Meteo 3: Chapter 8Meteo 3: Chapter 8

Stability and Cloud TypesStability and Cloud Types

Read Chapter 8Read Chapter 8

Parcel Theory Parcel Theory

Parcel:Parcel: volume of air assumed to behave volume of air assumed to behave separately from the air surrounding itseparately from the air surrounding it

Environment:Environment: everything outside the air parcel everything outside the air parcel

Hydrostatic Balance means small vertical Hydrostatic Balance means small vertical motionsmotions

Hydrostatic Balance can be violated on small space Hydrostatic Balance can be violated on small space and time scales…requires instability (buoyancy)and time scales…requires instability (buoyancy)

Role of Stability in Cloud FormationRole of Stability in Cloud Formation

We already have relationships to determine We already have relationships to determine if net condensation is occurring (e.g. LCL, if net condensation is occurring (e.g. LCL, vapor pressure vs. saturation vapor vapor pressure vs. saturation vapor pressure)pressure)

If we can determine how air moves If we can determine how air moves vertically, we can determine whether clouds vertically, we can determine whether clouds are likely to form and how deep they might are likely to form and how deep they might be!be!

Concept of BuoyancyConcept of BuoyancyThe concept of buoyancy

Archimedes Principle: the (upward-directed) buoyancy force is equal to theweight of the fluid displaced by the object—the result is that objects float in fluidsthat they are less dense (lighter) than

Archimedes Principle also can be extended to the atmosphere—an air parcelrises if it is less dense (warmer) than its surroundings, known as being positivelybuoyant [and sinks if it is denser (colder) than its surroundings, negativelybuoyant].

Lifting a Parcel of AirLifting a Parcel of Air

As an air parcel rises, itencounters lower ambientpressures, and thus itexpands and cools.

If the parcel is warmer thanthe environment (as in thefigure to the right), it willcontinue to rise (positivelybuoyant)

If the parcel becomescooler than environment, itsinks (negatively buoyant)

A rising warm bubble

How high a parcel can rise depends on the rate at which the parcel coolscompared to the rate at which the ambient environment cools. (Rates oftemperature change with height are called lapse rates.)

Unstable equilibrium: TUnstable equilibrium: Tparcelparcel > T > Tenvironmentenvironment

Stable equilibrium: TStable equilibrium: Tparcelparcel < T < Tenvironmentenvironment

Neutral equilibriumNeutral equilibrium

A parcel is given a push upward and comes A parcel is given a push upward and comes to a stop after a short rise to a stop after a short rise

(T(Tparcelparcel = T = Tenvironmentenvironment))

The key to determining stability is The key to determining stability is comparing a parcel’s temperature to the comparing a parcel’s temperature to the atmospheric temperature at the altitude atmospheric temperature at the altitude of interestof interest

Dry Adiabatic Lapse Rate (DALR)Dry Adiabatic Lapse Rate (DALR)Dry Adiabatic Lapse Rate

As long as a rising parcel isunsaturated, the rising parcelcools at ~10°C/km (5.5°F/1000ft)a quantity known as the dryadiabatic lapse rate

(Lapse Rate= Rate of temperature decrease withheight)

More on DALRMore on DALR

An unsaturated sinking air parcel warms at An unsaturated sinking air parcel warms at 10°C/km10°C/km

These temperature changes result from molecules These temperature changes result from molecules in the parcel doing work for parcel expansion in the parcel doing work for parcel expansion when rising and gaining kinetic energy via when rising and gaining kinetic energy via compression when sinkingcompression when sinking

Adiabatic:Adiabatic: no exchange of heat energy between a no exchange of heat energy between a parcel and its surroundingsparcel and its surroundings

Even moreEven more

Environmental lapse rate is what is measured by a Environmental lapse rate is what is measured by a weather balloonweather balloon– Determined locally by height above ground and Determined locally by height above ground and

temperature advection, among other thingstemperature advection, among other things

On average, is ~6.5°C/kmOn average, is ~6.5°C/km If temperature decreases with height, lapse rates If temperature decreases with height, lapse rates

are positiveare positive If lapse rate is negative (temperature increases If lapse rate is negative (temperature increases

with height), an inversion is presentwith height), an inversion is present

Moist Adiabatic Lapse Rate (MALR)Moist Adiabatic Lapse Rate (MALR)Moist Adiabatic Lapse Rate

Once condensationbegins, only approx6 degrees Celsius ofcooling per 1 km ofascent occurs (thisis the moistadiabatic lapse rate)

(the MALR is less thanDALR because of latentheating due tocondensation offsettingsome of the cooling due toexpansion)

Importance of MALRImportance of MALR

Since rising, saturated air parcels cool at a slower rate, Since rising, saturated air parcels cool at a slower rate, the parcel has a better chance of staying warmer than the parcel has a better chance of staying warmer than its environment longer, thus has a better chance of its environment longer, thus has a better chance of being positively buoyantbeing positively buoyant

Not constantNot constant

Conditional InstabilityConditional InstabilityConditionally unstable: lapserate of environment is lessthan DALR but greater thanMALR; thus, we haveinstability if a parcel issaturated but stability if it isunsaturated

