Transcript PPT - UW Atmospheric Sciences

Announcements
Exam THIS Friday in Discussion Section
Review session Thursday during lecture
Advanced notice is required if you know you will
miss Fri’s exam (we only offer early exams)
NO MAKEUP exams without official
documentation of an unexpected emergency!
GHG Ranking Factors
1. Amount: more there is, more radiation can
be absorbed
2. Ability: depends on the wavelength
3. Location: both where in the atmosphere and
where () in the outgoing radiation spectrum
Absorptivity versus Amount
Absorptivity ()
“Absorptivity is saturating”
1
3
2
2
1
1
m m
m1 m2 m3
A   1 (band saturation),
increasing amount of GHG gives
smaller and smaller increases in 
e.g.: 2 << 1
Amount of GHG
(i.e. concentration)
GHG Ranking: Wavelength Location
Water Level
~ Ts
Plug a small leak
small increase in Ts
Plug a big leak
big increase in Ts
GHG Ranking: “Emission Height”
Altitude (z)
Tb4
z2
Ta4
Emission to space from
z1 carries much more
energy than from z2
z1
Temperature
Ts
GHG Ranking: Emission Height (T)
Tafter
Tbefore
A GHG which causes a
higher emission altitude
is a larger constriction
of energy flow out.
Doubling CO2 will basically double the amount of
radiation absorbed by CO2.
1. True
2. False
ls
e
0%
Fa
Tr
ue
0%
Water Vapor
• The global warming skeptics’ favorite GHG
Fact: The Greenhouse Effect is mostly due to
absorption of OLR by water vapor
Fiction: “Therefore, Earth’s surface T will only
respond to changes in the amount of water vapor”
Saturation Vapor Pressure of Water
PH2O = 0
PH2O increases
PH2O “saturates”
H2Ogas H2Ogas
H O H2Ogas
2
gas
H2Ogas
wait
H2Oliq
Start: H2Oliq
w/dry air above
H2Oliq
Middle: net
evaporation
wait
H2Ogas H2Ogas
H2Ogas
H2Ogas
H2Ogas
H2Ogas
H2Oliq
End: condensation
equals evaporation
“Equilibrium”
Saturation Vapor Pressure of Water
The maximum amount of
water vapor air can hold
increases exponentially
with air T
Suppose we stop emitting CO2 but emit more
H2O vapor from our tail pipes in a H2powered economy. Should we worry about
continuing to enhance the GHE?
1. Yes
2. No
o
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N
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0%
The dependence of the saturation vapor
pressure of water on T is a negative
feedback on climate change
50%
50%
ls
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Fa
Tr
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1. True
2. False
Water Vapor as a GHG
• Water vapor increases for an increase in T
• Water vapor’s increase causes an additional
change in T
• An example of a positive feedback on climate
What have we accomplished?
1 Developed a physical basis of present day global
climate
2 Described the mechanisms that can cause global
climate change.
Tsf


