Transcript Lectures Chap 5-7 - Saint Leo University Faculty

Module Six
Chapter 5, What Holds the Atmosphere Up?
How the greenhouse effect works
within the temperature structure
of Earth’s atmosphere
 The greenhouse effect is powered by the lapse rate
 Atmospheric scientists call the change in temperature
of the air with altitude the lapse rate
 It is about 6°C colder per kilometer of altitude
 The lower part of the atmosphere is called the
troposphere
Atmosphere
 The troposphere is the lower part of the atmosphere
 It contains about 90% of the air
 It contains all of the weather
 The boundary of low temperature is about 17 km high
on average
 The boundary where the air temperature reaches its
coldest point is the tropopause
 Commercial airplanes fly in the tropopause
Atmosphere with altitude
Atmospheric layers
 Troposphere – about 10 km high, contains 90% of air
and all of the weather
 Tropopause – boundary where air is the coldest,
commercial aircraft area
 Stratosphere – air begins to warm up because of ozone
content
 Mesosphere – not much effect on the weather
 Exosphere – ditto
No temperature contrast,
no greenhouse effect
 Remember the layer model with a skin temperature
 Think of the skin altitude for the air column as some
kind of average altitude from which the IR escapes to
space
 The idea of a skin layer in the atmosphere is fuzzier
than using a glass pane in the layer model but it still a
useful concept
Lapse rate vs. strength of GH effect
 If we increase the GHG concentration of the
atmosphere, the IR radiation to space will originate
from a higher altitude (skin altitude).
 The increase in skin altitude increases the ground
temperature.
 If the temperature of the atmosphere was the same at
all altitudes, then raising the skin temperature would
have no impact on the ground temperature.
More CO2 → higher skin altitude → warmer ground
Pressure as a function of altitude
 The pressure in the atmosphere depends primarily on
the weight of the air over your head
 The weight of the overhead air at sea level is more than
 The weight of the overhead air at the top of a
mountain
 The pressure of the air is non-linear with altitude
(unlike scuba diving, where the pressure is linear with
depth)
Pressure is a non-linear
(exponential) function
What to remember…
 When a gas is depressurized (less pressure) the gas
expands
 When a gas expands, it cools
 When you pressurize a gas it heats up
Expansion, Compression and Heat
 If we had a gas inside a container with a piston, and
pressurized the gas, it would heat up, even in an insulated
container with no heat entering or leaving
 A closed system with no heat coming in or out is called
adiabatic
 If gas is compressed adiabatically, it warms up. It takes
work to compress a gas, the work energy is transferred to
heat
 When it expands, it cools, reversing the process and the gas
cools down
Water vapor and latent heat
 Remember chemistry and the phase change diagram,
where energy is added, the substance stayed at the
same temperature until it completely changed phase,
solid to liquid, or liquid to gas.
 The energy that was added is called latent heat
 Latent heat of fusion between solid and liquid
 Latent heat of vaporization between liquid and gas
In one direction the heat is added,
in the other direction the heat is released.
Phase changes
Solid + heat → liquid (latent heat of fusion) melting
Liquid + heat → gas (latent heat of vaporization) boiling
When the phase change goes in the other direction, the
same amount of energy is released during condensation
or freezing
Vapor → liquid + heat released
Liquid → solid + heat released
Latent heat
 You charge up an air parcel with latent heat when you
evaporate water into it (vapor contains the latent heatnot sensible heat)
 You get the heat back when the water condenses and
the latent heat is released
 A thermometer does not measure latent heat
 A thermometer measures sensible heat (what you can
sense)
Equilibrium conditions
When water is in equilibrium between liquid and vapor,
it’s called saturated, or 100% relative humidity, and the
equilibrium vapor pressure of water will be high.
