Phasedarray Weather Radar Interferometry to Measure Crossbeam Wind, Shear and Turbulence

1
Phased-array Weather Radar Interferometry to
Measure Crossbeam Wind, Shear and Turbulence

• Guifu Zhang and Richard J. Doviak
• University of Oklahoma
• National Severe Storms Laboratory
• Thanks to NSSL/LM engineers
• January 17, 2007

2
Overview

• Radar interferometry
• Complementary to Doppler method
• Used in MST community for a half century
• Weather application
• NCARs Multiple Antenna Profiling Radar (MAPR)
• Umasss Dual-polarization Spaced Antenna (DPSA)
  system
• National Weather Radar Testbed (NWRT)
• Phased Array Radar (PAR) Good opportunity to
  revisit spaced antenna interferomtry (SAI)
• Pattern measurement and calibration
• SAI theory in the presence of shear and turbulence

3
Possible configurations of SAI

• SPY-1 three channels
• Sum
• Azimuth difference
• Election difference

Azimuth SA
Elevation SA
SAI
Dual-beams to measure Crossbeam wind, shear and
turbulence
courtesy of A. Zahrai
Cross-correlation peak shifts due to signal
delay passing over antennas from R1 to R2
4
Monopulse Antenna Pattern Measurement versus
Theory

• Sidelobe level of -25 dB
• N5 (N the number of close-in sidelobes at -25
  dB level)

5
Pattern calibration
Sum
Azimuth Difference
Balance factor dB Phase center
separation m
?S0.73o ?1S1.72o ?R1.06o ?1R 2.50o
6
General formulation

• Tilted coordinate system
• Received signals

7
Derivation of cross correlation function

• Definition
• Velocity approximation
• Derived cross-correlation function

8
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
9
Physics explanation
Transverse wind Transverse shear of radial wind

• Time delay in both cases
• Configuration shifted or rotated

10
Apparent wind versus angular shear

• Apparent wind in the azimuth direction
• Angular shear in the azimuth direction
• Wind estimation using cross correlation ratio
• Apply to vertical wind/shear

11
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
12
Summary and Conclusions

• It has been shown that
• SAI can enhance the sensing capability of NWRT
• SAI measures angular shear of radial velocities
  crossbeam wind shear
• Shear and turbulence can be separated within V6
• Limitations
• Uniform wind and reflectivity required
• Long dwell times (i.e., seconds) for accurate
  crossbeam measurements

13
End of Slide Show
14
Angular shear estimation using dual-beam method

• Auto-correlation for narrow beam
• Auto-correlation for broad beam
• Shear
• Turbulence

15
Theoretical performance

• More than 10 seconds needed for valid wind
• SA better than DBS when the angular separation lt
  1 degree

16
Sample calculations
17
Separating shear and turbulence(dual beamwidth
method)
Transmit beam
Azimuth receive beam
Elevation receive beam
18
Comparison of SAI and DBS

• SAI better than DBS if angular separation lt Beam
  Width

19
Separating shear turbulence

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

20
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)