Updates to the SZ-2 Algorithm Sebastián Torres CIMMS/NSSL Technical Interchange Meeting Fall 2007

Over 25 years ago…

Today… KTLX radar in Oklahoma City 30 Mar 2007, 0.5 deg elevation (VCP 12/VCP 221) Courtesy of Jami Boettcher (WDTB)

SZ-2 is Operational! KCRI 20 June 2007 VCP 121 SZ-2 tilt Courtesy of Dave Zittel (ROC)

Outline SZ-2 Upgrades –4 th -trip overlaid echoes –Recovery region censoring Recommendations

20 June 2007 Case Observed “noisy velocities”

Sanity Check KCRI Level II Data KCRI Level I Data processed with MATLAB Note: Velocity color scales are different

Where is it Coming From? Long PRT: r a = 471 kmShort PRT: r a = 119 km Storm cell between 400 and 450 km: 4 th trip overlaid echoes

A Little Detective Work 4 th 3 rd 2 nd 1 st None 4 th 3 rd 2 nd 1 st None Noisy velocities correspond to strong 4 th trip and weak 1 st trip

The SZ-2 Processing Notch Filter S 1 = 10 dB v 1 = 15 m/s w 1 = 2 m/s S 2 = 0 dB v 2 = -15 m/s w 2 = 2 m/s Weak signal is in 2 nd trip 3 rd trip 4 th trip PNF width 0.75 M 0.50 M 0.75 M Overlaid signals Sidebands Sidebands don’t bias, but contribute to higher errors of velocity estimates Two replicas of the modulated weak signal are needed for proper spectral reconstruction

Back to Our Case… Strong signal is in the 4 th trip Weak signal is in the 1 st trip Recovered 1 st trip signal spectrum does not seem to have the expected mainlobe with decaying sidelobes! AZ = 266 deg r = 64 km Strong trip

SZ(8/64) Modulation Codes 1 st to 2 nd trip mod. code 1 st to 3 rd trip mod. code 1 st to 4 th trip mod. code Cohere to strong  Apply von Hann window  Apply PNF  Cohere to weak

Weak Trip Velocity Recovery 2 nd trip 3 rd trip 4 th trip Standard deviation of weak-trip velocity estimates If strong and weak signals are 3 trips apart (e.g., 1 st and 4 th trips), recovery of weak- trip velocities is not possible with the current algorithm

Why are the errors so large? PNF’s NW = 0.75M PNF’s NW = 0.50M  vn = 0.53  vn = Change

A New PNF Notch Width 2 nd trip 3 rd trip 4 th trip Standard deviation of weak-trip velocity estimates If strong and weak signals are 3 trips apart (e.g., 1 st and 4 th trips), recovery of weak velocities is possible with a small change to the current algorithm

Changes to SZ-2 ≠1

Back to Our Case with the Fix Current SZ-2 Algorithm Proposed SZ-2 Algorithm

19 June 2007 Case Current SZ-2 Algorithm Proposed SZ-2 Algorithm

Still Noisy Velocities! Weak Trip Number 4 th 3 rd 2 nd 1 st None

Recovery Region Censoring Weak-trip velocity estimates exhibit large errors for wider strong-trip spectrum widths and large power ratios –In this case, weak-trip velocities “cannot be recovered” –Weak-trip velocities are censored if we are outside the “recovery region” CTCT CICI CSCS Accept Censor Extracted from NSSL Report 2

Current SZ-2 Thresholds Narrow weak- trip widths Wide weak- trip widths Weak trip:  v = 4.2 m/s Nominal parameters f = 2800 MHz DELTA PRI C Nominal parameters f = 2800 MHz DELTA PRI C Strong trip:  v = 4.8 m/s  v = 3.7 m/s  v,s (m/s) P s /P w (dB) dB/(m/s) dB/(m/s) 10.9  v,s (m/s) P s /P w (dB) Narrow weak-trip widthsWide weak-trip widths

2 nd Trip Overlay Requirement: SD(v w ) < 2 m/s 4.5 m/s 3.5 m/s dB s/m 45 dB 4.25 m/s dB s/m 45 dB 4.5 m/s dB s/m 45 dB 4.5 m/s dB s/m 45 dB 4.5 m/s w w <3.5m/s3.5m/s<w w <4.5 m/s CTCT 45 dB CSCS dB s/m CICI 4.5 m/s4.25 m/s Conditions?

3 rd Trip Overlay Requirement: SD(v w ) < 2 m/s 6.5 m/s w w <6.5m/s CTCT 45 dB CSCS dB s/m CICI 2.75 m/s dB s/m 45 dB 2.75 m/s dB s/m 45 dB 2.75 m/s dB s/m 45 dB 2.75 m/s dB s/m 45 dB 2.75 m/s dB s/m 45 dB 2.75 m/s dB s/m 45 dB 2.75 m/s

4 th Trip Overlay Requirement: SD(v w ) < 2 m/s 4.5 m/s 3.5 m/s dB s/m 45 dB 2 m/s dB s/m 45 dB 2.25 m/s dB s/m 45 dB 2.25 m/s dB s/m 45 dB 2.25 m/s w w <3.5m/s3.5m/s<w w <4.5 m/s CTCT 45 dB CSCS dB s/m CICI 2.25 m/s2 m/s

Proposed thresholds Depend on type of overlay Current thresholds Same for every type of overlay w wn < w 0 w 0 < w wn < w 1 w w > w 1 CTCT 45 dB -∞ CSCS -772 N/A CICI N/A CTCT 45 dB-∞ CSCS N/A-772N/A CICI N/A CTCT 45 dB -∞ CSCS -772 N/A CICI N/A Summary of Recovery Regions One-trip difference w 0 = w 1 = Two-trip difference w 0 = 0 w 1 = Three-trip difference w 0 = w 1 =

Changes to the SZ-2 Algorithm Define thresholds for each overlay type Make recovery region censoring a function of the strong-to-weak trip number difference Define recovery region using three zones depending on the weak-trip spectrum width –Immediate censoring for large weak-trip widths

Back to our cases (I) 20 Jun 2007 Current Censoring (mild) Proposed Censoring (normal) Proposed Censoring (aggressive)

Back to our cases (II) 19 Jun 2007 Current Censoring (mild) Proposed Censoring (normal) Proposed Censoring (aggressive)

Summary Analysis of operational SZ-2 fields revealed noisy velocities Three-trip-difference overlay situations are not handled well in SZ-2 –This affects weak-trip velocities for those cases in which 1 st trip is weak, 4 th trip is strong, and there are no significant signals in the 2 nd and 3 rd trips –Effective recovery of weak-trip velocities in this situation requires a narrower PNF notch

Summary (cont’d) Censoring based on power-ratio recovery region is not aggressive enough –This affects weak-trip velocities only Higher errors of estimates –Different overlay situations require different recovery regions Current algorithm based on one-trip difference overlay case One-, two-, and three-trip difference overlay cases need to be considered –Recovery for wide weak-trip widths is very difficult

Recommendations Fix PNF notch width –Verified issue with clear-cut fix –Very simple, transparent change –Recommend implementation for ORDA Build 11 Upgrade “Recovery Region” Censoring –Change is relatively simple Will result in more purple haze –Should look at more cases before proposing new thresholds Involve DQ Team? –Recommend implementation ASAP