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Cloud Development and Precipitation-I

GEOL 1350: Introduction To Meteorology

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Stability and Cloud Formation

1.1. Definitions and causes of stable and unstable atmosphereDefinitions and causes of stable and unstable atmosphere

2.2. Processes that cause instability and cloud developmentProcesses that cause instability and cloud development

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Stability of Atmosphere

• The ability of the air to return to its origin after displacement

• Depends on the thermal structure of the atmosphere

• Classified into three categories– Stable– Neutral– Unstable

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Stable

• Returns to original position afterdisplacement

(courtesy F. Remer)

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Neutral

• Remains in new position after beingdisplaced

(courtesy F. Remer)

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Unstable

• Moves farther away from its originalposition

(courtesy F. Remer)

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Stability & MovementStability & Movement

A rock, like a parcel of air, that is in stable equilibrium will return to its originalA rock, like a parcel of air, that is in stable equilibrium will return to its originalposition when pushed.position when pushed.

If the rock instead departs in the direction of the push, it was in unstableIf the rock instead departs in the direction of the push, it was in unstableequilibrium.equilibrium.

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Stability in the atmosphere

Stable Unstable Neutral

If an air parcel is displaced from its original height it can:Return to its original height - StableAccelerate upward because it is buoyant - UnstableStay at the place to which it was displaced - Neutral

An InitialPerturbation

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Stability of air parcel under the givenenvironment

• A property of the environment• Stable - displacement results in

return to initial condition• Unstable - displacement results in

further displacement from initialcondition

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• Stability strongly influence vertical motion in theatmosphere

• Without vertical motion there would be no clouds, noprecipitation, no storms - weather as we know itwould simply not exist.

There are two types of vertical motion: forced motion such as forcing air up over a hill, over

colder air, or from horizontal convergence buoyant motion (convection) in which the air rises

because it is less dense than its surroundings -stability is especially important here

Stability is important because

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• An air parcel rises in the atmosphere when it’s densityis less than its surroundings

• Since density is not measured directly, we need toconvert this criterion to something easy to measure.

• Let ρenv be the density of the environment. From the Equation of State (P=ρdRdT): ρenv = P/RdTenv

• Let ρparcel be the density of an “dry” air parcel. Thenρparcel = P/RdTparcel

• Since both the parcel and the environment at the sameheight are at the same pressure when Tparcel > Tenv ρparcel < ρenv (positive buoyancy) when Tparcel < Tenv ρparcel > ρenv (negative buoyancy)

Buoyancy

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Stability is determined by comparing parcel’stemperature with that of its environment

Simply speaking,T parcel > T env unstable

T parcel < T env stable

T parcel = T env neutral

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Lapse Rate• The rate of temperature decrease with height is known

as lapse rate, (Γ = - ΔT / Δz )• There are two kinds of lapse rates:

Environmental Lapse Rate• What you would measure with a weather balloon; rate

at which the surrounding air temperature decreasesas we climb up.

Parcel Lapse Rate• The change of temperature that an air parcel would

experience when it is displaced vertically• This is assumed to be an adiabatic process (i.e., no

heat exchange occurs across parcel boundary)

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Dry adiabatic lapse rate – the rate of temperaturedecrease of a rising unsaturated air parcel

Γd= -ΔT/ Δz = 9.8 o C km-1 ≈ 10 o C km-1

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A rising unsaturated air parcel coolsaccording to the dry adiabatic lapse rate

If this air parcel is• warmer than surrounding air it is less

dense and buoyancy accelerates theparcel upward

• colder than surrounding air it is moredense and buoyancy forces oppose therising motion

Lifting a dry parcel

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Dry Adiabatic LapseDry Adiabatic LapseRate and StabilityRate and Stability

A parcel of risingA parcel of risingdry air cools atdry air cools atapproximatelyapproximately10° C/km10° C/kmBecause theBecause theenvironmental airenvironmental aircools morecools moreslowly, the parcelslowly, the parcelof air held by theof air held by thehelicopter ishelicopter isalways cooleralways coolerand sinks back toand sinks back toits originalits originalposition.position.

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• If a rising air parcel becomes saturatedcondensation occurs• Condensation warms the air parcel dueto the release of latent heat• So, a rising saturated parcel cools lessthan unsaturated air parcel• The cooling rate for a lifted saturated airparcel is called moist adiabatic lapse rate

Lifting a moist parcel

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Moist adiabatic lapse rateDecrease of temperature with height for saturated air

Near ground under humid condition, Γs = 4 oC / km

In lower to mid troposphere, Γs = 6 - 7 oC / km

Γs always < Γd

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Moist adiabatic lapserate and stability

–A rising saturated airparcel cools accordingto the moist adiabaticlapse rate

–When theenvironmental lapserate is smaller than themoist adiabatic lapserate, the atmosphereis termed absolutelystable.

