Earth Science 1111

1
Earth Science 1111

• Lecture 2 Electromagnetic Radiation and the
  Global Energy Budget

2
Means of Energy Transfer

• Energy can be transferred from one area to
  another using three means
• Conduction
• Convection
• Radiation

3
Conduction

• Conduction is the transfer of energy directly
  from molecule to molecule
• This mode of energy transfer is most important in
  solids
• An example of this is a metal spoon left in a pot
  on the stove. Energy will travel slowly up the
  spoons handle.
• It is important in the atmosphere only in the
  contact layer, or the lowest 2 mm of the air.

4
Convection

• Convection is the means of energy transfer
  through fluid motions
• Most important in fluids, such as liquids and
  gases
• Hot fluid rising and cold fluid sinking in a pot
  of boiling water is an example
• Convection is very important in the troposphere
  (thunderstorms are a form of convection).

5
Radiation

• Radiation is the transfer of energy by
  electromagnetic waves, which can travel through
  empty space
• The earth receives the majority of its energy
  from the sun, and it is received by radiation
• All objects having a temperature above absolute
  zero emit electromagnetic radiation
• Electromagnetic radiation can be viewed either as
  waves or particles (photons)

6
Electromagnetic Waves

• Wavelength refers to the distance between one
  part of a wave (peak) and the same part of the
  next wave
• There is a huge range of possible wavelengths
  from the very short gamma rays (ten-millionth of
  a millimeter) to the very long radio waves
  (thousands of meters)

7
Electromagnetic Spectrum

• FIGURE 2.2, PAGE 19

8
Electromagnetic Spectrum

• The complete set of all possible wavelengths,
  from radio to gamma rays, is called the
  electromagnetic spectrum
• Climatologically important radiation is between
  0.1 to 100 µm
• The human eye is sensitive to a small portion
  between 0.4 µm (violet) and 0.7 µm, called
  visible light
• Wavelengths immediately shorter than violet light
  are called ultraviolet, and wavelengths
  immediately longer than red are called infrared

9
Radiation Laws

• The amount of radiation emitted by an object at a
  particular wavelength depends on the temperature
  of that object.
• The basic law of radiation that governs the
  amount of radiation emitted by an object at a
  particular wavelength is called Plancks Law.

10
Black Bodies

• If an object were to emit precisely the amount of
  energy specified by Plancks Law, that object is
  referred to as a black body.
• Black bodies are theoretical. The emissivity is
  the degree to which that object behaves like a
  perfect black body.
• Solids usually have emissivities between 0.9 and
  1.0.
• Gases dont behave like black bodies. They
  radiate only at specific wavelengths.

11
Planck Curves

• FIGURE 2.3, PAGE 20

12
Wiens Law

• The wavelength of maximum emission (where the
  peak in the Planck curve exists for that object)
  is given by Wiens Law
• ?max 2897
• T
• Since temperature is in the denominator, an
  increase in temperature results in a decrease in
  the maximum wavelength emitted.

13
Stefan-Boltzmann Law

• The total area underneath the Planck curve
  represents the total energy emitted by that
  object.
• The total energy emitted by an object is given by
  the Stefan-Boltzmann Law
• E sT4
• s is a constant value
• Note that the total energy emitted is
  proportional to the temperature to the 4th power!
• If temperature were to double, the total energy
  emitted would be 16 times greater!

14
Kirchhoffs Law

• At a particular wavelength, a good emitter is
  also a good absorber.
• Ozone is a good absorber of ultraviolet light.
  Theoretically, it would also emit ultraviolet
  light as well. However, the temperature of ozone
  doesnt allow for the emission of ultraviolet
  light, so it is simply absorbed and not
  re-emitted by ozone.

15
Planck Curves

• FIGURE 2.3, PAGE 20

16
Radiation Laws

• Plancks Law, Wiens Law, and Stefan-Boltzmanns
  Law all relate to black bodies.
• The emissivity must be included in each equation
  for any object that is not a black body.

17
Solar Radiation

• The Sun has a surface temperature and Planck
  curve of an object at 5800 Kelvin
• Kelvin temperature is equal to the temperature in
  Celsius 273
• The peak color (Wiens Law) is blue-green for the
  Sun, but we see yellow because of a variety of
  factors (eye sensitivity, interactions with the
  atmosphere, and the shape of the curve is thicker
  on the yellow end.

