Advances in polarimetric X-band weather radar

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Advances in polarimetric X-band weather radar

Tobias Otto

Delft
University of
Technology Remote Sensing of the Environment

A
T Contents
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O
S
• motivation
• weather radar polarimetry
• X-band challenge
• radar data processing
• attenuation correction
• differential phase processing
• raindrop-size distribution
• quantitative precipitation estimation (QPE)
• further applications
• limitations of X-band weather radar
• radar technologies for polarimetric X-band weather radar

Delft
University of

A
T Why X-band*?
M
O Compact, easily deployable and cheaper than the usual S- or C-band weather radars.
S
Used for dedicated, short-range (< 60km) applications such as
• gap-filling radars in complex terrain such as moutainous areas, e.g.
- RHyTMEE project of Météo France
• high-resolution precipitation measurement in densly populated areas in order to
improve urban water management and flood prediction, e.g.
- polarimetric X-band radar network in Tokyo, Japan (http://www.bosai.go.jp/kiban/radar)
- RAINGAIN project in Paris, Rotterdam, London and Leuven (http://www.raingain.eu)
- CASA Dallas Fort Worth Urban Demonstration Network (http://www.casa.umass.edu/)
• improve the low-altitude radar coverage
They can provide a higher temporal and spatial resolution than standard operational
weather radars due to the reduced range coverage and less stringent requirements on
the scanning strategy due to their focused application.
But
• attenuation due to rain is stronger than at S- or C-band, total signal extinction within
few kilometres is possible in a cloudburst (instantaneous rain rates >100 mmh-1)
• resonance scattering (Mie scattering) occurs in moderate to strong rain
*electromagnetic frequency band from 8 – 12 GHz

Delft
University of

A
T The two X-band weather radar worlds
M
O
S

Marine radars turned into weather radars. Dedicated polarimetric weather radars.

 usually power measurement only  beside power also Doppler and polarimetric
 with fan beam antenna coarse resolution in elevation measurements
 good for a spatial overview of precipitation but  very good for quantitative precipitation estimation
not for quantitative precipitation estimation (QPE)  not that cheap
 cheap

gematronik.com

radar.dhigroup.com

metek.de
novimet.com

Delft
University of

A
T Why polarimetry?
M
O
S
Most hydrometeors are not spherical, and
they show distinct polarimetric signatures at microwave frequencies.

- ice particles

- hail

Beard, K.V. and C. Chuang: A New Model for the Equilibrium Shape of
Raindrops, Journal of the Atmospheric Sciences, vol. 44, pp. 1509 – 1524, June
1987. http://commons.wikimedia.org/wiki/Category:Hail
- raindrops

Delft
University of

A
T Which polarisations are used?
M
O
S linear horizontal / vertical polarisations (H and V)

Motivation:
- easier to understand especially for the weather radar user community
- close to the characteristic / principal polarisations for measurements
at low elevations, i.e. low depolarisation
- differential measurements (power, phase) between H and V are directly
linked to the anisotropy (oblateness) of the hydrometeors
What to measure?
- ideally the complex polarisation scattering matrix which links the incident electric
field vector Ei with the backscattered electric field vector Es

 Eh   S hh
s
S hv  Eh  e − jkr
i
 s=  i 
E  S S vv  Ev  r
 v   vh  
Delft
University of

A
T Measurement principle
M
O (alternate polarisation mode)
S
transmit

Zhh (dBZ) Zhv (dBZ)
receive

Zvh (dBZ) Zvv (dBZ)

Data: C- Band POLDIRAD (DLR, Oberpfaffenhofen, Germany), Prof. Madhu Chandra

Delft
University of

A
T Differential reflectivity
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S
transmit

Zhh (dBZ) Zhv (dBZ)
receive

-
Zvh (dBZ) Zvv (dBZ)

= Zdr
differential
reflectivity


Delft
University of

A
T Differential reflectivity
M
O
S

rain
aggregateslayer
ice crystals
melting (snow)

Reflectivity Differential Reflectivity

Phh
Z hh = 10 log CR 2 Phh ( dBZ) Z dr = 10 log ( dB)
Pvv

Delft
University of

A
T Linear depolarisation ratio
M
O
S
transmit

Zhh (dBZ) Zhv (dBZ)

