Clouds
Spring 2012, Lecture 8
1
Problems with Clouds
• Clouds are a major problem for climatologists
trying to understand how the atmospheric
system works, and to make predictions about
how GHG will change the climate system
• The IPCC has indicated clouds as a major
problem for their predictions, and as an area
needing much more research
2
Interaction with Radiation
• Clouds interact with both visible and infrared
radiation
o Clouds reflect visible light, should act as a
negative feedback, helping to cool the planet
o Clouds trap infrared radiation, and should act as a
positive feedback, warming the planet
3
Altitude
• Clouds can occur at different altitudes within
the troposphere
• The interaction with radiation depends greatly
on the cloud altitude
o High altitude clouds play a large role in blocking
IR radiation, but reflect little visible light, so they
heat the planet
o Low clouds reflect a great deal of visible radiation,
but are ineffective at trapping IR, so they cool the
planet
4
Clouds and Altitude
• Clouds are good examples of blackbody radiators
• They radiate to space at whatever temperature the
cloud top is at
• They trap IR radiation emitted from the much warmer
ground
• They radiate back to the ground at the temperature of
the bottom of the cloud
• This means they are net contributors of energy to the
atmosphere below the cloud, and high altitude clouds
are much more effective than low altitude clouds
5
Scattering of Visible Light
• The electric field of incoming visible light
excites vibrations in the water molecules
• They then reradiate the same frequency light,
mostly in a direction similar to the direction of
the incoming radiation
o The shape of the droplet influences scattering
direction
• The process is called scattering
6
Scattering vs. Absorption
• In scattering, the reradiated light is visible, at
the same frequency as the incoming radiation
• In absorption, the incoming light is converted
to thermal motion of the absorbing molecule,
and is reradiated in the IR
• There is always competition between
scattering and absorption
o Some light is scattered, some is absorbed
7
Scattering Fraction
• The amount of light scattered depends on a
number of factors
o The most critical factor is the length scale of all of
these structural features relative to the wavelength
of the light being scattered
o Density
o Anisotropy (unequal variation of physical
properties within a particle)
o Concentration fluctuations
o And others
8
Inability to Model
• The light scattering problem is complex, and
our understanding of this and other factors
associated with clouds means that no
physically realistic models have been
developed for clouds, or how clouds might
change as conditions change
• This is a major roadblock, and IPCC has urder
research in this area
9
Climate Feedbacks Video
10
Factors Affecting Cloud Albedo
• Factors that increase albedo
o Low clouds contain more water than high clouds
o Clouds about to rain have more water than clouds
that are not raining
o Ice crystals are better at reversing the direction of
light, sending it back into space
11
Formation of Clouds
• Clouds should start to form when the relative
humidity reaches 100%
• Often, this doesn’t happen
• It is very hard for water vapor molecules to adhere to
each other and condense to form a droplet
• Initially, all droplets have high surface area: volume
ratios
• The molecules on the surface tend to escape,
destroying the droplet
12
Condensation Nuclei
• Water will form droplets much more readily if there is
something in the air to which vapor molecules can
attach themselves
• The “something” are known as cloud condensation
nuclei
o
o
o
o
Dust particles
Smoke
Salt particles (from ocean spray)
Small biological organisms
13
Droplet Size
• Condensation nuclei need to be about 0.1
microns in size in order to be effective
• Once nucleated, droplets grow to around 5
microns
o Smaller droplets will join larger droplets – Archer
suggests this is urbanization of water droplets
• Rain clouds will have larger droplets
14
How Much Water is There in a Cloud?
• More than most people would think!
• Animation from NPR
15
Types of Clouds
• There are three main types of clouds
• Low-altitude
o Cumulus
o Stratus
• High altitude
o Cirrus
16
Cloud Type
Diagram:
Cumulus
• Cumulus clouds form when there is strong upward
convection, and they resemble towers
• They are low, usually between 1 and 4 kilometers
• They are difficult to model because their size is
smaller then the grid size of climate models
17
Cumulonimbus
• If convention is strong they form cumulonimbus
storm clouds
18
Cloud Type
Diagram:
Stratus
• Stratus clouds are layered, and occur over large
geographic areas where air motion is upward but slow
• Slow motion makes modeling quite difficult
• They are typically quite low, up to about 2 kilometers
19
Stratus Clouds
• Dull, feature-less low clouds
• Layered clouds at slightly
different elevations
20
Cloud Type
Diagram:
Cirrus
• Cirrus clouds form at higher altitudes, usually 8-12
kilometers
• They contain only about 1-10% as much water as
low altitude clouds
• They are often composed of ice crystals, either by
freezing of water, or by direct condensation from
vapor
21
Cirrus Clouds
• Cirrus cloud images
• Image on the right is at low sun angle, when
clouds may be illuminated after the sun is not
reaching the ground
22
Effect of Cirrus Clouds
• Cirrus clouds are thin, and do not block
incoming radiation much, so they have little
effect on visible albedo
• They absorb IR radiation from the ground
o Because they are cold, they re-emit the radiation at
much lower temperatures
• Intensity is less
• Half of the re-emission is directed downward
23
Clouds in the Future
• It has often been said that as climate warms, there
will be more clouds
• Warmer temperatures will mean more water will
evaporate
• However, warmer temperatures also mean that the
amount of water vapor the air can hold will go up, so
relative humidity may not reach 100%
• This also contributes to the difficulties with modeling
cloud behavior correctly
24
How Does Cloud Temperature
Affect Albedo?
