air would have if

moved adiabatically to a

pressure level of 1000 mb.

R/Cp= 0.286 for air

Tmust be in Kelvin

CpR

PT

1000


7/19ENVI 1400 : Meteorology and Forecasting 7

Adiabatic Lifting

As a parcel of air is lifted, the

pressure decreases & the parcel

expands and cools at the dry

adiabatic lapse rate.

As the parcel cools, thesaturation mixing ratio

decreases; when it equals the

actual water vapour mixing ratio

the parcel becomes saturated

and condensation can occur.

The level at which saturation

occurs is called the lifting

condensation level.

Lifting

condensation

level

Saturation mixing ratio

equal to actual watervapour mixing ratio of parcel

Dew point

at surface



If the parcel continues to rise, it

will cool further; the saturation

mixing ratio decreases, and

more water condenses out.

Condensation releases latentheat; this offsets some of the

cooling due to lifting so that the

saturated air parcel cools at a

lower rate than dry air.

The saturated (or wet)

adiabatic lapse rateis NOT

constant, but depends upon

both the temperature and

pressure.



Stability

If adiabatic ascent of a parcel of air

results in a temperature less than

the environmental temperature at

any given level, then the air parcel

will be more dense than the

surrounding air, and will fall backtowards its original level.

Such conditions are described as

(statically) stable. Similarly a parcel

forced downward, under stable

conditions will warm adiabatically to

a temperature greater than thesurrounding air, will be less dense,

and will rise back towards its

original level.

Environmental

Lapse Rate

Dry adiabatic

ascent of surface

air parcel

Environment warmer

than lifted parcel

stable



Lifted air is warmer

than environment

unstable

Environmental

Lapse RateDry Adiabatic

Lapse Rate

If adiabatically lifted air is warmer

than the surrounding environment, it

will be less dense, and therefore

buoyant, and will continue to rise.

Such conditions are described as

statically unstable, or convective.

This is common near the surface

when heated by sunlight.



Theoretical maximum

altitude to which parcel

may overshoot

Equal areas

Equal areas on a tephigram represent equal amounts of energy. The buoyant

potential energyavailable is represented by the area between the

environmental temperature curve and the adiabatic lapse rate. As the parcel

rises, this is converted to kinetic energy. The rising parcel may overshoot its

level of neutral buoyancy by an amount that just uses up all the kinetic energy.



Absolute Stability

Adiabatic lifting (dry & wet) never

results in the air temperature

exceeding that of the environment.

Lifting can only take place if forced,

and at the expense of using energy.

This is sometimes called forcedconvectionand may occur due to

mechanical mixing of stable air in

strong winds.

Cloud is formed if air lifted above the

lifting condensation level (LCL), but

remains limited to extent of parcellifted from below.

LCL

Temperature at surface

Dew point at surface

stable


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Absolute Instability

Any adiabatic lifting results in air that

is warmer than its environment, and

thus in buoyant convection. The

buoyancy force increases at the

lifting condensation level due

warming by the release of latentheat.

Strong solar heating of the surface,

or advection over a warmer surface

often results in unstable, or

convective, conditions in the

boundary layer. Cumulus cloudsfrequently form in such conditions.

LCL



Cloud overshoots level

of neutral stability

unstable

stable


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Conditional Instability

Forced adiabatic lifting of an air

parcel through a region of static

stability such that wet adiabatic lifting

succeeds in raising the temperature

above the environmental

temperature. At this point, the parcelbecomes convectively unstable and

continues to lift under its own

buoyancy.

LCL



unstable

stable

stable


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Convective Instability

The column of air A-B has alapse rate less than the dryadiabatic lapse rate, and isthus stable.

If the column is forced to liftadiabatically, the whole

columncools. If the lower partof the column reachessaturation [A'], it starts to coolat the wet adiabatic lapse rateif this is less than the lapserate of the column A'-B, the

column becomes unstable.This type of instability mayoccur during large scale liftingup frontal surfaces or flow overmountain ranges.

A'

B

B'

ALCL


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