18
Solar Radiation

• We receive only 4.5 x 10-10 of the total energy
  emitted by the Sun, but this energy drives the
  climate system
• 99 of the solar radiation emitted has a
  wavelength between 0.15 and 4 µm.
• 9 is in the ultraviolet band
• 45 is visible light
• 46 is infrared

19
Solar Constant

• The solar constant is the amount of energy
  passing in a unit time through a unit surface
  perpendicular to the Suns rays at the outer edge
  of the atmosphere at the average Earth-Sun
  distance.
• The solar constant is about 1372 Watts per square
  meter.
• The Sun is a very stable star, but the solar
  constant is known to vary on a number of time
  scales due to solar evolution, sunspots, etc.

20
Earths Orbit of the Sun

• The Earth revolves around the Sun in an
  elliptical orbit in the period of one year.
• The eccentricity is a measure of departure from a
  perfect circle.
• For the Earth, the eccentricity is small.
• The eccentricity changes very slowly over time,
  but has a very minor impact on climate over our
  lifetimes.
• We are closest to the Sun in January
  (perihelion), and farthest from the Sun in July
  (aphelion).
• Obviously, eccentricity is not what drives the
  seasons.

21
The Earths Axis of Rotation

• The Earth makes one complete spin about its axis
  over the course of 24 hours.
• The Earths axis is tilted 23 ½ relative to the
  plane that we revolve around the Sun.
• Because of the tilt, the duration of daylight and
  the height of the Sun in the sky changes during
  the course of a year.

22
The Earths Orbit

• FIGURE 2.4, PAGE 22

23
The Earths Tilted Axis

• On the summer solstice, the Suns rays are
  striking the Tropic of Cancer (23 ½ North
  Latitude) directly.
• On the winter solstice, the Suns rays are
  striking the Tropic of Capricorn (23 ½ South
  Latitude) directly.
• On the vernal and autumnal equinoxes, the Suns
  rays are striking the Equator directly.
• It is because of the tilt of the Earths axis,
  and the resultant change in sunlight, that we
  experience seasons.

24
INSOLATION

• FIGURE 2.5, PAGE 22

25
Insolation

• Insolation is an acronym that stands for INcoming
  SOLar radiATION.
• The North Pole gets more insolation than any
  other latitude on the summer solstice.
• The North Pole gets NO direct insolation between
  the equinoxes
• The Equator receives a consistently high amount
  of insolation throughout the course of a year.
• This spatial and temporal variation is vital for
  creating our climate system.

26
Radiation/Atmosphere Interactions

• Radiation passing through the atmosphere can only
  have one of three things happen
• Scattering When radiation interacts with small
  particles, the direction of travel for the photon
  changes (reflection is a special case of
  scattering where the particle is BIG and there is
  a complete change in direction).
• Transmitted The photon passes through
  unimpeded.
• Absorbed The photon is captured by an atom or
  molecule in the atmosphere, causing the energy of
  that atom or molecule to increase.

27
Scattering

• The direction of scattering can be basically
  broken down into two directions
• Upward unless it is scattered back down again,
  the photon is lost to space.
• Downward the photon is still in the climate
  system and able to interact later on with the
  climate system.

28
Rayleigh Scattering

• When the scattering particles are small compared
  to the wavelength of radiation.
• This is the case for air molecules.
• The amount of scattering is inversely
  proportional to wavelength (shorter wavelengths
  are scattered more).
• Scattering of blue light is 10 times greater than
  the scattering of red light.
• Rayleigh scattering is why the daytime sky is
  colored blue.
• For an evening red sky, the longer path length
  through the atmosphere results in most of the
  wavelengths being scattered out, including red
  being scattered out slightly.

29
Mie Scattering

• When the size of the particles are comparable to
  the wavelength of radiation.
• Cloud water droplets and pollution particles
  cause Mie scattering.
• All wavelengths are scattered in the same manner,
  which results in a light blue or greyish sky

30
Scattering

• FIGURE 2.6, PAGE 24

31
Multiple Scattering

• A photon may be scattered numerous times.
• Between or within clouds is a prime area for
  multiple scattering.
• Whiteout pertains to the condition where low
  clouds and snow cover results in multiple
  scattering which doesnt allow the horizon to be
  distinguished.

