DUAL-POL AND YOUWhat to make of this new radar technology

Luke Madaus, UW Atmospheric Sciences11/2/2011

Outline

• Dual-Pol background• New dual-pol variables and products• A few examples from Pacific Northwest

weather• Further information

What is Dual-Pol?

Dual-Pol Variables

Differential ReflectivityMETEOR SIZE

Correlation CoefficientHOMOGENEITY

(Specific) Differential PhaseMETEOR CONCENTRATION

Differential Reflectivity (ZDR)

• ZDR = ZH-ZV

• Units of dB

Differential Reflectivity (ZDR)

•Rather good relation between major-axis rain drop diameter and ZDR•(Large) hail tumbles spherical (0 dB ZDR)•Ice and snow poorly understood•Other echoes—even worse

Dual-Pol Variables

Differential ReflectivityMETEOR SIZE

Correlation CoefficientHOMOGENEITY

(Specific) Differential PhaseMETEOR CONCENTRATION

Correlation Coefficient (CC or rhv)

• “Measure of how similarly the horizontal and vertical pulses are behaving within a pulse volume”

• Unitless, should range from 0 to 1

Correlation Coefficient (CC or rhv)

Melting layer in Correlation Coefficient

MeltingLayer

All rain

All snow/ice

High CC

Low CC

High CC

Melting Layer shows up as a (roughly) circular band of lower correlation coefficients around the radar

SNOW & ICE

MELTING LAYER

RAIN

Melting Layer “Bright Band” in Correlation Coefficient

Dual-Pol Variables

Differential ReflectivityMETEOR SIZE

Correlation CoefficientHOMOGENEITY

(Specific) Differential PhaseMETEOR CONCENTRATION

Differential Phase (FDP)• FDP=FH-FV

• Think of it as the difference in speed between the horizontal and vertical pulses

• The more dense the precipitation, the more the horizontal pulse tends to get slowed down

Specific Differential Phase (KDP)• Range derivative of Differential Phase

Differential Phase

Both in phase

Passing through lots of fat drops causes horizontal pulse to slow down more than vertical pulse; no longer in phase

Phase difference preserved as pulses exit rain

Encountering more fat drops causes phase difference to increase again.

FDP 0 deg. 45 deg. 45 deg.0 deg. 90 deg.

Horizontal beamVertical beam

Estimating rain in complex terrain

• Radar reflectivity biased by ground returns – it will be high• KDP only affected by precipitation in the volume

Partial beam blockage

KDP-derived rainfall should give more numerous and accurate rainfall estimates in the mountains

Dual-Pol Derived Products

Dual-Pol RainfallACCUMULATED RAIN

Hydrometeor Class. AlgorithmLIKELY TARGET TYPE

Dual-Pol Rain RateINSTANTANEOUS RAIN RATE

Hydrometeor Classification Algorithm

• Abbreviated HCA• Represents a “best guess” of

the type of object (hydrometeor or not) at a particular location on radar

Rain Snow

Corr. Coeff.

ZDR

Reflectivity

Sigma Of Ref

KDP

HYDROMETEOR CLASSIFICATION ALGORITHM

Pacific Northwest Examples

• Light Rain Case• Frontal Passage• Another, detailed frontal passage (GR2A)• Bird Migration (GR2A)

More training

• NSSL Dual-Pol Info (Terry Schuur)– http://www.cimms.ou.edu/~schuur/dualpol/

• ROC Dual Pol Training for NWS Partners– http://www.wdtb.noaa.gov/courses/dualpol/outreach/

• Luke’s Animations of Dual-Pol Products from KLGX and KATX– http://www.atmos.washington.edu/~lmadaus/radars/

Actual NWS KDP Algorithm

Is 5-bin median CC > 0.90?

Compute 5-bin median of PHI

Is associated reflectivity < 40

dBZ?

Compute 25-bin average of PHI

Take 25-bin least-squares derivative

Compute 9-bin average of PHI

Take the 9-bin least-squares

derivative

No KDP Calculated

Take Median

Take Average

Take large-range derivative

Not so straight-forward!

Bird Migration

CC

Bird Migration

ZDR

Dual-Pol Upgrade Drawbacks

• Slow deployment• Loss of clutter filtering until next build release

--Dual-pol derived clutter filtering not for many years

• Loss of sensitivity – “3 dB down”

3)5.0(log10 10