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

http://cimms.ou.edu/~schuur/jpole/http://cimms.ou.edu/~schuur/jpole/

Presentation ContentsPresentation Contents

Principles of polarization diversityPrinciples of polarization diversity What can be measuredWhat can be measured Benefits of dual polarizationBenefits of dual polarization

– Precipitation estimationPrecipitation estimation– Error reductionError reduction– Hydrometeor classificationHydrometeor classification

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

ZZHH NoNo NoNo NoNo YesYes NoNo

ZZDRDR YesYes NoNo NoNo YesYes YesYes

KKDPDP YesYes YesYes YesYes YesYes NoNo

CorrelationCorrelation YesYes YesYes NoNo NoNo YesYes

DeltaDelta YesYes NoNo YesYes NoNo YesYes

LDRLDR YesYes NoNo NoNo NoNo YesYes

From Zrnic and Ryzhkov, 1999

Error reduction (Error reduction (ρρHVHV))

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

Hydrometeor ClassificationHydrometeor ClassificationSpeciesSpecies ZZHHHH (dB) (dB) ZZDRDR (dB) (dB) ρρHVHV KKDPDP (deg km (deg km-1-1)) LLDRDR (dB) (dB) Temp (C )Temp (C )

DrizzleDrizzle 10 to 2510 to 25 -0.2 to 0.2-0.2 to 0.2 > 0.99> 0.99 0 to 0.050 to 0.05 < -34< -34 -15 to 40-15 to 40

RainRain 25 to 6025 to 60 0.5 to 40.5 to 4 > 0.97> 0.97 0 to 200 to 20 -27 to 34-27 to 34 -15 to 40-15 to 40

Snow (Dry, Snow (Dry, low density)low density)

-10 to 35-10 to 35 0 to 0.50 to 0.5 > 0.99> 0.99 0 to 10 to 1 < -34< -34 -90 to 0-90 to 0

Snow (dry Snow (dry high density)high density)

-10 to 25-10 to 25 -0.5 to 1-0.5 to 1 > 0.95> 0.95 0.1 to 20.1 to 2 -25 to -34-25 to -34 -90 to 0-90 to 0

Snow (wet Snow (wet melting)melting)

20 to 45 20 to 45 -0.5 to 3-0.5 to 3 0.8 to 0.8 to 0.950.95

0.2 to 40.2 to 4 -13 -18-13 -18 0 to 50 to 5

Graupel, dryGraupel, dry 20 to 5020 to 50 -0.5 to 1 .0-0.5 to 1 .0 > 0.99> 0.99 -1 to 1-1 to 1 < -30< -30 -90 to 0-90 to 0

Graupel, wetGraupel, wet 40 to 5540 to 55 -0.5 to 3-0.5 to 3 > 0.99> 0.99 -1 to 4-1 to 4 -20 to -25-20 to -25 0 to 50 to 5

Hail, small Hail, small wet < 2cmwet < 2cm

50 to 6050 to 60 -0.5 to 0.5-0.5 to 0.5 > 0.95> 0.95 -1 to 1-1 to 1 < -20< -20 -7 to 5-7 to 5

Hail, large Hail, large wet> 2cmwet> 2cm

55 to 7055 to 70 -4 to –0.5-4 to –0.5 >0.96>0.96 -2 to 2-2 to 2 -10 to -15-10 to -15 -7 to 5-7 to 5

Rain& hailRain& hail 55 to 7055 to 70 -1 to 1-1 to 1 >0.99>0.99 0 to 200 to 20 -20 to -10-20 to -10 -7 to 5-7 to 5

Graupel?Graupel?

Hail and hail sizeHail and hail size

Combining Z and ZCombining Z and ZDRDR gives information on gives information on

thisthis Also knowledge of freezing layer height Also knowledge of freezing layer height

helps (see next talk!)helps (see next talk!)

Lightning detection / predictionLightning detection / prediction

KDP is negative: Vertically aligned ice crystals

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

WAFICUS 2: December 2003WAFICUS 2: December 2003