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Dual-Polarization Radar Variables . Differential Reflectivity ( Zdr ). Ratio of returned power in the horizontal to returned power in the vertical Can also be thought of as difference between horizontal reflectivity factor and vertical reflectivity factor

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differential reflectivity zdr
Differential Reflectivity (Zdr)
  • Ratio of returned power in the horizontal to returned power in the vertical
    • Can also be thought of as difference between horizontal reflectivity factor and vertical reflectivity factor
      • Meteorologists typically look at reflectivity factors (dBZ)
  • Allows for determination of particle shape
    • Spherical (drizzle or tumbling hail) or teardrop shape (rain)
    • Near zero Zdr spherical
    • Positive Zdr values  more teardrop shaped
      • Larger values with larger raindrops
    • Negative  vertically oriented ice crystals
  • Can locate thunderstorm updrafts
    • Zdr columns
differential reflectivity zdr1
Differential Reflectivity (Zdr)
  • Use in concert with reflectivity (at least)
    • High reflectivity, near zero Zdr hail
    • Low reflectivity, near zero Zdr  drizzle or dry snow
    • High reflectivity, positive Zdr  rain
differential reflectivity zdr2
Differential Reflectivity (Zdr)


Large rain drops

Small rain drops and drizzle

differential reflectivity zdr3
Differential Reflectivity (Zdr)
  • Zdr Column (location of updraft)
    • Higher positive values (rain) embedded in near zero values (dry snowor graupel)
correlation coefficient hv
Correlation Coefficient (ρhv)
  • How similar the precipitation particles are in a given volume
    • Values near 1  all particles are similar
      • All rain or all snow in the volume
    • Values between .8 and .97  mixed precipitation, hail, wet snow
    • Values less than .8  birds, buildings, tornadic debris
  • Use in concert with reflectivity and other dual-pol variables
correlation coefficient hv1
Correlation Coefficient (ρhv)
  • Very useful for determining:
    • Winter precip, melting layers (bright banding)
    • Non-meteorological objects (i.e. bugs)
    • Large hail
    • Tornadic debris signature (TDS)
correlation coefficient hv2
Correlation Coefficient (ρhv)

Melting layer/bright banding

correlation coefficient hv3
Correlation Coefficient (ρhv)





  • Be careful when using operationally!
    • Only tells you precip type at height of radar beam
correlation coefficient hv4
Correlation Coefficient (ρhv)
  • Tornado Debris Signature
    • High reflectivity, low correlation coefficient, and velocity couplet
specific differential phase kdp
Specific Differential Phase (KDP)
  • Differential phase
    • Difference between horizontal and vertical phase shifts
    • Depends on horizontal and vertical cross sections of precip particle
      • Positive for rain drops, near 0 for drizzle and hail
  • Specific differential phase is just the change in differential phase with distance
    • Useful for distinguishing between liquid (rain) and ice (hail, snow, etc.)
    • Good for rainfall estimation
    • Positive  rain drops
    • Near zero  ice particles
    • Negative  vertically oriented ice particles
what is the precip type
What is the Precip Type?


Differential Reflectivity (Zdr)

what is the precip type1
What is the Precip Type?


Correlation Coefficient (CC)

what is the precip type2
What is the Precip Type?