Upload
others
View
4
Download
0
Embed Size (px)
Citation preview
Course on radiation and climate change
• Lecturer: Martin Wild ([email protected]) (CHN L16.2)
• Language: English
• Please everybody register (otherwise no course info, no grades)
• Copies of lecture slides will be provided
• Complementary practical work (computer lab, NO D39, 3 exercices), provisoric dates: 20.3., 24.4., 22.5. (to be confirmed).
• Course Assistants: Christoph Heim ([email protected] CHN L11) and RuoyiCui ([email protected] CHN L16.1)
• 3 credit points
• Semester test to obtain credit points (benotete Semsterleistung / graded semester performance):
Date of exam: 29.5.2020, written exam. Exam will cover material presented in lectures/exercises
• Website:
http://www.iac.ethz.ch/edu/courses/master/modules/radiation-and-climate-change.html
Website for this course
PDFs of slides available for download
Further reading material is made available on the website
http://www.iac.ethz.ch/edu/courses/master/modules/radiation-and-climate-change.html
Introduction
Global Mean Energy Balance
Wild et al. 2013 IPCC AR5
Radiation and Climate Change FS 2020 Martin Wild
Radiation and Climate Change FS 2020 Martin Wild
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 agriculture, tourism, renewable energy, solar power
Solar power production
Solar power production
Radiation and Climate Change FS 2020 Martin Wild
Radiation and Climate Change FS 2020 Martin Wild
Solar power production
Projected Use of Solar Power 21th Century
Radiation and Climate Change FS 2020 Martin Wild
source: German Advisory Council on Global Change
2000 2100
x 10
18J
Source: Berner Fachhochschule Burgdorf
Insolation on horizontal and tilted (45°) panels 1992-2011
Measured at Burgdorf (Switzerland)
Tilted 45°South
Horizontal plane
Stability of solar energy source
Radiation and Climate Change FS 2020 Martin Wild
• 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• Present day radiation balance of the Earth (observations,
modeling approaches) surface, atmosphere, TOA• Examples of radiation and climate change over Earth’s History• Anthropogenic perturbations of the Earth radiation balance
(greenhouse effect, global dimming)• Impacts of radiative changes on climate system components
Radiation and climate change: contents
Radiation and Climate Change FS 2020 Martin Wild
Radiation and Climate Change FS 2020 Martin Wild
LiteratureGeneral overview:IPCC Reports, since 1990 (www.ipcc.ch)e.g. IPCC 5th assessment report (2013): Climate Change 2013: the physical science basis, Cambridge University Press: Freely available on www.ipcc.ch
6th IPCC assessment report (AR6): published in 2021
Radiation and Climate Change FS 2020 Martin Wild
LiteratureState of the art research is found in peer reviewed journals:Journals of major relevance for this course:
Radiation and Climate Change FS 2020 Martin Wild
LiteratureState of the art research is found in peer reviewed journals:Journals of major relevance for this course:
J. Climate Bullletin of the American Meteorological SocietyJ. Geophys. Res.Geophysical Research LettersACP (Atmospheric Chemistery and Physics)
A selection of relevant articles will be provided on the website
Radiation and Climate Change FS 2020 Martin Wild
1. Physical basis of radiation
- terminoloy and definitions
- basic radiation laws
Energy can be transported by electromagnetic radiation. Electromagnetic waves can be characterized by 3 parameters:
λ n = c
λ : wavelength (m): distance between individual peaks in the oscillation. n: 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/ l): number of wave peaks (or troughs) counted within a fixed length: Unit m-1
Radiation and Climate Change FS 2020 Martin Wild
Electromagnetic waves
Radiation can be described in terms of electromagnetic waves (classical physics), but also in terms of particles (photons) (quantum physicsEinstein 1905)
Energy per photon:E(n)=hn The higher the frequency, the higher the energy of a photon
h=Planck constant, 6.62606957×10−34 J·sn = frequency (s-1)
Energy per frequency interval dn:E(n)=N(n)hndn
N(n)=Number of photons per frequency
Energy per frequency interval equals the number of photons times the energy per photon
Radiation and Climate Change FS 2020 Martin Wild
Particle representation of radiation
Electromagnetic spectrum: classification of the electromagnetic waves according to their wavelengths:
In climatology, only electromagnetic waves with wavelengths between about 0.1 μm and 100 μm (uv, visible light and infrared radiation) are relevant.