 Undersaturated occurs when it is cold, the amount of
water vapor is lower than the equilibrium value
 Supersaturated occurs when vapor pressure is higher
than equilibrium, and the vapor tends to condense
into precipitation
Convection
Convection occurs when you heat a fluid from below or
cool it from above (either a liquid or a gas)
 Fluid expands as temperature increases, density
decreases
 Unstable condition causes the fluid column to turn
over
 Warm fluid rises to the top
 The Atmosphere tends to mix when it convects
Air is compressible
 The air is not all the same temperature
 Pressure is higher at the bottom because o f the weight
of the air column
 Compressed air at the bottom heats up
 Because the air is well mixed, the moving air will
always find itself at the same temperature as the rest of
the air in the column
 This is what static stability looks like in a column of
compressible air – the same temperature as the rest of
the column
Convection in the atmosphere
 Driven by sunlight hitting the ground
 Warms the air at the bottom of the column
 Warm air begins to rise, as it rises, it expands, and
cools
 While ascending, it remains lighter and warmer than
the air around it
 If it does not mix on the way up, the air can get all the
way to the top of the column
 If it mixes on the way up, the whole column warms up
uniformly
Moist Convection
The latent heat in water vapor
drives most of the drama in
our weather
Moist convection
 Air at the surface of the Earth with a relative humidity
of 100% rises due to convection
 As the temperature drops, the equilibrium amount of
water vapor decreases
 Supersaturation drives water to condense into droplets
or ice
 The story of cloud formation will continue in chapter 7
Water vapor
 It changes the temperature of the air
 It systematically changes the lapse rate
 Dry convection has a lapse rate of about 10°C
temperature change per km of altitude
 Add the latent heat in moist convection, the lapse rate
decreases to about 6°C per km
 It is possible that the lapse rate of the atmosphere
could be different in a changing climate
Take home points, Chapter 5
 Air in the upper troposphere is colder than air at the
ground because of the process of moist convection.
The process includes the following:
 Convection is driven by sunlight heating the air near the
ground
 The air rises and cools because it expands
 Water vapor condenses, releasing heat as the air rises
Continued…
 The moist convecting air gets colder with altitude, but
not as much as if it were dry
 If the air did not get colder with altitude at all, there
would be no greenhouse effect
Revisit the layers of the
atmosphere
 Troposphere
 Stratosphere
 Mesosphere
Entering outer space:
 Ionosphere
 Exosphere
Chapter 6, Weather and Climate
How the Weather Affects the Climate
Chaos
 10 days is the limit for predicting weather because
weather is chaotic – an extreme sensitivity to initial
conditions, so that small differences between two
states tend to amplify, and the states diverge from each
other
 The butterfly effect, a puff of air from a butterfly’s
wing eventually resulting in a giant storm somewhere
that would not have happened if the butterfly had
never existed
Butterfly effect
 First observed in a weather simulation model
 The model stopped running
 Edward Lorenz restarted it by typing in the variables
like temperature and wind speed
 He had small, insignificant changes, such as rounding
errors
 The model diverged completely from the results of the
initial simulation
Edward Norton Lorenz
 Mathematician
 Edward Norton Lorenz was an American
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mathematician and meteorologist, and a pioneer of
chaos theory. He discovered the strange attractor
notion and coined the term butterfly effect. Wikipedia
Born: May 23, 1917, West Hartford, CT
Died: April 16, 2008, Cambridge, MA
Books: The essence of chaos
Education: Massachusetts Institute of Technology,
Dartmouth College, Harvard University
Weather
 Forecasts rely on computer models
 Small imperfections in the initial conditions and the
model cause the model weather to diverge from the
real weather
 By about 10 days the prediction is worthless
 To overcome the error, run the model may times with
tiny variation in initial conditions
 “an ensemble of model runs”
Climate
 Defined as some time average of the weather
 Climatological January (or any other month) would be
the average of many Januaries
 The weather is chaotic, but the climate generally is not
 The weather would predict rain on a particular day,
whereas the climatologist may predict a rainy season
Averaging
Layer Model
Real World
 Some places much hotter
 Warm and cold
 Some place much colder
 Summers and winters
 Radiative energy budget at
 Day and night
some place could be wildly
out of balance
 Will averaging change the
answer to something
unreasonable?
