# electromagnetic spectrum and laws of radiation satellite meteorology/climatology professor menglin...

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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
• 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