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Transcript of Staggered PRT Update Part I Sebastián Torres and David Warde CIMMS/The University of Oklahoma and...
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Staggered PRT Update
Part I
Sebastián Torres and David Warde CIMMS/The University of Oklahoma
and National Severe Storms Laboratory/NOAA
NEXRAD TACNorman, OK
29 August, 2012
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Why Staggered PRT?• Less “Purple haze”: larger ra
• Less Velocity aliasing: larger va
Doppler Velocity from KTLXBatch Mode
Doppler Velocity from KOUNStaggered PRT
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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
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SPRT Range Coverage
ra,2
ra,1
ra,2
ra,1
ra,2-ra,1
Batc
hS
PR
T
ra,L 2ra,S
ra,S
2ra,S
ra,S
ra,2
ra,1
Reflectivity Doppler Var. Polarimetric Var.
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No “purple haze” possible
Some “purple haze” possible
key: vs.
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SPRT Velocity Dealiasing• 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
T1 T2
timeT1 T2
…R1
R2 R1 R2
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“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
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ORPG Legacy Velocity Dealiasing
KCRI - 31 March 2008
3.1 degNo Dealiasing
KCRI - 31 March 2008
3.1 degBaseline
Dealiasing
KCRI - 31 March 2008
3.1 degModified
Dealiasing
(courtesy of David Zittel, ROC)
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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
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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
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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
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Batch PRT vs. Staggered PRT Reflectivity
(courtesy of D. Saxion, ROC)
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BATCHSPRT
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Batch PRT vs. Staggered PRT Doppler Velocity
(courtesy of D. Saxion, ROC)
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BATCHSPRT
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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
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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 Cuts2 scans at each elevation angle
Batch PRT
1 scan at each elevation angle
Uniform PRT
1 scan at each elevation angle7.0°
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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 angle7.0°
Alternating long- and short-PRT pulses.
Range-unfolded reflectivities and velocities with good maximum unambiguous velocity (ORDA Build 14+)
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Staggered PRT
1 scan at each elevation angle
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What does a good VCP look like?
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T1 = 1.6 msT2 = 2.4 msM = 28
SPRT
T1 = 1.23 msT2 = 1.84 msM = 40
SPRTSPRT
T1 = 1.6 msT2 = 2.4 msM = 28
TL = 3.11 msTS = 0.98 msML = 6, MS = 41
Batch(~ same dwell times)
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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
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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
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Standard vs. Proposed VCP 221
Same update time Larger va
Reduced “purple haze” for v and v
No “purple haze” for Z, ZDR, DP, and HV
Lower variance of Z compared to batchSlightly increased variance of v
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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
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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
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Backup Slides
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Another great example
Batch Mode SPRT
Doppler Velocity (04 Mar 2004)
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Case I: April 22, 2004 – 2.5 deg
Batch ModeVCP 11
ra = 147 kmva = 28.8 m/s
Doppler Velocity
Staggered PRT(2/3, same DT)
ra = 184 kmva = 45.1 m/s
Doppler Velocity
Reflectivity
ra = 276 km
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Case II: June 30, 2004 – 1.5 deg
Staggered PRT(2/3, same DT)
ra = 240 kmva = 34.7 m/s
Doppler Velocity
Batch ModeVCP 11
ra = 147 kmva = 28.8 m/s
Doppler Velocity
Reflectivity
ra = 360 km
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Case III: March 3, 2004 – 2.5 deg
Batch ModeVCP 11
ra = 147 kmva = 28.8 m/s
Doppler Velocity
Staggered PRT(2/3, same DT)
ra = 184 kmva = 45.1 m/s
Doppler Velocity
Reflectivity
ra = 276 km
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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!
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T1 T2
I II I II III I
= 2/3R1 R2R1
R2R1R2