 Completely balanced energy
budget
 Averaging is valid
Averaging a non-linear system
Top panel – averaging radiative
energy flux (S-B equation) over a
large temperature range introduces
a large bias.
Bottom panel – over the
temperature range of normal Earth
conditions, the blackbody
radiation energy flux is closer to
linear, so averaging over a small
range would be less of a problem
The Fluctuating Heat Budget
 Not stable like a model, but fluctuates widely
 Solar energy comes in only during the day
 Sunshine varies seasonally and by location
 Infrared is radiated day and night
 The energy budget is in balance over a 24 hour period
 But at any time in any spot on the planet, the energy is
usually out of balance
Seasonal variations
The seasons are caused by the tilt of the
Earth relative to its orbit around the sun,
the obliquity
 Wintertime, days are shorter and the sun is lower
 Added over a day – the winter hemisphere has less
sunlight
Seasons are NOT caused by the
Earths distance from the sun
 The eccentricity cycle refers to the shape of the Earth’s
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orbit around the sun
It varies from elliptical, to circular
Currently we are in a near circular orbit
The Earth is actually closer to the sun in January than
it is in July
Seasons are not caused by proximity to the sun
Earth’s seasons are caused by the
tilt of the poles relative to the
orbit, and not by its distance to the
Sun
Incoming flux depends on latitude
and day of the year
Northern
hemisphere
summer is in the
middle of the plot,
which shows flux
as a function of
latitude and time
of the year.
Interesting to note from the plot
 Highest daily fluxes are at the poles during the
summer
 Poles get six months of sunlight
 Sun whirls around in a circle above the horizon (not
overhead)
 Why isn’t it a tropical garden in the summer?
Thermal Inertia
 Damps out the temperature swing between day and
night
 Damps out the temperature swing as the seasons
change
 Even damps out the temperature change of global
warming
Oceans
 Has a tremendous capacity to absorb and release heat
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from the atmosphere
Land – not so much – diffusion through the soil is slow
and only affects the first meter or two
Cool water surface turns over and has convective
mixing to about 100 meters
Maritime areas have milder seasons
Middle of large continents have more intense seasonal
cycles
Averaging a seasonal cycle
 Out of balance because of the heat distribution from
the water and from the wind
 The outgoing heat in the tropics can’t keep up with the
incoming solar radiation
 The heat is carried to cooler, higher latitudes by water
and winds
 The Earth can vent the excess heat to space from the
higher latitudes
Heat carried to higher latitudes for
venting to space
The Coriolis Acceleration
 http://www.youtube.com/watch?v=i2mec3vgeaI
 http://www.youtube.com/watch?v=aeY9tY9vKgs
 http://www.youtube.com/watch?v=iqpV1236_Q0
Two clips on the Coriolis Effect and one shows a Foucault
Pendulum, demonstrating the rotation of the Earth.
Coriolis Effect
 The water and the air feel the most effect at the poles
(incredibly high tides in higher latitudes, nearly no
tide difference at the equator)
 At the equator there is no apparent rotation
 The middle latitudes fall somewhere between these
two extremes
Modeling the Weather
 Fluids are governed by Newton’s Laws of Motion
because fluid has mass and inertia
 Inertia is the sluggishness of matter to resist changes
in motion
 Tendency to keep moving if it’s moving, or remain
stationary if it is already stationary
 To change speed or direction, motion requires a force
such as gravity or a change in pressure (weather)
Bathtub vs. Earth
 Bathtub flows more quickly than the Earth rotates, so
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does not feel the Earth’s rotation
Flows in the atmosphere and ocean persist long
enough to feel the effect of a rotating Earth
Ocean flows can be driven by friction with the wind
Coriolis acceleration tries to deflect the flow to the
right in the northern hemisphere
After a few rotations, a steady state is reached where
the fluid flows 90 degrees to the wind
The eventual
steady state
Top – the fluid initially
flows in the direction of
the wind. Middle – after
a while the Coriolis force
swings the fluid to the
right. Eventually, the
fluid itself flows 90
degrees to the wind or
pressure force, and the
Coriolis force just
balances the wind or the
pressure force. Bottom
– the steady state where
the flow stops changing
and remains steady.
