outline further reading: chapter 04 of the text book - matter-energy interactions - shortwave...

Outline

Further Reading: Chapter 04 of the text book

- matter-energy interactions

- shortwave radiation balance

- longwave radiation balance

Natural Environments: The AtmosphereGE 101 – Spring 2007

Boston University

MyneniLecture 07: Atmosphere-Surface-Energy-Budget

Jan-31-07(1 of 12)

- global radiation balance

Introduction


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Jan-31-07(2 of 12)

• Previously, we studied

Insolation (a) Seasonal/latitudinal patterns in incident radiation (b) Latitude zones

Composition of the Atmosphere (a) Evolution (b) Current composition (c) State variables

• Today, we begin by looking at the fate, transformations, and exchanges of absorbed radiation within system

To do this we begin to look at how radiation interacts with matter


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Jan-31-07(3 of 12)

Radiation and Matter

Absorption: • Radiation is absorbed by matter (liquid and gases) in the atmosphere• Leads to heating

Transmission: • Radiation passes through the atmosphere and does not interact with matter• Does not affect the atmosphere

Scattering: • Radiation is reflected and scattered by matter in the atmosphere, but is not absorbed• Also does not affect the atmosphere• Results in a change of wavelength and/or direction of travel

Interactions depend on wavelength and type of matter


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Jan-31-07(4 of 12)

Example: The Ozone Hole

• In the upper part of the atmosphere, there is a layer with high concentrations of ozone• Although transparent to most solar radiation, ozone selectively absorbs high-energy UV solar radiation• This is actually vital for life on earth - high-energy UV radiation can easily break molecular bonds in biological organisms, leading to mutations, particularly in the form of cancer • When this disappears, this UV energy gets through to the ground


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Jan-31-07(5 of 12)

Incoming Solar Radiation

• It turns out that the clear atmosphere is relatively transparent to solar radiation• However, when we introduce clouds, we find that the transmission is cut down. In addition there is more reflection as well as more absorption in the atmosphere• For right now though, we will just be considering average conditions

• As the above example suggests, what happens to radiation depends on its wavelength Atmosphere is transparent to solar radiation (i.e. it reaches the surface and heats it) Atmosphere absorbs longwave radiation (i.e. it heats the atmosphere)

Lets first consider solar radiation


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Jan-31-07(6 of 12)

Shortwave Radiation Budget• Typically, to aid with discussion, we use ‘dimensionless units’ where 100 represents an amount equal to what comes from the sun

Reflection (Albedo):• 3 reflected to space by atmosphere• 19 reflected by clouds• 9 reflected by the surface

Absorption in the atmosphere:• 18 absorbed by atmosphere• 3 absorbed by clouds

Absorption by the surface:• 48 absorbed at the surface• This radiation is absorbed, heats the surface and is converted to

Longwave radiation Sensible and latent heat


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Jan-31-07(7 of 12)

Longwave Radiation

• The surface emits longwave radiation (A and B), some of which escapes to the free space (A)• As opposed to solar radiation, the atmosphere is strongly absorptive to longwave radiation• Hence, the atmosphere “traps” outgoing longwave flux emitted by the surface (B)• This absorbed long-wave radiation goes to heating the gas molecules in the atmosphere• In turn, the atmosphere emits longwave radiation

Some of this radiation is emitted to space (C) But some of it is also emitted back towards the earth (D)

• Hence emission of longwave radiation from the atmosphere back to the surface represents another form of radiative heating of the surface


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Jan-31-07(8 of 12)

Longwave Radiation Budget

• We will continue use the dimensional units

From surface: 6 escapes to space 107 is radiated by surface and absorbed by atmosphere

From Atmosphere: 97 is re-radiated by atmosphere and is absorbed by the surface 63 is re-radiated to atmosphere and escapes

Why is more radiation emitted from the surface of the earth than is absorbed through solar radiation?

Because of the “Natural Greenhouse Effect”


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Jan-31-07(9 of 12)

The Greenhouse Effect

• Refers to the emission of longwave radiation by the surface, absorption by the atmosphere, and re-radiation back to the surface

• Because of the re-radiated longwave energy, the earth receives more than just energy from the sun

• Note that the earth receives more longwave energy from the atmosphere than it does directly from the sun

• Can consider this to be ‘recycling’ of energy within the earth system

• This is why the surface temperature of the earth is greater than 255K expected by simple radiation balance considerations


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Jan-31-07(10 of 12)

Surface Radiation Balance

48

Solar Radiation

HeatingIncoming Solar Radiation 48Incoming Longwave Radiation 97

Total 145

CoolingOutgoing Longwave Radiation 113Latent Heat 22Sensible Heat 10Total 145


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Jan-31-07(11 of 12)

Atmospheric Radiation Balance

Incoming Solar Radiation 21Incoming Longwave Radiation 107Latent Heat 22Sensible Heat 10Total 160

Outgoing Longwave Radiation(to space) 63

Outgoing Longwave Radiation(to Earth) 97

Total 160

Heating Cooling


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Jan-31-07(12 of 12)

Global Radiation Balance

Incoming Solar Radiation 100

Total 100

Reflected to space 31Longwave Radiation from Earth 6Longwave Radiation from Atmos. 63Total 100

outline further reading: chapter 04 of the text book - matter-energy interactions - shortwave...

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