Introduction to the Dual-Polarized WSR-88D

Don BurgessOU CIMMS/NSSL (Ret.)

Storm-Scale Data Assimilation Workshop October 2011

Explaining Dual-Polarization

Dual-polarization radars emit EM waves with horizontal and vertical polarizations.

- Alternating H & V Transmission requires an expensive fast switch and longer acquisition times

WSR-88D Dual-Polarization Upgrade

• Simultaneous Transmission And Reception (STAR); Slant 45

• Transmit at 45o, receive at both horizontal and vertical

• There is a PROBLEM!

WSR-88D Dual-Polarization Upgrade

• Simultaneous Transmission And Reception (STAR); Slant 45

• Transmit at 45o, receive at both horizontal and vertical

• There is a PROBLEM! Split the power; 3-dB sensitivity loss

List of New WSR-88D Dual-Pol Outputs• 3 New Variables (like moments)

– Differential Reflectivity (ZDR; Zdr)

– Correlation Coefficient (CC; Rhv)

– Specific Differential Phase (KDP; Kdp)

• 3 New Algorithms – Melting Layer– Hydrometeor Classification– QPE

• 9 NEW Precipitation Estimation Display Products

New Product #1: Differential Reflectivity (ZDR)

^

^

10log10

v

h

Z

ZZDR

Definition Possible Range of

Values

Units Abbreviated Name

Measure of the log of the ratio of the horizontal to vertical power

returns

-4 to 10 Decibels (dB)

ZDR

Horizontal Reflectivity

Vertical Reflectivity

ZDR Physical Interpretation

Spherical (drizzle, small hail, etc.)

Horizontally Oriented

(rain, melting hail, etc.)

Vertically Oriented

(i.e. vertically oriented ice crystals)

ZDR ~ 0 dB ZDR > 0 dB ZDR < 0 dB

Pv

Ph

Pv

Ph

Pv

Ph

Zh ~ Zv Zh > Zv Zh < Zv

0log10 10

v

h

Z

Z0log10 10

v

h

Z

Z0log10 10

v

h

Z

Z

Typical ZDR Values for Various Targets

New Product #2 Correlation Coefficient (CC)

2/122/12

*

vvhh

hhvv

SS

SSCC

Definition Possible Range of

Values

Units Abbreviated Name

Measure of similarly of the horizontally and vertically

polarized pulse behavior within a pulse volume

0 to 1 None CC (AWIPS)ρHV (Literature)

CC Physical Interpretation

Non-Meteorological (birds, insects, etc.)

Metr (Uniform)(rain, snow, etc.)

Metr (Non-Uniform)(hail, melting snow, etc.)

Shapes are complex and highly variable. Horizontal and vertical pulses will behave very differently with these objects

Shapes are fairly simple and do not vary much. Horizontal and vertical pulses behave very similarly with these objects

Shapes can be complex and are mixed phase. Horizontal and vertical pulses behave somewhat differently with these objects

Low CC (< 0.85) High CC (> 0.97) Moderate CC (0.85 to 0.95)

Correlation Coefficient (CC) Typical Values

PrecipNon-Precip

• Definition: gradient of the difference between phase shift in the horizontal and vertical directions

• Units: degrees per kilometer (o/km)

New Product #3: Differential PhaseSpecific Differential Phase Shift (KDP)

VHDP Differential phase shift

What ΦDP Means

t6

ΦDP

KDP Has Big Advantages

• Immune to partial beam blockage, attenuation, radar calibration, presence of hail

• Used primarily for rainfall estimation and locating heavy rain

Gradients Most Important

= KDP!!!

Typical Values :: KDP

The main difference between ZH and KDP is that ZH gets contributions from all hydrometeors, including those comprised of ice, whereas KDP is not sensitive to ice particles.

Almost linearly related to rainfall rate KDP is great at indicating high amounts of liquid precipitation

• Run in the RPG• Mixed phase hydrometeors: Easy

detection for dual-pol!– Z typically increases (bright band)– ZDR and KDP definitely increase

– Coexistence of ice and water will reduce the correlation coefficient (CC ~0.9-0.8)

• Algorithm overlay product for top and bottom of melting layer

• User Selectable MLDA, RUC, Sounding

Melting Layer Detection Algorithm

ML Product in AWIPS

• Run in RPG• Algorithm makes best guess of dominant

radar echo type for each gate– Display Product for each radar

elevation angle• Based on Fuzzy Logic• Tornado debris category to be added

Hydrometeor Classification Algorithm (HCA)