Geostrophic Flow
In a rotating world the fluid will eventually
end up flowing completely crossways to the
direction that It’s pushed. This condition is
called geostrophic flow.
A geostrophic flow balances the forces on it
against each other.
Geostrophic cells on weather maps
 Cells of high pressure and low pressure with flow
going around them
 Low pressure, pressure force points inward, 90° to the
right of that the winds flow counterclockwise in the N.
hemisphere – cyclonic direction of flow
 High pressure, pressure force points outward, and the
flow is clockwise around the high pressure –
anticyclonic direction
Surface wind field from a climate
model (computer generated)
Parameterization,
assumptions in models
 Assume that cloud formation is a function of humidity
in the air, humidity is a parameter that would control
cloudiness
 Effects of turbulent mixing
 Air-sea processes such as heat transfer
 Biology modeling
Take home points chapter 6
 The energy budget to space of a particular location on
Earth is probably out of balance, fluctuating through
the daily and seasonal cycles and with the weather,
This is in contrast to the Layer Model.
 The annual average energy budget for some location
on Earth may not balance either, because excess heat
from the tropics is carried to high latitudes by winds
and ocean currents.
 The global warming forecast requires simulating the
effects of weather, which is a really difficult
computational challenge.
Chapter 7, Feedbacks
Complexity in the Earth system arises form the way
pieces of it interact with each other
Positive and Negative Feedbacks
 A feedback is a loop of cause and effect
 At the center of a feedback is a state variable (average
temperature of the Earth)
 A positive feedback makes the temperature change
larger than it would be without the feedback
 A negative feedback counteracts some of the
external forcing, and tends to stabilize the state
variable
Feedbacks:
A positive feedback is an amplifier
A negative feedback is a stabilizer
Stefan-Boltzmann Feedback
 Negative feedback – a stabilizer
 The radiated infrared heat attempts to pull
the temperature back down
Ice Albedo Feedback
 Positive feedback – an amplifier
 Ice albedo feed works on the state variable of
temperature. An input perturbation, such as a rise
GHG, drives temperature up. Ice melts, reducing the
albedo, and warming the ground up a bit. The
direction of the input and the feedback loop agree
with each other. It can also go in the other direction,
perturbation cools things down and feedback agrees.
Water Vapor Feedbacks
Positive
Negative
 Water is involved in a positive
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feedback loop acting on
global temperature
Warming allows more water
to evaporate before it rains
Water vapor is a GHG
Doubles the climate impact
of rising CO2 concentrations
Without the water vapor
feedback, climate would be
less sensitive to CO2
 There is a negative feedback
loop that controls the amount
of water vapor in the
atmosphere at any given
temperature, having to do
with rainfall and evaporation
(the hydrological cycle)
At the
center of
the
feedback
loop is a
state
variable
Runaway Greenhouse Effect
 It is possible for the water-vapor feedback to feed into
itself
 Means the end of a planets water
 Earth’s climate uses the high latitudes as cooling fins
to avoid the runaway greenhouse effect
 A runaway greenhouse effect stops if the vapor
concentration in the air reaches saturation with liquid
water or ice, so that any further evaporation would just
lead to rainfall or snow
Phase diagram shows that Venus had a
runaway GH effect, but not Earth and
Mars
Triple point of
water
Pressure:
0.006207 atm
Temperature:
0.01°C (273.16 K)
Earth retained it’s water
 Earth has it’s water because of the structure of the
atmosphere
 The tropopause acts as a cold trap, making sure that
water vapor rains or snows out before getting too close
to space
 The oceans are protected by a thin layer of cold air for
billions of years now
 The Hadley circulation controls the distribution of
atmospheric water vapor – warm air rises at the
equator, it cools and water condenses
Clouds
