Course on radiation and climate change
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Lecturer:
Martin Wild ([email protected]) (CHN L16.2)
Will Ball ([email protected]) (CHN P14)
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Language: english
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Please everybody register!! (otherways no course infos, no grades)
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Copies of lecture slides will be provided
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Complementary practical work (computer lab, NO D39, 3 sessions, dates will be
announced, likely dates: 15.4, 13.5., 27.5, to be confirmed)
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Course Assistants: Sebastiaan Crezee ([email protected]) (CHN M16.1)
and Matthias Schwarz ([email protected]) (CHN L14)
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There is lecture on Friday 6.5. following ascension day (Auffahrt)
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3 credit points
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Semester test for credit points (benotete Semsterleistung / graded semester
performance):
Website for this course
PDFs of slides available for
download
Further reading material is
made available on the website
Date of exam: 3.6.2016, written exam. Exam will cover material presented in lectures/
exercises and notes distributed during lecture
• 
Website:
http://www.iac.ethz.ch/edu/courses/master/modules/radiation-and-climate-change.html
http://www.iac.ethz.ch/edu/courses/master/modules/radiation-and-climate-change.html
Global Mean Energy Balance
Introduction
Wild et al. 2013
IPCC AR5
Radiation and Climate Change FS 2016 Martin Wild
Radiation and climate change over Earth history
Why study radiation in the climate system?
•  Radiation provides the energy for all climate processes as well as
for the foundation of life on our planet
•  The temporal and spatial variations in the radiation balance are the
major determinants of the thermal and hydrological conditions on
Earth, and the drivers of the atmospheric general circulation and the
global water cycle
•  Anthropogenic interference with the climate system occurs first of
all though a perturbation of the radiation balance (e.g., greenhouse
effect, air pollution, land use change)
•  Radiation key driver of climate evolution over Earth history
•  Practical application in the area of renewable energy, solar power
Radiation impacts climate evolution throughout Earth’s history
Radiation and Climate Change FS 2016 Martin Wild
Radiation and Climate Change FS 2016 Martin Wild
Solar power production
Projected Use of Solar Power 21th Century
233GW
x 1018 J
source: German Advisory Council on Global Change
2015
Radiation and Climate Change FS 2016 Martin Wild
2000
Radiation and Climate Change FS 2016 Martin Wild
2100
Stability of solar energy source
Radiation and climate change: contents
•  Basic radiation laws and definitions
•  Sun-Earth relations
•  Radiative transfer trough the atmosphere and greenhouse
effect
•  Role of radiation in a hierarchy of climate models
•  Radiation and climate change over Earth’s History (faint Sun
paradox, Snowball Earth, Milankovich theory)
•  Present day radiation balance of the Earth (observations,
modeling approaches) surface, atmosphere, TOA
•  Anthropogenic perturbations of the Earth radiation balance
(greenhouse effect, global dimming)
•  Impacts of radiative changes on climate system components
Radiation and Climate Change FS 2016 Martin Wild
Radiation and Climate Change FS 2016 Martin Wild
Literature
Literature
State of the art research is found in peer reviewed journals:
General overview:
IPCC Reports
Journals of major relevance for this course:
(www.ipcc.ch)
e.g. IPCC 5th assessment report (2013): Climate Change 2013: the physical science basis,
Cambridge University Press.
5th IPCC assessment
report (AR5):
AR
Freely available on
www.ipcc.ch
Radiation and Climate Change FS 2016 Martin Wild
Radiation and Climate Change FS 2016 Martin Wild
Literature
State of the art research is found in peer reviewed journals:
Journals of major relevance for this course:
1. Physical basis of radiation
- terminoloy and definitions
- basic radiation laws
J. Climate
Bullletin of the American Meteorological Society
J. Geophys. Res.
Geophysical Research Letters
ACP (Atmospheric Chemistery and Physics)
A selection of relevant articles will be provided on the website.
Radiation and Climate Change FS 2016 Martin Wild
Electromagnetic waves
Energy can be transported by electromagnetic radiation.
Electromagnetic waves can be characterized by 3 parameters:
λν=c
Radiation and Climate Change FS 2016 Martin Wild
Particle representation of radiation
Radiation can be described in terms of electromagnetic waves (classical
physics), but also in terms of particles (photons) (quantum physics
Einstein 1905)
Energy per photon:
E(ν)=hν
The higher the frequency, the higher the energy of a photon
h=Planck constant, 6.62606957×10−34 J·s
ν = frequency (s-1)
λ : wavelength (m): distance between individual peaks in the oscillation.
