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  • Slide 1
  • Electromagnetic Spectrum and Laws of Radiation Satellite Meteorology/Climatology Professor Menglin Jin
  • Slide 2
  • n How much energy is emitted by some medium? n What kind of energy (what frequency/wavelength) is emitted by some medium? n What happens to radiation (energy) as it travels from the target (e.g., ground, cloud...) to the satellites sensor?
  • Slide 3
  • Frequency and wavelength v = c Frequency (Hz) Wavelength Speed of light 1 hertz (Hz) = one cycle per second c = 3.0 x 10 8 ms -1
  • Slide 4
  • Electromagnetic spectrum 0.001 m1m1m1000 m 1m1000m 1,000,000 m = 1m GammaX rays Ultraviolet (UV) Infrared (IR)MicrowaveRadio waves Red (0.7 m) Orange (0.6 m) Yellow Green (0.5 m) Blue Violet (0.4 m) Visible Longer waves Shorter waves
  • Slide 5
  • Blackbody radiation n Examine relationships between temperature, wavelength and energy emitted n Blackbody: A perfect emitter and absorber of radiation... does not exist
  • Slide 6
  • Measuring energy n Radiant energy: Total energy emitted in all directions (J) n Radiant flux: Total energy radiated in all directions per unit time (W = J/s) n Irradiance (radiant flux density): Total energy radiated onto (or from) a unit area in a unit time (W m -2 ) n Radiance: Irradiance within a given angle of observation (W m -2 sr -1 ) Spectral radiance: Radiance for range in Spectral radiance: Radiance for range in
  • Slide 7
  • Radiance Toward satellite Solid angle, measured in steradians (1 sphere = 4 sr = 12.57 sr) Normal to surface
  • Slide 8
  • Electromagnetic radiation n Two fields: Electrical & magneticElectrical & magnetic n Travel perpendicular & speed of light n Property & behaves in predictable way n Frequency & wavelength n Photons/quanta C=3*10 8 =v *
  • Slide 9
  • Stefan-Boltzmann Law M BB = T 4 Total irradiance emitted by a blackbody (sometimes indicated as E*) Stefan-Boltzmann constant The amount of radiation emitted by a blackbody is proportional to the fourth power of its temperature Sun is 16 times hotter than Earth but gives off 160,000 times as much radiation
  • Slide 10
  • Plancks Function n Blackbody doesn't emit equal amounts of radiation at all wavelengths n Most of the energy is radiated within a relatively narrow band of wavelengths. n The exact amount of energy emitted at a particular wavelength lambda is given by the Planck function:
  • Slide 11
  • Plancks function B (T) = c 1 -5 exp (c 2 / T ) -1 Irridance: Blackbody radiative flux for a single wavelength at temperature T (W m -2 ) Second radiation constant Absolute temperature First radiation constantWavelength of radiation Total amount of radiation emitted by a blackbody is a function of its temperature c 1 = 3.74x10 -16 W m -2 c 2 = 1.44x10 -2 m K
  • Slide 12
  • Planck curve
  • Slide 13
  • Weins Displacement Law m T = 2897.9 m K Gives the wavelength of the maximum emission of a blackbody, which is inversely proportional to its temperature Earth @ 300K: ~10 m Sun @ 6000K: ~0.5 m
  • Slide 14
  • Intensity and Wavelength of Emitted Radiation : Earth and Sun
  • Slide 15
  • Rayleigh-Jeans Approximation B (T) = (c 1 / c 2 ) -4 T When is this valid: 1. For temperatures encountered on Earth 2. For millimeter and centimeter wavelengths At microwave wavelengths, the amount of radiation emitted is directly proportional to T... not T 4 (c 1 / c 2 ) -4 Brightness temperature (T B ) is often used for microwave and infrared satellite data, where it is called equivalent blackbody temperature. The brightness temperature is equal to the actual temperature times the emissivity. B (T) T B =
  • Slide 16
  • Emissivity and Kirchoffs Law Actual irradiance by a non-blackbody at 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 temperatureradiatedblack body absorbed / incident Absorptivity (r, reflectivity; t, transmissivity)
  • Slide 17
  • Kirchoffs Law Materials which are strong absorber at a particular wavelength are also strong emitter at that wavelength
  • Slide 18
  • Solar Constant n The intensity of radiation from the Sun received at the top of the atmosphere n Changes in solar constant may result in climatic variations n http://www.space.com/scienceastronomy/ 071217-solar-cycle-24.html
  • Slide 19
  • Solar Constant n While there are minor variations in solar output n the amount of solar radiation at the top of the Earths atmosphere is fairly constant ~1367 W/m 2. n Its called the solar constant
  • Slide 20
  • The wavelengths we are most interested in for climatology and meteorology are between 0.01 and 100 m The wavelengths we are most interested in for climatology and meteorology are between 0.01 and 100 m
  • Slide 21
  • Radiative Transfer What happens to radiation (energy) as it travels from the target (e.g., ground, cloud...) to the satellites sensor?
