Transcript Clouds and Climate

Radiative Properties of Clouds
SOEE3410
Ken Carslaw
Lecture 3 of a series of 5 on clouds and climate
• Properties and distribution of clouds
• Cloud microphysics and precipitation
• Clouds and radiation
• Clouds and climate: forced changes to clouds
• Clouds and climate: cloud response to climate
change
Content of This Lecture
•
•
•
•
•
Global radiation balance and the role of clouds
Radiation interaction with cloud particles
Shortwave radiation (Cloud albedo)
Longwave radiation (emissivity)
Net radiative effect of clouds
You should understand the role of clouds in the climate system, the
different behaviour of long and shortwave radiation, and the different
radiative effects of different cloud types
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Atmospheric Radiation Streams
Sun’s shortwave
energy arriving at
Earth
SW
Earth’s emitted
longwave (infrared)
energy at the top of
the atmosphere
LW
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
• Blackbody
emission
spectrum
• E=sT4
1
weak IR
Absorption in the 8-14
mm “window”
weak near-IR
absorption
strong IR
absorption
(GH effect)
no visible light
absorption
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Global Energy Budget
~75% by clouds
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Problems to Solve
• How much solar SW radiation is reflected by a cloud and
what physical properties of the cloud control the albedo?
• Is any solar radiation absorbed by a cloud?
• How much terrestrial (Earth) LW radiation is absorbed by
clouds?
• What is the net effect of clouds on Earth’s energy
balance and future changes to that balance?
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Radiation Interaction With Clouds
• Rays A-D are scattered (no loss of radiative energy)
• Ray E is absorbed (converted to heat)
• Scattering + absorption = extinction
Scattered light gives
cloud white
appearance
scattering
absorption
Intensity of direct beam
progressively
reduced inside cloud
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Microphysical Factors Affecting
Scattering
• Scattering cross-section defined as
s   r Q sca
2
• Qsca = scattering efficiency (fraction of light
scattered relative to “shadow area”)
• Qsca depends on
– size of particle relative to wavelength of light
– Index of refraction
• For large cloud drops Qsca  2
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Scattering Versus Drop Size for
Constant Water Content
•
If Liquid Water Content (LWC [kg m-3]) is constant, then
1
LWC  N
4
3
r
3

3
LWC


 r 
 4 N 
 3

N=drops/volume
2

3
LWC
2


r  
;
 4 N 
 3

•
•
•
•
•
N 
LWC
4
3
r
3
2
Total light scattered in a thin cloud depends on N  r Q sca
1
Therefore, scattering efficiency of a cloud depends on N 3 or r
Therefore, doubling N (r decreases) increases albedo by 25%
doubling r (N decreases) decreases albedo by 50%
But thickness is also important
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1
1
Cloud Reflectivity in Solar Spectrum
(Shortwave, SW)
100
80
4
60
8
16
Typical
clouds
40
20
16
2
0
10
•
•
•
Absorptivity (%)
2
100
1000
liquid water path (g m-2)
Reflectivity (%)
80
Reflectivity (%)
100
drop radius (mm)
60
cloud thickness (m)
1500
500
Typical
clouds
40
100
20
50
0
LWC = 0.3 g m-3
10
100
1000
cloud drop concentration (cm-3)
Visible light absorption negligible
Some weak absorption in near-IR part of solar spectrum
Cloud drop size and number are important in global energy balance
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Mean Liquid Water Path (LWP)
(measured in g m-2)
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Solar Radiation Intensity Through a
Cloud
Upward and downward SW radiation streams
through a cloud above a surface of albedo = 0
1000
S
Height (m)
800
S
600
Note rather slow
decrease:
Clouds need to be fairly
thick to have a high
albedo
400
200
0
0 200 400 600 800 1000
SW intensity (W m-2)
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Clouds and Longwave (LW)
Radiation
25
Wavelength / mm
15
10
•
8
blackbody curves
for different
temperatures
Measured infrared
spectrum of Earth
from above a
cloudless Sahara
Desert
Clouds absorb in
the atmospheric
window
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Cloud Absorptivity of LW Radiation
1.0
Absorptivity
0.8
Some scattering remains,
but cloud becomes
close to a perfect
emitter/absorber above
quite low LWP
0.6
0.4
0.2
0
0
10
100
1000
liquid water path (g m-2)
• Clouds are very efficient absorbers of LW across
the entire terrestrial spectrum
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Solar (SW) and Terrestrial (LW)
Radiation Intensity Through a Cloud
1000
1000
Height (m)
S
800
Height (m)
S
800
strong LW cooling
at cloud top
600
400
200
0
0 200 400 600 800 1000
SW intensity (W m-2)
L
L
600
L and L in
400 balance
high LW
200
downward flux
below cloud
0
280 300 320 340 360 380
LW intensity (W m-2)
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Consequences of Different SW and
LW Behaviours
• Clouds need to be relatively thick to have an
albedo approaching 1.0
• Even relatively thin clouds are good absorbers of
LW radiation
• Thin cirrus clouds are effective LW absorbers
but poor SW reflectors
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210
Altitude/km
Temp/K
Consequences
15
Cb: Large effect on albedo,
large effect on OLR
Ci: Small effect on albedo,
large effect on OLR
Clear sky
OLR through
“atmospheric
window”
250 5
280 1
290 0
Sc: Large effect on albedo,
small effect on OLR
E emit  s T
4
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Albedo and Outgoing Longwave
Radiation (OLR) From ERBE
ERBE = Earth Radiation Budget
Experiment satellite
Deep Cb
cirrus
low Sc
Ocean surface
Deep Cb
low Sc
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Cloud Forcing (net radiative effect of
clouds)
Deep Cb small net radiative effect
(SW cooling, LW heating)
Cirrus warming
low Sc cool in summer and
Warm in winter
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Net Effect of Clouds on global
energy balance
• SW cooling
• LW heating
• Not complete cancellation, and depends on
cloud type and season
• Net effect is global mean –15 to 20 Wm-2
(cooling)
• About 4-5 times radiative effect of CO2 doubling
• Changes in cloud type/cover/properties have
potential to affect climate
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Reading/Further Investigation
• Read a description of ERBE
• Examine and understand further images
• http://cimss.ssec.wisc.edu/wxwise/homerbe.html
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Questions for this lecture
1. What are the approximate wavelengths of solar shortwave and
terrestrial longwave radiation streams?
2. Which radiation stream (SW or LW) is absorbed more in the
atmosphere?
3. Which radiation stream (SW or LW) is absorbed more in a cloud?
4. What is the main process that attenuates SW radiation as it passes
through a cloud?
5. As you leave this lecture, what clouds can you see, what is their
approximate droplet concentration, and what effect are they likely
to be having on climate?
6. Why is the “atmospheric window” important for climate change?
7. Explain the reason why thin cirrus clouds can warm the climate.
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