Lecture 11: Passive Microwave Remote Sensing

Transcription

1 Passive Microwave Radiometry Satellite Remote Sensing SIO 135/SIO 236 Lecture 11: Passive Microwave Remote Sensing Helen Amanda Fricker Microwave region: GHz ( cm) Uses the same principles as thermal remote sensing Multi-frequency/multi-polarization sensing Weak energy source so need large IFOV and wide bands Related more closely to classical optical and IR sensors than to radar (its companion active 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 atmosphere) Atmospheric attenuation of microwave radiation is primarily through absorption by H 2 0 & O 2 - absorption is strongest at the shortest wavelength. 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. The microwave portion of the electromagnetic spectrum includes 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. The microwave range is approx. 300 GHz to 0.3 GHz. Most radiometers operate in the range GHz ( cm). 1

2 Thermal Radiation Thermal radiation is emitted by all objects above absolute zero In many cases the spectrum of this radiation (i.e. intensity vs wavelength) follows the idealized black-body radiation curve Rayleigh-Jeans approximation Convenient and accurate description for spectral radiance for wavelengths much greater than the wavelength of the peak in the black body radiation formula i.e. λ >> λ max Stefan-Boltzmann law: Total energy emitted over time by a black body is proportional to T 4 Wiens displacement law: The wavelength of the spectral peak is proportional to T -1 Approximation is better than 1% when hc/λkt << 1 or λt > 0.77 m K. For example, for a body at 300 K, the approximation is valid when λ > 2.57 mm; in other words this approximation is good when viewing thermal emissions from the Earth over the microwave band. a constant Rayleigh-Jeans Approximation L " = # 2kcT " 4 spectral radiance is a linear function of kinetic temperature Planck s law 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 radiance per unit wavelength as a function of wavelength λ and L ν(ν) is the radiance per unit frequency as a function of frequency ν. The first form is: k is Planck s constant, c is the speed of light, ε is emissivity, T is kinetic temperature This approximation only holds for λ >> λ max (e.g. λ > K) 2

3 Planck s law To relate the two forms and establish L ν(ν), we take the derivative of L with respect to ν using the chain rule: Since λ = c/ν, so that which gives: Microwave Brightness Temperature Microwave radiometers can measure the emitted spectral radiance received (L λ ) This is called the brightness temperature and is linearly related to the kinetic temperature of the surface The Rayleigh-Jeans approximation provides a simple linear relationship between measured spectral radiance temperature and emissivity Microwave Brightness Temperature At the long wavelengths, of the microwave region, the relationship between spectral emittance and wavelength can be approximated by a straight line. εt is also called the brightness temperature typically shown as T B T B = "4 2kc L " 3

4 Microwave Brightness Temperature Brightness temperature can be related to kinetic temperature through the emissivity of the material, i.e. its ability to emit radiation. T b = "T kin So passive microwave brightness temperatures can be used to monitor temperature as well as properties related to emissivity brightness temperature Snow Emissivity Example dry snow (2) Soil snow water equivalent Dry Snow Wet Snow In the microwave region, materials have large variations in emissivity (1) Soil (3) Soil Wet snow is a strong absorber/emitter Microwave Radiometers Advanced Microwave Sounding Unit (AMSU) 1978-present Scanning Multichannel Microwave Radiometer (SMMR) Special Sensor Microwave/Imager (SSM/I) 1987-present Tropical Rainfall Measuring Mission (TRMM) 1997-present Advanced Microwave Scanning Radiometer (AMSR-E) 2002-present 4

5 Passive Microwave Radiometry Passive microwave sensors use an antenna ( horn ) to detect photons at microwave frequencies which are then converted to voltages in a circuit Scanning microwave radiometers mechanical rotation of mirror focuses microwave energy onto horns Comparative Operating Characteristics of SMMR, SSM/I, and AMSR Parameter (Nimbus-7) (DMSP-F08,F10, (Aqua) SMMR F11,F13) SSM/I AMSR-E Time Period 1978 to to Present 2002 to Present Frequencies (GHz) Sample Footprint Sizes (km): 6.6, 10.7, 18, 21, , 22.3, 36.5, , 10.7, 18.7, 23.8, 36.5, x 95 (6.6 GHz) 37 x 28 (37 GHz) 74 x 43 (6.9 GHz) 27 x 18 (37 GHz) 15 x 13 (85.5 GHz) 14 x 8 (36.5 GHz) 6 x 4 (89.0 GHz) Passive Microwave Applications Example radiometer Soil moisture Snow water equivalent Sea-ice extent, concentration and type (and lake ice) Sea surface temperature Atmospheric water vapor Surface wind speed only over the oceans Cloud liquid water Rainfall rate sin φ r = λ/d R = 2 H λ /D H = 800 km λ = 1cm D = 1m H R D Φ r ==> R = 16 km 5

6 Monitoring Temperatures with Passive Microwave Sea surface temperature Land surface temperature Passive Microwave Sensing of Land Surface Emissivity Differences 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 Emissivity Differences SSM/I Northern Hemisphere snow water equivalent (mm of water) 6