 Iin 1  A  


 4  1    
2


1
4
3 Now know how changes in solar radiation, albedo,
and, [GHGs] can/should alter global average T.
This Week and Next: Regional Climates
• Part 1 – Atmospheric Motions
–
–
–
–
Wet tropics, major deserts
Temperate but stormy mid-latitudes
Dry/Cold Poles
Continentality
• Part 2 – Ocean Circulation
Regional Climates
• The average weather of a particular
latitude and longitude zone
Spatial Radiation Imbalance
Spatial Radiation Imbalance
300
W/m2
EOUT (LW)
EIN (SW)
100
W/m2
Where on Earth receives the largest daily
average solar flux
1. Equator
2. Mid-latitudes
3. Poles
66%
22%
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Today: Atmospheric Motions – Part 1
• Horizontal Motions: parallel to
Earth’s surface (the everyday wind)
• Vertical Motions: perpendicular to
Earth’s surface (up/down)
Important Concepts in Vertical Motions
• An air parcel that rises  expands
due to lower surrounding pressure
• EXPANSION requires WORK by
the air parcel
• Air parcel doing WORK, loses
Energy, temperature goes down.
Day After Tomorrow
In “Day After Tomorrow”, a super-storm
with a very low pressure center draws very
cold lower stratospheric air to the surface
causing instant freezing.
83%
1. Consistent with 1st Law
2. Violates 1st Law
C
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tw
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1s
1s
tL
tL
aw
aw
17%
The movie’s science advisor, if there was
one, should have his/her PhD revoked.
92%
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8%
Ye
1. Yes
2. No
Announcements
• EXAM FRIDAY, Review session Thursday
– Through yesterday:
– i.e. T(z) = Tsurf –z*(10K/km) if dry
– T(z) = Tsurf – z*(6.5K/km) if wet
• Start forming a final project group (3-4),
– see course website for project ideas
– Use discussion board for finding group members
What is Buoyancy?
PGF
Fluid
(rf)
PGF
object
(rob)
Fg
Fg
Fbuoyancy = PGF – Fg  0 if rob  rfluid
Heating a Fluid From Below
Warm surface air less dense “buoyant” rises
Convection: transfer of heat via motions of a fluid
Making Air Move
Pressure Gradient Force
Air/water will move from a region of high
pressure to low pressure
Fluid with horizontal pressure gradient
Horizontal Pressure Gradients
PGF
The picture is roughly symmetric
for the Southern Hemisphere (SH)
General Circulation of the Atmosphere
An early picture based on Hadley’s 1735 paper:
Tropopause
divergence
subsidence
Upwelling
subsidence
convergence
90
45
0
-45
-90
Cold high lats
Warm Tropics Cold high lats
higher pressure Lower pressure higher pressure
Phase Transitions in Rising/Sinking Air
In subsiding air, condensation and
precipitation suppressed.
In rising air, condensation and
precipitation more likely
rising
subsiding
Intertropical Convergence Zone
The (original) Hadley Circulation (1735)
COLD
HOT
COLD
Explains:
Intertropical
Convergence Zone
(ITCZ)
Wet Tropics
Dry Poles
But it has some
problems.
Earth Rotates from West to East
Circumference decreases from
equator to pole
To complete a full rotation in
1 day, equator rotates faster
than higher latitudes
coriolis effect movie
Coriolis Effect in Northern Hemisphere
Object moving from equator has greater easterly
velocity than surface at northern latitudes
 so object curves to right of initial direction.
Coriolis Cartoon
Trajectory seen
by thrower
An object moving south from the equator
curves to the ______ of its initial forward
direction
53%
1. Left
2. Right
t
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47%
Coriolis/Centrifugal Effect Summary
Applies to objects moving North or South
relative to Earth’s surface
A similar effect occurs for objects traveling in
E-W directions (centrifugal force)
Objects (air or water, e.g.) moving in the NH
will curve to the right of their forward direction
and to the left in SH.
Suppose Earth were a cylinder. Where would
there be a coriolis effect?
56%
40%
B
B
A
an
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A
1. A only
2. B only
3. A and B
A
Upper-level Flow Tends to be Geostrophic
PGF
Coriolis
Flow direction
Low Pressure
Geostrophic Flow: When PGF
balanced by Coriolis Effect
High Pressure
Coriolis Effect Modifies Hadley Circulation
Modified Hadley Circulation
*
Horizontal motions
convergence: coming together
divergence: spreading apart
Vertical motions
upwelling: rising air
subsidence: sinking air
“Seeing” Hadley Circulation
Mean Surface Pressure Contours
Lines of constant
pressure (isobars).
More closely spaced
lines: steeper change
in pressure
H
H
H
H
H
H
Hadley Circulation and Regional Climates
• World’s largest deserts
• Wet and dry seasons in the Tropics
• The Trade Winds
World’s Deserts
Not shown: Polar Regions!
World’s Deserts
Desert dust blows from
W. Sahara and N.
Morocco over Canary
Islands.
Desert dust is a source
of nutrients to ocean
and land biota (often a
world away).