1. Wave Measurements, Low Frequency Energy, and Hmax Bridget Thomas Atlantic Climate Centre, Meteorological Services Canada - Atlantic 2 nd EC/NOAA Marine Collaboration Workshop, Halifax, NS, 11-13 Feb 2014

2. Outline Issues –What is the low frequency cutoff (LFC) (EC buoys), how does it affect Hs and Tp? –How valid are reports of Hmax >> Hs? JCOMM Wave Measurement Evaluation Test –Comparison of Hs and Hmax for NOMAD and 3D with co-located WR EC Operational Buoy Wave Measurements Access to Historical Buoy Data and Metadata Hmax EC Low Frequency Cut-off Cases of low freq. storm wave energy including tabular wave spectrum for Post Tropical Noel with Hs calculated starting from different frequency bands Cases of other sources of v. low frequency energy/ spurious large Hmax Summary

3. Weather Buoy – Datawell Waverider Intercomparison JCOMM Wave Evaluation and Test Project: compare wave measurements from different hull/sensor types to co-located standard (reference): Datawell Directional Waverider (WR) with vertically-stabilized wave sensor (Jenson et al 2011, 2012) http://www.jcomm.info/index.php?option=com_content&view=article&id=62 http://www.jcomm.info/index.php?option=com_content&view=article&id=62 Report that Canadian buoy wave heights biased low in comparison to NDBC (relative to altimeter data) (Durant et al 2009) [confirmed by Ray and Beckley 2012]; is accuracy in measurements limiting wave model improvement? EC 3D and NOMAD buoys use strapped-down accelerometer wave sensors; Pender et al (2010): waves from strapped-down accelerometer in 3D buoy in Hurricane Katriona biased due to sustained tilt (heel) of hull in extreme winds and currents EC Buoy locations and deployment period of co-located Waverider –6N 44255 Jun 2010 – Feb 2011 (top Hs 7m) –3D 46185, Sep 2010 – Sep 2011 (top Hs 10m) –3D 44258, May 2011 – Jun 2012 (top Hs 7m) –3D 46206, May 2012 – May 2013 (top Hs 8 m) Each EC buoy also had TriAXYS 3D wave sensor on board

4. NOMAD: Burgeo Bank 6N 44255 (179 m) and S. Ramea Is. WR 44235 (CDIP170), Jun 2010-Feb 2011 (top Hs 7m, Sep 5, 2010 Earl)

5. NOMAD Hs to Waverider Hs (44255 Burgeo Bank, June 2010-Feb 2011) Relatively little scatter, NOMAD Hs ~ 6% lower than WR Hs Thomas, 2011b

6. NOMAD Hmax to Waverider Hmax (44255 Burgeo Bank, June 2010-Feb 2011, QC’d) fairly good agreement (NOMAD Hmax ~3% lower than WR Hmax) up to about Hmax 8m, more scatter above

7. Hmax to Hs (44255 Burgeo Bank, June 2010 – Feb 2011) NOMAD and Waverider: Hmax ~ 1.6 * Hs, up to Hs 7-8 m Similar slopes at this location, but Waverider Hmax to Hs - more scatter NOMAD Waverider

8. Halifax Harbour (Approaches) 3D 44258 (58 m) and WR 44172 (CDIP176) May 2011 – Jun 2012 (top Hs 7m, Oct 30, 2011)

9. 3D Hs - Waverider Hs (44258 Halifax Harbour approaches, May 2011 – June 2012) 3D Hs ~ 3 % > WR Hs to 6 m, a few higher 3D Hs

10. 3D - Waverider Hmax (44258) May 2011-Jun 2012

11. La Perouse Bank 3D 46206 (72 m) and WR 46139 (CDIP195), May 2012 – May 2013 (top Hs 8 m, Feb 23, 2013)

12. La Perouse Bank 3D 46206 Hs against WR Hs 3D Hs ~ 2 % > WR Hs to 8 m

13. La Perouse Bank 3D 46206 Hmax against WR Hmax 3D Hmax ~ 12 % > WR Hmax, more scatter at higher Hs

14. La Perouse Bank 3D Buoy 46206, WR, Hmax against Hs 3D WR

15. South Hecate Strait 3D 46185 (230 m) and South Hecate WR 46138 (CDIP174), Sep 2010 – Sep 2011 (top Hs 10m, Nov 29, 2010)