32
Albedo

• A measure of the amount of scattering and
  reflection of radiation.
• The textbook equation (2.10) has an error.
  Albedo is the amount of shortwave energy sent up
  divided by the total amount coming down (it
  represents the percentage reflected).
• Planetary albedo is the albedo at the top of the
  atmosphere for the entire globe (about 30).
• Surface albedo is the albedo for a local area
  which is highly dependent upon the nature of the
  surface (significant for local climates).

33
Absorption

• Absorption of shortwave radiation is a relatively
  small amount compared to the amount of
  scattering.
• The atmosphere is transparent to the visible
  wavelengths.
• Strong absorption of ultraviolet radiation
  happens with ozone and oxygen in the stratosphere
  (ozone layer). This leads to a strong heat
  source (why the stratosphere has temperature
  increasing with height).
• Only about 18 of the insolation entering the top
  of the atmosphere is absorbed by the atmosphere
  (most UV). Clouds absorb a remaining 2 for a
  20 total absorption.

34
Energy at the Surface

• Direct radiation are those photons that were not
  scattered (direct solar beam).
• Diffuse radiation are those photons that have
  been scattered (skylight).
• Both direct and diffuse radiation behave in the
  same way energetically at the surface.
• A maximum amount of insolation is received at the
  surface in the subtropical deserts (high sun
  angle combined with a lack of cloud cover)
• A minimum amount of insolation is received at the
  poles (low sun angle with a high albedo surface).
• The radiation that reaches the surface must
  either be reflected or absorbed.

35
Insolation at the Surface

• FIGURE 2.7, PAGE 29

36
Surface Albedo

• The percentage of insolation reflected by the
  Earths surface
• Varies with the type of surface
• Not wavelength dependent (single value)
• Natural surfaces have a 10-25 albedo
• Water albedo depends on sun angle
• Snow has a high albedo. Clean snow will last
  longer than dirty snow.
• Global average is 15 (water dominates the
  surface of the Earth)

37
Albedos and Emissivities at Surface

• TABLE 2.1, PAGE 29

38
Surface Absorption

• Because most surfaces have a low albedo, the
  majority of insolation reaching the surface will
  be absorbed.
• Of the entire insolation that reaches the top of
  our atmosphere, 50 will be absorbed by the
  surface (recall 20 will be absorbed by the
  atmosphere).
• The remaining 30 is reflected back out to space
  is results in our planetary albedo of 30.

39
Short-wave Budget

• FIGURE 2.1(A), PAGE 18

40
Surface Heating

• The primary source of heating for the lower
  atmosphere is the underlying surface of the Earth
• With the heat source from beneath,
• Vertical air motions are established (convection)
• Vertical air motions are critical for cloud
  formation
• Different surface types can result in regional
  weather and climate differences

41
Terrestrial Radiation

• Absorption of solar radiation at the surface
  leads to heating
• Almost all surface types have an emissivity
  greater than 0.9, which means the Earths surface
  is fairly close to a blackbody for long wave
  (infrared) radiation.

42
The Atmosphere

• The atmosphere is not a blackbody
• The values of absorptivity and emissivity vary
  depending on wavelength
• Each individual gas will absorb only specific
  wavelengths, and this appears as spectral
  absorption lines on the radiation curves
• Lines of spectral absorption can be grouped
  together to form absorption bands
• The location of the bands depend on the atomic
  structure of the gas
• Major absorbers of infrared are water vapor,
  carbon dioxide, and to a lesser extent, ozone and
  methane

43
The Atmosphere and Infrared

• At wavelengths greater than 3 µm are almost all
  absorbed by one gas or another
• The exception is a narrow band between 8 µm and
  11 µm, called the atmospheric window
• Therefore, most long wave radiation emitted by
  the surface is absorbed by the atmosphere

44
Long-Wave Budget

• FIGURE 2.1(B), PAGE 18

45
Atmospheric Heating

• Atmospheric heating is the total short wave
  radiation absorbed directly by sunlight, plus the
  absorbed long wave radiation emitted by the
  Earths surface
• This atmospheric heating causes the atmosphere to
  also emit long wave radiation
• Upward Outgoing long-wave radiation that is
  eventually lost to space
• Downward Incoming long-wave radiation can be
  eventually received by the surface again

