VCPs for Staggered PRT Sebastián Torres and David Warde CIMMS/NSSL Data Quality MOU – Technical Interchange Meeting Fall 2008

2 Outline Designing VCPs for Staggered PRT (recap) –Performance indicators Test VCPs 14 and 15 (recap) –Design criteria Benefits of using shorter PRTs –Trade-offs A new test VCP –Performance analysis Recommendation and Future Work Staggered PRT r a = 184 km, v a = 45.1 m/s

3 VCP Performance Indicators Acquisition time –Dwell times Maximum unambiguous range –Surveillance (Z) –Doppler (V & W) Maximum unambiguous velocity Spectrum width saturation Errors of estimates –Statistical errors –Dealiasing errors Clutter suppression

4 PRT Choice Shorter PRTs –Larger Nyquist –Larger maximum spectrum width –More samples for the same dwell times –Lower errors of estimates –Lower rate of catastrophic errors –Better clutter suppression Longer PRTs –Longer unambiguous range

5 VCP Design Criterion Make T 1 as short as possible –No-overlay condition r a,1 ≥ r max –T 1 ≥ 2r max / c –Algorithm limitation r a,2 ≥ r max –T 1 ≥ 4r max / 3c r a,D = r a,1 and r a,S = r a,2 –The no-overlay condition is more restrictive, but not a requirement –The current SPRT algorithm does not handle range unfolding

6 Test VCPs for Staggered PRT 2007: Staggered PRT subcommittee –Two test VCPs based on VCP 12 Intermediate and high elevations running SPRT (  = 2/3) Design criterion –VCP 14: Same dwell times as VCP 12 –VCP 15: Double dwell times as VCP 12 for SPRT EL < 4 degEL > 4 deg

7 VCPs 14 and 15

8 If we allow some overlay –r a,1 ≤ r max ≤ r a,2 No overlay: r a,1 = r max Maximum overlay: r a,2 = r max –Shorter PRTs –Must have technique to recover overlaid echoes Overlay Trade-off r a,2 r a,1 r max

9 New VCP Design Criterion We can trade shorter PRT for increased likelihood of overlaid echoes –Benefits are realized for elevations above 2.4 deg Solid: old Dashed: new

10 Choosing the Dwell Times Dwell times must be chosen to… –Meet error requirements for reflectivity, velocity, and spectrum width –Meet clutter filtering requirements –Satisfy operational needs

11 Dwell Times for SACHI (I) Maximum clutter suppression based on Reflectivity requirements

12 Dwell Times for SACHI (II) Maximum clutter suppression based on Spectrum Width requirements

13 Dwell Times for SACHI (III) Maximum clutter suppression based on Velocity requirements

14 Performance of SACHI Excessively long dwell times seem to be needed to meet clutter filtering velocity requirements –Feasible, but operationally not acceptable Recommend using dwell times to meet error requirements –Longer dwell times than with VCP 12, but operationally acceptable We have anecdotal evidence of operators switching from VCP 12 to VCP 21 to get what they want

15 Performance of SACHI Only Doppler velocities are affected corresponding to gates … –… with strong clutter contamination and … –… with velocities around 0, ±va/5, ±2va/5 and … –… where SACHI doesn’t pick the right wx “replica” and … –… where the ORPG velocity dealiasing algorithm fails to fix spatial discontinuity This has not been quantified! Censoring of “bad velocities” is possible –Reflectivities are still good for rainfall rate estimates Standard errors are more than 3x better than with the Batch mode –Spectrum widths are still good for turbulence

16 Proposed VCP T 1 for maximum allowable overlay Dwell times to meet error requirements –VCP time ~6 min

17 Proposed VCP Performance Acquisition time –Longer dwell times lead to VCP time ~6 min Maximum unambiguous range and velocity –Great improvement w.r.t legacy VCPs Spectrum width saturation –Acceptable Errors of estimates –Statistical errors meet requirements –Catastrophic errors effectively handled by ORPG’s VDA Clutter suppression –Meets requirements for Z and W –V dealiasing errors may be handled by ORPG’s VDA

18 Summary Staggered PRT provides significant benefits … –Longer unambiguous range –Larger Nyquist velocity –More accurate reflectivity estimates … that must be traded for … –Longer acquisition times (~VCP 21) Operationally acceptable –Increased rate of velocity dealiasing errors Handled by ORPG’s VDA

19 Future Work Recommendation –Implement proposed test VCP –Collect more data cases –Process data using recommended SPRT algorithm including SACHI and modified ORPG velocity dealiasing algorithm –Assess data quality Future work –Complete evaluation of technique to recover overlaid echoes for SPRT –Research ways to improve SACHI

Back up slides

21 VCP 12