16. 3D 46185 Hs vs. WR Hs (South Hecate Strait) 3D Hs ~ 2% > WR Hs

17. S.Hecate Strait, Hmax vs Hs 3D (with strapdown) shows spurious high Hmax with increasing Hs: Hs 7-10 m -> reported Hmax to 25 m 3D WR

18. Summary (3D and NOMAD, strapped-down accelerometer: Intercomparison with WR) Hs NOMAD ~ 6% < Hs-WR (limited data) Hs 3D ~ 2% > Hs WR Typical ratio Hmax to Hs: 1.6 (WR), 1.6-1.7 (NOMAD), 1.6 to 3 (3D) – much scatter! 3D results worse at S. Hecate Strait Hmax from 3D (strapped-down) unreliable, often too high

19. EC Operational Buoy Wave Measurements use a heave sensor (1-D accelerometer) (aka strap-down or fixed accelerometer) Sampling interval 34 minutes at 1Hz; wave sampling ends before 10 minute met sampling begins (in the Atlantic, wave sampling starts 20 minutes after the hour; in Pacific, depends on transmit time) calculations by wave module in buoy processor (Watchman 100 (WM), by Axys) integrates acceleration twice to get time series of displacement Peak wave limits +/-15 m [+/-20 m at NDBC buoys & Datawell WRs] Calculates spectral energy from time series, using FFT, in frequency bands Hs (or Hm o ) from the spectral energy Hm o =4*sqrt(m o ) (m o is sum of energy in each frequency band) peak wave period, Tp, corresponds to the frequency band at the peak in the energy spectrum [also called dominant period – NDBC] Hmax: maximum peak to trough displacement during the sampling interval [Vertically-stabilized wave sensors used in field experiments (e.g. Datawell Waverider (WR) at S. Hecate); on 3 offshore Pacific NOMADS (late 1980s to late1990s); on DFO wave buoys (1970s to 1990s) & at Atlantic offshore oil & gas platforms (1980s to present) – WR & Triaxys wave buoys; wave buoys sample for 20 min]

20. Access to Archived Buoy Data Department of Fisheries and Oceans (DFO) Integrated Science Data Management (ISDM) online archive ( http://www.meds-sdmm.dfo- mpo.gc.ca/isdm-gdsi/waves-vagues/index-eng.htm ) allows download of entire period of record of met data and summary wave data for individual buoy stations, or one year at a time of met/wave data including spectral data http://www.meds-sdmm.dfo- mpo.gc.ca/isdm-gdsi/waves-vagues/index-eng.htm ISDM sets a QC flag based on transmitted spectral data (unless the flag indicates that buoy is off-station), so it does not directly apply to met data or buoy-reported Hs (VWH$) and Tp (VTP$) ISDM uses own low frequency cutoff for calculating Hs (VCAR) and Tp (VTPK): value is in spectral data files (LCF$) Caution: ISDM date/time indicates the start of the wave sample, nearly 45 min before the end of the met sample - may need to round up to the next hour when matching with data valid for the top of the hour NDBC has realtime tabular data for last 45 days Canadian buoy data – link to historical data goes to DFO/ISDM: http://www.ndbc.noaa.gov/http://www.ndbc.noaa.gov/ Weather Buoy System (WBS) (Internal EC site) (http://thetis.pyr.ec.gc.ca/) archives raw (SXVX KWAL) bulletins and ship format (SMVD) bulletins, can download one day at a time; also archives GPS coordinates, can download one day at a timehttp://thetis.pyr.ec.gc.ca/)

21. Historical Buoy Status Reports Example: part of report for Atlantic Region, 2014-02-06 (gives hull type, sensor config, service dates, etc) ISDM website (http://www.meds-sdmm.dfo-mpo.gc.ca/isdm-gdsi/waves-vagues/index-eng.htm)http://www.meds-sdmm.dfo-mpo.gc.ca/isdm-gdsi/waves-vagues/index-eng.htm Weather Buoy System (WBS) (Internal EC site) (http://thetis.pyr.ec.gc.ca/)http://thetis.pyr.ec.gc.ca/) Link from NDBC goes to current status report: http://shylock.pyr.ec.gc.ca/~wbs/bplatstat.htmlhttp://shylock.pyr.ec.gc.ca/~wbs/bplatstat.html 44251 Nickerson Bank, 44255 NE Burgeo Bank, 44258 Halifax Harbour