-
receive

Zvh (dBZ) Zvv (dBZ)

= LDR (dB)
linear depolar-
isation ratio


Delft
University of

A
T Linear depolarisation ratio
M
O
S

melting clutter
ground layer

Reflectivity Linear Depolarisation Ratio
Phv
Z hh = 10 log CR 2 Phh ( dBZ) LDR = 10 log ( dB)
Pvv

Delft
University of

A
T Differential phase
M
O
S range-normalised microwave propagation through rain
phase difference between H and V
differential phase Φdp (deg)

range r

The slope of the differential phase is called
specific differential phase:

Φ dp (r2 ) − Φ dp (r1 )
range (
K dp deg km −1 =) 2 ⋅ ( r2 − r1 )
The measurement of the differential phase is crucial for polarimetric X-band weather radars because it is:
- independent from radar calibration
- independent from partial beam blocking and attenuation as long as the signal is not totally extinct
- almost linearly related to rain attenuation
- very useful at X-band for rainfall rate estimation when R ≥ 3 mm h-1

Delft
University of

A
T X-band challenge
M
O
S Power and differential phase measurements by X-band weather radars are always a
combination of propagation and backward-scattering effects that need to be separated
before analysing the weather radar data.

propagation backward-

Z ' ( rn ) = Z ( rn ) − 2 ∫ α ( r )dr
(forward-scattering) scattering
rn−1

attenuation A reflectivity Z r =r1
rn−1
differential propagation differential backscatter
phase Φdp phase δco Ψ dp ( rn ) = δ co ( rn ) + 2 ∫ K dp (r )dr
r = r1

Delft
University of

A
T X-band challenge
M 0.5°
O
S A clutter-filtered polarimetric X-band
reflectivity (dBZ) weather radar measurement. differential reflectivity (dB)

differential attenuation

differential phase (deg)

differential backscatter phase
(an indicator of resonance/Mie scattering)

Data: TU Delft X-band IDRA, data freely available at http://data.3tu.nl/repository/collection:cabauw

Delft
University of

A
T Contens
M
O
S
• motivation
• weather radar polarimetry
• X-band challenge
• radar data processing
• attenuation correction
• differential phase processing
• raindrop-size distribution
• quantitative precipitation estimation (QPE)
• further applications
• limitations of X-band weather radar
• radar technologies for polarimetric X-band weather radar

Delft
University of

A
T Estimation of attenuation
M
O
S • attenuation can be estimated via the specific differential phase Kdp:
X-band scattering computation using measured drop-size distributions
(by 2D-video disdrometer) and several raindrop-shape models

αhh specific one-way attenuation at
horizontal polarisation (dB km-1)

αh-v differential attenuation (dB km-1),
i.e. αh-v=αhh- αvv

• rule of thumb for S-, C- and X-band:
whenever microwave attenuation due to rain is substantial, the differential phase
accumulation is significant enough that Kdp can be estimated
Delft
University of

A
T Estimation of attenuation
M
O
S • a more complex attenuation correction method relies on the determination of the
path-integrated attenuation (PIA), e.g. by
• differential phase (no estimation of Kdp required),
• power measurement of a fixed target at far range (ground clutter), …
• the PIA is distributed over the range bins weighted by the reflectivity
 z ' ( rn )  × 100.1×b×PIA − 1)
 (
b
 α specific one-way attenuation (dB km-1)
α ( rn ) =
I ( r1 : rN ) + ( 100.1×b×PIA − 1) I ( rn : rN ) z reflectivity in linear units (mm6m-3)
z′ attenuated reflectivity (mm6m-3)
rN

I ( rn : rN ) = 0.46 ×b × ∫  z ' ( rn )  dr
b

α = a ⋅ zb
 
r = rn

PIA (dB)

Delft
University of

A
T Differential phase processing
M
O
S
Goal is the estimation of the slope of the differential propagation phase Kdp.
rn−1

Ψ dp ( rn ) = δ co ( rn ) + 2 ∫ K dp (r )dr
r = r1

2011-09-10 19:45:19UTC, az. 324.4 deg

most likely differential
backscatter phase

Delft
University of

A
M
O Goal is the estimation of the slope of the differential propagation phase Kdp.
S
2011-09-10 19:45:19UTC, az. 324.4 deg

Most common method:
Linear regression with a running
window length of about 1-3km.

