Stability in Meteorology: Lapse Rates, Adiabatic Processes, and Environmental Conditions, Lecture notes of Meteorology

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Chapter 6: StabilityChapter 6: Stability

• Concept of Stability• Lapse Rates

Lapse Rates

• Determine Stability and Stability Indices

Concept of StabilityConcept of Stability

Air Parcel Expands As It Rises…Air Parcel Expands As It Rises…

-^

Air pressure decreases with elevation.

-^

If a helium balloon 1 m inIf a helium balloon 1 m indiameter is released at sealevel, it expands as it floats

d b

f th

upward because of thepressure decrease. Theballoon would be 6.7 m in diameter as a height of 40km.

(from

The Blue Planet

)

What Happens to the Temperature?What Happens to the Temperature?What

Happens to the Temperature?

What Happens to the Temperature?^ •

Air molecules in the parcel (or the balloon) have to use theirAir molecules in the parcel (or the balloon) have to use theirkinetic energy to expand the parcel/balloon.

-^

Therefore the molecules lost energy and slow down their

-^

Therefore, the molecules lost energy and slow down theirmotions Î

The temperature of the air parcel (or balloon) decreases withl

ti^

Th

l^

t^

i^

d t

i^

th

t^

ti l

elevation. The lost energy is used to increase the potentialenergy of air molecular.Si

il^

l^

h^

h^

i^

l d

d^

h^

i l

f

•^

Similarly when the air parcel descends, the potential energy ofair molecular is converted back to kinetic energy. Î

Air temperature rises.

Diabatic ProcessDiabatic Process

•^

Involve the direct addition or removal of heat

•^

Involve

the direct addition or removal of heat

energy.

•^

Example: Air passing over a cool surface loses

•^

Example: Air passing over a cool surface losesenergy through conduction.

Dry Adiabatic Lapse RateDry Adiabatic Lapse Rate

(from

M

eteorology: Understanding the Atmosphere

)

• Air parcels that do not contain cloud (are not saturated) cool at

p^

(^

the dry adiabatic lapse rate as they rise through the atmosphere.

• Dry adiabatic lapse rate = 10°C/1km

Dry Adiabatic Lapse RateDry Adiabatic Lapse RateDry

Adiabatic Lapse Rate

Dry Adiabatic Lapse Rate

Moist Adiabatic Lapse RateMoist Adiabatic Lapse Rate

(from

Meteorology: Understanding the Atmosphere

)^

• Air parcels that get saturated as

they rise will cool at a rate

ll^

h^

h^

d^

di b

i

smaller than the dry adiabaticlapse rate due the heatingproduced by the condensation of water vapor.

• This moist adiabatic lapse rate

is not a constant but determinedis not a constant but determinedby considering the combinedeffects of expansion coolingand latent heatingand latent heating.

• In the lower troposphere, the rate is 10°C/km – 4°C/km = 6°C/km.• In the middle troposphere, the rate is 10°C/km – 2°C/km = 8°C/km.

p^

p^

• Near tropopause, the rate is 10°C/km – 0°C/km = 10°C/km.

Environmental Lapse RateEnvironmental Lapse Rate

pp

•^

The environmental lapse rate is referred to as the

p

rate at which the air temperature surrounding us(or the air parcels) would be changed if we were toclimb upward into the atmosphere.

•^

This rate varies from time to time and from place

•^

This rate varies from time to time and from placeto place.

•^

A rawinsonde’s thermometer measures theenvironmental lapse rate.

Environmental Lapse RateEnvironmental Lapse Rate

-^

The environmental (orambient) lapse rate is

f^

d

h

i^

l

referred to the verticalchange in temperaturethrough still air.through still air.

-^

The environmentallapse rate is not fixedlapse rate is not fixed.It changes from day today and from place to

y

p

place.

environmental lapse rate =0 5

° C/100m

environmental

lapse rate =0.5 C/100m

(from

Understanding Weather & Climate

)

How to Determine StabilityHow to Determine StabilityHow

to Determine Stability

How to Determine Stability

• How do we determine where the

atmosphere is unstable – under which

p

convective clouds and storms may form?

Î

A

C

th

i^

t l l

Î

A

nswer

: Compare the environmental lapse

rate with the dry/moist lapse rate

Static Stability of the AtmosphereStatic Stability of the AtmosphereStatic

Stability of the Atmosphere

Static Stability of the Atmosphere

e = environmental lapse rate

p

d = dry adiabatic lapse rate Γ

m = moist adiabatic lapse rate

• Absolutely Stable

e <

m

e <

m

• Absolutely Unstable

Γe >

d

Γe >

d

• Conditionally Unstable

Γm <

e <

d

(from

Meteorology Today)

Absolutely Unstable AtmosphereAbsolutely Unstable AtmosphereAbsolutely

Unstable Atmosphere

Absolutely Unstable Atmosphere (from

Meteorology Today)

Conditionally Unstable AtmosphereConditionally Unstable Atmosphere

y

p

y

p

(from

Meteorology Today)