course on radiation and climate change - eth z...course on radiation and climate change •...

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

http://env.ethz.ch

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


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

Projected Use of Solar Power 21th Century


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


• 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



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


LiteratureState of the art research is found in peer reviewed journals:Journals of major relevance for this course:


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


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


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


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.


Electromagnetic spectrum

Terminologies and definitions


Shortwave versus longwave radiation

Shortwave often known as solar

Longwave often known as thermal / terrestrial/ (far) infrared



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)


Source: Sun

Direct radiation

Diffuse radiationReflected radiation

Global radiation=

sum of direct + diffuse

Separation according to originshortwave (< 4 μm)




Global, direct and diffuse radiation during a cloud-free day



Direct and diffuse radiation during the course of a year

Site in Scotland Site in South Africa

60% diffuse 25% diffuse


Measurements from Odessa, Ukraine

Global, direct and diffuse radiation over decades


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 definitionsOutgoing longwave radiation at the Top of Atmosphere (TOA)


Quantification of Radiation


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




Radiance (Strahldichte) I:Radiative flux from a specific direction and area on the celestial sphere

(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 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)



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).


I = potential irradiancesolid angle

=ΔFθΔω

=ΔF

Δω cosθ


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.


ΔF = IΔω cosθ = Fθ cosθ Cosine law

If surface is horizontal: θ = zenith angle


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


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


Illustration of cosine law


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


Normal angle

(Cosine) Irradiance collectorcollects radiation from a 180°solid anglePyranometer

Radiance collectorcollects radiation from a specified solid anglePyrheliometer

Measuring irradiances and radiances



Measurements from Mauna Loa Observatory Hawaii

Pyrheliometer

Pyranometer with shading disk


Radiation field with radiance distribution I(ϕ, θ)

Dependent on:ϕ: Azimuthθ: Zenith angle


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


Figure 1: Geometry of radiation fields and solid angles

1

sin θ

solid angle element dω = dθ dΦsinθ


dω


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


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°?



course on radiation and climate change - eth z...course on radiation and climate change •...

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