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Lecture 11: Passive Microwave Remote Sensing

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Passive Microwave Radiometry Satellite Remote Sensing • Microwave region: 1-200 GHz (0.15-30cm) SIO 135/SIO 236 • Uses the same principles as thermal remote sensing • Multi-frequency/multi-polarization sensing • Weak energy source so need large IFOV and wide Lecture 11: Passive Microwave bands Remote Sensing • Related more closely to classical optical and IR sensors than to radar (its companion active Helen Amanda Fricker microwave sensor) Passive Microwave Radiometry Passive Microwave Radiometry microwave microwave • Recall the "windows" of low opacity, which allow the transmission of only certain EMR (caused by the absorption spectra of the gasses in the • The microwave portion of the electromagnetic spectrum includes atmosphere) wavelengths from 0.1 mm to > 1 m. It is more common to refer to microwave radiation in terms of frequency, f, rather than wavelength, λ. • Atmospheric attenuation of microwave radiation is primarily through • The microwave range is approx. 300 GHz to 0.3 GHz. absorption by H 0 & O - absorption is strongest at the shortest wavelength. 2 2 Most radiometers operate in the range 0.4-35 GHz (0.8-75 cm). Attenuation is very low for λ > 3 cm (f < 10 GHz). In general µwave radiation • is not greatly influenced by cloud or fog, especially for λ > 3 cm. 1 Thermal Radiation Rayleigh-Jeans approximation • Thermal radiation is emitted by all objects above absolute zero Convenient and accurate description for spectral radiance for • In many cases the spectrum of this wavelengths much greater than the wavelength of the peak in radiation (i.e. intensity vs wavelength) the black body radiation formula i.e. λ >> λmax follows the idealized black-body radiation curve Stefan-Boltzmann law: Total energy emitted Approximation is better than 1% when hc/λkT << 1 over time by a black body is proportional to T4 or λT > 0.77 m K. For example, for a body at 300˚K, the approximation is valid Wiens displacement law: The wavelength when λ > 2.57 mm; in other words this approximation is good -1 of the spectral peak is proportional to T when viewing thermal emissions from the Earth over the microwave band. Planck’s law Rayleigh-Jeans Approximation Describes the amplitude of radiation emitted (i.e., spectral radiance) from a black body. It is generally provided in one of two forms; Lλ(λ) is the a constant radiance per unit wavelength as a function of wavelength λ and L ν(ν) is 2kcT spectral radiance is the radiance per unit frequency as a function of frequency ν. L = # a linear function of The first form is: " "4 kinetic temperature • k is Planck’s constant, c is the speed of light, ε is emissivity, T is kinetic temperature • This approximation only holds for λ >> λmax • (e.g. λ > 2.57mm @300 K) ! 2 Planck’s law Microwave Brightness Temperature To relate the two forms and establish L ν(ν), we take the derivative of L with respect to ν using the chain rule: • Microwave radiometers can measure the emitted spectral radiance received (Lλ) • This is called the brightness temperature and is linearly related to the kinetic temperature of the surface Since λ = c/ν, so that • The Rayleigh-Jeans approximation provides a simple linear relationship between measured spectral which gives: radiance temperature and emissivity Microwave Brightness Temperature At the long wavelengths, of the microwave region, the relationship between εT is also called the “brightness spectral emittance and temperature” typically shown as T wavelength can be B approximated by a straight line. "4 T = L B 2kc " ! 3 e r Snow Emissivity Example u t a Microwave Brightness Temperature r e p m Brightness temperature can be related to kinetic • e t Dry temperature through the emissivity of the material, i.e. its s s e Snow ability to emit radiation. n t h g i r dry snow (2) Soil T = "T b b kin snow water equivalent • So passive microwave brightness temperatures can be used to monitor temperature as well as properties related to Wet emissivity Snow • In the microwave region, materials have large variations in (1) Soil (3) Soil emissivity Wet snow is a strong ! absorber/emitter Microwave Radiometers • Advanced Microwave Sounding Unit (AMSU) 1978-present • Scanning Multichannel Microwave Radiometer (SMMR) 1981- 1987 • Special Sensor Microwave/Imager (SSM/I) 1987-present • Tropical Rainfall Measuring Mission (TRMM) 1997-present • Advanced Microwave Scanning Radiometer (AMSR-E) 2002-present 4 Comparative Operating Characteristics of SMMR, SSM/I, and AMSR Passive Microwave Radiometry Parameter (Nimbus-7) (DMSP-F08,F10, (Aqua) SMMR F11,F13) SSM/I AMSR-E • Passive microwave sensors use an antenna (“horn”) to Time Period 1978 to 1987 1987 to Present 2002 to Present detect photons at microwave frequencies which are then converted to voltages in a circuit Frequencies 6.6, 10.7, 18, 21, 37 19.3, 