Passive Microwave Remote Sensing

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Passive Microwave Remote Sensing

• Lecture 11

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Principals

• While dominate wavelength of Earth is 9.7 um
  (thermal), a continuum of energy is emitted from
  Earth to the atmosphere. In fact, the Earth emits
  a steady stream of microwave energy as well,
  though it is relatively weak in intensity due to
  its long wavelength.
• The spatial resolution usually low (kms) since
  the weak signal.
• A suit of radiometers can record it. They measure
  the brightness temperature emitted from the
  terrain or the atmospheric gasses, dusts. This is
  much like the thermal infrared radiometer for
  temperature measurement as we discussed before.
• A matrix of brightness temperature values can
  then be used to construct a passive microwave
  image.
• To measure soil moisture, precipitation, ice
  water content, sea-surface temperature, snow-ice
  temperature, and etc., based on brightness
  temperature images.

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Rayleigh-Jeans approximation of Plancks law
Thermal infrared domain (Plancks law)
Microwave domain (Rayleigh-Jeans approximation)
Recall
Let
We have
We have
Unit is Wm-2Hz
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• For a Lambertian surface, the surface brightness
  radiation B(v,T),
• The really useful simplification involves
  emissivity and brightness temperature in
  microwave range

Unit is Wm-2Hzsr
In comparison with thermal infrared (TB)4 e?
(T)4
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Some important passive microwave radiometers

• Special Sensor Mirowave/Imager (SSM/I)
• It was onboard the Defense Meterorological
  Satellite Program (DMSP) since 1987
• It measure the microwave brightness temperatures
  of atmosphere, ocean, and terrain at 19.35,
  22.23, 37, and 85.5 GHz.
• TRMM microwave imager (TMI)
• It is based on SSM/I, and added one more
  frequency of 10.7 GHz.

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AMSR-E

• Advanced Microwave Scanning Radiometer EOS
• It observes atmospheric, land, oceanic, and
  cryospheric parameters, including precipitation,
  sea surface temperatures, ice concentrations,
  snow water equivalent, surface wetness, wind
  speed, atmospheric cloud water, and water vapor.
• At the AMSR-E low-frequency channels, the
  atmosphere is relatively transparent, and the
  polarization and spectral characteristics of the
  received microwave radiation are dominated by
  emission and scattering at the Earth surface.
• Over land, the emission and scattering depend
  primarily on the water content of the soil, the
  surface roughness and topography, the surface
  temperature, and the vegetation cover.
• The surface brightness T (TB ) tend to increase
  with frequency due to the absorptive effects of
  water in soil and vegetation that also increase
  with frequency. However, as the frequency
  increase, scattering effects from the surface and
  vegetation also increase, acting as a factor to
  reduce the TB

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AMSR-E
Najoku et al. 2005
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Example1 Snow depth or snow water equivalent
(SWE)

• The microwave brightness temperature emitted from
  a snow cover is related to the snow mass which
  can be represented by the combined snow density
  and depth, or the SWE (a hydrological quantity
  that is obtained from the product of snow depth
  and density).

?Tb Tb19V-Tb37V
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Large grains tend to scatter microwave radiation
more than smaller grains
Volume fraction () snow density/900 From
fresh snow to packsnow, the snow density increase
from lt100 kg m-3 to between 200-400 kg m-3
Kelly et al. 2003
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3. Study Area (1)
Example 1
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Impact of snow density (4)-mean SD
Snow density 0.4 g/cm3 or 400 kg m-3
Multi-snow density
Wang, Xie, and Liang 2006
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Results AMSR-E vs ground- SD at individual
stations (snow density 0.4 g/cm3)
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Results AMSR-E vs ground- SD at individual
stations (snow density 0.4 g/cm3)
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Results Annual change of SWE in YWR
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Antarctic sea ice
Example 2
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Footprint size 58 km 37 km 21
km 11 km 5 km
Level 2 data
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Footprint size 58 km 37 km 21
km 11 km 5 km
AMSR Bootstrap/ Ice temperature
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Footprint size 58 km 37 km 21
km 11 km 5 km
AMSR Bootstrap/ Ice temperature
Bootstrap
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Footprint size 58 km 37 km 21
km 11 km 5 km
AMSR Bootstrap/ Ice temperature
Bootstrap
NASA Team 2/ Snow depth
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AMSR-E derived sea ice concentrations
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Ice Concentration/area
Based on SMMR-SSM/I (http//nsidc.org)
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Compare AMSR-E ice concentration and NIC ice edge
Cicek et al. 2009
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Snow cover

• Idea
• Radiation from the ground is scattered by the
  snow cover.
• The more snow the more scattering.
• Scattering efficiency is frequency dependent.
• hs c (T37GHz-T19GHz)
• Difficulties
• Different terrain forms (e.g., tundra, mountains,
  plains) different ice properties (FY/MY icel,
  ridges)
• Scattering varies with snow physical properties
  (e.g., grain size, density, wetness)

(From C.L. Parkinson, Earth from above,1997)
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Monthly snow depth data are derived from
satellite passive microwave data
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A Weddell Sea 7/86 - 9/86
(Wadhams et al., 1986) B East Antarctic
10/88 - 12/88 (Allison et al.,
1993) C Weddell Sea 9/89 -
10/89 (Eicken et al., 19994) D East
Antartic 11/91
(Worby and Massom, 1991) E Weddell Sea
6/92 - 7/92 (Drinkwater and Haas,
1994) F East Antarctic 10/92 -
11/92 (Worby and Massom, 1995) G East
Antarctic 3/93 - 5/93 (Worby
and Massom, 1995) H Bellingshausen
8/93 - 9/93 (Worby et al., 1996) I
Amundsen 9/94 - 10/94
(Sturm et al., 1998) J East Antarctic
9/94 - 10/94 (Jeffries et al.,
1995) K Ross Sea 5/95
- 6/95 (Sturm et al.,1998) L Ross
Sea/Bellingshausen 8/95-9/95 (Sturm et al.,
1998)
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Inter-annual variability of September snow
depth (on a pixel-by-pixel basis)
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Radio-frequency interference contaminate the 6.9
and 10.7 GHz channels
Example 3

• Radio-frequency interference (RFI) includes the
  cable television relay, auxiliary broadcasting,
  mobile. RFI is several orders of magnitude higher
  than natural thermal emissions and is often
  directional and can be either continuous or
  intermittent.
• Radio-frequency interference (RFI) is an
  increasingly serious problem for passive and
  active microwave sensing of the Earth.
• The 6.9 GHz contamination is mostly in USA,
  Japan, and the Middle East.
• The 10.7 GHz contamination is mostly in England,
  Italy, and Japan
• RFI contamination compromise the science
  objectives of sensors that use 6.9 and 10.7 GHz
  (corresponding to the C-band and X-band in active
  microwave sensing) over land.

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radio-frequency interference (RFI) index (RI)
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Li et al. 2004
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6.9 GHz contamination
Najoku et al. 2005
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