22. Hmax Hmax reported in raw buoy data, not in FM13 ship code report Hmax: maximum peak to trough displacement (Watchman) Old definition of Hmax (Zeno): twice the maximum positive displacement (Zeno replaced by Watchman processors in late 1990s) Rule of thumb: Hmax up to 2 times Hs, but…. Sometimes see large ratios of Hmax to Hs (3 times Hs) in storms, especially from 3D buoys – are these real waves? – how valid are these Hmax? anemometer damage/failure in storms sometimes coincides with report of a very large Hmax Often see Hmax 10 – 25 m when the 3D buoy is put into the water; with old definition of Hmax, would see Hmax ~ 30 m –horizontal displacement of 3D buoy with strap-down while moved on its side from the ship is measured as a large wave –Hmax 30 m corresponds to twice 15 m, top of displacement range –must flag these deployment related reports to exclude from analysis

23. V. large Hmax during deployment or related to anemometer damage in storms Deep water mooring Strap-down accelerometer Buoys start reporting just before put into water; swung out while tilted (3D) produces large Hmax (+ve displacement) Needed to flag reports from before full deployment, & exclude from analysis Storm damage to anemometer with report of large Hmax may result from similar event (horizontal translation while tilted), rather than true Hmax video of 3D deployment http://axystechnologies.com/ video-gallery/ http://axystechnologies.com/ video-gallery/ From Thomas 2011a 3D 46207 (East Dellwood)

24. Hmax vs Hs for 3D Buoy 46185 (S. Hecate Str.), 230 m, inverse catenary mooring See same pattern of few occurrences of large Hmax with old defn, many more with new defn, even though not a shallow water location, suggests more frequent large negative displacements - due to mooring restoring force Zeno (Hmax = 2x positive displ.) WM (Hmax = max +ve to –ve displ.)

25. EC Low Frequency Cut-off and Other Factors Affecting Measurement of Hs and Tp LFC designed to exclude very low frequency energy (spurious wave energy, noise) from calculation of Hs and Tp Low frequency noise comes from use of strap-down accelerometers (measuring energy from horizontal displacements while tilted) and mooring interactions (mooring strain) Not just a problem for 3D buoys in shallow water – also a problem for 3Ds in deep water, and more rarely, for 6N buoys Why does it matter? – in cases of extreme storms or long period swell at buoys exposed to open ocean, the LFC if set too high has the effect of erroneously reducing Hs (and sometimes Tp) Trade off between detection/height estimation of long period storm waves and elimination of low frequency noise ISDM using slightly different values than buoys, so sometimes get differences between reported and archived Hs (and Tp) NDBC uses one cutoff (0.033 Hz or about 30 s) for their buoys regardless of depth but filters out mooring and electronic noise

26. How to Find the Frequency Band (bb) used by WMAN Processor for LFC Halifax Harbour (Approaches) 3D 44258 Raw Data (SXVX40 KWAL), (Oct 30, 2011, 16UTC) SXVX40 KWAL 301637 474A30CE 303163711 25051 1555 44258 46/// /062204(/071198) 10088 49825(49825) 22200 00117 1107074 333 912249(912242) WQ08215 A10999 A2002 A3137 A4127 A52626 A62010 A7125 A94430.092,06324.265 A16+0000 A17002 A19000 A200 B20B11111815 $|DE`wOAa@`@`@`@`@`@`@`@`Sp}pwk@PGusAYXAn]N{nqOT oWonMINpNeLVLVLfLslck]jKkNLalMLJN[NsoOfONGOonxmuM{L AmMLxkhK|jCjTiYh}Fde/ 41-0NN 33E bb=08 (bin number) Hmax=21.5m Q=questionable, Hmax>2Hs Hs=7.4m

27. Examples of valid low freq wave energy: Wave spectra in PT Noel (2007) and Juan (2003), Halifax Harbour (Approaches) Buoy 44258 Low frequency cutoff excluded long period (>15 s) hurricane generated wave energy, Hs reduced Very low frequency energy separated from peak wave energy by a minimum 3D (in PT Noel) showed more very low freq energy than 6N (in Hurr Juan); anchor was dragged in both cases From Mercer and Thomas 2009