Radiation and Climate Change FS 2020 Martin Wild
Electromagnetic spectrum
Terminologies and definitions
Radiation and Climate Change FS 2020 Martin Wild
Shortwave versus longwave radiation
Shortwave often known as solar
Longwave often known as thermal / terrestrial/ (far) infrared
Radiation and Climate Change FS 2020 Martin Wild
Terminologies and definitions
Separation according to wavelengthUltraviolet (UV) radiation
q UV-C 0.20-0.28 µm (completely absorbed/scattered by O3)q UV-B 0.28-0.32 µm (genetic damage, dangerous for skin cancer)
q UV-A 0.32-0.40 µm (skin browning, strengthening of the immune system)
Visible radiation 0.40-0.74 µm
Near Infrared 0.74-4.0 µm
Far Infrared 4.0-100 µm(Longwave)
Radiation and Climate Change FS 2020 Martin Wild
Terminologies and definitions
Separation according to wavelengthUltraviolet (UV) radiation
q UV-C 0.20-0.28 µm (completely absorbed/scattered by O3)q UV-B 0.28-0.32 µm (genetic damage, dangerous for skin cancer)
q UV-A 0.32-0.40 µm (skin browning, strengthening of the immune system)
Visible radiation 0.40-0.74 µm
Near Infrared 0.74-4.0 µm
Far Infrared 4.0-100 µm(Longwave)
Radiation and Climate Change FS 2020 Martin Wild
Source: Sun
Direct radiation
Diffuse radiationReflected radiation
Global radiation=
sum of direct + diffuse
Separation according to originshortwave (< 4 μm)
Terminologies and definitions
Radiation and Climate Change FS 2020 Martin Wild
Terminologies and definitions
Global, direct and diffuse radiation during a cloud-free day
Radiation and Climate Change FS 2020 Martin Wild
Terminologies and definitions
Direct and diffuse radiation during the course of a year
Site in Scotland Site in South Africa
60% diffuse 25% diffuse
Radiation and Climate Change FS 2020 Martin Wild
Measurements from Odessa, Ukraine
Global, direct and diffuse radiation over decades
Terminologies and definitions
Global
Direct
Diffuse
Source: Earth surface + Atmosphere
Outgoing longwave radiation at TOA:
Origin: Earth surface + Atmosphere
Surface downward longwave radiationOrigin: Atmosphere
Surface upward longwave radiation Origin: Earth surface
Separation according to originlongwave (> 4 μm)
Terminologies and definitions
Radiation and Climate Change FS 2020 Martin Wild
Radiation and Climate Change FS 2020 Martin Wild
Terminologies and definitionsOutgoing longwave radiation at the Top of Atmosphere (TOA)
Radiation and Climate Change FS 2020 Martin Wild
Quantification of Radiation
Terminologies and definitions
Term Unit Description
Radiative energy J EnergyRadiative flux W Power, Energy per time (J/s)Irradiance Wm-2 Power per AreaRadiative emittance Wm-2 Power per Area
Radiance Wm-2sr-1 Power per Area per solid angle
Irradiance (Bestrahlungsstärke) F
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)
Similarly: Radiative Emittance: Total amount of radiation emitted from a unit surface per unit time
Irradiance F = total radiative energyarea∗ time
=H
ΔAΔT
Terminologies and definitions
Radiation and Climate Change FS 2020 Martin Wild
Radiation and Climate Change FS 2020 Martin Wild
Radiance (Strahldichte) I:Radiative flux from a specific direction and area on the celestial sphere
(cf. Irradiance: independent of direction of radiation)
Terminologies and definitions
• 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 w
Solid angle w (Raumwinkel)
Apparent area of a radiating element of the celestial sphere
The solid angle is equal to the area of a segment of a unit sphere
surface of the unit sphere: 4π=> w = 2π for the half sphere visible above a given surface
Unit: steradian sr-1 (dimensionless)
Terminologies and definitions
Radiation and Climate Change FS 2020 Martin Wild
Radiance (Strahldichte) I:
Units Wm-2sr-1
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).
