Dual Polarization Martin Hagen, Elena Saltikoff Deutsches Zentrum für Luft- und Raumfahrt (DLR) Oberpfaffenhofen, Germany Finnish Meteorological Institute

Dual Polarization

Martin Hagen, Elena Saltikoff

Deutsches Zentrum für Luft- und Raumfahrt (DLR)Oberpfaffenhofen, Germany

Finnish Meteorological Institute (FMI)Helsinki, Finland

Radar course 2010/11, Class room phase 2

Precipitation is directly related to atmospheric motion.

Hydrometeors are displaced Doppler shift of radar waves

Cloud and precipitation particles have different shape, phase, size, and falling behaviour

scattering properties Polarization

Why Doppler and Polarization?

Dynamics and Microphysics of precipitation


Dual Polarization

• Who did work through the pre-reading material?

• Are you ready for a short test?


Usage of dual-polarization

• Survey by Elena

January 2010

• What isyour answer?

What do you think dual-polarization will do for you(one ore more options)?

Better quality of dataBetter rainfall estimations

Better hail detection

Identificat. non-met. echoes

Rain-snow bounda. detect.Particle classification

All of aboveI do not know

31.4%33.3%37.3%

15.7%

31.4%23.5%31.4%35.2%


Usage of dual-polarization

• Survey by Elena

January 2010

• What isyour answer?

Does your institute use or plan to use dual-polarization for operational forecasting?

Which dual-polarization parameters you measure orwill measure (one or more options)?

We already doYes, before 2012

Yes, after 2012No confirmed plans yet

I do not know

ZDRrhoHV

KDPphiDP

LDRNone

I do not know

25.5%13.7%9.8%

11.8%39.2%

20.4%18.4%12.2%16.3%4.1%4.1%

75.5%


1960 1985 1995 2002

Doppler Radar Doppler Radar bistatatic dual-Doppler+Lidar (research) (operational Doppler Radar Assimilation in NWP in Europe)

1976 1986 1990 2004

polarimetric Radar polarimetric Radar polarimetric Radar (research) (operational, without (operational,

(R-Z-ZDR) (R-KDP) substantial success) MeteoFrance)

Polarization and Doppler Radar History

DLRPoldirad


Weather Radarsin Europe

194 weather radars

166 have Doppler

34 have dual- polarization

http://www.knmi.nl/opera


Shape of Falling Raindrops

Raindrops falling with their terminal velocity are oblate due to the air flow from below.

Drops can be described as rotationalellipsoids with the axis a and b

Observations in a vertical wind tunnel

(Pruppacher & Klett)

2.70 mm 3.45 mm 5.30 mm

5.80 mm 7.35 mm 8.00 mm


Internal Motion of Raindrops

Raindrops do oscillate and tumble during fall

• 5 mm raindrop in a vertical wind tunnel (Univ. Mainz)

• Oscillation60 Hz for a5 mm drop

• tumbling


Rain Drop ShapesVarious studies in wind tunnels or observations in free atmosphere.

Parameterization by Pruppacher and Beard (1970):

a/b = 1.03 – 0.062 Deq with Deq in mm

0.5

0.6

0.7

0.8

0.9

0 1 2 3 4 5 6Drop diameter in mm

Axi

s ra

tio

Goddard et al., 1994

1.0

Keenan et al., 1997

Andsager et al., 1999

Kubesh&Beard (1993)Beard et al. (1991)Jones (1959)Sterlyadkin (1988)Chandrasekar et al. (1988)

Beard, 1976Pruppacher &

Beard, 1970

Deq

equivalent volumetric diameter


Polarimetric Radar Observations

• The polarization of an electromagnetic wave is defined by the orientation of the electrical field vector E

• Conventional Doppler radars use horizontal linear polarization only

The orientation of the wave guide at the feed defines the polarization

H

E



• Modesimultaneous H and V transmit and receive

- simple technical realization

- possible contamination by strong depolarization in melting layer

Rain Graupel Hail



• Modealternating H and V transmit (pulse to pulse), simultaneous H and V receive

- expensive and sensitive switch

required

- can measure full scattering matrix (research radars)

Rain Graupel Hail


Dual-Polarization Modes

polarization transmit receive

simultaneous transmit and receiveSTAR-mode, hybrid

H

V

H co-polar

V co-polar

alternating from pulse to pulse

1. pulse: H

2. pulse: V

H co-polar

V cross-polar

V co-polar

H cross-polar

fixed

(“LDR-mode”)

H H co-polar

V cross-polar

= 45°

weather services use the STAR mode, some additionally LDR mode

Dual-Polarization Radar Products

- Differential Reflectivity (ZDR)

- Copolar Correlation Coefficient (ρHV(0))

- Differential Phase, specific Differential Phase

- Linear Depolarization Ratio


Differential Reflectivity (ZDR)

Differential reflectivity is the ratio between horizontal and vertical reflectivity factor

using zH, zV in mm6 m3, or ZH, ZV in dBZ.

