Rain Detection

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Rain Detection Attenuation for Remote sensing
auxiliary sensors

• Dr. Sandra Cruz-Pol
• INEL 5995
• DCAS network weather radars

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Outline

• Radar Equation for Meteorology
• Raindrops
• Volume scattering
• Mie scattering
• Rayleigh approximation
• Radar reflectivity reflectivity
• Dielectric factor, K
• Ancillary Sensors
• Rain gauges
• disdrometers

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Radar equation for Meteorology

• For weather applications
• for a volume

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Radar Equation

• For power distribution in the main lobe assumed
  to be Gaussian function.

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Rain drops
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Clouds Types in our Atmosphere
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Sizes for hydrometeors
cloud drops rain drops
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EM interaction with Single drop
Si

• Absorption
• Cross-Section, Qa Pa /Si
• Efficiency, xa Qa /pr2
• Scattered
• Cross-section , Qs Ps /Si
• Efficiency, xs Qs /pr2
• Total power removed by sphere from the incident
  EM wave, xe xs xa
• Backscatter, Ss(p) Sisb/4pR2
• Efficiency, xb sb /pr2

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Mie Scattering (necessary if d/l?1),

• Mie theory A complete mathematical-physical
  theory of the scattering of electromagnetic
  radiation by spherical particles, developed by G.
  Mie in 1908.
• In contrast to Rayleigh scattering, the Mie
  theory embraces all possible ratios of diameter
  to wavelength. The Mie theory is very important
  in meteorological optics, where
  diameter-to-wavelength ratios of the order of
  unity and larger are characteristic of many
  problems regarding haze and cloud scattering.
• When d/l 1 neither Rayleigh or Geometric
  Optics Theory applies. Need to use Mie.
• Scattering of radar energy by raindrops
  constitutes another significant application of
  the Mie theory.

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Mie Solution

• Mie solution
• where am bm are the Mie coefficients

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Mie coefficients
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Backscattering from metal sphere

• Rayleigh Region defined as
• For conducting sphere (n )

Where,
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Single Particle Cross-sections vs.c for Rayleigh
region
For small drops, almost no scattering, i.e. no
bouncing from drop since its so small.

• Scattering cross section
• Absorption cross section
• In the Rayleigh region (ncltlt1) gtQa is larger, so
  much more of the signal is absorbed than
  scattered. Therefore

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Scattering from Hydrometeors
Rayleigh Scattering
Mie Scattering

• comparable to particle size
• --when rain or ice crystals are present.

95GHz (3mm)
33GHz (9mm)
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Volume Scattering

• Two assumptions
• particles randomly distributed in volume--
  incoherent scattering theory.
• Concentration is small-- ignore shadowing.
• Volume Scattering coefficient is the total
  scattering cross section per unit volume.

Np/m
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Total number of drops per unit volume
in units of mm-3
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Volume backscattering
Using...

• Its also expressed as
• or in dB/km units,

Np/m
s,e,b stand for scattering, extinction and
backscattering.
dB/km