Crossbeam Wind Measurements with Phased-Array 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 ERAD2006

2. Spaced Antenna Interferometry (Overview) • Interferometry: • Complementary to the Doppler method • Used by the MST community for a half century • Weather applications: • NCAR’s Multiple Antenna Profiling Radar (MAPR) • UMass’s Dual-polarization Spaced Antenna (DPSA) system • National Weather Radar Testbed (NWRT); (phased-array weather radar) • Good opportunity to revisit spaced antenna interferometry ERAD2006

3. Phase Array Radar(scanning diversity;multi-mission*; etc.) *ARSR;ASR;TDWR;WR ERAD2006

4. National Weather Radar Testbed Monopulse Antenna on the University of Oklahoma’s Campus (2) (1) ERAD2006

5. Monopulse Antenna Patterns(Sum and Azimuth Difference) SUM Azimuth Difference ERAD2006

6. 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(τ) CDD(τ) CSD(τ) C11(τ) C12(τ) ERAD2006

7. Possible configurations of SAI R 1 R R 1 2 R 2 s q R s s q T f T s f R 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: ERAD2006

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 ERAD2006

9. Tilted Cartesian Coordinate System ; First order perturbations ; Mean wind ERAD2006

10. Azimuth cross correlation coefficient(to obtain horizontal component of crossbeam wind) Where, are apparent crossbeam winds ERAD2006

11. Apparent wind versus angular shear • Apparent wind in the azimuth direction: • Angular shear in the azimuth direction: • Wind estimation using cross correlation ratio: ERAD2006

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 ERAD2006

13. Auto & cross-correlation coefficients c11 (a) (b) c12 Auto- and cross-correlation coefficients for the NWRT PAR. Meteorological parameters are:vy ′(0) = 20, vz ′(0) = 5,σtx ′ = 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; ERAD2006

14. s q R s s q T f T s f R Separating shear and turbulence(dual beamwidth method) Transmit beam Azimuth receive beam Elevation receive beam ERAD2006

15. Separating shear & turbulence • Auto-correlation for narrow (Sum) beam • Auto-correlation for broad beam (left or right side of array • Shear • Turbulence ERAD2006

16. 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 ERAD2006

17. Comparison of SAI and DBS • SAI better than DBS if angular separation < Beam Width ERAD2006

18. 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 ERAD2006

19. End of Slide Show ERAD2006

20. Differences between current weather surveillance and PAR Technology A wide transmit beam and Multiple receive beams ERAD2006

21. 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 ERAD2006

22. 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; λ = 10.cm • Pulsewidths = 1.57 s, 4.71 s • Maximum Duty Factor = 0.002 ERAD2006

23. General formulation • Configuration sketch • Received signals ERAD2006

24. Derivation of cross correlation function • Definition • Velocity approximation • Derived cross-correlation function ERAD2006

25. Physics explanation Transverse wind Transverse shear of radial wind • Time delay in both cases • Configuration shifted or rotated ERAD2006