Lgt/modrain

Heavyrain Hail “Big

drops” Graupel Ice crystals

Drysnow

Wetsnow Unknown AP or

Clutter Biological

Current Classification Options

• SOO-DOH Images\kcri_0.5_HC_20080408_0638.png20000 ft MSL

• Run in the RPG

• Verification Limitations: We need the A10 aircraft

• “Fuzzy” Logic; assumes Zdr Accuracy

• Typical Radar sampling limitations (snow at 2000 ft AGL may not be snow at the surface)

Hydrometeor Classification Algorithm Challenges

• Run in the RPG• Uses Z, Zdr, Kdp• 9 new products

–Match Legacy PPS– Instantaneous Rate–User Selectable (Up to 10 durations)

for the NWS–Difference products

• Legacy Products still available

Dual-Pol QPE Algorithm

The WSR-88D Dual-Pol Upgrade

WSR-88D Dual-Pol Calibration

• Dual-Pol calibration is more complex than Legacy calibration– Zdr calibration – Initial System PHIdp calibration– Components/outputs are temperature sensitive

• Initial system PHIdp calibration working well• Zdr calibration still under investigation

– Full system calibration• Vertical pointing = NO• Cross-polar calibration = Not Yet

– We think Zdr not calibrated to < 0.1 dB

“Investigate System ZDR” Basics

• Part of “ZDR” comes from the system– Different losses in H & V transmit and receive paths– ZDRtrue = ZDRmeasured - ZDRsys

• ZDRsys initially measured during off-line calibration, then adjusted for drift over time

ZDRsys = Initial ZDRsys + drift compensationUpdated each

volume scanOffline calibration Retrace + 8 hr check

0.99<Rhv ; Range: 20-60 km; Elevation: 2.4 deg; Height < 2.5 km

0.23 dB expected value

0.65 dB observed value

Zdr sys error = .42 dB

0.23 dB expected value

0.45 dB observed value

Zdr sys error = .22 dB

0.99<Rhv ; Range: 20-60 km; Elevation: 2.4 deg; Height < 2.5 km

The WSR-88D Upgrade Deployment

• All WSR-88Ds upgraded 2010-2012 = NO

• 10-14 days radar downtime during upgrade

• System Test: Apr 10 – Sep 10• KOUN: April 2010

• Ops Test: Sep 10 – May 11• Vance: Feb 2011

• Beta Test: Jun 11 – Aug 11• Wichita: June 2011• Phoenix: June 2011• Pittsburgh: July 2011• Morehead City: July 2011

• Full Deployment: Sep 2011 to Apr 2013

Deployment Complete

Deployment In Progress

Deployment Scheduled

Legend

Radar coverage shown is at 10,000 ft AGL or below

Sep 26, 2011

Deployment Complete

Deployment In Progress

Deployment Scheduled

Legend

Radar coverage shown is at 10,000 ft AGL or below

Jan 2, 2012

Deployment Complete

Deployment In Progress

Deployment Scheduled

Legend

Radar coverage shown is at 10,000 ft AGL or below

Jun 18, 2012

Deployment Complete

Deployment In Progress

Deployment Scheduled

Legend

Radar coverage shown is at 10,000 ft AGL or below

Dec 01, 2012

Deployment Complete

Deployment In Progress

Deployment Scheduled

Legend

Radar coverage shown is at 10,000 ft AGL or below

Apr 22, 2013

Life Gets More Complicated With 5-cm and 3-cm Radar

The Result for 5-CM & 3-CM

• Scattering response different for

shorter wavelengths (above)

• ZDR and KDP are different at the shorter wavelengths (right)

Summary

• WSR-88D Dual-Pol deployment underway; lots of data in 2012

• More work needed on dp calibration and dp algorithms

• Some model verification work with dp data can be done soon

• Lots of model verification work with dp data can be done with time

• How will dual-pol information be assimilated into models?• Assimilate dual-pol variables?• Assimilate dp algorithm output?• Drop size and ice distributions?• Something else?

Questions?

Impacts of radar wavelength

Taken from: Gu, et al. (conditionally accepted to JAMC)

Fields of measured Z, ZDR, and ΦDP at C and S bands for the storm on 03/10/2009 at 0309 UTC.

El(C) = 0.41°, El(S) = 0.48°. C-band radar is at X = 0, Y = 0.

The areas of visible negative bias of Z caused by attenuation at C band are marked as A and B (left top panel).

Those Darn Laws of Physics Again

• The Full Radar Equation is Ugly• Radar Scattering Cross-Section Equation is

Ugly• We Simplify Things at 10-cm Wavelength• 10-cm: Rayleigh Approximation

– Scattering by particles whose radii are ~1/10 of the radar wavelength or smaller

• 5-cm & 3-cm we must use the full MieScattering Equation…the Ugly Equation

Important Information• The fundamentals of this presentation and other

Dual-Polarization training materials for outreach (NWS, media, others) are at:– http://www.wdtb.noaa.gov/modules/dualpol/

index.htm