7 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 Over the ocean: Microwave emissivity of rain (liquid water) is about 0.9 Emissivity of the ocean is much lower (0.5) Changes in emissivity (as seen by the measured brightness temperature) provide and estimate of surface rain rate Over the land surface: Rainfall from passive microwave sensors: Accumulated precipitation from the Tropical Rainfall Measuring Mission (TRMM) Similar to SSM/I 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

8 Passive Microwave Remote Sensing from Space Advantages Penetration through nonprecipitating clouds Radiance is linearly related to temperature (i.e. the retrieval is nearly linear) Highly stable instrument calibration Global coverage and wide swath Disadvantages Larger field of views (10-50 km) compared to VIS/IR sensors Variable emissivity over land Polar orbiting satellites provide discontinuous temporal coverage at low latitudes (need to create weekly composites) Sea-ice Sea ice is frozen seawater floating on the ocean surface Sea-ice has an insulating effect on the ocean (traps heat) & affects the Earth s albedo Some sea ice is semi-permanent, persisting from year to year, and some is seasonal, melting and refreezing from season to season. The sea ice cover reaches its minimum extent at the end of each summer and the remaining ice is called the perennial ice cover. Passive microwave data have shown that the spatial extent of the Arctic sea-ice cover is shrinking Passive Microwave Remote Sensing from Space Measures thermal emissions - as for Thermal IR, but at longer wavelengths. Rayleigh-Jeans approximation: T B = T s ε (λ, θ) Large contrast in ε of open ocean GHz) & sea ice 18 GHz) Sea Ice Extent Combine 19 & 37GHz data Sea Ice Concentration Lubin & Massom (2007), after Comiso (1985) 8

9 Emissivities of sea-ice types and open water at 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. Emissivities: seawater = 0.4 young ice = 0.95 old ice = 0.85 T b Massom (in press) after Svendsen et al. (1993) 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. The Passive Microwave Radiometer is the Bread and Butter Sensor for Measuring Sea-Ice Concentration and Extent DMSP SSM/I Monthly Means ~3 million km 2 ~19 million km 2 In Operation Since 1973 Poor Spatial Resolution (25km) But Penetrates Cloud and Darkness, + Complete Daily Coverage Courtesy Leanne Armand Including the annual growth and decay cycle & its variability. February March April May June July August September October November December January 9

10 First views of seasonal waxing and waning in Almost daily since. Arctic: ~8 to 15 million km 2 March June Sept. Dec. Satellite-derived maps of Sea Ice Concentration February 2002 Oct million km 2 19 million km 2 Satellite AMSR-E data (courtesy J. Comiso, NASA GSFC) Antarctic: ~3 to 19 million km 2 Carsey, 1992 Sea-ice extent and concentration SSM/I, 25 km res. (NSIDC) Hemispheric time series back to 1978, uninterrupted by cloud & darkness. Routine availability (NSIDC), uninterrupted by cloud & darkness Different algorithms Bootstrap & NASA Team see recommendations in Report. SSM/I 25 km res. Aug 31, 2006 Ross Sea AMSR-E, 6.25 km res. (U Bremen) Since 2002, also AMSR-E km res (NASA/NSIDC) 6.25 km res (Univ Bremen) More structural detail Different datasets recommend GSFC combined SMMR-SSM/I (internal consistency + good quality controls). Data courtesy NSIDC 10

11 Monthly Mean DMSP SSM/I Ice Concentration and Motion Map, July 1999 Climatological Day of Ice Advance + Retreat ( ) Ice Season Length SSM/I & AMSR 12.5/25 km Resolution East Wind Drift Mertz Glacier Polynya Stammerjohn et al., Relatively long annual expansion (Feb-Oct), most rapid March-June, then rapid decay (Nov-Jan) Ross Sea. NB Apparent recent redistribution to the Ross Sea from the Amundsen-Bellingshausen Seas. Parkinson, 2005 Massom et al., 2003 We are losing the ice cover fast Summer 2007: A new record low Climatology ( ) Stroeve et al

12 Climate models suggest once the sea ice cover is thinned sufficiently, a strong kick from natural variability can initiate a rapid slide towards ice-free conditions in summer (e.g. Holland et al., 2006). Ice Extent (million sq-km) CCSM3 model simulation Observations September Sea Ice Extent Model drop 1.8 million sq km, Observed drop 1.6 million sq km, Year Mean thickness (70-90N) in CCSM3 before abrupt change: 1.71 m Mean thickness (70-90N) from ICESat in Spring 2007: 1.75 m (data from D. Yi and J. Zwally) Predictions for the future Yet, no new record low But the in 2008 trend accelerates further from to -11.8%/decade 12

13 Ice sheet surface melt monitoring PMW sensors detect dramatic rise in emissivity associated with the onset of melt Amount of surface melting on Antarctic ice shelves 13