Transcript 7_GC1_Atmosphere_an..

Atmosphere structure, Solar Inputs
and the Transport of Heat
Heat, winds, and currents
We will address the following topics....
• Why do the winds blow?
The source of the winds is ultimately the Sun. We’ll discuss how
heating by the Sun generates air flow.
• What influence does the Earth’s rotation have on winds?
Earth’s rotation causes the winds and currents to turn…without this
rotation the climate would be very different.
Solar Insolation controls almost everything:
But it is not just what we get, it is what we keep that defines the
thermal balances.
Structure of the Modern Atmosphere
• Pressure: force exerted
per unit area by the
weight of overlying air
(1 mb = 100 Pa; 1000
mb ~ 1bar ~1 atm)
• Temperature: measure
of the molecular kinetic
energy.
Thermosphere
Thermosphere: upper atmospheric layer with
temperature increasing with altitude
• Heated by absorption of high-energy radiation by oxygen
• Atmosphere is extremely thin, nearly a vacuum. As a result,
Sun’s energy can heat air molecules to very high temperatures
(2500 °C) during the day. But there are so few, it doesn’t really
matter
Auroras occur in thermosphere
Mesosphere
Mesosphere: middle atmospheric layers where
temperature decreases with altitude
• Temperatures as low as -100 C
• Million of meteors burn up daily in the mesosphere, due to
collision with air molecules
Noctilucent clouds (blue-white) over Finland.
Stratosphere
Stratosphere: temperature increases with altitude
due to absorption of UV by ozone
• Ozone is concentrated around an altitude of 25 km in the “ozone
layer”
• Ozone layer protects surface from harmful UV radiation
Troposphere
Troposphere: lowest layer in atmosphere, temperature
decreases with altitude
• Temperature determined
by surface heating
• Well mixed by weather
Shortwave radiation
• Earth receives more solar radiation at low latitudes than high
latitudes.
• Ultimately, it is this solar insolation that provides the heat
that controls weather and climate. It is the imbalance across
the Earth’s surface that controls winds and currents.
ANNUAL
Shortwave radiation
3 factors influence the shortwave radiation
received at Earth’s surface
• Beam spreading: each unit of shortwave radiation is spread
over a larger area away from the equator
Shortwave radiation
3 factors influence the shortwave radiation
received at Earth’s surface
• Beam spreading: each unit of shortwave radiation is spread
over a larger area away from the equator
• Beam depletion: radiation is absorbed and reflected as it
passes through atmosphere
Shortwave radiation
3 factors influence the shortwave radiation
received at Earth’s surface
• Beam spreading: each unit of shortwave radiation is spread
over a larger area away from the equator
• Beam depletion: radiation is absorbed and reflected as it
passes through atmosphere
• Day length: hours of daylight varies with latitude and season
Shortwave radiation
Earth has seasons because its axis is tilted 23.5º
with respect to the plane of the ecliptic
No tilt
Tilted
Why do we have seasons?
Seasonal variations in insolation are greatest at
high latitudes
Shortwave radiation
Earth receives more solar radiation at low
latitudes than high latitudes
Dec-Jan-Feb.
Jun-Jul-Aug
Longwave radiation
Earth emits more longwave radiation at low
latitudes than high latitudes
Longwave radiation
Earth emits more longwave radiation at low
latitudes than high latitudes
Dec-Jan-Feb.
Jun-Jul-Aug
Why is there a difference between Winter and Summer?
Net radiation
Net radiation: total radiation
• Net radiation: shortwave - longwave
• There is an energy imbalance!
Global Energy Balance
Global Energy Balance
Global Energy Balance
Global Energy Balance
Energy Transport
• To make the energy balance, there must
be a transport of energy from low to high
latitudes.
• Radiation is converted to other forms of
energy that can be transported by winds
and currents
• Sensible heat: heat that you can feel
(stored in a substance as a change in
temperature)
• Latent heat: heat required to changes
phases (solid --> liquid --> gas)
Fluid Flow
Newton’s first law: A body at rest remains at rest and
a body in motion remains in constant motion unless
acted upon by an external force
• Fluid flow is driven by forces
• Forces include
- Pressure
- Coriolis
- Friction
Fluid Flow
Pressure: force exerted against a surface due to the
weight of air
• Pressure gradient force: fluid flows from high pressure to low
pressure
Fluid Flow
Pressure: force exerted against a surface due to the
weight of air
• Pressure gradient force: fluid flows from high pressure to low
pressure
- Flow in direction from H to L
- Larger gradient = faster flow
Pressure Differences
Pressure differences arise from temperature
differences.
A
Heating air causes it to expand
B
Equal masses of air
In this example, the masses of
the 2 air columns, A and B,
are equal
COLD
HOT
SURFACE
Pressure Differences
Pressure differences arise from temperature
differences.
A
Top of Atmos.
B
COLD
COLD
COLD
HOT
> mass
The mass of air overlying
column A is greater than
that overlying column B
SURFACE
< mass
Pressure Differences
Pressure differences arise from temperature
differences.
A
Because the mass
is greater in
column A, the surface
pressure (i.e., the weight
of the overlying air) is
greater.
Top of Atmos.
COLD
B
COLD
BONUS!
COLD
HIGH
HOT
LOW
Pressure Differences
Pressure differences arise from temperature
differences.
General Circulation of the Atmosphere
Circulation on a non-rotating Earth
Fluid Flow
Coriolis force: an apparent deflection of moving
objects when observed from a rotating reference frame
Coriolis force is an artificial force
that arises because we are riding
on a rotating rock.
Fluid Flow
Coriolis force: an apparent deflection of moving
objects when observed from a rotating reference frame
Stationary Observer’s Perspective
Consider two children throwing
a ball on a moving merry-goround.
Rotating Observer’s Perspective
Fluid Flow
Coriolis force: an apparent deflection of moving
objects when observed from a rotating reference frame
Stationary observer
The stationary observer sees
the ball moving in a straight
line, and Johnny and Jill
moving in a circle.
Fluid Flow
Coriolis force: an apparent deflection of moving
objects when observed from a rotating reference frame
Moving observer
Johnny and Jill on the merrygo-round perceive that they are
stationary. They see the ball
move to the right.
Fluid Flow
Coriolis force: an apparent deflection of moving
objects when observed from a rotating reference frame
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/crls.rxml
General Circulation of the Atmosphere
Circulation on a rotating Earth
• Trade winds
• Mid-latitude westerlies
• Polar easterlies
DESERTS – THE CONVERGENCE OF ATMOSPHERIC CIRCULATION
CELLS AND THE SURFACE OF THE EARTH
Net radiation
Net radiation: total radiation
• Net radiation: shortwave - longwave
• There is an energy imbalance!
• Cold Poles and Hot Tropics – the drive for circulation in both
the atmosphere and the ocean systems
Circulation of the Ocean
Thermohaline Driving Mechanism
THE ATLANTIC GULF STREAM
WARMING THE POLES – COOLING THE TROPICS
MAJOR OCEAN CURRENT SYSTEMS
GLOBAL SCALE CIRCULATION OF OCEANS – A THERMAL TRANSFER.
SURFACE TEMPERATURE ANOMOLIES