- Principles of Radiative Transfer Principles of Remote Sensing Marianne König EUMETSAT marianne.koenig@eumetsat.int
Remote Sensing All measurement processes which perform observations/measurements of parameters which carry information about properties at the location of interest, far from the location of interest Opposite: In-situ measurements, i.e. at the location of interest For Meteorology: most remote sensing relies on electromagnetic waves Slide: 2 WSN-12 Rio de Janeiro 06-10 August 2012
Remote Sensing "Remote Sensing" is for us not such a strange principle we have several remote sensing devices (which?) Slide: 3 WSN-12 Rio de Janeiro 06-10 August 2012
Remote Sensing - Principle In order to obtain meaningful information from remote sensing images, the probed radiation field must have some interaction with the parameter of interest Example: A fish seen in "visible" wavelengths (by humans) Slide: 4 WSN-12 Rio de Janeiro 06-10 August 2012
Remote Sensing - Principle In order to obtain meaningful information from remote sensing images, the probed radiation field must have some interaction with the parameter of interest Example: A fish seen in "visible" wavelengths (by humans) And seen by x-rays Slide: 5 WSN-12 Rio de Janeiro 06-10 August 2012
Remote Sensing - Principle Another example: photograph and infrared picture of a house Slide: 6 WSN-12 Rio de Janeiro 06-10 August 2012
Electromagnetic Waves Characteristics: Wavelength λ Propagation velocity c Frequency ν = c / λ Wavenumber =1/ λ (cm -1 ) Slide: 7 WSN-12 Rio de Janeiro 06-10 August 2012
Electromagnetic Spectrum 1m 1mm 1µm Slide: 8 WSN-12 Rio de Janeiro 06-10 August 2012
Sources of Radiation (Met Applications) Slide: 9 WSN-12 Rio de Janeiro 06-10 August 2012
Electromagnetic Radiation Units and Concepts Irradiance Watts/meter 2 Total energy which falls onto 1 sqm of surface Slide: 10 WSN-12 Rio de Janeiro 06-10 August 2012
Electromagnetic Radiation Units and Concepts Radiance Watts/meter 2 /ster (W/m 2 /ster) Energy which falls onto 1 sqm of surface, coming from a certain direction Can also be expressed as radiance per wavelength or wavenumber: W/m 2 /ster/µm W/m 2 /ster/cm -1 Slide: 11 WSN-12 Rio de Janeiro 06-10 August 2012
Fundamental Radiation Law: Planck s Law 3 2hν B( ν, T ) = 2 c 1 e h ν kt 1 hc B( λ, T ) = 5 λ 2 1 e hc λkt 1 k T h = Boltzmann s constant = Temperature = Planck s constant Slide: 12 WSN-12 Rio de Janeiro 06-10 August 2012
Fundamental Radiation Law: Planck s Law Slide: 13 WSN-12 Rio de Janeiro 06-10 August 2012
Spectral Distribution of Energy Radiated from Blackbodies at Various Temperatures Slide: 14 WSN-12 Rio de Janeiro 06-10 August 2012 P. Menzel, 2007
Concept of a Blackbody Concept of Emissivity A Blackbody is an object of temperature T which radiates energy according to Planck s Law. Nature does not have perfect blackbodies: Slide: 15 WSN-12 Rio de Janeiro 06-10 August 2012
Satellite Orbits What Can We Measure? Geostationary orbit: 36000 km height Usable energy in solar and infrared bands Low earth / polar orbit: ~800-900 km height Usable energy in solar, infrared and microwave bands Slide: 16 WSN-12 Rio de Janeiro 06-10 August 2012
Remote Sensing of the Atmosphere What do we measure? Solar radiation: reflected by the surface, by clouds, scattered by molecules (wavelengths?) Thermal radiation: emitted by the earth / clouds / atmosphere (wavelengths?) What about thermal radiation from the sun??? Slide: 17 WSN-12 Rio de Janeiro 06-10 August 2012