• positives ZDR is caused by oblate particles falling orientated parallel to the polarization basis.

• ZDR is weighted with reflectivity. • ZDR depends on particle shape,

orientation and falling behaviour.

dB unit = orlog VHzz ZZZDR10=ZDR

V

H

ZH ZV

VH



Indication for oblate particles falling horizontally orientated



• ZDR can be used to identify insects and birds in clear air echoes– Rain: ZDR 0 – 5 dB– Insects ZDR 5 – 10 dB

Z ZDR

PO

LDIR

AD

at W

alte

nhei

m-s

ur-Z

orn



• ZDR should be zero or positive• negative ZDR is an indication for a failure

differential attenuation in C-band by strong rain

Z ZDR



• ZDR should be zero or positive• negative ZDR is an indication for a failure

“monster snow flakes” Mie-scatter, large particles

Z ZDR


Co-polar Correlation Coefficient (ρHV(0))

The correlation coefficient ρHV(0) describes the correlation of the scattering signal between horizontal and vertical polarization.– a high correlation is expected if the orientation of particles

does not change between pulses– a low correlation is expected if the orientation of particles

changes irregular between pulses

high correlation between H and V low correlation between H and V

V-Pol.H-Pol.V-Pol.H-Pol.

pulses pulses

log.

pow

er


Co-polar Correlation Coefficient (ρHV(0))

• ρHV(0) it almost 1 in rain, in strong rain 0.98 to 0.97.• ρHV(0) below 0.90 indicates particles with irregular shapes,

ice/water mixtures and strong tumbling during fall.• Used for the identification of irregular shaped particles.


Co-polar Correlation Coefficient (ρHV(0))ρHV(0)

ZDR

ZH

Poldirad13 Jan. 2011 09:15 towards 52°

(-1 dB offset)


Differential Propagation Phase

H

V

H

V

0(r)

0(r)

0(r)

0(r)

0(r)

0(r)

0(r) +

H

0(r) +

V

VHDP

Measurements of the differential phase describe properties of the propagation path, it is not a property of the backscattering media.

• Different propagation speed of waves ( phase shift) in rain and air. Refractive index n = 1.0003 (air), n = 1.33 (water).

• Non-spherical particles will have different phase shift depending on polarisation plane.


• The differential propagation phase shift on forward scatter DP occurs on the way towards the scattering particle.

• The backscatter for particles with D << λ is without phase shift.

• The backscattered wave will receive again a differential phase shift.

• The specific differential propagation phase KDP describes the phase shift between horizontal and vertical polarized wave.


degree/km) (unit)(2

)()(

12

12

rr

rrK DPDP

DP

DP(r1)

DP(r2)r1 r2


Differential Phase



POLDIRAD


Summary Differential Propagation Phase

• KDP is a measure for the mass of the particles.

• Phase measurements are absolute measurements. They are independent of the calibration of the radar.

• Phase measurements are not affected by attenuation.

• KDP and DP is used for the estimation of rain rate and the correction of attenuation.


Linear Depolarization Ratio (LDR)

The linear depolarization ratio LDR describes the ratio of cross-polar reflectivity to co-polar reflectivity

• using zVH, zH in mm6 m-3 or ZVH, ZH in dBZ.

• LDR is caused by particles which are rotated to the polarization plane.

• LDR is weighted by reflectivity.

• LDR depends on the shape of the particles, their orientation and their falling behaviour.

dB) unit( = LDRorlog HVHzz ZZ10 = LDR

H

VH


Linear Depolarization Ratio (LDR)Indication for oblate particlesfalling irregularor canted


Melting Layer Stratiform Precipitation


Melting Layer Convective Precipitation

Application of Dual-Polarization

- Rain rate estimation

- Hydrometeor classification

- Quality control


Rain rate and radar reflectivity

Empirical relation between rain rate Rand reflectivity z

z in mm-6 m-3

R in mm/h

Coefficients a and b depend ondrop size distribution.

bz a = R

7000 1-minute drop size distribution,Oberpfaffenhofen, 1996

Z=44.6 dBZ

10-1100101102103104105106

0 1 2 3 4 5Diameter (mm)

N(D

) (m

m

m

)-1

-3 Z=33.8 dBZ

10-1100101102103104105106


N(D

) (m

m

m

)-1

-3

R=13.6 mm/h R=13.6 mm/h


Rain rate and polarimetric radar measurements

Additional information about raindropsize distribution by differential reflectivity: sensitive to large drops.