  • Slide 22
  • Processes: transmissionreflectionscatteringabsorptionrefractiondispersiondiffraction
  • Slide 23
  • transmission n the passage of electromagnetic radiation through a medium n transmission is a part of every optical phenomena (otherwise, the phenomena would never have occurred in the first place!)
  • Slide 24
  • reflection n the process whereby a surface of discontinuity turns back a portion of the incident radiation into the medium through which the radiation approached; the reflected radiation is at the same angle as the incident radiation.
  • Slide 25
  • Reflection from smooth surface angle of incidence angle of reflection light ray
  • Slide 26
  • Scattering n The process by which small particles suspended in a medium of a different index of refraction diffuse a portion of the incident radiation in all directions. No energy transformation results, only a change in the spatial distribution of the radiation.
  • Slide 27
  • Molecular scattering (or other particles)
  • Slide 28
  • Scattering from irregular surface
  • Slide 29
  • Absorption (attenuation) n The process in which incident radiant energy is retained by a substance. A further process always results from absorption:A further process always results from absorption: The irreversible conversion of the absorbed radiation goes into some other form of energy (usually heat) within the absorbing medium.
  • Slide 30
  • substance (air, water, ice, smog, etc.) incident radiation absorption transmitted radiation
  • Slide 31
  • Refraction n The process in which the direction of energy propagation is changed as a result of: A change in density within the propagation medium, orA change in density within the propagation medium, or As energy passes through the interface representing a density discontinuity between two media.As energy passes through the interface representing a density discontinuity between two media.
  • Slide 32
  • Refraction in two different media less dense medium more dense medium
  • Slide 33
  • Refraction in two different media less dense medium more dense medium tt tt
  • Slide 34
  • Gradually changing medium ray wave fronts low density high density
  • Slide 35
  • Dispersion n the process in which radiation is separated into its component wavelengths (colors).
  • Slide 36
  • The classic example white light prism
  • Slide 37
  • Diffraction n The process by which the direction of radiation is changed so that it spreads into the geometric shadow region of an opaque or refractive object that lies in a radiation field.
  • Slide 38
  • light Solid object shadow region
  • Slide 39
  • Atmospheric Constituents: empty space molecules dust and pollutants salt particles volcanic materials cloud droplets rain drops ice crystals
  • Slide 40
  • Optical phenomena process + atmospheric constituent optical phenomena atmospheric structure light
  • Slide 41
  • Atmospheric Structure temperature gradient humidity gradient clouds layers of - pollutants, clouds layers of stuff - pollutants, clouds
  • Slide 42
  • Optical phenomena process + atmospheric constituent optical phenomena atmospheric structure light
  • Slide 43
  • White clouds n scattering off cloud droplets ~ 20 microns
  • Slide 44
  • Dark clouds n scattering and attenuation from larger cloud droplets and raindrops
  • Slide 45
  • Slide 46
  • Blue skies n scattering from O 2 and N 2 molecules, dust violet light is scattered 16 times more than redviolet light is scattered 16 times more than red
  • Slide 47
  • Molecular scattering (nitrogen and oxygen) [blue scatters more than red]
  • Slide 48
  • Hazy (milky white) sky n Scattering from tiny particles terpenes

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