Stability & Cloud DevelopmentStability & Cloud Developmentdwdubois/geog390_lecture08b_ch06b.pdf · Adiabatic ChartsAdiabatic Charts • Adiabatic charts show how various atmospheric

Stability & Cloud DevelopmentStability & Cloud Development

Chapter 6Part 2Part 2

Reversible/IrreversibleReversible/Irreversible

• Reversible adiabatic process- no precipitationReversible adiabatic process no precipitation falls and leaves parcel- parcel can return to original temperature and pressure

• Irreversible pseudoadiabatic process-precipitation fall out and parcel cannot return to it’s original temperature and pressure (warms)

Irreversible pseudoadiabatic process - precipitation falls out and parcel cannot return to it’s original temperature

3500

p g pand pressure (warms)

PrecipitationSinking parcel becomes

2500

3000

)Precipitation starts

becomes unsaturated (cloud dissipates)- dry lapse rate

1500

2000

Hei

ght (

m)

sinking Trising T

500

1000

H

00 10 20 30 40

Temperature (C)

Condensation occurs- wet lapse rate

Temperature (C)

StabilityStability• Determine stability by comparing temperature of rising

parcel to surroundings- if colder than surroundings moreparcel to surroundings if colder than surroundings, more dense, parcel sinks toward original position- stable, if warmer, less dense, continues to rise-unstable

• Need to know temperature of parcel and it’s environment• Need to know temperature of parcel and it s environment (surrounding air) at different heights in atmosphere

• Environmental lapse rate is lapse rate of atmosphere-b d t i d b di d d t f thcan be determined by radiosonde data from weather

balloons • If environmental lapse rate is less than wet adiabatic p

rate→absolutely stable• Greater that dry adiabatic rate→ absolutely unstable• Between wet & dry adiabatic → conditionally unstable• Between wet & dry adiabatic → conditionally unstable

Absolutely stable atmosphere- environmental lapse rate less than moist adiabatic rate

Absolutely unstable atmosphere- lapse rate t th d di b ti tgreater than dry adiabatic rate

Conditionally unstable - stable if rising air t t t d t bl if i i i t t dnot saturated, unstable if rising air saturated

Summary of stability conditionsSummary of stability conditions

Stable air• Stable air resists vertical motion - if clouds form

from forced lifting, flat tops and bases• Atmosphere becomes more stable if air aloft

warms or air below coolsS bsidence in ersion sinking air arms at dr• Subsidence inversion – sinking air warms at dry adiabatic lapse rate, air below often cooler- very common Pacific Coast in summer, with cool ,marine layer below- high air pollution potential

• Cooling in lower layers - radiational cooling; cold i d ti i i ld fair advection; air moving over cold surface

• Cold air drainage into valleysI i l t lid t t ti l• Inversion layer acts as lid to prevent vertical mixing

• Neutral stability → environmental lapse rate = dryNeutral stability → environmental lapse rate dry adiabatic rate – would occur in well mixed atmosphere such as windy conditions or much of lower atmosphere i ftin sunny afternoons

• Stability changes diurnally and varies with height• Unstable- usually only very near ground during intenseUnstable- usually only very near ground during intense

solar radiation- air quickly rises and mixes resulting in near neutral stability

• Conditionally unstable- between wet & dry adiabatic lapse rates- commonly the case

• Average tropospheric lapse rate ≈6 5 º C per Km• Average tropospheric lapse rate ≈6.5 C per Km, typically near or greater than the wet adiabatic rate

• Show example radiosonde temperature profiles 0z, 12z

Adiabatic ChartsAdiabatic Charts• Adiabatic charts show how various atmospheric

variables change with height: P, T, humidityg g , , yThe right white line is T profile. The left line is the Td.

The pressure lines are plotted horizontally in blueThe pressure lines are plotted horizontally in blue and are also on an inverse log scale.

The concept of Skew T means that the temperature i t l tt d ti ll b t l ff t th i ht tis not plotted vertically but angles off to the right at a 45 degree angle. The temperature lines of the Skew T are in blue.

The green lines are called dry adiabats.

The light blue dashed lines are saturation adiabats.

The yellow dashed lines are lines of constant mixing ratio.

Morning sounding shows ground-based inversion

Afternoon sounding shows near dry adiabatic lapse rate to 5000 m

Causes of Instability• Cooling aloft- winds bringing in colder air (cold air

advection) – radiational cooling from clouds or airf f f• Warming of surface air- daytime solar heating of the

surface (ground); warm air advection by winds; air moving over warm surface

• Lifting layer of air- top cools more than bottom, becomes less stable- often conditionally unstable and clouds may formform

• Convective instability- if lower layer portion moist, upper layer portion dry, lower layer becomes saturated and cools at wet adiabatic rate upper portion at dry ratecools at wet adiabatic rate, upper portion at dry rate, upper portion becomes much cooler and get instability-often associated with thunderstorms and tornadoes

Cumulus clouds formingg

Condensation begins

Rising air cooling to dewpoint

Rising, cooling, and expanding air

Hot unstable air

Mixing tends to steepen lapse rate

Cloud DevelopmentCloud Development

• Clouds develop as an air parcel rises andClouds develop as an air parcel rises and cools below the dew point.

• Usually a trigger or process is need to• Usually a trigger or process is need to initiate the rise of an air parcel.

Cloud DevelopmentCloud Development

• ConvectionConvection– Differential land surface heating creates areas

of high surface temperatureof high surface temperature.– Air above warm land surface heats, forming a

‘bubble’ of warm air that rises or convection.bubble of warm air that rises or convection.– Cloud base forms at level of free convection.

Development of Cumulus CloudsDevelopment of Cumulus Clouds

Stability influences Cumulus CloudsStability influences Cumulus Clouds

Cloud DevelopmentCloud Development

• TopographyTopography– Orographic uplift

Orographic clouds– Orographic clouds– Windward, leeward, rain shadow

Lenticular clouds– Lenticular clouds

Orographic UpliftOrographic Uplift

Wave clouds from mountainsWave clouds from mountains

Mountain Wave CloudsMountain Wave Clouds

Lenticular CloudsLenticular Clouds

Cloud DevelopmentCloud Development

• Changing cloud formsChanging cloud forms– Stratus clouds can change to cumulus clouds

if the top of the cloud cools and the bottom ofif the top of the cloud cools and the bottom of the cloud warms.

– Alto cumulus castellanus: towers on altoAlto cumulus castellanus: towers on alto stratus

– If moist stable air without clouds is mixed or stirred it can form stratocumulus clouds.

Billow clouds from wind shearBillow clouds from wind shear

Stratocumulus FormationStratocumulus Formation