22.3, 36.5, 85.5 6.9, 10.7, 18.7, (GHz) 23.8, 36.5, 89.0 Sample 148 x 95 (6.6 GHz) 37 x 28 (37 GHz) 74 x 43 (6.9 GHz) • Scanning microwave radiometers Footprint 27 x 18 (37 GHz) 15 x 13 (85.5 GHz) 14 x 8 (36.5 GHz) Sizes (km): 6 x 4 (89.0 GHz) – mechanical rotation of mirror focuses microwave energy onto horns Passive Microwave Applications Example radiometer • Soil moisture sin φ = λ/D • Snow water equivalent r D • Sea-ice extent, concentration and type (and lake ice) R = 2 H λ /D • Sea surface temperature Atmospheric water vapor • H = 800 km H • Surface wind speed only over the oceans Φr • Cloud liquid water λ = 1cm • Rainfall rate D = 1m R ==> R = 16 km 5 Monitoring Temperatures with Passive Microwave Passive Microwave Sensing of Land Surface • Sea surface • Land surface Emissivity Differences temperature temperature • Microwave emissivity is a function of the “dielectric constant” • Most earth materials have a dielectric constant in the range of 1 to 4 (air=1, vegetation=3, ice=3.2) • Dielectric constant of liquid water is 80 • Thus, moisture content strongly affects emissivity (and therefore brightness temperature) • Surface roughness also influences emissivity Passive Microwave Sensing of Land Surface SSM/I Emissivity Differences Northern Hemisphere snow water equivalent (mm of water) 6 Atmospheric Effects Atmospheric Mapping • Mapping global water vapor • 85 GHz • At frequencies less than 50 GHz there is little effect of clouds and fog on EMR (it “sees through” clouds) • So PM can be used to monitor the land surface under cloudy conditions • In atmospheric absorption bands, PM is used to map water vapour, rain rates, clouds etc. Passive Microwave Sensing of Rain Rainfall from passive microwave sensors: • Over the ocean: Accumulated – Microwave emissivity of rain (liquid water) is about 0.9 precipitation from – Emissivity of the ocean is much lower (0.5) the Tropical – Changes in emissivity (as seen by the measured brightness Rainfall Measuring temperature) provide and estimate of surface rain rate Mission (TRMM) Similar to SSM/I • Over the land surface: – Microwave scattering by frozen hydrometeors is used as a measure of rain rate – Physical or empirical models relate the scattering signature to surface rain rates 7 Passive Microwave Remote Sensing from Space Sea-ice Sea ice is frozen seawater floating on the ocean surface Advantages Disadvantages • • Penetration through non- • Larger field of views (10- • Sea-ice has an insulating effect on the ocean (traps heat) & precipitating clouds 50 km) compared to affects the Earth’s albedo • Radiance is linearly related VIS/IR sensors Some sea ice is semi-permanent, persisting from year to year, to temperature (i.e. the Variable emissivity over • • and some is seasonal, melting and refreezing from season to retrieval is nearly linear) land season. • Highly stable instrument • Polar orbiting satellites calibration provide discontinuous • The sea ice cover reaches its minimum extent at the end of • Global coverage and wide temporal coverage at each summer and the remaining ice is called the perennial ice swath low latitudes (need to cover. create weekly composites) • Passive microwave data have shown that the spatial extent of the Arctic sea-ice cover is shrinking Passive Microwave Remote Sensing from Space Sea-ice monitoring Measures thermal emissions - as for Thermal IR, but at longer wavelengths. Rayleigh-Jeans approximation: TB = Ts ε (λ, θ) Large contrast in ε of open ocean (~0.4 @18 GHz) & sea ice (~0.9 @ 18 GHz) Sea Ice Extent Combine 19 & 37GHz data Sea Ice Concentration Lubin & Massom (2007), after Comiso (1985) 8 Emissivities of sea-ice types and open water at Sea-ice monitoring microwave frequencies Suppose we measure the thermal emissions at 10 GHz in a polar ocean which has a mixture of open seawater, young sea ice, and old sea ice. It is a warm day so both the ice and water are at the melting point. At 10 GHz (~3 cm), the EMR waves penetrate ~1 mm into the seawater and ~1 m into the ice. Tb Emissivities: seawater = 0.4 young ice = 0.95 old ice = 0.85 Brightness temperature observed by the radiometer aboard the spacecraft will reflect the variations in the emissivity of the surface. This is an excellent way to monitor the ice cover of the polar oceans and discriminate first-year ice from multi-year ice. Massom (in press) after Svendsen et al. (1993) The Passive Microwave Radiometer is the “Bread and Butter” Sensor Sea-ice monitoring for Measuring Sea-Ice Concentration and Extent DMSP SSM/I Monthly Means Including the February annual growth March and decay cycle April May & its variability. June July ~3 million km2 ~19 million km2 August September October In Operation Since 1973 November Poor Spatial Resolution (25km) December January But Penetrates Cloud and Darkness, + Complete Daily Coverage Courtesy Leanne Armand 9 First views of seasonal waxing and waning in 1973.
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