1. Staggered PRT Update Part I Sebastián Torres and David Warde CIMMS/The University of Oklahoma and National Severe Storms Laboratory/NOAA NEXRAD TAC Norman, OK 29 August, 2012

2. Why Staggered PRT? Less “Purple haze”: larger r a Less Velocity aliasing: larger v a Doppler Velocity from KTLX Batch Mode Doppler Velocity from KOUN Staggered PRT 2

3. Early SPRT Concerns Extending the range coverage –Overlaid echoes Extending the Nyquist velocity –Velocity dealiasing algorithm –Catastrophic errors Defining scanning strategies –PRT selection –Dwell-time selection Mitigating ground-clutter contamination 3

4. SPRT Range Coverage r a,2 r a,1 r a,2 r a,1 r a,2 - r a,1 Batch SPRT ra,Lra,L 2r a,S r a,S 2r a,S r a,S r a,2 r a,1 ReflectivityDoppler Var.Polarimetric Var. 4 No “purple haze” possibleSome “purple haze” possible key:vs.

5. SPRT Velocity Dealiasing Velocities are estimated for each PRT –v 1 are estimated from the short-PRT pairs –v 2 are estimated from the long-PRT pairs Maximum unambiguous velocities for the short and long PRT pairs are different –Velocities v 1 and v 2 alias in different ways The true (de-aliased) velocity can be obtained by “analyzing” how v 1 and v 2 alias T1T1 T2T2 time T1T1 T2T2 … R1R1 R2R2 R1R1 R2R2 5

6. “Catastrophic” Velocity Errors A “catastrophic error” occurs if errors of estimates are so large that v 1 and v 2 cannot be properly dealiased –These appear as speckles in the velocity fields –Catastrophic errors are more likely for wider normalized spectrum widths –The ORPG velocity dealiasing algorithm has been modified to mitigate these 6

7. ORPG Legacy Velocity Dealiasing KCRI - 31 March 2008 3.1 deg No Dealiasing KCRI - 31 March 2008 3.1 deg Baseline Dealiasing KCRI - 31 March 2008 3.1 deg Modified Dealiasing (courtesy of David Zittel, ROC) 7

8. Milestones 2003: 2/3 PRT ratio with DC filter –NSSL Report 7 2005: SACHI filter (standalone) –NSSL Report 9 2007: Informal presentation to ROC DQ Team 2008: Any PRT ratio with DC filter –Last NEXRAD TAC informational briefing –NSSL Report 12 2009: 2/3 PRT ratio with SACHI filter –Stand-alone algorithm description delivered on 03/09 2009: 2/3 PRT ratio with SACHI filter and overlaid echo recovery –Stand-alone algorithm description delivered on 07/09 –NSSL Report 13 2010: Extension to dual polarization –Last NSSL/NCAR/ROC TIM briefing –NSSL Report 14 2012: CLEAN-AP –NSSL Report 16 8

9. 2012 SPRT Algorithm Description Dual-pol extension –H- and V-channel processing –Spectral moments and polarimetric variables CLEAN-AP integration –Detection and filtering functions –Modular description to facilitate future uniform-PRT implementation Recovery of overlaid echoes –Allows using shorter PRTs for better performance 9

10. ROC’s 3-Phase Approach SPRT (NSSL) DPT2 (SIGMET) Algorithm Selection Engineering Implementation Operational Implementation DC Removal GCF RPG Velocity Dealiasing Algorithm Spectral GCF Scanning Strategies 10

11. Batch PRT vs. Staggered PRT Reflectivity (courtesy of D. Saxion, ROC) 11 BATCH SPRT

12. Batch PRT vs. Staggered PRT Doppler Velocity (courtesy of D. Saxion, ROC) 12 BATCHSPRT

13. VCPs for SPRT Careful VCP design for SPRT is needed –SPRT algorithm performance is more intimately tied to acquisition parameters Ground clutter suppression Catastrophic errors –More trade-offs than with other techniques We want everything we have and more! SPRT can complement SZ-2 in VCPs that achieve a “complete solution” for the mitigation of R/V ambiguities 13

14. Legacy Mitigation Strategy 0.5° 1.5° 19.5° Long-PRT scan followed by short-PRT scan. Long-PRT reflectivities are used to unfold short-PRT velocities (at most, strongest overlaid trip can be recovered) Alternating batches of long- and short-PRT pulses. Long-PRT reflectivities are used to unfold short-PRT velocities (at most, strongest overlaid trip can be recovered) Split Cuts 2 scans at each elevation angle Batch PRT 1 scan at each elevation angle Uniform PRT 1 scan at each elevation angle 7.0° 14