Z=44.6 dBZ

10-1100101102103104105106


N(D

) (m

m

m

)-1

-3 Z=33.8 dBZ

10-1100101102103104105106


N(D

) (m

m

m

)-1

-3

R=13.6 mm/h R=13.6 mm/h

ZDR=2.6 dB ZDR=0.3 dB

cb ZDRz a = R

7000 1-minute drop size distribution,Oberpfaffenhofen, 1996

Small errors in polarimetric quantities can give large errors in rain rate estimation.


Summary Rain Rate Estimation

Polarimetric quantities are only available for rainfall rates above a certain value, since small raindrops are spherical.

• As a first step a quality control is necessary

• Polarimetric estimates are only valid in the rain layer

• An optimized procedure would use different methods depending on rain intensity (numbers are approximate C-band)

– rain rates below 2 mm/h use z-R relation

– rain rates 2 – 10 mm/h use z-ZDR-R relation

– rain rates above 10 mm/h use KDP-ZDR-R relation


Classification of Hydrometeors

Forecasters want to see this and not that

Differential Reflectivity

Range (km)

Hei

ght (

km)

60 65 70 75 80

2

4

6

8

10

12

14

40 55 600

Hei

ght (

km)

2

4

6

8

10

12

14

Range (km)60 65 70 75 80

ZDR (dB)

-3 -0.5 +0.5 4.5

Hei

ght (

km)

2

4

6

8

10

12

14

60 65 70 75 80

Range (km)

LDR (dB)

-35 -28 -19 -13

Reflectivity

Depolarization Ratio

R Large Raindrops

G

S S

rR

GH

H

HW HW

HLW

Hydrometeor Type

2

4

6

8

10

12

14

Heig

ht (k

m)

S SnowG GraupelH HailHW Wet HailHLW Large Wet Hailr Smal RaindropsR

Reflectivity (dBZ)

Range (km)60 65 70 75 80



• From observations and theoretical or practical considerations we know:

Z (dBZ)

ZDR(dB)

KDP(°/km)

ρHV(0) LDR (dB)

Rain 10 – 55 0 – 5 0 – 10 ≈ 1.0 < -30

Ice crystals < 15 0 – 2 0 ≈ 0.99 < -30

Snow aggregates

< 25 0 – 2 0 ≈ 0.99 < -30

Graupel up to 40

≈ 0 ≈ 0 > 0.95< 0.95 melting

< -30< -25 melting

Hail up to 70

≈ 0 ≈ 0 0.9 – 0.95< 0.9 melting

> -25-25 – -15 melting



• From observations and theoretical or practical considerations we know:

Z (dBZ)

ZDR(dB)

KDP(°/km)

ρHV(0) LDR (dB)

Insects < 5 5 – 10 ? 0.9 – 1.0 ? < -30 ?

Birds < 5 3 – 6 ? 0.9 – 1.0 ? < -30 ?

Chaff < 5 0 – 6 ? < 0.3 > -20

Ground clutter

any noisy noisy 1 stopped < 0.6 rotating antenna

> -20



Based on thresholds(Höller et al., 1994)

Based on fuzzy logic(Vivekanandan et el., 1999)

LD

R

Each manufacturer (researcher) has her/his own algorithm,display, hydrometeor classes, parameters to adjust




Example of Fuzzy Logic: Rain – SnowA

. D

alp

hin

et,

Me

teo

Fra

nce

- D

LR

snow atground

rain atground

Verification using MRRprecipitation fall velocities

snowrain

13 14 UTC

2500

AGL

0 m

Reflectivity

21 Nov. 2008 13:30 UTC


Quality control for polarimetric radar products

Quality control for rain rate estimation:

rain only, good beam filling, low beam blockage, low attenuation, ...

bad good


Dual-Polarization Conclusion

• Additional information available– useful for hydrometeor classification– improved rain rate estimation– improved quality control– attenuation correction possible

• Dual-polarization– makes attenuation visible (at C- and X-band)– requires very high data quality (calibration issue)– many algorithms are only available for rain

Documents

Dual Polarization Martin Hagen, Elena Saltikoff Deutsches Zentrum für Luft- und Raumfahrt (DLR) Oberpfaffenhofen, Germany Finnish Meteorological Institute