– Recall that the dryadiabatic lapse rate islarger than the moistadiabatic lapse rate,so when theenvironmental lapserate is less than themoist adiabatic lapserate, it also smallerthan the dry adiabaticlapse rate

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The atmosphere is absolutely stable whenthe environmental lapse rate is less than themoist adiabatic lapse rate, Γ < Γs

What conditions makes atmosphere stable?Warming aloft and cooling below

Air near surface may be cooled by Nighttime radiational cooling Cold air advection Air moving over a cold surface

Air aloft may be warmed by

warm air advection and subsidence warming

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AbsolutelyAbsolutelyUnstableUnstable

AtmosphereAtmosphere

AbsolutelyAbsolutelyunstableunstableconditionsconditionsindicate that aindicate that alifted parcel of air,lifted parcel of air,whether dry orwhether dry ormoist, will bemoist, will bewarmer than thewarmer than thesurroundingsurroundingenvironmental air,environmental air,and henceand hencecontinue to rise.continue to rise.

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Absolute instability

• The atmosphere is absolutely unstable ifthe environmental lapse rate exceeds thedry adiabatic lapse rates Γ > Γd

• This situation is usually short-lived Usually results from surface heating

and is confined to a shallow layer nearthe surface

Vertical mixing can eliminate it

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What makes atmosphere unstable?

Cooling aloft and warming below

The cooling of air aloft may be due to

Cold air advection

Cloud radiational cooling

The warming of the surface air may be due to Solar heating

Warming air advection Air moving over a warm surface (e.g., warm body ofwater, contributing to the lake effect of snow

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Heat as Instability TriggerHeat as Instability Trigger

As the environmental lapseAs the environmental lapserate rate steepenssteepens, it becomes, it becomesmore unstable.more unstable.

Heating air below, throughHeating air below, throughcontact with a hot surface orcontact with a hot surface orfire, will cause thefire, will cause theenvironmental lapse rate toenvironmental lapse rate tosteepensteepen..

Cooling air aloft by cloudsCooling air aloft by cloudsradiating energy or coldradiating energy or coldadvection will also increaseadvection will also increaseinstability.instability.

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ConditionallyConditionallyUnstableUnstable

The atmosphere isThe atmosphere isconditionally unstableconditionally unstablewhen the environmentalwhen the environmentallapse rate is between thelapse rate is between thedry and moist adiabaticdry and moist adiabaticlapse rates.lapse rates.

Γs < Γ < Γd

The atmosphere is stablewith respect tounsaturated air, but isunstable with respect tosaturated air

Moistadiabaticlapserate

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EnvironmentalEnvironmentaltemperaturestemperaturesdetermine stabilitydetermine stabilityfor rising parcels.for rising parcels.

The atmosphere isThe atmosphere isabsolutely unstableabsolutely unstablewhen thewhen theenvironmental lapseenvironmental lapserate is steeper thanrate is steeper thanthe dry lapse rate,the dry lapse rate,absolutely stableabsolutely stablewhen the lapse rate iswhen the lapse rate isless steep than theless steep than themoist lapse rate, andmoist lapse rate, andconditionallyconditionallyunstableunstable otherwise. otherwise.

Stability Summary

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Absolutely stable Γ < Γs

Absolutely unstable Γ > Γd

Conditionally unstable Γd >Γ > Γs

Neutral

Γ =Γd for unsaturated air

Γ =Γs for saturated air

Stability and lapse rate

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Cloud Development• Active

– occurs in unstable air– buoyancy force stimulates and sustains rising motion– air cools to saturation, cloud forms– condensation enhances buoyancy– large vertical dimension

• Passive– In stable air that suppresses vertical development– Saturation through

• Large scale lifting -fronts, lows• Forced lifting - topography• Radiative cooling

– Stratiform (layered) clouds

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Convection & CloudsConvection & CloudsPockets ofPockets ofwarm air rise aswarm air rise asthermals withthermals withinvisible waterinvisible watervapor, and atvapor, and atthe dew pointthe dew pointtemperaturetemperaturecondensationcondensationcreates thecreates thecloud base.cloud base.

Rising air fromRising air frombelow isbelow isreplaced byreplaced bysinking air fromsinking air fromabove, creatingabove, creatingareas of blueareas of bluesky.sky.

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Topography & CloudsTopography & Clouds

Winds blowing moist air toward a mountain will experience Winds blowing moist air toward a mountain will experience orographic orographic uplift to anuplift to anelevation where dew point is reached and clouds are formed.elevation where dew point is reached and clouds are formed.

When the condensed moisture falls as rainfall, the leeward side of the mountain is keptWhen the condensed moisture falls as rainfall, the leeward side of the mountain is keptin a rain shadow.in a rain shadow.

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1. Stable air tends to resist upward vertical motions, clouds forming in astable air will spread horizontally and have a stratified appearance.

2. An unstable atmosphere tends to favor vertical air currents andproduce cumuliform clouds.

3. In a conditionally unstable atmosphere, rising unsaturated air may belifted to a level where condensation begins, latent heat is released, andinstability results.

4. The atmosphere is absolutely stable when the air at the surface iseither cooler than the air aloft (an inversion), or the temperaturedifference between the warmer surface air and the air aloft is not verygreat, i.e., the environmental lapse rate is less than the moist adiabaticrate.

5. The atmosphere is absolutely unstable when the surface air is muchwarmer than the air aloft, i.e., the environmental lapse rate is greaterthan the dry adiabatic rate.

SUMMARY