10 C

Simple Model of StabilitySimple Model of Stability

Instability results when an air parcel is warmer than its Instability results when an air parcel is warmer than its environment (warm below cool ideal)environment (warm below cool ideal)– Solar heating & low-level warm advection increase instability…Solar heating & low-level warm advection increase instability…

sometimes lapse rate near ground is greater than DALR = sometimes lapse rate near ground is greater than DALR = superadiabaticsuperadiabatic

– Cooling ground increases stability (radiational Cooling ground increases stability (radiational cooling/evaporational cooling)cooling/evaporational cooling)

– Upper-level cooling (warming) increases (decreases) instabilityUpper-level cooling (warming) increases (decreases) instability

Warm, moist low levels and chilly mid-levels are an ideal Warm, moist low levels and chilly mid-levels are an ideal setup for an unstable atmosphere (thunderstorms)setup for an unstable atmosphere (thunderstorms)

Warm below, cold aboveWarm below, cold above

Cloud ClassificationCloud Classification

Basic cloud classification

• High (cirrocumulus, cirrostratus, cirrus)

• Middle (altocumulus, altostratus)

• Low (cumulus, stratus, stratocumulus)

• Clouds of significant depth (cumulonimbus,cumulus congestus)

• Stratiform (layered) clouds suggest a stableatmosphere (wider than tall) & produce steady,long-lived precipitation over a large area

• Cumuliform (heap) clouds suggest an unstableatmosphere & produce intense, short-livedprecipitation over a small area

High CloudsHigh Clouds

Composed of ice crystalsComposed of ice crystals Cirrostratus can lead to formation of halo Cirrostratus can lead to formation of halo

around sun or moonaround sun or moon

Cirrus

Cirrostratus

nebulosis

fibratus

Middle cloudsMiddle clouds

If cirrostratus lower and thicken into If cirrostratus lower and thicken into altostratus, that’s a good sign precipitation altostratus, that’s a good sign precipitation may be on the waymay be on the way

Alto-, strato- & cirro- cumulus form from Alto-, strato- & cirro- cumulus form from destabilization of cloudy region in a destabilization of cloudy region in a generally stable environmentgenerally stable environment

Altostratus

translucidus

opacus

Altocumulus / Cirrocumulus

perlucidus

undulatus

Altocumulus castellanus

Low cloudsLow clouds

Stratiform clouds to north of warm front where Stratiform clouds to north of warm front where overrunning is occurringoverrunning is occurring

““NimbusNimbus” means rain” means rain

Stratus, Nimbostratus

Stratus, Nimbostratus

Stratocumulus

Cumuliform cloudsCumuliform clouds

Form via convectionForm via convection ““Clouds of vertical development”Clouds of vertical development” Can last for minutes-few hoursCan last for minutes-few hours Cumulonimbus produce thunderstorms, Cumulonimbus produce thunderstorms,

tornadoes, hailtornadoes, hail Updrafts range from tens of cm/s (cumulus) to Updrafts range from tens of cm/s (cumulus) to

tens of m/s (cumulonimbus)tens of m/s (cumulonimbus)

Cumulus humilis

Cumulonimbus

overshooting topanvil

Mammatus Clouds

wave clouds

Lenticular clouds

Wave CloudsWave Clouds

Indicate a stable atmosphereIndicate a stable atmosphere

Kelvin-Helmholtz instability

Smoke as a stability clueSmoke as a stability clue

Summertime afternoons and Summertime afternoons and wind wind

How does precipitation form?How does precipitation form?

Liquid water can become supercooled => drops Liquid water can become supercooled => drops of water exist as liquid with temperatures below of water exist as liquid with temperatures below 0°C0°C

At temperatures >-40°C, tiny particles are At temperatures >-40°C, tiny particles are needed to start ice crystal formation (ice nuclei)needed to start ice crystal formation (ice nuclei)

Ice, liquid, and vapor coexist in cloudsIce, liquid, and vapor coexist in clouds Equilibrium vapor pressure over ice is less than Equilibrium vapor pressure over ice is less than

the equilibrium vapor pressure over water at the the equilibrium vapor pressure over water at the same sub-freezing temperaturesame sub-freezing temperature

Bergeron ProcessBergeron Process

Saturation is reached over Saturation is reached over ice but not over water, thus ice but not over water, thus ice crystals grow at the ice crystals grow at the expense of liquid expense of liquid drops….eventually drops….eventually become large and fallbecome large and fall

Collision-Coalescence ProcessCollision-Coalescence Process

Occurs in “warm” cloudsOccurs in “warm” clouds Falling ice crystals melt into rainFalling ice crystals melt into rain Bigger raindrops fall faster than Bigger raindrops fall faster than

smaller onessmaller ones Larger drops collide with Larger drops collide with

smaller drops smaller drops Grow as all or part of smaller Grow as all or part of smaller

drop’s water coalesces with drop’s water coalesces with larger droplarger drop

VirgaVirga