Disadvantage:
• leads to negative Kdp in the presence
of differential backscatter phase
• reduced range resolution of the
resulting Kdp
• Kdp peaks are underestimated

Delft
University of

A
M
S
2011-09-10 19:45:19UTC, az. 324.4 deg
• the difference of Ψ between the ranges ra
dp
X-band scattering computations based on
and a can be distributed
raindrop-size distributions measured by rb disdrometer among the range
bins including a weighting with the reflectivity
zhh and the differential reflectivity zdr
ΔΨdp = Ψdp(rb) – Ψdp(ra) 1
K dp ( rn ) = ×∆Ψ dp ×w
2∆r
with
−0.42
 zhh ( rn )   zdr ( rn ) 
0.69

w=    
ra rb
∑ zhh zdr
0.69 −0.42

range
(coefficients valid for rain, X-band, zhh and zdr in linear units)

• the differential reflectivity is closely related to
the backscatter phase,

Delft
University of

A
M
S
2011-09-10 19:45:19UTC, az. 324.4 deg
• the difference of Ψdp between the ranges ra
and rb can be distributed among the range
bins including a weighting with the reflectivity
zhh and the differential reflectivity zdr
ΔΨdp = Ψdp(rb) – Ψdp(ra) 1
K dp ( rn ) = ×∆Ψ dp ×w
2∆r
with
−0.42
 zhh ( rn )   zdr ( rn ) 
0.69

w=    
ra rb
∑ zhh zdr
0.69 −0.42

range
(coefficients valid for rain, X-band, zhh and zdr in linear units)

• the differential reflectivity is closely related to
the backscatter phase,
ra and rb can be chosen such that
Zdr(rb) - Zdr(ra) ≈ 0, therefore δco(rb) - δco(ra) ≈ 0,
in this case, ΔΨdp is due to the differential
propagation phase only.
Delft
University of

A
T X-band challenge
M attenuated reflectivity (dBZ)
corrected reflectivity (dBZ) attenuated differential reflectivity (dB)
corrected differential reflectivity (dB)
O A clutter-filtered polarimetric X-band
S weather radar measurement.

The separation of the forward- and
backward-scattering components is
crucial at X-band.
Only after a separation of both
components, the data can be further
processed and analysed (rainfall rate
retrieval, hydrometeor classification).

specific differential phase (deg km-1) differential phase (deg) differential backscatter phase (deg)

Delft
University of

A
T Raindrop-size distribution
M
O
S The weather radar measurements are connected via the raindrop-size distribution (RDSD)
to meteorological parameters such as liquid water content or rainfall rate.

• Raindrop-size distribution normalised
with respect to the liquid water content:
µ D
 D  -(3.67 + µ) D0
N ( D) = N w f ( µ)  ÷ e
 D0 

6 (3.67 + µ) µ+ 4
f ( µ) =
3.67 4 Γ( µ + 4)
Nw .. concentration parameter
D0 .. median volume diameter
µ .. shape parameter

• for simplicity, often µ = 0 is assumed
such that the RDSD becomes a two-
parameter exponential distribution

Delft
University of

A
T Raindrop-size distribution
M
O
S
Meteorological parameters:
π
LWC = 109 ∫ D N ( D)dD
3
• liquid water content (mm3m-3)
6D
raindrop volume

π
• rainfall rate (mm h-1) R = 3.6 × 106 × ∫ D3 v( D) N ( D)dD
6D
terminal fall velocity (m s-1)

Polarimetric weather radar measurements:
valid for Rayleigh wavelength
scattering λ4
• reflectivity (mm6m-3) z = 10 ×∫ D N ( D)dD =
18 6
2
1018 ×∫ σ ( D ) N ( D)dD
D π5 K D
radar coss-section
dielectric factor

180
K differential ×λ × ℜ(deg( D) −) f vv ( D) ] N ( D )dD
specificdp = 10 phase [ f hh km-1
3
∫
π D
forward-scattering
amplitudes
Delft
University of