• It had been thought that warmer clouds would
have higher albedo, because they contain more
water vapor
• George Tselioudis, William Rossow and David
Rind of GISS examined satellite data
measuring the amount of sunlight reflected by
low-level clouds
• They found these clouds were actually brighter
when the air was cold than when it was warm
25
Why Are Cold Clouds Brighter
• Anthony Del Genio and Audrey Wolf helped to explain why
o “The atmosphere above cold ground tends to remain calm, like a pot of
water on a stove before the flame is turned on
o Water vapor molecules evaporated into such air, or blown in from other
places, can build up and form a thick layer of clouds that reflects
sunlight well
o However, the air above warm ground becomes turbulent, like a pot of
hot or boiling water
o The warm currents of rising air (the kind that gliders use to stay afloat)
carry water vapor molecules up and away, preventing clouds from
forming close to the ground
o Instead, only a thin layer of not very reflective clouds develops a
kilometer or two above the ground, at the altitude where rising air
currents terminate”
26
How Will This Effect Climate Change?
• Yao et al. used the GISS global climate model to predict
changes in different types of clouds, and the sensitivity of the
climate, to a doubling of carbon dioxide concentration
• They found that low-level clouds in the model behaved much
the same as anticipated from satellite and surface data.
o At midlatitudes, clouds became a bit thinner and less reflective in the
simulated warmer climate
o Clouds became less reflective in the tropics as well, but for a different
reason: clouds in the warmer climate lost more of their water due to
greater rainfall
o Other types of clouds did not behave in the same way - For example,
the big "anvil" clouds that accompany thunderstorms at high altitudes
became more extensive and brighter in the warmer climate, instead
27
Cancellation of Effects
• Changes in low- and high-level clouds mostly
cancelled each other out, so the net global effect of
the clouds did not differ very much in the warmer
climate scenario from that in today's climate
• This scenario differs considerably from what many
climate scientists had been assuming in the 1990s
o It had been thought that brighter clouds would partly "save"
us from significant global warming, by reflecting more
energy into space
o Instead, these results suggest that clouds are not necessarily
the white knight that will rescue us from climate change
28
TC4 Project
• NASA’s study of the Tropics
• TC4 stands for Tropical Composition, Cloud and Climate
Coupling
29
Anthropogenic Modification of Clouds
• Humans can modify, or create, clouds in a variety of ways
• Cirrus clouds may be created by jet contrails
• In the absence of condensation nuclei, air may remain
supersaturated almost indefinitely
• Water may be inserted into the stratosphere
• As jets fly, they emit water vapor, formed by combustion of
hydrogen in the hydrocarbon fuel, and particles, some of
which can serve as condensation nuclei
30
Jet Contrails
• The white contrail is caused by condensation of water
as the emissions cool off
• The contrail grows with time due to droplets serving as
condensation nuclei, taking water from the air
• The cloud particles spread with time, becoming
indistinguishable from cirrus clouds
• Particle matter and CO2 are usually not visible
31
Jet Contrails vs. CO2
• Contrails would dissipate within a few days if
we stopped creating them
• They warm the planet, just as CO2 does
• CO2 will remain in the atmosphere for a
century or so
32
Pollution and Low-Altitude Clouds
• Smoke particles can affect low clouds
• The most effective particles are those
composed of sulfur compounds, usually from
coal
• Sulfur is usually emitted as sulfur dioxide, SO2
• It oxides in air, with a time frame of about 1
week
o SO2 + 2 H2O → H2SO4
• H2SO4 is sulfuric acid
33
Sulfuric Acid Properties
• Sulfuric acid is hygroscopic, which means it as a great affinity
for water
• Sulfuric acid rapidly forms droplets in the atmosphere
• These droplets are weaker solutions of sulfur acid, but they
attract still more water, and the droplets grow
• When it rains, the sulfuric acid droplets are washed out of the
atmosphere as acid rain
• Acid rain is a serious environmental problem
• It is typically found in areas downwind of major industrial or
population centers, especially those using coal
34
Other Condensation Nuclei
• Natural condensation nuclei include sea salt,
dust, pollen, and smoke
• Anthropogenic sources include dust and smoke
generated by man
• The next slide shows an example of manmade
cloud formation, as observed by the MODIS
satellite
35
Ship Tracks from Space
36