 Cirrus – high altitude – thin and wispy, barely
noticeable, and made of ice crystals
 Cumulus clouds – storm clouds – are towers, the
result of focused upward blasts of convection
 Stratus clouds – low clouds – layered, formed by
broad diffuse upward motion spread out over large
geographical areas
Clouds:
 Interfere with both incoming visible light, and
outgoing IR light
 In the IR, clouds act as blackbodies, warming the
planet
 Incoming visible light is reflected back to space,
cooling the planet
 The overall impact of a cloud depends on which of
these two effects is stronger, which in turn depends on
what type of cloud it is
Earths Energy Budget
The difference between Earth’s
energy budget between absorbed
and scattered sunlight is that when
light is scattered back to space, its
energy is never converted to heat, so
it never enters into the planets heat
budget
Clouds:
 Vary by meteorological conditions and human
pollution
 Cloud droplet size is an important factor
 The smaller the drop, the better it scatters light
 Rain clouds look dark because they have large droplets,
and are optically thick
 Cloud droplets are affected by cloud condensation
nuclei (seeds) that help droplets form
 Sea salt, pollen, dust, smoke, and sulfur compounds
from phytoplankton
Human footprints
 Sulfate aerosols from coal fired power plants
 Internal combustion engines
 Forest fires, heating fires and cooking fires
 Contrails (short for condensation trails) – jet airplanes
passing through clean air containing water vapor
 Persistent spreading contrails are thought to have a
significant effect on global climate
Generalities  You can’t see through low clouds meaning they are
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optically thick
You can see through high clouds, optically thin
High clouds warm, low clouds cool
Clouds that form in dirty air tend to be better light
scatterers with a higher albedo, cooling the planet
Clouds are the largest source of uncertainty in climate
models
Ocean Currents,
el Niño climate oscillation
 Periodic flip flop between two states of the ocean
called el Niño and la Niña
 Ocean interaction with the atmosphere,
corresponding atmospheric cycle called the Southern
oscillation
 ENSO – el Niño Southern Oscillation
 The state of the ENSO affects climate patterns around
the world
El Niño climate oscillation
La Niña
El Niño
 Cool surface water
 Warm surface water
 Productivity high
 Less fertile
 Fisheries good
 Fisheries collapse
 Equatorial E→W wind
 Winds diminish
 Tilted thermocline
 Thermocline collapses
 Wetter weather
 Drier weather
Meridional overturning circulation
in the North Atlantic
 Gulf stream carries warm water from tropics to the
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North Atlantic
Water cools and sinks, making more room for warm
water
Greenland ice cores show instability in Meridional
overturning synchronous with large temperature
swings (~ 10°C) within a few years
8.2k event (8200 years ago) catastrophic freshwater
release to the North Atlantic
Circulation will slow down with melting ice
Terrestrial Biosphere Feedbacks
 Changes in vegetation could alter the albedo of the
land surface when ice melts
 Land surface stores carbon
 Trees evaporate water through transpiration (a selfreplicating cycle)
 Droughts, vegetation dies, soil dries, and the water
shortage is a positive feedback
Carbon Cycle Feedbacks
The subject of the next
three chapters (Module 7)
Feedbacks in the
Paleoclimate Record
 Models tend to under-predict the extremes
of climate variation in the real world climate
 The future may surprise us
Take home points chapter 7
 Positive feedbacks act as amplifiers of variability,
whereas negative feedbacks act as stabilizers.
 The water-vapor feedback doubles or triples the
expected warming owing to rising CO2
concentrations.
 The ice albedo feedback amplifies the warming in high
latitudes by a factor of three or four.
Continued…
 Clouds have a potentially huge impact on climate.
Clouds are expected to exert an amplifying feedback to
climate warming, although the strength of this
feedback is uncertain. Clouds are the largest source
of uncertainty in model estimates of the climate
sensitivity.