Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online...

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Lecture 5: Thermal Emission Chapter 6, Petty k Prof. Cheng-ta Cheng for sharing his online notes on this subject
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Transcript of Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online...

Page 1: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Lecture 5: Thermal Emission

Chapter 6, Petty

We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Page 2: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Thermal Emission• Principal source of longwave radiation. Emission is the process by

which some of the internal energy of a material is converted into radiant energy.

• We absorb longwave radiation that is emitted by our environment and our own body also lose heat energy via

emission of radiation. (but why don’t we feel it)

• Possible to derive the relationship between temperature and thermal emission from first principle (quantum mechanics and statistical thermodynamics).

• Here we will just directly explain the general characteristics of thermal emission.

Page 3: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

• In thermodynamics, the internal energy is the total energy contained by a thermodynamic system

• Internal energy has two major components, kinetic energy and potential energy. The kinetic energy is due to the motion of the system's particles (translations, rotations, vibrations), and the potential energy is associated with the static constituents of matter, static electric energy of atoms within molecules or crystals, and the static energy of chemical bonds

• First Law of Thermodynamics/ Internal Energy

http://www.youtube.com/watch?v=Xb05CaG7TsQ

Page 4: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Key facts about thermal emission• Planck’s function

Page 5: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Planck curve

Page 6: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Planck’s Function

• Blackbody doesn't emit equal amounts of radiation at all wavelengths

• Most of the energy is radiated within a relatively narrow band of wavelengths.

• The exact amount of energy emitted at a particular wavelength lambda is given by the Planck function:

Page 7: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Planck’s function

• Planck’s function is seen to have its peak at a wavelength that is inversely proportional to absolute temperature.

- At any given wavelength, emission increases monotonically with increasing temperature.

- Emission is not symmetrically distributed about its peak.

Page 8: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Planck’s Function (cont.)

Page 9: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Solar Spectrum

Page 10: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Intensity and Wavelength of Emitted Radiation : Earth and Sun

Page 11: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Wein’s Displacement Law

mT = 2897.9 m K

Gives the wavelength of the maximum emission of a blackbody, which is inversely proportional to its temperature

Earth @ 300K: ~10 mSun @ 6000K: ~0.5 m

Page 12: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

What is a “blackbody”

- An object that absorbs radiation perfectly

- absorptivity, a=1

Page 13: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.
Page 14: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Answer:

Page 15: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Stefan-Boltzmann Law

Page 16: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Stefan-Boltzmann Law

Page 17: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Emissivity and Kirchoff’s Law

Actual irradiance bya non-blackbodyat wavelength

Emittance: Often referred to as emissivity

Emissivity is a function of the wavelength of radiation and the viewing angle and) is the ratio of energy radiated by the material to energy radiated by a black body at the same temperature

absorbed/ incident

Absorptivity (r , reflectivity; t , transmissivity)

Page 18: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Stefan-Boltzmann Law

Answer:

Page 19: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Rayleigh-Jeans Approximation

Page 20: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.
Page 21: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Rayleigh-Jeans Approximation

Page 22: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Emissivity

• What is a “emissivity”?

-the ratio of what is emitted by a given surface to what would be emitted if it were a blackbody.

-emissivity at a single wavelength (remote sensing, intensities)

- emissivity over a broad range of wavelengths (energy transfer, fluxes).

Page 23: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Monochromatic Emissivity

Page 24: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Graybody Emissivity

Page 25: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.
Page 26: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

(Jin and Liang, 2006 J. of Climate)MODIS broadband emissivity

Jan 2003

July 2001

Page 27: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Kirchhoff’s Law

Page 28: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Kirchhoff’s Law• Local Thermodynamic Equilibrium

Page 29: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Brightness temperature

Page 30: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Brightness temperature

Page 31: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Brightness temperature

Answer:

Page 32: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Brightness temperature

Answer:

Page 33: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Brightness temperature

Answer:

Page 34: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

When Des Thermal Emission Matter?

• When one can and can’t ignore thermal emission from the earth and the atmosphere?– At what wavelength? Solar radiation only,

Both solar radiation and thermal emission, Thermal Emission only?

4μm

Page 35: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

When Does Thermal Emission Matter?

Page 36: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Applications to Meteorology, Climatology and Remote Sensing

• graybody approximation to SW and LW radiation.

• SW absorptivity, asw=(1-A).• LW absorptivity, alw=ε LW emissivity.• Emitted LW flux = εσT4, and ε≈1.• Emission in SW band is negelected.• Radiative equilibrium is all fluxes balance at

each point in the system.• If not equilibrium, the radiative imbalance may

be interpreted as a heating or cooling rate

Page 37: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Applications to Meteorology, Climatology and Remote Sensing

Page 38: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Answer:

Page 39: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

• Radiative Equilibrium in a Vacuum

Page 40: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

• - Radiative Equilibrium in a Vacuum

Page 41: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Top-of-the-Atmosphere Global Radiation Balance

Page 42: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

Top-of-the-Atmosphere Global Radiation Balance

Page 43: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.
Page 44: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

The skin temperature used in calculating heat fluxes and radiation:G = f( Tskin

- Tsoil) Eq. (1)H = CDHU(Taero-Ta) Eq. (2)LE =CDEU(qTskin*-qa) Eq. (3)

(1-α)Sd +LWd-εσTskin4 -H-LE - G= 0

Rn

Page 45: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

IR Imaging from Space

Page 46: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

IR Imaging from Space

Page 47: Lecture 5: Thermal Emission Chapter 6, Petty We thank Prof. Cheng-ta Cheng for sharing his online notes on this subject.

IR Imaging from Space