Staggered PRT Update for Severe Storm Detection

1. Staggered PRT UpdatePart I Sebastián Torres and David Warde CIMMS/The University of Oklahoma and National Severe Storms Laboratory/NOAA NEXRAD TACNorman, OK29 August, 2012

2. Why Staggered PRT? • Less “Purple haze”: larger ra • Less Velocityaliasing: larger va Doppler Velocity from KOUNStaggered PRT Doppler Velocity from KTLXBatch Mode

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

4. SPRT Range Coverage Reflectivity Doppler Var. Polarimetric Var. ra,2 ra,2 ra,L ra,1 ra,1 Batch 2ra,S 2ra,S ra,2 ra,1 ra,S ra,S SPRT ra,2-ra,1 key: vs. No “purple haze” possible Some “purple haze” possible

5. SPRT Velocity Dealiasing T1 T2 T1 T2 … time • Velocities are estimated for each PRT • v1 are estimated from the short-PRT pairs • v2 are estimated from the long-PRT pairs • Maximum unambiguous velocities for the short and long PRT pairs are different • Velocities v1 and v2 alias in different ways • The true (de-aliased) velocity can be obtained by “analyzing” how v1 and v2 alias R1 R2 R2 R1

6. “Catastrophic” Velocity Errors • A “catastrophic error” occurs if errors of estimates are so large that v1 and v2 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

7. ORPG Legacy Velocity Dealiasing KCRI - 31 March 20083.1 degNo Dealiasing KCRI - 31 March 20083.1 degBaseline Dealiasing KCRI - 31 March 20083.1 degModified Dealiasing (courtesy of David Zittel, ROC)

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

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

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

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

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

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 VCPsthat achieve a “complete solution” for the mitigation of R/V ambiguities

14. Legacy Mitigation Strategy 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) Uniform PRT 1 scan at each elevation angle Batch PRT 1 scan at each elevation angle Split Cuts2 scans at each elevation angle 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) 19.5° 7.0° 1.5° 0.5°

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

16. What does a good VCP look like? T1 = 1.6 ms T2 = 2.4 ms M = 28 T1 = 1.23 ms T2 = 1.84 ms M = 40 TL = 3.11 ms TS = 0.98 ms ML = 6, MS = 41 T1 = 1.6 ms T2 = 2.4 ms M = 28 SPRT SPRT SPRT 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 • ra,2 = rmax  T2 = 2rmax /c

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

19. Standard vs. Proposed VCP 221 Same update time Larger va Lower variance of Z compared to batch No “purple haze” for Z, ZDR, FDP, and rHV Slightly increased variance of v Reduced “purple haze” for v and sv

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

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 sv , 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

22. Backup Slides

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

24. Case I: April 22, 2004 – 2.5 deg ra = 147 kmva = 28.8 m/s ra = 184 km va = 45.1 m/s ra = 276 km Doppler Velocity Doppler Velocity Batch ModeVCP 11 Staggered PRT(2/3, same DT) Reflectivity

25. Case II: June 30, 2004 – 1.5 deg ra = 147 kmva = 28.8 m/s ra = 240 km va = 34.7 m/s ra = 360 km Doppler Velocity Doppler Velocity Batch ModeVCP 11 Staggered PRT(2/3, same DT) Reflectivity

26. Case III: March 3, 2004 – 2.5 deg ra = 147 kmva = 28.8 m/s ra = 184 km va = 45.1 m/s ra = 276 km Doppler Velocity Doppler Velocity Batch ModeVCP 11 Staggered PRT(2/3, same DT) Reflectivity

27. Staggered PRT Processing T2 T1 k = 2/3 • 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! R2 R2 R2 R1 R1 R1 I II I II III I