ν: frequency, units (s−1): number of oscillations that occur within a fixed
(1 sec) period of time.
c: speed of light (ms−1), constant in vacuum c = 299′792′458 ms−1.
In climatology , sometimes wavenumbers rather than wavelengths are
used: wavenumber (= 1/ λ): number of wave crests (or troughs)
counted within a fixed length: Unit m-1
Radiation and Climate Change FS 2016 Martin Wild
Energy per frequency interval:
E(ν)=N(ν)hνdν
N(ν)=Number of photons per frequency
Energy per frequency interval equals the number of photons times the
energy per photon
Radiation and Climate Change FS 2016 Martin Wild
Electromagnetic spectrum
Electromagnetic spectrum: classification of the electromagnetic waves
according to their wavelengths:
Terminologies and definitions
Shortwave versus longwave radiation
Shortwave often known as solar
In climatology, only electromagnetic waves with wavelengths between
about 0.1 µm and 100 µm (uv, visible light and infrared radiation) are
Radiation and Climate Change FS 2016 Martin Wild
relevant.
Longwave often known as thermal / terrestrial/ (far) infrared
Radiation and Climate Change FS 2016 Martin Wild
Terminologies and definitions
Terminologies and definitions
Separation according to wavelength
Separation according to origin
Ultraviolet (UV) radiation
shortwave (< 4 µm)
!  UV-C
0.20-0.28 µm (completely absorbed/scattered by O3)
!  UV-B
0.28-0.32 µm (genetic damage, dangerous for skin cancer)
Source: Sun
!  UV-A
0.32-0.40 µm (skin browning, strengthening of the immune
system)
Direct radiation
Diffuse radiation
Visible radiation 0.40-0.74 µm
Reflected radiation
Near Infrared
0.74-4.0 µm
Global radiation=
Far Infrared
(Longwave)
4.0-100 µm
sum of direct + diffuse
Radiation and Climate Change FS 2016 Martin Wild
Radiation and Climate Change FS 2016 Martin Wild
Terminologies and definitions
Terminologies and definitions
Site in Scotland
Site in South Africa
60% diffuse
Global, direct and diffuse radiation during a cloud-free day
Radiation and Climate Change FS 2016 Martin Wild
Terminologies and definitions
Measurements from Odessa, Ukraine
Direct and diffuse radiation during the course of a year
Radiation and Climate Change FS 2016 Martin Wild
Terminologies and definitions
Separation according to origin
longwave (> 4 µm)
Source: Earth surface + Atmosphere
Outgoing longwave radiation at TOA:
Origin: Earth surface + Atmosphere
Surface downward longwave radiation
Origin: Atmosphere
Surface upward longwave radiation
Global, direct and diffuse radiation over decades
Radiation and Climate Change FS 2016 Martin Wild
25% diffuse
Origin: Earth surface
Radiation and Climate Change FS 2016 Martin Wild
Terminologies and definitions
Outgoing longwave radiation at the Top of Atmosphere (TOA)
Radiation and Climate Change FS 2016 Martin Wild
Terminologies and definitions
Irradiance (Bestrahlungsstärke) F
Terminologies and definitions
Quantification of Radiation
Term
Unit
Description
Radiative energy
J
Energy
Radiative flux
W
Power, Energy per time
Irradiance
Wm-2
Power per Area
Radiative emittance
Wm-2
Power per Area
Radiance
Wm-2sr-2
Power per Area per solid angle
Radiation and Climate Change FS 2016 Martin Wild
Terminologies and definitions
Radiance (Strahldichte) I:
Radiative flux from a specific direction and area on the celestial sphere
Irradiance F =
total radiative energy
H
=
area ∗ time
ΔAΔT
Total amount of radiative energy incident on a unit surface per unit time
Measured in units (Jm-2s-1) or (Wm-2) (Energy per square meter received
per second)
(cf. Irradiance: independent of direction of radiation)
•  Direction defined by the angle θ
between the direction to the source
of the radiation and the vector
normal to the surface
•  If surface is horizontal: θ = Zenith
angle
•  Area defined as solid angle ω
Similarly: Radiative Emittance:Total amount of radiation emitted from a
unit surface per unit time
Radiation and Climate Change FS 2016 Martin Wild
Radiation and Climate Change FS 2016 Martin Wild
Terminologies and definitions
Terminologies and definitions
Solid angle ω (Raumwinkel)
Apparent area of a radiating element of the celestial sphere
Radiance (Strahldichte) I:
I=
potential irradiance ΔFθ
ΔF
=
=
solid angle
Δω Δω cosθ
Units Wm-2sr-1
The solid angle is equal to the area of a segment of a unit sphere
surface of the unit sphere: 4π
=> ω = 2π for the half sphere visible above a given surface
ΔFθ : potential irradiance, if the surface is oriented (with its normal vector)
towards the solid angle element from which the radiation is coming
(surface optimally oriented towards the radiation source).