Explanation At our Earth s distance from the sun, the radiation received from the sun is approximately on the same energy level as the radiation emitted from the earth/atmosphere Slide: 18 WSN-12 Rio de Janeiro 06-10 August 2012
Visible (Reflective Bands) Infrared / microwave (Emissive Bands) microwave P. Menzel, 2007 Slide: 19 WSN-12 Rio de Janeiro 06-10 August 2012
Processes for Solar Radiation Why is grass green? Slide: 20 WSN-12 Rio de Janeiro 06-10 August 2012
Processes of Thermal Radiation Q Slide: 21 WSN-12 Rio de Janeiro 06-10 August 2012
Earth Spectrum and Planck Curves O3 CO2 H20 CO2 Slide: 22 WSN-12 Rio de Janeiro 06-10 August 2012
Radiative Processes Absorption: Energy of the electromagnetic wave is taken up by matter (e.g. change in atomic state) Emission: Energy change in the matter (e.g. change in the atomic state) releases electromagnetic radiation Emission = Absorption!! Absorption coefficient = property of matter Slide: 23 WSN-12 Rio de Janeiro 06-10 August 2012
Radiative Processes Absorption: Energy of the electromagnetic wave is taken up by matter (e.g. change in atomic state) Emission: Energy change in the matter (e.g. change in the atomic state) releases electromagnetic radiation Emission = Absorption!! Absorption coefficient = property of matter Scattering/reflection: Radiation is geometrically forced to deviate from a straight line Slide: 24 WSN-12 Rio de Janeiro 06-10 August 2012
Illustration: Beam at 11 µm wavelength ( Window ) Sensor Temperature Profile Slide: 25 WSN-12 Rio de Janeiro 06-10 August 2012 Earth Surface
Illustration: Beam at 6.5 µm wavelength (WV Absorption) Sensor Temperature Profile Slide: 26 WSN-12 Rio de Janeiro 06-10 August 2012 Earth Surface
Weighting Functions height Absorption Channel: peaks high in the atmosphere 0 1 Window Channel: High contribution from surface Slide: 27 WSN-12 Rio de Janeiro 06-10 August 2012
Question: What happens for higher viewing angles? A) Satellite measures the same brightness temperatures B) Satellite measures higher brightness temperatures C) Satellite measures lower brightness temperatures Sensor Temperature Profile Earth Surface Slide: 28 WSN-12 Rio de Janeiro 06-10 August 2012
Question: What happens for strong absorption channels, at higher viewing angles? A) Satellite measures the same brightness temperatures B) Satellite measures warmer brightness temperatures C) Satellite measures colder brightness temperatures Sensor Q Temperature Profile Earth Surface Slide: 29 WSN-12 Rio de Janeiro 06-10 August 2012
Radiative Processes Can Be Modelled - RTMs The equation of radiative transfer simply says that as a beam of radiation travels, it loses energy to absorption, gains energy by emission, and redistributes energy by scattering. The equation is a differential equation, numerical models exist which provide a solution (Radiative Transfer Models, RTMs) Slide: 30 WSN-12 Rio de Janeiro 06-10 August 2012
Practical Example: MODIS Imagery, 03 April 2011 Solar Bands 0.6 µm 0.9 µm 1.6 µm
Practical Example: MODIS Imagery, 03 April 2011 Thermal Bands 266.9 K 237.6 K 249.3 K 218.5 K 218.8 K 220.9 K 11 µm 13.2 µm 7.3 µm
Scattering - Scattering by particles which are much smaller than the electromagnetic wavelength ("Rayleigh Scattering") - Scattering by particles which are of same size and larger than the electromagnetic wavelength ("Mie Scattering") Distribution for all angles: phase function Slide: 33 WSN-12 Rio de Janeiro 06-10 August 2012
