Spaceborne Weather Radar

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Spaceborne Weather Radar
http//www.nasa.gov/mpg/126762main_cloudsat-animat
ion.mpg
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• Spaceborne weather radars have only been
  operational since 1997, however the idea has been
  around since the early 1960s.
• Technological advances in signal processing,
  power requirements, and antenna design brought
  the cost to feasible levels during the 1990s.
• This, along with increased awareness for the
  importance of quality rainfall measurements for
  climate applications, led the impetus for the
  design and launch of the Tropical Rainfall
  Measuring Mission and its precipitation radar
  (PR) in 1997, which is a Ku-band (frequency 14
  GHz, wavelength 2.2 cm) scanning radar.
• Now, CloudSat is the first spaceborne cloud
  radar, which will allow the mapping of clouds and
  light precipitation beyond the capabilities of
  TRMM. CloudSat is a W-Band (frequency 95 GHz,
  wavelength 3 mm) nadir pointing system.
• The planned Global Precipitation Mission
  dual-wavelength precipitation radar (DPR),
  planned for launch in 2011, will have two
  frequencies at Ku (same frequency as TRMM) and Ka
  (frequency 35 GHz, wavelength 8.5 mm), which
  will allow retrieval of the drop size
  distribution through dual-wavelength techniques,
  will have higher sensitivity at Ku band. This
  will allow improved rainfall retrievals.

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• Comparison with ground-based radar
• Same principle, less infrastructure
• In low earth orbit, so 350 (TRMM)-750 km
  (CloudSat)
• Spaceborne radar generally has
• Lower transmit power
• Lower sensitivity
• Lower azimuthal resolution, higher vertical
  resolution
• No Doppler or Polarimetric Capability (yet)
• Moving at 10s of km/s, so only get snapshots of
  precipitation (vs. volume scans)
• Cross-track scanning, or nadir (straight down)
  pointing angles

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• Spaceborne radar design considerations
• There are many more careful engineering
  considerations and trade-offs when designing a
  spaceborne radar system compared with a
  ground-based system
• Ideally, one would like to have high spatial
  resolution, good sensitivity, large coverage area
  (swath), little attenuation and a wavelength
  sensitive to the parameter of interest without
  being susceptible to Mie effects
• These issues include
• Duty-cycle of scanning antenna vs. wide swath
• Power available aboard spacecraft
• Signal processing and data communications
• Size and weight vs. wavelength, spatial
  resolution, sensitivity, beam filling, swath
  width, and sidelobes
• Movement of satellite during pulse volume
  (satellite speed 10 km s-1)

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Geometry and spatial sampling considerations for
TRMM
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• Design considerations
• The minimum detectable reflectivity is determined
  according to the radar equation
• Where
• Pr is the return power
• Pt is the transmit power
• C is the radar constant
• r is range
• G is the antenna gain
• Z is the equivalent radar reflectivity factor

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• For a spaceborne radar, the transmit power,
  wavelength, and antenna gain are limited by the
  size of the antenna (3 m), power available (from
  solar panels), receiver sensitivity, and range
  from the ground (750 km)
• These factors limit the minimum detectable
  reflectivity of the TRMM radar to 17-18 dBZ

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Raw TRMM Reflectivity Product - 1C21
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Attenuation-corrected TRMM Reflectivity Product -
2A25
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The fraction of the beam filled, plotted as F
in this diagram, shows that at an incidence angle
(q30º), the amount of beam filling is strongly
determined by the radars beam width.
Iguchi and Meneghini (1990)
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• Partial Beam Filling

Often times, the radar suffers from partial beam
filling either by precipitation or by the
surface clutter. This introduces a non-linear
averaging problem because the power returned is
partitioned in an unknown fashion.
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• The surface return is both a blessing and a curse
  for spaceborne radar.
• It can be used as a reference for attenuation
  correction and calibration, since it is a stable
  quantity that can be compared over time and space
• Its reliability is good apart from at high
  incidence angles with high wind speed) over ocean
• Over land its standard deviation is generally
  larger than over ocean
• It also introduces non-precipitating echo, which
  increases in depth away from nadir in the main
  lobe of the radar, as well as sidelobes (which
  are lower in returned power, but are higher in
  altitude

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Good range resolution is required to optimize
vertical resolution for scans away from nadir -
must weigh against number of samples at each gate.
Iguchi and Meneghini (1990)
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Sidelobes are a problem because ground return is
so intense. These plots show the ratio of the
rain to surface return as a function of rain rate
and altitude.
Iguchi and Meneghini (1990)
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The surface return masks out rain return near the
surface due to the main lobe and side lobe
clutter. Here are results from two incidence
angles 10 (left) and 30 (right) for 4
different rain rates.
Iguchi and Meneghini (1990)
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• Sidelobes and Surface Effects

Hanado and Ihura (1992)
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• Sidelobe illustration - the serial sidelobes
  appear to radiate up from the nadir point due to
  the increasing inclination angle.

Hanado and Ihura (1992)
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Mirror Image Power making 4 hops back to the
receiver
Schematic diagram of the mirror image
Mirror return
TRMM PR observed rainfall reflectivity cross
section
Courtesy K. Nakamura
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• Cloud Radar in Space CloudSat
• While TRMM has been a successful precipitation
  radar, its 17-18 dBZ minimum detectable signal
  does not allow views of light precipitation
  and/or clouds (except some anvils) due to
  wavelength and sensitivity
• Going to a higher frequency increases sensitivity
  to smaller particles (D6)
• However, Mie effects (cutoff at ) are more likely
  to occur, so there is some tradeoff
• W-Band (mm-wave) is an attractive option, since
  it is sensitive to many large cloud particles
• It has been demonstrated as an excellent airborne
  (Wyoming King Air) and ground-based platform, in
  combination with lidar, to estimate IWC and LWC
  in clouds
• Attenuation and Mie effects in precipitation
  limit the maximum retrievable rain rate
  (depending on the DSD) to about 15-25 dBZ
  (overlap with TRMM?)

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• Radar energy becomes more susceptible to Mie
  scattering as wavelength gets shorter or
  particles get larger (cutoff at x pD/? ltlt1 for
  Rayleigh regime) - above this cutoff, Rayleigh
  assumption breaks down and backscattered energy
  (in this case, Z) is reduced

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• Attenuation vs. Wavelength
• While CloudSat is more sensitive to smaller
  particles at W-Band vs. Ku-Band for TRMM, it is
  susceptible to attenuation and Mie scattering
  effects

Attenuation Rates for TRMM (left) Vs.
Cloudsat (right)
Stephens et al. (2002)
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• Attenuation (or absorption) is related to
  reflectivity by a relationship k a Z b
• Assuming a and b for various hydrometeor species
  has uncertainty, but safer than for Z-R
  relationship

Rain at 10C
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Stephens et al. (2002)
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(No Transcript)
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http//www.cloudsat.cira.colostate.edu/dpcstatusQL
.php
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(No Transcript)
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• Estimating precipitation from spaceborne radar
  (single wavelength - TRMM)
• Need to invert Z to R (use Z a R b)
• Z is attenuated (by gases, cloud, and
  precipitation), so need to correct
• Use surface reference technique (estimate path
  integrated attenuation or PIA and redistribute k
  in vertical) or k-Z relationship to correct Z for
  each assumed hydrometeor type
• Then use corrected Z to estimate R via Z-R
  relationship (with adjustment from attenuation
  correction if available)
• Reference Iguchi et al. (2000, Journal of
  Applied Meteorology)