Unit: steradian sr-1 (dimensionless)
Radiation and Climate Change FS 2016 Martin Wild
Radiation and Climate Change FS 2016 Martin Wild
Terminologies and definitions
From Radiance to Irradiance:
Zenith angle θ: angle between the vector normal to the horizontal surface and
the vector pointing to the radiation source (e.g., sun).
Fraction of irradiance ΔF onto a surface coming
from a specific solid angle element Δω, from a
direction, defined by the angle θ.
Fθ * A = F * B
If surface is horizontal: θ = zenith angle
ΔF = IΔω cosθ = Fθ cosθ
A
= cosθ
B
A
⇒ F = Fθ = Fθ cosθ
B
Fθ
with
Cosine law
Units Wm-2sr-1 (steradian).
ΔFθ : potential irradiance
ΔF: energy arriving on the surface in question (irradiance)
I : Radiance
Radiation and Climate Change FS 2016 Martin Wild
Zenith Angle and the cosine law
A
θ
F
θ
B
Potential irradiance Fθ on the surface A equals
Irradiance F on the horizontal surface B
Radiation and Climate Change FS 2016 Martin Wild
Zenith Angle and the cosine law
Illustration of cosine law
F = Fθ cosθ
Zenith angle θ: angle between the vector normal to the horizontal surface and
the vector pointing to the radiation source (e.g., sun).
Fθ * A = F * B
A
with = cosθ
B
A
⇒ F = Fθ = Fθ cosθ
B
Zenith angle Ɵ
Fθ
A
Fθ cosθ
B
Irradiance F on horizontal surface:
only vertical component of potential irradiance Fθ counts
Radiation and Climate Change FS 2016 Martin Wild
Normal angle
Radiation and Climate Change FS 2016 Martin Wild
Measuring irradiances and radiances
Normal angle: angle between the vector normal to the illuminated
surface, and the vector pointing to the radiation source (e.g., sun).
Zenith angle special case of normal angle with horizontal surface
Normal angle
(Cosine) Irradiance collector
collects radiation from a 180°solid angle
Pyranometer
Radiation and Climate Change FS 2016 Martin Wild
Radiance collector
collects radiation from a specified solid angle
Pyrheliometer
Measuring irradiances and radiances
Measuring irradiances and radiances
Pyrheliometer
Pyranometer with
shading disk
Measurements from Mauna Loa Observatory Hawaii
Radiation and Climate Change FS 2016 Martin Wild
Radiation and Climate Change FS 2016 Martin Wild
Geometrical relations
Geometrical relations
Radiation field with radiance distribution I(ϕ, θ)
Definition Radiance: I =
Dependent on:
ϕ: Azimuth
θ: Zenith angle
ΔFθ
ΔF
=
Δω Δω cos θ
Fraction of irradiance dF onto a sensor surface dA coming from a specific
solid angle element dω = dθ dΦsinθ is equal to
€
dF = I(φ, θ )cosθ dω = I(φ, θ )cosθ sin θ dφ dθ
and thus from a given celestial area with a solid angle G
FG =
∫∫
G
I(φ,θ )cos θ sin θdφdθ
and correspondingly from the half sphere above the sensor
π /2 2 π
Figure 1: Geometry of radiation fields and solid angles
Radiation and Climate Change FS 2016 Martin Wild
€
FH =
π /2
∫ ∫ I(φ,θ )cosθ sinθ dφ dθ = ∫
0
0
0
2π
"
%
$ cosθ sin θ ∫ I(φ, θ )d φ 'dθ
#
&
0
Radiation and Climate Change FS 2016 Martin Wild
Geometrical relations
Exercices
1) Calculate the total irradiance FH from the half sphere above a plane
for an isotropic radiance I(ϕ, θ) = I0.
2) What is the solid angle of the full lunar disk with an angular diameter
of 0.5°?
Radiation and Climate Change FS 2016 Martin Wild