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Staggered PRT Update Part I

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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

- Less “Purple haze”: larger ra
- Less Velocityaliasing: larger va

Doppler Velocity from KOUNStaggered PRT

Doppler Velocity from KTLXBatch Mode

- 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

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

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

- Velocities v1 and v2 alias in different ways

R1

R2

R2

R1

- 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

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)

- 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

- 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

Algorithm Selection

Engineering Implementation

Operational Implementation

SPRT (NSSL)

DC Removal GCF

Spectral GCF

DPT2 (SIGMET)

RPG Velocity Dealiasing Algorithm

Scanning Strategies

BATCH

SPRT

(courtesy of D. Saxion, ROC)

SPRT

BATCH

(courtesy of D. Saxion, ROC)

- 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 algorithm performance is more intimately tied to acquisition parameters
- SPRT can complement SZ-2 in VCPsthat achieve a “complete solution” for the mitigation of R/V ambiguities

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°

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°

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)

- 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

- Dwell times must be chosen to…
- Meet meteorological-variable variance requirements
- Meet ground-clutter suppression requirements
- Satisfy operational needs for short update times

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

- 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

- 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

- range coverage
- SPRT is expected to replace Batch and Doppler waveforms in R/V ambiguity mitigation VCPs

Backup Slides

Doppler Velocity (04 Mar 2004)

Batch Mode

SPRT

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

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

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

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!

- Segment I: overlaid echoes on one sample of every pair

R2

R2

R2

R1

R1

R1

I II I II III I