30. Band No. (Bin No) Xmit'd Band No. Band Centre (Hz) Band Width (Hz) energy density in band (m 2/ Hz) energy in band (m 2 ) Band Centre (Period) (s) Hs excl lower freq bins (m) Wave length, L(m) Interm Depth (m) (L/4) Shallow Depth (m) (L/25) Ratio Hmax/Hs 12 0.003906 111.50000 0.435519256.015.71022492556240901.72 23 0.0078130.00390672.66000 0.283810128.015.525556638910221.74 34 0.0117200.00390696.32000 0.37622685.315.41135728394541.76 45 0.0156300.003906151.20000 0.59058764.015.2638615962551.78 56 0.0195300.003906124.80000 0.48746951.214.8409010221641.82 67 0.0234400.003906169.30000 0.66128642.714.628397101141.85 78 0.0273400.003906152.90000 0.59722736.614.22087522831.90 89 0.0312500.003906126.30000 0.49332832.013.91597399641.95 910 0.0351600.003906106.60000 0.41638028.413.61262315501.99 1011 0.0390600.00390627.86000 0.10882125.613.31022256412.03Optimum? 1112 0.0429700.00390624.89000 0.09722023.313.3845211342.04 1213 0.0468800.00390644.74000 0.17475421.313.2710177282.04 1314 0.0507800.003906144.50000 0.56441719.713.1605151242.06 1415 0.0546900.003906271.80000 1.06165118.312.8522130212.12 1516 0.0585900.003906407.90000 1.59325717.112.1454114182.24energy pk 1617 0.0625000.003906232.10000 0.90658316.011.0399100162.46 1718 0.0664100.003906364.40000 1.42334615.110.335488142.63Buoy 1819 0.0703100.003906200.50000 0.78315314.229.131679132.97ISDM 1920 0.0742200.003906163.60000 0.63902213.58.428371113.22 2021 0.0781300.00390659.31000 0.23166512.87.725664103.48 2122 0.0820300.003906101.90000 0.39802112.27.52325893.60 2223 0.0859400.00390665.64000 0.25639011.67.12115383.82 2324 0.0898400.00390659.31000 0.23166511.16.81934883.99 2425 0.0937500.00390659.31000 0.23166510.76.51774474.16 2526 0.0976600.003906107.80000 0.42106710.26.21644174.35 Wave Spectral Data, 3D 44258, 2007-11-04 9UTC (PT Noel), Hs Calculated Excluding Lower Freq Bins Hmax27.0, WM Hs 10.3 m, Tp 15.1 s; ISDM Hs 9.1 m, Tp 14.22 s

31. Band No. Xmit'd Band No. Band Centre (Hz) Band Width (Hz) energy density in band (m 2/ Hz) energy in band (m 2 ) Band Centre (Wave Period) (s) Hs (m) using band cutoffs 12 0.003906 111.50000 0.435519256.015.7 23 0.0078130.00390672.66000 0.283810128.015.5 34 0.0117200.00390696.32000 0.37622685.315.4 45 0.0156300.003906151.20000 0.59058764.015.2 56 0.0195300.003906124.80000 0.48746951.214.8 67 0.0234400.003906169.30000 0.66128642.714.6 78 0.0273400.003906152.90000 0.59722736.614.2 89 0.0312500.003906126.30000 0.49332832.013.9 910 0.0351600.003906106.60000 0.41638028.413.6 1011 0.0390600.00390627.86000 0.10882125.613.3 1112 0.0429700.00390624.89000 0.09722023.313.3 1213 0.0468800.00390644.74000 0.17475421.313.2 1314 0.0507800.003906144.50000 0.56441719.713.1 1415 0.0546900.003906271.80000 1.06165118.312.8 1516 0.0585900.003906407.90000 1.59325717.112.1 1617 0.0625000.003906232.10000 0.90658316.011.0 1718 0.0664100.003906364.40000 1.42334615.110.3 1819 0.0703100.003906200.50000 0.78315314.29.1 1920 0.0742200.003906163.60000 0.63902213.58.4 2021 0.0781300.00390659.31000 0.23166512.87.7 2122 0.0820300.003906101.90000 0.39802112.27.5 Frequency Bands, Energy & Hs at 44258 Noel, 9z

32. Example 1 of Very Low Freq Energy, v. large Hmax Wave Spectral Data, 3D Buoy 46185 (S. Hecate), 1991-12-20 20UTC, Hmax 30.4 m ISDM: LCF$.041211 Hz Hs VCAR 13.56 m Tp VTPK 12.19 s Buoy: Hs VWH$ 14.9 m Tp VTP$ 32.0 s Hmax VCMX 30.4 m ISDM used LFC that excluded very low freq energy, at the time, Watchman processer did not

