Earth’s Energy Balance and the
Atmosphere
Topics we’ll cover:
• Atmospheric composition
– greenhouse gases
• Vertical structure and radiative balance
– pressure, temperature
• Global circulation and horizontal energy
transport
Main questions part 1- composition
• What are the main constituents of the atmosphere?
• Which gases are most important for the greenhouse effect?
• Which gases are short-lived and which are long-lived?
• At what wavelengths is the “atmospheric window” found?
• How do “greenhouse gases” interact with different kinds of E-M radiation?
• What is a blackbody? How is a gray body different? How is a non-gray
body different?
Main questions part 2 – vertical structure
and radiative balance
• How does pressure change with altitude? Why is it a simple function?
• What is the adiabatic lapse rate? What are the two types?
• Explain what the temperature structure of the Earth looks like, and what occurs in the different layers to cause this. Why is the temperature structure more complex than the pressure structure?
• What is the Stefan-Boltzman equation? What is a blackbody? What is a gray body? What is Wien’s Law? What is Kirchhoff’s Law?
• What is the effective emission temperature of the Sun? How about the Earth? What are the characteristic wavelengths for each?
• What is the surface T of Earth using a single (perfect) GHG layer? Calculate this value.
• How do clouds form?
• How do clouds affect the radiative balance?
Main questions part 3 – horizontal
structure, a.k.a. circulation
• How is incoming solar radiation distributed? How is outgoing longwave
radiation distributed? What does this tell us about horizontal transport of
energy?
• How is this energy transported? In what forms?
• How does the three-cell model of atmospheric circulation work? How does
it relate to energy transport?
• How is the three-cell model inaccurate? What is a better model?
• What are the different climatic zones and associated cloud types?
• How does a global map of shortwave reflectance and longwave emission
relate to these features?
N.B. Any short-answer questions from this lecture for the final exam will be
taken directly from the past three slides – no surprises! The answers to
these questions are found either in the reading or in this presentation.
Atmospheric Composition
Which gases are reactive?
Which are greenhouse gases?
Division between
shortwave and
longwave around 3 to
4 um
5800 K 288 K
Blackbodies emit the maximum
amount of radiation at a given
wavelength
Gray bodies emit less than the
maximum radiation – but it’s a
constant fraction across wavelengths
Non-gray bodies (here called a
“selective radiator”) has an emissivity
that depends on wavelength.
The integral under the black body
curve is given by the Stefan-
Boltzmann equation (reading).
Same is true for gray bodies, where
the emissivity must now be factored
in.
Note that the units are
weird – the absorption
increases downwards.
This is the “atmospheric
window” – the low absorption
region between 8 and 12 um
that permits Earth’s IR to
escape directly to space.
Atmospheric Structure and
Radiative Balance
This occurs mainly because
pressure is determined by the
integral of all the air above
some altitude, so the pressure
is insensitive to local changes.
Pressure decreases exponentially with altitude
Temperature structure more complex than pressure structure
In contrast to pressure, temperature is
determined by the local energy
balance. Therefore, local changes will
be expressed and thereby cause the
temperature structure to be more
complex.
Dry adiabatic lapse rate is a result of trading gravitational
potential energy for internal energy.
Adiabatic = no exchange of energy
between a “system” and its environment.
Dry = no liquid or solid water is present at
any time.
[Water vapor can be present.]
For a “dry” air parcel, its energy has two
components:
1. Internal energy (temperature of air)
2. Gravitational potential energy
If the air parcel rises, it gains (2); if it is
adiabatic, then it must lose (1), i.e. it must
cool.
Moist adiabatic lapse rate is smaller than dry version
because of phase change
If the dew point temperature of the air is
reached, then further increases in
altititude and temperature lead to
condensation.
Condensation releases energy, which
causes the cooling rate to be lower.
Water condenses into small droplets, thus
forming clouds.
Temperature structure more complex than pressure structure
Temperature increase with height due
to absorption of UV by O2.
Temperature decrease with height due
to adiabatic lapse rate
Temperature increase with height due
to absorption of UV by O3.
Temperature decrease with height due
to adiabatic lapse rate
A simple 1-D radiative balance model illustrates the basic
features of the greenhouse effect (see reading)
Assumes:
1. Earth radiates as a blackbody
2. an isothermal GHG layer that radiates like a gray body
Note: this diagram has an
error so don’t solve it.
[Come talk to me after is
you want to know what’s
wrong.]
This is a more sophisticated view of the
Earth’s energy balance. The real world
is more complicated than a simple 1-D
model!
Global Circulation
Differences in temperature lead to winds and global atmospheric
circulation.
However, the circulation affects patterns in temperature.
Hence radiative balance and circulation are coupled.
To maintain steady state
latitudinally, there must be
horizontal transport of energy.
Somewhere between 30 and 40
N/S latitude is the transition
between net energy export and net
energy import.
The atmosphere transports at
least 2/3 of the energy, with
remaining by the ocean. How
would Earth’s temperature change
if the ocean circulation simply shut
down?
[Note that we believe that if this
happened, the atmosphere would
take on most of the remaining
needed energy transport, so the
impact would not be as large as
implied here.]
The imbalance in energy (last
slide) causes atmospheric
circulation (winds).
The winds transport energy and
balance the energy budget.
Northwardheat transport (PW)
Region of
net energy
export
Region of
net energy
import
Region of
net energy
import
The Three-Cell model of
global circulation that is driven
by the pole-to-equator
temperature gradient. Winds
are then affected by coriolis.
<show movie one year simulation>
These images
depict average for
Mar 2000.
Data from CERES
satellite.
Upward longwave fluxes (top) and
shortwave fluxes (bottom) on Feb 26,
2000 (CERES satellite).
All data ~ late morning local time.
