Transcript Module 1

Module 9
Atmospheric Stability
MCEN 4131/5131
Preliminaries
• 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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MCEN 4131/5131
Review
Module 7 Educational
Objectives
• 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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MCEN 4131/5131
Learning
Objectives
for Today
Module 8 Educational
Objectives
• General circulation patterns
– Coriolis force
• Stability and vertical mixing
– Temperature gradient in
atmosphere
• Lapse rate
• Temperature inversions
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MCEN 4131/5131
Learning
Objectives
Circulation patterns
Vertical mixing
Lapse rate
Temperature inversions
Circulation of the
Atmosphere
• 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
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MCEN 4131/5131
Learning
Objectives
Wind profiles in lower
atmosphere
Circulation patterns
Vertical mixing
Lapse rate
Temperature inversions
• 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
– WindGeostrophic
profile determined
layerby surface drag and
300-500 m
50-100 m
temperature gradient and pressure gradient
• Ekman layer
Planetary boundary
Ekman– layer
Wind profile determined by surface drag,
pressure
layer
gradient and Coriolis
Surface layer
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MCEN 4131/5131
Clicker Question
•
This force results from the
earth’s rotation and deflects air
movement to the right in the N.
hemisphere
a.
b.
c.
d.
Friction force
Coriolis force
Rotational atmospheric force
Centrifugal force
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MCEN 4131/5131
Learning
Objectives
Circulation patterns
Vertical mixing
Lapse rate
Temperature inversions
Coriolis 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
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MCEN 4131/5131
Learning
Objectives
Coriolis Cont’d
Circulation patterns
Vertical mixing
Lapse rate
Temperature inversions
W
E
N
Equator
E
W
N
rotation
Earth from above
Earth from the side
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MCEN 4131/5131
Learning
Objectives
Circulation patterns
Vertical mixing
Lapse rate
Temperature inversions
Ekman 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
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MCEN 4131/5131
Learning
Objectives
Clicker Question?
Circulation patterns
Vertical mixing
Lapse rate
Temperature inversions
Wind speed as a function of stability
and surface
• The relationship between wind
velocity and height in the
atmosphere are described by
which function?
Height above ground (m)
120
100
80
Stability A, Rough
Stability F, Rough
60
a.
b.
c.
d.
40
20
0
0
Exponential
Logarithmic
Power
Linear
5
Stability A, Smooth
Stability F, smooth
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p


u2
z
  2 
u1 z1 
Wind Speed (m/s)
Typically u1 is measured at z1 = 10 m.

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MCEN 4131/5131
Learning
Objectives
Temperature structure of
the lower atmosphere
Circulation patterns
Vertical mixing
Lapse rate
Temperature inversions
•
•
•
•
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
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MCEN 4131/5131
Learning
Objectives
Circulation patterns
Vertical mixing
Lapse rate
Temperature inversions
Temperature of
Atmosphere
• In the troposphere
normally the temperature
decreases as you go up in
QuickTime™ and a
TIFF (Uncompressed) decompressor
altitude
are needed to see this picture.
• 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
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MCEN 4131/5131
Learning
Objectives
Circulation patterns
Vertical mixing
Lapse rate
Temperature inversions
Lapse 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
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MCEN 4131/5131
Learning
Objectives
Circulation patterns
Vertical mixing
Lapse rate
Temperature inversions
Adiabatic lapse rate
• Rate at which temperature of dry
air changes with height in the
atmosphere due to adiabatic
expansion or compression
dT
g

  d
dz
Cp
 0.98 C per 100 m
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MCEN 4131/5131
Learning
Objectives
Group clicker question
Circulation patterns
Vertical mixing
Lapse rate
Temperature inversions
•
If the lapse rate is equal to the
dry adiabatic lapse rate, the
stability condition is:
a. Unstable
b. Neutral
c. Stable
•
And what if the lapse rate is less
than d?
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MCEN 4131/5131
Learning
Objectives
Circulation patterns
Vertical mixing
Lapse rate
Temperature inversions
Atmospheric 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
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MCEN 4131/5131
Learning
Objectives
Circulation patterns
Vertical mixing
Lapse rate
Temperature inversions
Pasquill 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
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MCEN 4131/5131
Learning
Objectives
Circulation patterns
Vertical mixing
Lapse rate
Temperature inversions
Stability Classes
Stability class
A (extremely unstable)
B (moderately unstable)
C (slightly unstable)
D (neutral)
E (slightly stable)
F (moderately stable)
Lapse rate (C/100 m)
< -1.9
-1.9 to -1.7
-1.7 to -1.5
-1.5 to -0.5
-0.5 to 1.5
> 1.5
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MCEN 4131/5131
Learning
Objectives
Circulation patterns
Vertical mixing
Lapse rate
Temperature inversions
Temperature Inversions
• When there is cold air near the
ground, and a layer of warmer air
above
Temperature
profile as a
function of
height
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
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MCEN 4131/5131
Learning
Objectives
Circulation patterns
Vertical mixing
Lapse rate
Temperature inversions
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
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MCEN 4131/5131
Learning
Objectives
Circulation patterns
Vertical mixing
Lapse rate
Temperature inversions
•
• WhenQuestion?
there is cold
air near
Clicker
Which
of thethe
ground,Inversions
and a layer
of warmer
air
following
plays
the most
above role in cause smog
important
problems?
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
a. Subsidence
b. Frontal
c. Radiation
And what about for wood-burning in the winter?
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