Crossbeam Wind Measurements with PhasedArray Doppler Weather Radar

1
Crossbeam Wind Measurements with Phased-Array
Doppler Weather Radar

• Richard J. Doviak
• National Severe Storms Laboratory
• Guifu Zhang
• School of Meteorology, University of Oklahoma
• Norman, Oklahoma

2
Spaced Antenna Interferometry (Overview)

• Interferometry
• Complementary to the Doppler method
• Used by the MST community for a half century
• Weather applications
• NCARs Multiple Antenna Profiling Radar (MAPR)
• UMasss Dual-polarization Spaced Antenna (DPSA)
  system
• National Weather Radar Testbed (NWRT)
• (phased-array weather radar)
• Good opportunity to revisit spaced antenna
  interferometry

3
Phase Array Radar(scanning diversitymulti-missio
n etc.)
ARSRASRTDWRWR
4
National Weather Radar Testbed Monopulse Antenna
on the University of Oklahomas Campus
(2)
(1)
5
Monopulse Antenna Patterns(Sum and Azimuth
Difference)
SUM
Azimuth Difference
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Monopulse Antenna Outputs 1) Sum 2) Elevation
difference 3) Azimuth difference
Correlations of Sum and Difference Signals
Correlations of Signals from the Left and
right halves of array
Weather Signals Vs(t)VD(t)
Within V6
CSS(t) CDD(t) CSD(t)
C11(t) C12(t)
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Possible configurations of SAI
voy
V2(t)
V1(t)

• three channels
• Sum
• Azimuth difference
• Elevation difference

Azimuth SA
Elevation SA
Dual-beams to separate shear and turbulence

Azimuth cross correlation
8
Auto and cross correlation coefficients
Cross-correlation peak shifts due to the delay
of diffraction pattern passing over antennas from
R1 to R2
c11
c12
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Tilted Cartesian Coordinate System
First order perturbations
Mean wind
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Azimuth cross correlation coefficient(to obtain
horizontal component of crossbeam wind)
Where,
are apparent crossbeam winds
11
Apparent wind versus angular shear

• Apparent wind in the azimuth direction
• Angular shear in the azimuth direction
• Wind estimation using cross correlation ratio

12
Showing why SAI cannot distinguish crossbeam wind
from crossbeam shear of along-beam axis wind
vy(0)
Beam axis
Beam axis
vy(0)
Crossbeam wind
Crossbeam shear of along-beam axis wind
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Auto cross-correlation coefficients
c11
(a)
(b)
c12
Auto- and cross-correlation coefficients for the
NWRT PAR. Meteorological parameters arevy '(0)
20, vz '(0) 5,stx ' 0.5 m s-1, sx ' 0. (a)
Dependence on r0, sy' 0, sz ' 0.002 s-1 (b)
Dependence on shear sy ' at r0 30 km
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Separating shear and turbulence(dual beamwidth
method)
Transmit beam
Azimuth receive beam
Elevation receive beam
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Separating shear turbulence

• Auto-correlation for narrow (Sum) beam
• Auto-correlation for broad beam (left or right
  side of array
• Shear
• Turbulence

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Theoretical performance
CCR
FCA
About 10 s needed for 2 m s-1 crossbeam wind
accuracy at near ranges for 0.5 m s-1 turbulence
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Comparison of SAI and DBS

• SAI better than DBS if angular separation lt Beam
  Width

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Summary and Conclusions

• It has been shown that SAI (NWRT)
• (1) measures angular shear of radial velocities
  within V6
• (2) IFF transverse shear of the Cartesian wind
  component parallel to the beam axis is
  negligible, can crossbeam wind within V6 be
  measured
• (3) separates shear and homogeneous turbulence
  so that turbulence within V6 can be measured
• Limitations of crossbeam wind measurements with
  SAI
• (1) Uniform wind and reflectivity required
• (2) Long dwell times (i.e., seconds) for
  accurate crossbeam measurements

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End of Slide Show
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Differences between current weather surveillance
and PAR Technology
A wide transmit beam and Multiple receive beams
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Advantages of a phased array weather radar

• 1) significant reduction in the time to make
  measurements over storm volumes
• 2) obtaining more frequent measurements of
  meteorological hazards, (e.g., tornado cyclones,
  etc.)
• 3) monitoring, at a lower revisit rate, areas
  void of weather
• 4) faster update rates of selected storms (i.e.,
  better retrieval of storm properties to predict
  developing hazards)
• 5) better ground clutter canceling and
  compensation for reflectivity biases
• 6) the angular resolution of a stationary beam
  (i.e., no smearing due to rotation)
• 7) Multiple mission (tracking aircraft weather
  etc.)
• 8) direct measurement of crossbeam wind using
  interferometric techniques

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Testbed Basic Radar Parameters

• Radar Antenna System
• 3.66 m diameter with 10 tilt-back 4,000
  elements
• Az/El Broadside Beamwidth 1.6(Tx) 1.8(Rx)
• Nominal Gain 41 dB
• Linear Vertical Polarization
• Scan volume (electronic) ? 45? Az, 0 - 55 El
• Transmitter WSR-88D (NEXRAD)
• Output Power 700 KW ? 10.cm
• Pulsewidths 1.57 ?s, 4.71 ?s
• Maximum Duty Factor 0.002

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General formulation

• Configuration sketch
• Received signals

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Derivation of cross correlation function

• Definition
• Velocity approximation
• Derived cross-correlation function

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Physics explanation
Transverse wind Transverse shear of radial wind

• Time delay in both cases
• Configuration shifted or rotated