Rayleigh Scattering Rayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. Scattering is ~ λ -4, i.e. scattering occurs for shorter wavelengths! Slide: 34 WSN-12 Rio de Janeiro 06-10 August 2012
Water Clouds: Scattering on Spherical Particles Size distribution Wavelength 0.87 µm Cloud droplets 1-10 µm Strong forward scattering Slide: 35 WSN-12 Rio de Janeiro 06-10 August 2012
Ice Clouds: Complex Scattering Depending on Ice Crystals' Shape Plates Columns Rosettes Aggregates Slide: 36 WSN-12 Rio de Janeiro 06-10 August 2012
Radiative Transfer Theory: Forward Problem ATMOSPHERIC VARIABLES BOUNDARY CONDITIONS T,q(z) Hydrometeors Liquid water Cloud type etc. RADIATIVE TRANSFER EQUATION RADIOMETER CHARACTERISTICS TB Slide: 37 WSN-12 Rio de Janeiro 06-10 August 2012
Radiative Transfer Theory: Inverse Problem ATMOSPHERIC VARIABLES BOUNDARY CONDITIONS T,q(z) Hydrometeors Liquid water Cloud type etc. RADIATIVE TRANSFER EQUATION RADIOMETER CHARACTERISTICS TB Slide: 38 WSN-12 Rio de Janeiro 06-10 August 2012
Inversion Problem p T(p) p T(p) p q(p) p T(p) q(p) T(p) q(p) p T(p) Retrieval/ Inversion Scheme TBs in different wavelengths q(p) p T(p) q(p) p T(p) ILL POSED PROBLEM q(p) p q(p) T(p) q(p)... Many possible states of Temperature Water vapour, etc. (or cloud parameters, aerosol information.) Slide: 39 WSN-12 Rio de Janeiro 06-10 August 2012
Inversion Problem p T(p) p T(p) p q(p) p T(p) q(p) q(p) T(p) q(p) p T(p) Retrieval/ Inversion Scheme Many TBs in many different wavelengths p T(p) q(p) p T(p) q(p) p q(p) T(p) q(p)... More channels = more information! Slide: 40 WSN-12 Rio de Janeiro 06-10 August 2012
Inversion Problem: Practical Example, 11µm RTM result 286 K Satellite Measurement: 286 K 286 K 286 K Little H 2 O Some more H 2 O Even more H 2 O 290 K ε = 0.99 291 K ε = 0.99 292 K ε = 0.99 Slide: 41 WSN-12 Rio de Janeiro 06-10 August 2012
Inversion Problem: Practical Example, 11µm RTM result 286 K Satellite Measurement: 286 K 286 K 286 K Q Which is the correct surface temperature? Little H 2 O Can we tell from this one measurement? Some more H 2 O Even more H 2 O 290 K ε = 0.99 291 K ε = 0.99 292 K ε = 0.99 Slide: 42 WSN-12 Rio de Janeiro 06-10 August 2012
Inversion Problem: Practical Example, 11µm RTM result 286 K Satellite Measurement: 286 K 286 K 286 K Little H 2 O No, we cannot tell! Possible: constrain humidity by forecast profile Or: combine with another channel Some more that His 2 Osensitive to surface temperature and humidity Even more H 2 O 290 K ε = 0.99 291 K ε = 0.99 292 K ε = 0.99 Slide: 43 WSN-12 Rio de Janeiro 06-10 August 2012
HIRS Ch01 ca. 23 km Slide: 44 WSN-12 Rio de Janeiro 06-10 August 2012
HIRS Ch02 ca. 19 km Slide: 45 WSN-12 Rio de Janeiro 06-10 August 2012
HIRS Ch03 ca. 17 km Slide: 46 WSN-12 Rio de Janeiro 06-10 August 2012
HIRS Ch04 ca. 7 km Slide: 47 WSN-12 Rio de Janeiro 06-10 August 2012
HIRS Ch05 ca. 4 km Slide: 48 WSN-12 Rio de Janeiro 06-10 August 2012
HIRS Ch06 ca. 2 km Slide: 49 WSN-12 Rio de Janeiro 06-10 August 2012
Outlook: Hyperspectral Measurements Surface Clouds Surface Clouds Temp (CO2) Surface Clouds Instruments like IASI measure the IR spectrum in 8461 different samples CO O3 Temp (CO2) H2O,CH4,N2O N2O, Temp (CO2) Slide: 50 WSN-12 Rio de Janeiro 06-10 August 2012
IASI Example Slide: 51 WSN-12 Rio de Janeiro 06-10 August 2012
The End Thank you for your attention! Consider yourself "remote sensing experts" now! Slide: 52 WSN-12 Rio de Janeiro 06-10 August 2012