15. Evolutionary Mitigation Strategy 0.5° 1.5° 19.5° Long-PRT scan followed by phase-coded short-PRT scan. Long-PRT reflectivities are used to unfold short-PRT velocities (two strongest overlaid trips can be recovered) (ORDA Build 9) Phase coding (SZ-2) 2 scans at each elevation angle Staggered PRT 1 scan at each elevation angle Uniform PRT (Legacy) 1 scan at each elevation angle 7.0° Alternating long- and short-PRT pulses. Range-unfolded reflectivities and velocities with good maximum unambiguous velocity (ORDA Build 14+) 15 Staggered PRT 1 scan at each elevation angle

16. What does a good VCP look like? 16 T 1 = 1.6 ms T 2 = 2.4 ms M = 28 SPRT T 1 = 1.23 ms T 2 = 1.84 ms M = 40 SPRT T 1 = 1.6 ms T 2 = 2.4 ms M = 28 T L = 3.11 ms T S = 0.98 ms M L = 6, M S = 41 Batch (~ same dwell times)

17. Choosing the PRTs PRT ratio: 2/3 –Not required but recommended for initial implementation Choose the PRTs as short as possible –The proposed SPRT algorithm can handle overlaid echoes –Maximum overlay condition result in shortest PRTs r a,2 = r max  T 2 = 2r max /c 17

18. Choosing the Dwell Times Dwell times must be chosen to… –Meet meteorological-variable variance requirements –Meet ground-clutter suppression requirements –Satisfy operational needs for short update times 18

19. Standard vs. Proposed VCP 221 Same update timeLarger v a Reduced “purple haze” for v and  v No “purple haze” for Z, Z DR,  DP, and  HV Lower variance of Z compared to batch Slightly increased variance of v 19

20. Future Work Document performance of CLEAN-AP for SPRT –Similar to what was done for SACHI Support implementation of SPRT on the ORDA Support engineering evaluation of SPRT Support ORPG 2D-VDA modifications for SPRT –Similar to what was done for legacy VDA Support VCP design for SPRT Investigate advantages of other PRT ratios –Algorithm already generalized to work with any PRT ratio 20

21. Summary SPRT is a mature technique –dual-pol extension –CLEAN-AP integration Algorithm description delivered to the ROC in April ‘12 SPRT improves –range coverage less purple haze for v and  v, no purple haze for the polarimetric variables –velocity measurements significantly less velocity aliasing –data quality reflectivity and polarimetric variables with lower variance SPRT is expected to replace Batch and Doppler waveforms in R/V ambiguity mitigation VCPs 21

22. Backup Slides

23. Another great example Batch ModeSPRT Doppler Velocity (04 Mar 2004) 23

24. Case I: April 22, 2004 – 2.5 deg Batch Mode VCP 11 r a = 147 km v a = 28.8 m/s Doppler Velocity Staggered PRT (2/3, same DT) r a = 184 km v a = 45.1 m/s Doppler Velocity Reflectivity r a = 276 km 24

25. Case II: June 30, 2004 – 1.5 deg Staggered PRT (2/3, same DT) r a = 240 km v a = 34.7 m/s Doppler Velocity Batch Mode VCP 11 r a = 147 km v a = 28.8 m/s Doppler Velocity Reflectivity r a = 360 km 25

26. Case III: March 3, 2004 – 2.5 deg Batch Mode VCP 11 r a = 147 km v a = 28.8 m/s Doppler Velocity Staggered PRT (2/3, same DT) r a = 184 km v a = 45.1 m/s Doppler Velocity Reflectivity r a = 276 km 26

27. Staggered PRT Processing Reflectivity and polarimetric-variable estimation –Segment I: short PRT samples –Segment II: short and long PRT samples –Segment III: long PRT samples Velocity and spectrum width estimation –Segment I: overlaid echoes on one sample of every pair No bias for “dominant” echo –Segment II: clean pairs –Segment III: overlaid echoes on one sample of every pair Non-dominant echo must be censored! 27 T1T1 T2T2 I II I II III I  = 2/3 R1R1 R2R2 R1R1 R2R2 R1R1 R2R2