46
Clouds and Infrared

• Clouds close the atmospheric window and absorb
  those extra infrared bands
• Cloudy nights dont cool down as rapidly as clear
  nights because the clouds act like an insulating
  blanket and prevent outgoing long-wave radiation
  from leaking out to space

47
Greenhouse Effect

• Solar energy passes mostly unimpeded through the
  atmosphere and is absorbed at the surface
• Outgoing long-wave radiation off the surface is
  absorbed by the atmosphere
• A portion of the absorbed long-wave radiation is
  sent back down to the surface
• The surface has a temperature 30 C warmer than
  it normally would have
• This process is termed the greenhouse effect

48
Greenhouse?!

• Greenhouses are certainly warm places on sunny
  days
• However, greenhouses are not warm because of the
  trapping of infrared radiation
• A greenhouse is warm because convection is
  inhibited by the glass (the warm air is stuck
  inside the greenhouse)
• Therefore, the term greenhouse effect is a
  misnomer and does not apply to the atmosphere

49
Greenhouse Effect vs. Global Warming

• The greenhouse effect is a good thing,
  otherwise we would be an ice planet
• The terms greenhouse effect and global
  warming do not mean the same thing
• Global warming is the concern that, by increasing
  the gases that trap infrared radiation in our
  atmosphere, we will increase the average surface
  temperature of the Earth

50
Global Radiation Budget

• Over long time scales, and over the entire globe,
  the amount of incoming short-wave radiation
  equals the amount of outgoing long-wave radiation
• This results in a global radiation balance
• Some minor variations can occur, and this drives
  climate variations

51
Smaller-Scale Radiation Budgets

• Within the long-term global energy balance, there
  are local and short-term imbalances
• These imbalances drive the weather and climate
  system
• In calculating radiation budgets, we are
  concerned with only two levels
• Top of the atmosphere
• Surface of the Earth

52
Top of the Atmosphere Budget

• The radiation budget for the top of the
  atmosphere looks like this
• Q K Lup
• Net Radiation absorbed -
  outgoing long-wave
• 
  short-wave from below
• 
  below
• We only have a balance when Q is zero (when K
  Lup)

53
Variations in K

• The absorbed solar radiation beneath the top of
  the atmosphere depends on
• Albedo
• Total amount of solar radiation incoming
• The poles receive less because of a low sun angle
  and high albedo
• There is a considerable difference between the
  Equator and the poles

54
Average Short/Long-Wave Budget by Latitude

• FIGURE 2.10, PAGE 33

55
Variations in L

• There is less of a difference between the amount
  of outgoing long-wave radiation at the Equator
  and at the poles
• The atmosphere and oceans are very efficient at
  moving energy from the tropics to the polar
  regions, and this makes the difference less

56
Seasonal Variations in K L

• Because of snow and ice, there is a seasonal
  variation in surface albedo, especially in the
  Northern Hemisphere
• Because K is less in the winter due to a higher
  albedo, L is also lower because of lower surface
  temperatures
• Changes in albedo also occur in the mid-latitudes
  and tropical regions due to changes in
  vegetation, but this is a smaller variation

57
Seasonal Variations in K L

• FIGURE 2.11, PAGE 34

58
Surface Energy Budget

• The surface energy budget also experiences a
  long-term global balance with local and
  short-term variations
• The energy budget for the surface is
• Q Kdown Kup Ldown Lup
• net radiation incoming outgoing
  incoming outgoing
• short-wave short-wave
  long-wave long-wave

59
Surface Energy Budget

• The values in the equation can change over many
  time scales, including daily
• The solar radiation term, which depends on
  latitude, season, time of day, clouds, etc., is
  the most variable of all the terms
• The long-wave term is less variable, but it does
  change with the surface temperature and humidity
  values, which can change by the wind
• At night, Q (net radiation) is most likely to be
  negative (only L is active), depending on clouds
  and winds
• Other factors need to be included later to
  balance it out

60
Diurnal Variation in Surface Budget

• FIGURE 2.12, PAGE 35