Terminologies and definitions
I = potential irradiancesolid angle
=ΔFθΔω
=ΔF
Δω cosθ
Radiation and Climate Change FS 2020 Martin Wild
Units Wm-2sr-1 (steradian). ΔFθ : potential irradianceΔF: energy arriving on the surface in question (irradiance)I : Radiance
From Radiance to Irradiance:
Fraction of irradiance ΔF onto a surface coming from a specific solid angle element Δω, from a direction, defined by the angle θ.
Terminologies and definitions
ΔF = IΔω cosθ = Fθ cosθ Cosine law
If surface is horizontal: θ = zenith angle
Radiation and Climate Change FS 2020 Martin Wild
Zenith Angle and the cosine law
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
with AB= cosθ
⇒ F = FθAB= Fθ cosθ
A
BPotential irradiance Fθ on the surface A equals Irradiance F on the horizontal surface B
Fθ
F θ
θ
Radiation and Climate Change FS 2020 Martin Wild
Zenith Angle and the cosine law
Zenith angle θ: angle between the vector normal to the horizontal surface and the vector pointing to the radiation source (e.g., sun).
A
B
Fθ Fθ cosθ
Irradiance F on horizontal surface: only vertical component of potential irradiance Fθ counts
Fθ *A = F *B
with AB= cosθ
⇒ F = FθAB= Fθ cosθ θ
Radiation and Climate Change FS 2020 Martin Wild
Illustration of cosine law
Radiation and Climate Change FS 2020 Martin Wild
F = Fθ cosθ
Zenith angle Ɵ
Normal angle
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
Radiation and Climate Change FS 2020 Martin Wild
Normal angle
(Cosine) Irradiance collectorcollects radiation from a 180°solid anglePyranometer
Radiance collectorcollects radiation from a specified solid anglePyrheliometer
Measuring irradiances and radiances
Radiation and Climate Change FS 2020 Martin Wild
Radiation and Climate Change FS 2020 Martin Wild
Measuring irradiances and radiances
Radiation and Climate Change FS 2020 Martin Wild
Measuring irradiances and radiances
Measuring irradiances and radiances
Measurements from Mauna Loa Observatory Hawaii
Pyrheliometer
Pyranometer with shading disk
Radiation and Climate Change FS 2020 Martin Wild
Radiation field with radiance distribution I(ϕ, θ)
Dependent on:ϕ: Azimuthθ: Zenith angle
Radiation and Climate Change FS 2020 Martin Wild
Figure 1: Geometry of radiation fields and solid angles
Geometrical relations
Radiation field with radiance distribution I(ϕ, θ)
Dependent on:ϕ: Azimuthθ: Zenith angle
Fraction ofirradiance on horizontal sensor ΔA from solid angle dω
Radiation and Climate Change FS 2020 Martin Wild
Figure 1: Geometry of radiation fields and solid angles
1
sin θ
solid angle element dω = dθ dΦsinθ
Geometrical relations
dω
Radiation and Climate Change FS 2020 Martin Wild
Definition Radiance:
Fraction of irradiance dF onto a sensor surface dA coming from a specific solid angle element dω = dθ dΦsinθ is equal to
and thus from a given celestial area with a solid angle G
and correspondingly from the half sphere above the sensor
Geometrical relations
dF = I(φ,θ )cosθdω = I(φ,θ )cosθ sinθdφdθ€
I =ΔFθΔω
=ΔF
Δω cosθ
€
FG = I(φ,θ)cosθ sinθdφdθG∫∫
FH = I(φ,θ )cosθ sinθ0
2π
∫0
π /2
∫ dφdθ = cosθ sinθ I(φ,θ )dφ0
2π
∫"
#$
%
&'
0
π /2
∫ dθ
Exercices1) 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 2020 Martin Wild
Geometrical relations