Please read Chapters 2 and 3 in Hartmann

1
Planetary Energy Balanceand Radiative Transfer

• Please read Chapters 2 and 3 in Hartmann
• I will mix and match among the two chapters

2
Electromagnetic Radiation

• Oscillating electric and magnetic fields
  propagate through space
• Virtually all energy exchange between the Earth
  and the rest of the Universe is by
  electromagnetic radiation
• Most of what we perceive as temperature is also
  due to our radiative environment
• May be described as waves or as particles
  (photons)
• High energy photons short waves lower energy
  photons longer waves

3
Electromagnetic Spectrum of the Sun
4
Spectrum of the sun compared with that of the
earth
5
Blackbodies and Graybodies

• A blackbody is a hypothetical object that absorbs
  all of the radiation that strikes it. It also
  emits radiation at a maximum rate for its given
  temperature.
• Does not have to be black!
• A graybody absorbs radiation equally at all
  wavelengths, but at a certain fraction
  (absorptivity, emissivity) of the blackbody rate
• The energy emission rate is given by
• Plancks law (wavelength dependent emission)
• Stefan Boltzmann law (total energy)
• Wiens law (peak emission wavelength)

6
Blackbody Radiation

• Plancks Law describes the rate of energy output
  of a blackbody as a function of wavelength
• Emission is a very sensitive function of
  wavelength
• Total emission is a strong function of
  temperature

7
Total Blackbody Emission

• Integrating Planck's Law across all wavelengths,
  and all directions, we obtain an expression for
  the total rate of emission of radiant energy from
  a blackbody
• E sT4
• This is known as the Stefan-Boltzmann Law, and
  the constant s is the Stefan-Boltzmann constant
  (5.67 x 10-8 W m-2 K-4).
• Stefan-Boltzmann says that total emission
  depends really strongly on temperature!
• This is strictly true only for a blackbody. For
  a gray body, E eE, where e is called the
  emissivity.
• In general, the emissivity depends on wavelength
  just as the absorptivity does, for the same
  reasons el El/El

8
Red is Cool, Blue is Hot

• Take the derivative of the Planck function, set
  to zero, and solve for wavelength of maximum
  emission

9
Solar and Planetary Radiation

• Earth receives energy from the sun at many
  wavelengths, but most is visible or shorter
• Earth emits energy back to space at much longer
  (thermal) wavelengths
• Because temperatures of the Earth and Sun are so
  different, it's convenient to divide atmospheric
  radiation conveniently into solar and planetary

10
Ways to label radiation

• By its source
• Solar radiation - originating from the sun
• Terrestrial radiation - originating from the
  earth
• By its proper name
• ultra violet, visible, near infrared, infrared,
  microwave, etc.
• By its wavelength
• short wave radiation ? ? 3 micrometers
• long wave radiation ? gt 3 micrometers

11
Molecular Absorbers/Emitters

• Molecules of gas in the atmosphere interact with
  photons of electromagnetic radiation
• Different kinds of molecular transitions can
  absorb/emit very different wavelengths of
  radiation
• Some molecules are able to interact much more
  with photons than others
• Different molecular structures produce
  wavelength-dependent absorptivity/emissivity

12
Conservation of Energy

• Radiation incident upon a medium can be
• absorbed
• reflected
• transmitted
• Ei Ea Er Et
• Define
• reflectance r Er/Ei
• absorptance a Ea/Ei
• transmittance t Et/Ei
• Conservation r a t 1

13
Absorption of Solar Radiation
14
Planetary Energy Balance

• Atmosphere of hypothetical planet is transparent
  in SW, but behaves as a blackbody in LW

15
Planetary Energy Budget
342 W/m2

• 3 Balances
• Recycling greenhouse
• Convective fluxes at surface
• LE gt H

16
The earths orbit around the sun is not quite
circular the earth is closer to the sun in
January than it is in July
Is this why we have seasons?
17
The Earths Orbit Around the Sun

• Seasonally varying distance to sun has only a
  minor effect on seasonal temperature
• The earths orbit around the sun leads to seasons
  because of the tilt of the Earths axis

18
Smaller angle of incoming solar radiation the
same amount of energy is spread over a larger area
High sun (summer) more heating Low sun (winter)
less heating Earths tilt important!
19
NH summer
June 21
Equinox
March 20, Sept 22
NH winter
Dec 21
20
Geometry of Solar Absorption

• Think about geometry of sunlight striking our
  tilted spherical Earth changes with latitude and
  seasons

21
Sun-Earth Geometry
(See appendix A in Hartmann)
22
Top of the Atmosphere Insolation
d Sun-Earth distanceS0 1367 W m-2
hour angle
SZA
lat
declination
(sunrise/sunset)
Total daily TOA Insolation
23
TOA Daily Insolation

• 75º N in June gets more sun than the Equator
• Compare meridional gradient of insolation by
  seasons
• Very little tropical seasonality

24
TOA Daily Insolation(zonal integral)

• Nearly flat in summer hemisphere
• Steep gradient from summer tropics to winter pole

25
Daily Average Solar Zenith Angle(insolation-weigh
ted)
26
Planetary Albedo
Annual Mean

• Global mean 30
• Not the same as surface albedo (clouds, aerosol,
  solar geometry)
• Increases with latitude
• Lower over subtropical highs
• Higher over land than oceans
• Bright spots over tropical continents
• Strong seasonality clouds, sea ice and snow
  cover
• dark shading gt 40light shading lt 20

JJA
DJF
27
TOA Outgoing Longwave Radiation
Annual Mean

• Given by esT4 (which T?)
• Combined surface and atmosphere effects
• Decreases with latitude
• Maxima over subtropical highs (clear air neither
  absorbs or emits much)
• Minima over tropical continents (cold high
  clouds)
• Very strong maxima over deserts (hot surface,
  clear atmosphere)

JJA
DJF
dark shading lt 240 W m-2 light shading gt 280 W
m-2
28
TOA Net Incoming Radiation
Annual Mean

• Huge seasonal switch from north to south
• Tropics are always positive, poles always
  negative
• Western Pacific is a huge source of energy (warm
  ocean, cold cloud tops)
• Saharan atmosphere loses energy in the annual
  mean!
• TOA net radiation must be compensated by lateral
  energy transport by oceans and atmosphere

JJA
DJF
dark shading lt 0 W m-2 light shading gt 80 W m-2
29
Energy Surplus and Deficit

• Absorbed solar more strongly peaked than the
  emitted longwave
• OLR depression at Equator due to high clouds
  along ITCZ
• Subtropical maxima in OLR associated with clear
  air over deserts and subtropical highs

Annual Mean Zonal Mean TOA Fluxes
TOA net radiation surplus in tropics and deficits
at high latitudes must be compensated by
horizontal energy transports in oceans and
atmosphere
30
Energy Budget Cross-Section

• Excess or deficit of TOA net radiation can be
  expressed as a trend in the total energy of the
  underlying atmosphereoceanland surface, or as a
  divergence of the horizontal flux of energy in
  the atmosphere ocean
• Cant have a trend for too long. Transport or
  RTOA will eventually adjust to balance trends.

31
Energy Transports in the Ocean and Atmosphere

• Northward energy transports in petawatts (1015 W)
  
• Radiative forcing is cumulative integral of
  RTOA starting at zero at the pole
• Slope of forcing curve is excess or deficit of
  RTOA
• Ocean transport dominates in subtropics
• Atmospheric transport dominates in middle and
  high latitudes

• How are these numbers determined?
• How well are they known?