A
T Rainfall rate estimation
M
O
S Variability due to:
X-band scattering computations based on
raindrop-size distribution measured by a disdrometer • raindrop-size distribution
numeric example assuming Rayleigh scattering
raindrop water volume
#/m3 Z
diameter per cubic meter
1 mm 4096 36 dBZ 2144.6 mm 3
reflectivity zhh 4 mm 1 36 dBZ 33.5 mm3
A fixed parameterisation of Z-R / Kdp-R relations
leads to uncertainties due to the natural variability
Logarithmic scale. of rainfall.
Z-R / Kdp-R relations are not linear!
• raindrop shape (Kdp)

specific differential Note:
phase Kdp
• Kdp can be estimated up to ~0.1 deg,
only useful for instantaneous rainfall
rates larger than ~3 mmh-1 at X-band
• Kdp – based rainfall rate estimates
tend to be more accurate also due to
its independence from radar
calibration and signal attenuation
Delft
University of

A
M
O
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Data processing and rainfall rate estimation of the TU Delft polarimetric X-band radar IDRA:
• spectral clutter suppression [1]
• estimation of the specific differential phase Kdp [2]
(reflectivity-weighted to overcome the coarse range-resolution of conventional Kdp estimators,
the estimated Kdp is unaffacted by signal attenuation and independent of the radar calibration)

• estimation of the one-way specific attenuation by αhh = 0.34∙Kdp with αhh (dB km-1) and Kdp (deg km-1) and
attenuation correction of the reflectivity
• the parametrisations for the rainfall rate estimation are based on 41530 raindrop-size distributions
measured by a 2D-video disdrometer data at Cabauw (Netherlands) in 2009:
• zhh = 243∙R1.24 with the rainfall rate R (mm h-1) and the reflectivity at horizontal polarisation zhh (mm6 m-3)

• R = 13∙Kdp0.75 with the rainfall rate R (mm h-1) and the one-way specific differential phase Kdp (deg km-1)

• for the final rainfall rate product, R(Kdp) is chosen if the reflectivity is above 30 dBZ, and the standard
deviation of Kdp is below 2 deg km-1, else R(zhh) is used

[1] C. Unal, 2009: Spectral Polarimetric Radar Clutter Suppression to Enhance Atmospheric Echoes,
J. Atmos. Oceanic Technol., 26, 1781–1797.
[2] T. Otto and H.W.J. Russchenberg, 2011: Estimation of Specific Differential Phase and
Differential Backscatter Phase from Polarimetric Weather Radar Measurements of Rain,
IEEE Geosci. Remote Sens. Lett., 8, 988-992.

Delft
University of

A
M corrected reflectivity (dBZ) corrected differential reflectivity (dB)
O A clutter-filtered polarimetric X-band
S weather radar measurement.

rainfall rate estimate (mm h-1)

specific differential phase (deg km-1) differential backscatter phase (deg)

Delft
University of

A
T Further applications of radar polarimetry
M
O
S

Hydrometeor classification
• the hydrometeors (snow, ice, rain, hail) show different polarimetric signatures
 a classification is possible and can improve rainfall rate estimation

Adaptive clutter suppression
• robust suppression of clutter (ground targets, birds, planes) is possible taking
advantage of the different polarimetric signatures
 see next presentation by Christine Unal

Raindrop-size distribution retrieval
• the polarimetric parameters can be combined to estimate the parameters of
the raindrop-size distribution and to improve the rainfall rate estimation

Delft
University of

A
T Limitations of X-band radar
M
O
S • major limitation of X-band weather radar systems is attenuation in heavy rain / wet hail:

ΔΨ = 180 deg, that corresponds
to ~60 dB round-trip attenuation
over 8 km distance!

• if the purpose of an X-band radar is the observation of heavy precipitation:
• instead of using a single X-band radar, use a network of X-band radars, or
• complement the X-band radar measurements with measurements of the
operational weather radar network (S- or C-band observations).

