Correcting for Attenuation in the TRMM PR

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Correcting for Attenuation in the TRMM PR

• Sarah Finn
• AT 741

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TRMM PR

• 13.8 GHz (Ku band) space borne precipitation
  radar
• Significant attenuation
• Non-sun-synchronous orbit
• Footprint diameter 4.3 km at nadir
• Pulse width of 1.67 ?s corresponds to range
  resolution 250 m

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Attenuation

• Must be corrected for before calculation of rain
  rate
• Algorithm uses Z-R relationship RaZeb
• Use k-Z relationship k?Ze?
• k is specific attenuation
• Values of coefficients chosen based on rain type,
  height of 0º isotherm, storm top, consistency
  with assumed DSD model

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Attenuation Correction Hitschfeld-Bordan Method
Need estimate of attenuation to calculate true
effective radar reflectivity assume power law
relationship k?Ze?
q 0.2ln(10)
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PROBLEM In heavy rain, ? ? 1, and the expression
for PIA becomes meaningless Another method?
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Surface Reference Technique

• Gives independent estimate of PIA
• Assumes decrease of apparent surface
  cross-section
• caused by propagation loss of radar signal by
  rain

Problem measurement of surface cross-section is
subject to error - if change is not much greater
than measurement error, introduce ridiculous error
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Solution Blend Techniques for Hybrid Algorithm

• Given two independent quantities related to PIA,
  find most probable value
• Maximize probability function
• p(PIA ?1,?2)
• ?1 function of ??0
• ?2 function of ?
• Use Bayess Theorem and much math for function
  blending methods

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If ? gt 1, no HB If ? small, assume small RR, ??0
set to 0.5dB before minimization
Put curve figure 2 from 2000 paper
Iguchi et al. 2000
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Use Attenuation Estimate

• Use calculated attenuation from hybrid algorithm
  to correct reflectivity factor
• Ze(r) Zm (r) /A (r)
• Plug in to Z-R relationship to retrieve rainfall