Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 08:Patterns of Radiation
Feb-07-05
(1 of 10)
Further Reading: Chapter 04 of the text book
Outline
- geographic patterns of energy balance
- net radiation
- meridional transport
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 08:Patterns of Radiation
Feb-07-05
(2 of 10)
Introduction
•
Previously, we discussed the energy balance on a global scale and found that there are
complex interactions between subsystems but that overall there is no net gain or loss of
energy within any subsystem
•
Has important implications:
– Global energy balance is the primary control on global climate
– The natural Greenhouse effect (i.e. absorption and re-emission of longwave radiation
by the atmosphere) is the key to temperature control of the entire system
– The concept of thermal equilibrium suggest that the system establishes an energy
balance by adjusting the temperature of the system
• If incoming energy is greater than outgoing energy, then the temperature increases,
increasing the outgoing radiation
• This is the key to understanding the global warming debate: if humans add CO2, the
Greenhouse effect increases, more re-radiated energy reaches the surface, hence incoming
radiation increases, thus the temperature increases, increasing outgoing radiation until the
system is balanced again
• Although the system strives for thermal equilibrium, changes in the system allow for
different climates -> “climate change”
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 08:Patterns of Radiation
Feb-07-05
(3 of 10)
Insolation
• Today, we will look at the geographic patterns in the radiation balance
 Insolation
 Albedo
 Longwave Radiation
 Net Radiation
Insolation:
• depends on latitude & time of the year
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 08:Patterns of Radiation
Feb-07-05
(4 of 10)
Albedo
• Albedo is the fraction of incident shortwave radiation that is reflected by a surface
• It determines the amount of solar radiation absorbed by a surface
• Albedo is high near the poles (snow & ice), low over water bodies, medium over land
• Albedo over equatorial lands can be high due to persistent cloud cover
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 08:Patterns of Radiation
Feb-07-05
(5 of 10)
Absorbed Shortwave Radiation
More solar radiation is absorbed by the oceans than adjacent lands
about the equator due to clouds on land
Less solar radiation is absorbed at the poles than at the equator (in general)
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 08:Patterns of Radiation
Feb-07-05
(6 of 10)
Outgoing Longwave Radiation
• Outgoing longwave radiation is very similar to absorbed shortwave radiation
• This is because longwave radiation is determined by the underlying temperature which
is determined by the amount of incoming solar radiation
 High in tropics
 Decreases towards poles
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 08:Patterns of Radiation
Feb-07-05
(7 of 10)
Net Radiation
– Remember, globally net radiation was zero, i.e. incoming balanced outgoing radiation
– This is not true for given latitudes
• At the tropics, the incoming is greater than the outgoing due to high insolation and low
albedo
• At the poles, outgoing is greater than incoming due to low insolation and high albedo
– Why does the temperature at the equator not rise and the temperature at the poles not
decrease?
– Because energy is physically transported from the equator to the poles by the atmosphere
and the ocean -> it is this difference in energy between the poles and equator which drives
almost all dynamics in the system
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 08:Patterns of Radiation
Feb-07-05
(8 of 10)
Meridional Transport
• Driven by gradient in TOA net radiation
• Net Radiation at TOA
 Low latitudes: Positive
 High latitudes: Negative
• Outgoing energy partly derived from tropics
• Transport accomplished via two mechanisms (Atmosphere & Oceans)
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 08:Patterns of Radiation
Feb-07-05
(9 of 10)
Meridional Transport via The Atmosphere
Transports warm tropical air to high latitudes; e.g., tropical air masses
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 08:Patterns of Radiation
Feb-07-05
(10 of 10)
Meridional Transport via The Oceans
Transports warm tropical water to high latitudes; e.g. Gulf Stream