module 9 atmospheric stability mcen 4131/5131 2 preliminaries i will be gone next week, mon-thur...

Module 9Module 9

Atmospheric Stability

MCEN 4131/5131

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PreliminariesPreliminaries• I will be gone next week, Mon-Thur• Tonight is design night, 7:30ish, meet in classroom• Next tues Tan and Nick will be in class to help you with your Projects - they

are graduate students who took class

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Review

Module 7 Educational Module 7 Educational ObjectivesObjectives

• Increased use of cars worldwide has altered the field of air pollution control

• The air ER is the actual air/fuel ratio divided by the stoichiometric air/fuel ratio.

– For gasoline, the AFR is 14.7• fuel rich for ER < 1

– major pollutant emissions are CO, HCs• fuel lean for ER > 1

– major pollutant emissions are NOx especially near ER = 1• The IC engine does not have complete combustion

because of the temperature distribution within the cylinder, and the walls are cooler, quenching reactions

• Add-on technologies that control emissions are the catalytic converter and the carbon canister

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LearningObjectivesfor Today Module 8 Educational Module 8 Educational

ObjectivesObjectives• General circulation patterns

– Coriolis force

• Stability and vertical mixing– Temperature gradient in

atmosphere

• Lapse rate• Temperature inversions

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Circulation of the Circulation of the AtmosphereAtmosphere

• Global circulation patterns due to – nonuniform heating of earth’s surface– Buoyancy (warm air rises)– Coriolis effect

• Nonuniform heating of earth’s surface– Greatest heating at equator– Air rises at equators, subsides at poles– Because of earth’s rotation, this

pattern is broken up

LearningObjectives

Circulation patternsVertical mixingLapse rateTemperature inversions

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Wind profiles in lower Wind profiles in lower atmosphereatmosphere

• geostrophic layer– inviscid (viscous effects are negligible)– Wind profile determined by pressure

gradient and coriolis effect

• planetary boundary layer– Effect of earth’s surface is important– Important in pollutant transport

• surface layer– Wind profile determined by surface drag and

temperature gradient and pressure gradient• Ekman layer

– Wind profile determined by surface drag, pressure gradient and Coriolis50-100 m

300-500 m

Planetary boundarylayer

Ekman layer

Surface layer

Geostrophic layer

LearningObjectives


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Clicker QuestionClicker Question

• This force results from the earth’s rotation and deflects air movement to the right in the N. hemisphere

a. Friction forceb. Coriolis forcec. Rotational atmospheric forced. Centrifugal force

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Coriolis ForcesCoriolis Forces

• Influences circulation in the geostrophic layer

• Think of wind blowing toward south in northern hemisphere– Surface velocity of earth

increases toward equator– From earth, wind gains a

velocity toward west

LearningObjectives


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Coriolis Cont’dCoriolis Cont’d

Earth from above Earth from the side

N

Nrotation

Equator E

EW

W

LearningObjectives


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Ekman SpiralEkman Spiral

• refers to winds near a horizontal boundary in which the flow direction rotates as one moves away from the boundary

• Happens within planetary boundary layer– Consequences: top of plumes can

move in directions as much as 50 degrees from the bottom of the plume

LearningObjectives


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Clicker Question?Clicker Question?

• The relationship between wind velocity and height in the atmosphere are described by which function? a. Exponentialb. Logarithmicc. Powerd. Linear

Wind speed as a function of stability and surface

0

20

40

60

80

100

120

0 5 10

Wind Speed (m/s)

Height above ground (m)

Stability A, Rough

Stability F, Rough

Stability A, Smooth

Stability F, smooth

€

u2

u1

=z2

z1

⎛

⎝ ⎜

⎞

⎠ ⎟

p

Typically u1 is measured at z1 = 10 m.

LearningObjectives


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Temperature Temperature structure of the lower structure of the lower

atmosphereatmosphere• Affects stability of troposphere• Controls vertical air movement• Disperses near-surface emissions• Troposphere: T decreases with

height– Warm air is less dense than cool air– Warm air under cool air results in

vertical mixing

LearningObjectives


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Temperature of Temperature of AtmosphereAtmosphere

• In the troposphere normally the temperature decreases as you go up in altitude

• Rate is on average 0.65 degrees C per 100 meters (called a lapse rate)

• This decrease in temperature helps to mix the air, dispersing pollutants

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

LearningObjectives


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Lapse RateLapse Rate

• Consider stationary mass of air governed by pressure forces and gravity (ignore viscous effects)– Large distortable volume– Slowly exchanges heat and mass

with surroundings– Pressure equilibrates rapidly– no energy is added or removed– Hydrostatics: -(dP/dz) = (MWag/RT)P– Solve for dT/dz

LearningObjectives


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Adiabatic lapse rateAdiabatic lapse rate

• Rate at which temperature of dry air changes with height in the atmosphere due to adiabatic expansion or compression

€

dT

dz= −

g

Cp= −γ d

= −0.98 C per 100 m

LearningObjectives


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Group clicker Group clicker questionquestion

• If the lapse rate is equal to the dry adiabatic lapse rate, the stability condition is:

a. Unstableb. Neutralc. Stable

• And what if the lapse rate is less than d?

LearningObjectives


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Atmospheric StabilityAtmospheric Stability

• Stable– buoyancy returns a parcel of air to its original position

after it has been displaced upward or downward– Atmospheric lapse rate < adiabatic lapse rate– Atmosphere cools less rapidly with height than parcel– Vertical mixing suppressed

• Unstable– buoyancy increases the displacement of the parcel of

air that has moved upward or downward– adiabatic lapse rate < atmospheric– Atmosphere cools more rapidly with height than parcel– Vertical mixing is promoted

• Neutral– the lapse rate is equal to the dry adiabatic lapse rate,

parcel of air stays where it has been displaced– Adiabatic = atmospheric lapse rate

LearningObjectives


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Pasquill stability classPasquill stability class

• Would like to predict atmospheric lapse rate from readily observable properties

• Pasquill (1961) introduced notion of stability class

• Based on 3 characteristics– Intensity of solar radiation– Near-surface wind speed– Extent of nighttime cloud cover

• Relationship of stability class to lapse rate

LearningObjectives


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Stability ClassesStability Classes

Stability class Lapse rate (C/100 m)

A (extremely unstable) < -1.9B (moderately unstable) -1.9 to -1.7C (slightly unstable) -1.7 to -1.5D (neutral) -1.5 to -0.5E (slightly stable) -0.5 to 1.5F (moderately stable) > 1.5

LearningObjectives


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Temperature Temperature InversionsInversions

• When there is cold air near the ground, and a layer of warmer air above



Temperatureprofile as a function of height

LearningObjectives


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LearningObjectives


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• When there is cold air near the ground, and a layer of warmer air above

• Clicker Question? Which of the following Inversions plays the most important role in cause smog problems?

a. Subsidenceb. Frontalc. Radiation

And what about for wood-burning in the winter?



LearningObjectives


module 9 atmospheric stability mcen 4131/5131 2 preliminaries i will be gone next week, mon-thur...

Documents

class slide

centrifugal force slide

surface drag

atmospheric stability

temperature distribution

air er

carbon canister slide

earths rotation