Delft
University of

A
T Simultaneous H/V mode
M
O
S • most commercially available polarimetric weather radars do not employ the alternate
polarisation mode, instead they use the “simultaneous H/V mode”:
 simultaneous transmission of a horizontally and a vertically polarised wave with equal amplitude
 they will combine dependening on their phase offset to an elliptically polarised wave
 the radar measures a combination of co- and cross-polarised scattering matrix components:

Ehs = S hh ×Eh + S hv ×Evi ≈ S hh ×Eh
i i

only in case of very low cross-polarisation!
E = Svh ×E + Svv ×E ≈ Svv ×E
s
v
i
h
i
v
i
v

Advantages
• no need of a high-power ferrite switch
• double unambiguous Doppler velocity interval
Disadvantages
• very demanding requirements on the radar cross-polarisation isolation
• depolarisation in the melting layer / ice clouds will deteriorate the measurements
• reduced accuracy of polarimetric weather radar measurements due to cross-pol component
• no measurement of the linear depolarisation ratio, instead cross-correlation coefficent
• loss of 3dB in sensitivity compared to alternate mode because the transmit power is split
equally over the H and V transmit channel
Delft
University of

A
T Phased-array antennas
M
O
S • important antenna specifications for polarimetric weather radars are
• high resolution in azimuth and elevation, i.e. pencil beam (large directional gain),
• ideally equal specifications for horizontal and vertical polarisation
(e.g. matched co-polarised beam patterns, S-parameters),
• low cross-polarisation levels.

• usually parabolic reflector antennas are employed by polarimetric weather radars

• there is some on-going research in order to use phased-array antennas,
e.g. by the Engineering Research Center for Collaborative Adaptive of the
Atmosphere (CASA, USA)[1]:
• 64 T/R modules with 1.25W transmit power each
• electronic phase steering in azimuth (±45 deg) and mechanical steering in elevation
• elevation beamwidth of 2.8 deg, azimuth beamwidth of 1.8 deg – 2.4 deg
• alternate polarisation mode due to limited cross-polarisation isolation

[1] J.L. Salazar, E.J. Knapp and D.J. McLaughlin, 2010: Dual-polarization performance of the phase-tilt antenna array
in a CASA dense network radar, Geoscience and Remote Sensing Symposium, IGARSS 2010, 3470-3473.

Delft
University of

A
T Solid-state transmitter
M
O
S • first commercial systems are on the market that use solid-state transmitter instead
of the traditionally used high-power microwave sources:
• long lifetime
• compact, no high-power microwave circuits (waveguides etc.)
• combined with an arbitrary waveform generator (e.g. direct digital-synthesizer), high
flexibility of the transmitted waveform  software-defined radar
• to retain the sensitivity of such systems, pulse-compression is employed
• e.g. alternate transmission of a modulated long pulse (~50 µs) for far-range
measurements and a short pulse (~1 µs) for close-range measurements
Txlong Rxlong Txshort Rxshort Txlong Rxlong

time
far-range measurement close-range
measurement

combination

Delft
University of

A
T TU Delft X-band weather radar: IDRA
M
O CESAR – Cabauw Experimental Site for Atmospheric Research
Specifications
S
• 9.475 GHz central frequency
• FMCW with sawtooth modulation
• transmitting alternately horizontal and vertical
polarisation, receiving simultaneously the co-
and the cross-polarised component
• 20 W transmission power
• 102.4 µs – 3276.8 µs sweep time
• 2.5 MHz – 50 MHz Tx bandwidth
• 60 m – 3 m range resolution
• 1.8° antenna half-power beamwidth

Reference
J. Figueras i Ventura: “Design of a High Resolution
X-band Doppler Polarimetric Weather Radar”,
PhD Thesis, TU Delft, 2009.
(online available at http://repository.tudelft.nl)
Near real-time display:
IDRA is mounted on
top of the 213 m high
http://ftp.tudelft.nl/TUDelft/irctr-rse/idra
meteorological tower. Processed and raw data available at:
http://data.3tu.nl/repository/collection:cabauw
Delft
University of

A
T
M
O
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Advances in polarimetric X-band weather radar
Tobias Otto

e-mail t.otto@tudelft.nl
web http://atmos.weblog.tudelft.nl
radar data http://data.3tu.nl/repository/collection:cabauw
references R. E. Rinehart, “Radar for Meteorologists”,
Rinehart Publications, 5th edition, 2010.
V. N. Bringi and V. Chandrasekar, “Polarimetric Doppler
Weather Radar: Principles and Applications”, Cambridge
University Press, 1st edition, 2001.
R. J. Doviak and D. S. Zrnić, “Doppler Radar and Weather
Observations”, Academic Press, 2nd edition, 1993.

Delft
University of

Advances in polarimetric X-band weather radar

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