What can Dual- What can Dual- Polarization Doppler Polarization Doppler Radar Do for You? Radar Do for You? Neil Fox Neil Fox Department of Atmospheric Department of Atmospheric Science Science University of Missouri - University of Missouri - Columbia Columbia
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What can Dual-Polarization Doppler Radar Do for You? Neil Fox Department of Atmospheric Science University of Missouri - Columbia.
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What can Dual-Polarization What can Dual-Polarization Doppler Radar Do for You?Doppler Radar Do for You?
Neil FoxNeil Fox
Department of Atmospheric ScienceDepartment of Atmospheric Science
University of Missouri - ColumbiaUniversity of Missouri - Columbia
The NSSL Joint Polarization The NSSL Joint Polarization ExperimentExperiment
The Nexrad radar at Norman, OK has been The Nexrad radar at Norman, OK has been retrofitted for dual-polarization capabilityretrofitted for dual-polarization capability
The project will determine the benefits of a The project will determine the benefits of a network wide upgradenetwork wide upgrade
This talk will present (some of) the expected This talk will present (some of) the expected benefitsbenefits
Principles of polarization diversityPrinciples of polarization diversity What can be measuredWhat can be measured Benefits of dual polarizationBenefits of dual polarization
Polarized electric fields of radiation Polarized electric fields of radiation are scattered differently by oblate are scattered differently by oblate
hydrometeorshydrometeors
ZZDRDR
Differential reflectivityDifferential reflectivity The ratio of returned The ratio of returned
power in the horizontal power in the horizontal and vertical polarized and vertical polarized channelschannels
Information on axial Information on axial ratio of precipitation ratio of precipitation particlesparticles
ZDR
ZHHZVV
dB 1 0 lo g ( )
Axial ratio of airborne particlesAxial ratio of airborne particlese.g. an elephante.g. an elephant
Axial ratio ~ 1
ZDR = 0
Real rain: this will not flyReal rain: this will not fly
Or Disney rain? Has even less Or Disney rain? Has even less chance of being in the airchance of being in the air
ZDR < 0
Tells us something is wrong!
Something this shape may be found Something this shape may be found in the airin the air
ZDR > 0
KKDPDP
Differential phaseDifferential phase Measures the difference in phase of the Measures the difference in phase of the
horizontal and vertical polarized return horizontal and vertical polarized return signalssignals
Affected by particle axial ratio as it impacts Affected by particle axial ratio as it impacts on forward propagating signal phaseon forward propagating signal phase
KD P k
fH H
fV V
N D dD
2
0
R e ( )
Rain rate estimationRain rate estimation
Z-R relationships: very sensitive to drop size Z-R relationships: very sensitive to drop size distributiondistribution
ZZDRDR-R relationships: inherent drop size -R relationships: inherent drop size
informationinformation KKDPDP-R relationships: Much less sensitive to -R relationships: Much less sensitive to
drop size distributiondrop size distribution KKDPDP, Z, ZDRDR, Z – R relationships: Use all the , Z – R relationships: Use all the
available information on DSDavailable information on DSD
Sample relationshipsSample relationships
Z = 200RZ = 200R1.61.6 or 300R or 300R1.51.5
– Reflectivity related to DReflectivity related to D66
R = 43 KR = 43 KDPDP0.80.8
– Many relationships close to linearityMany relationships close to linearity
R = 54 KR = 54 KDPDP0.910.91 Z ZDRDR
-0.42-0.42
– Combined relationshipCombined relationship
– Can have other combinations R(Z,ZCan have other combinations R(Z,ZDRDR,K,KDPDP))
From Schuur et al. (2001)From Schuur et al. (2001)
Parameter sensitivityParameter sensitivity
Independent Independent of Calibrationof Calibration
Immune to Immune to Propagation Propagation EffectsEffects
Immune to Immune to Noise BiasNoise Bias
Used forUsed for
Quantitative Quantitative
EstimationEstimation
Independent of Independent of ConcentrationConcentration
Cross-correlation of horizontal and vertical Cross-correlation of horizontal and vertical polarized return signalspolarized return signals
For most precipitation For most precipitation ρρHVHV is close to 1 is close to 1
For melting snowflakes (e.g. in the bright band) For melting snowflakes (e.g. in the bright band) it is lowerit is lower
For non-meteorological targets it is even lowerFor non-meteorological targets it is even lower
H V
SH H
D SV V
N D dD
SH H
N D dD SV V
N D dD
* ( ) ( )
( ).
( ).2 0 5 2 0 5
Bright band detectionBright band detection Bright band is characterized by the Bright band is characterized by the
presence of axially asymmetric melting presence of axially asymmetric melting snow particlessnow particles
These produce large values of ZThese produce large values of ZDRDR not seen not seen elsewhere in radar dataelsewhere in radar data
This provides a means of detecting areas of This provides a means of detecting areas of overestimation of rainfall and correcting overestimation of rainfall and correcting themthem
Also provides a means of finding the 0Also provides a means of finding the 0C C isothermisotherm
ZZDRDR Brightband detection Brightband detection
Differentiating Brightband from Differentiating Brightband from embedded convectionembedded convection
KDP is positive: Horizontally aligned ice crystals
If it’s so great, why test it?If it’s so great, why test it?
Theory is great, but how does it work Theory is great, but how does it work operationally?operationally?
How does the processing workHow does the processing work Where are the real benefits and how do Where are the real benefits and how do
these compare to costs?these compare to costs? Testing different scan strategiesTesting different scan strategies Application in other areasApplication in other areas