33. Examples 2 and 3: Very Low Freq Energy, v. large Hmax Buoy 46183 Storm Wave Spectra right: hourly spectra 24 Sept 2010, 1 UTC-24 UTC) single spectrum 11 Feb 1999 14z

34. 3D-StrapDown - Waverider Hmax (44258) May 2011-Jun 2012 Storm Oct 30, 2011 16 UTC

35. 3D Halifax Harbour, 44258 Storm Oct 30, 2011 17 UTC 16 UTC

36. LCF$0.066864 Hz VCAR6.9 m VWH$7.4 m VCMX21.5 m VTPK10.67 s VTP$10.7 s Courtesy of Bruce Bradshaw, DFO/ISDM

37. Wave Spectrum at Buoy 44251 (Nickerson Bank, 6N, 71 m) in Hurricane Igor, 21 Sept 2010, 17z -during this ob the buoy moved the furthest distance, to SE, 2 hrs after min press (952 mb) & wind shift fm SE to NW, winds G37m/s

38. Summary To assess EC buoy wave reports, it would help forecaster to have tools to plot the wave spectra and plot buoy motion around its watch circle LCF may be set as a function of depth, with higher cut-off for shallower buoys – new guidelines should make exceptions to this procedure. LFC for intermediate depth buoys exposed to open ocean swell and storm waves should include wave energy to periods of ~30 s in storms, spurious very low frequency energy (periods > 30s) can also appear in deep water buoys (more often with 3D than 6N), often coincide with reports of large Hmax in comparison to Hs, Hmax 3-5x > Hs, Hmax suspect –in some cases mooring under strain, buoy at furthest point of its watch circle due to storm/currents (eg 44258 (anchor dragged – PT Noel 2007), 46183), buoy likely heeled over –in other cases after rapid wind shift when buoy is moving back across watch circle (eg 44251, 44139 in Hurricane Igor 2010)

39. References Axys Environmental Consulting Ltd. 1996. Meteorological and oceanographic measurements from the Canadian weather buoys. A review of sensors, data reduction, transmission, quality control and archival methods. Final report for Environment Canada, Downsview, Ontario, April 1996. Axys Environmental Systems. 2000. The Canadian buoy network technical meeting. A review of sea surface temperature measurements, metadata, and wave sensing and processing. Summary report and action items prepared for Environment Canada, Downsview, Ontario, Canada, February 2000. Bender, L.C. et al. 2009. A Comparison of Two Methods for Determining Wave Heights from a Discus Buoy with a Strapped- Down Accelerometer. 11 th International Workshop on Wave Hindcasting and Forecasting. Halifax, NS, Canada, October 24- 29, 2009. [http://www.waveworkshop.org/11thWaves/index.htm ]http://www.waveworkshop.org/11thWaves/index.htm Bender, L.C., Guinasso Jr., Walpert, J.N., Howden, S.D. 2010. A Comparison of Two Methods for Determining Wave Heights – Applied to a 3-m Discus Buoy during Hurricane Katrina. J. of Atmospheric and Oceanic Tech., 27, 1012-1028. Durrant, T. H., D. J. M. Greenslade, and I. Simmonds. 2009. Validation of Jason-1 and Envisat remotely sensed wave heights. J. Atmospheric and Oceanic Technology, 26: 123–134. Jensen, R., Swail, V., Lee, B., and O’Reilly, B. 2011. Wave Measurements Evaluation and Testing. 12 th International Workshop on Wave Hindcasting and Forecasting. Hawaii, Oct 30-Nov 4, 2011. (pdf 12p.) Jensen, R., Swail, V. Lee, B., and Hesser, T. 2012. Measurement Evaluation and Testing Phase II. Scientific and Technical Workshop of the Data Buoy Cooperation Panel, 2 Oct 2012, Fremantle, AU. Mercer, D. and B. Thomas. 2009. Significant Wave Height and Low Frequency Cutoffs at Canadian Moored Buoys During Extreme Storms. 11 th International Workshop on Wave Hindcasting and Forecasting. Halifax, NS, Canada, October 24-29, 2009. [http://www.waveworkshop.org/11thWaves/index.htm ]http://www.waveworkshop.org/11thWaves/index.htm Ray R.D. and Beckley, B.D. 2012. Calibration of ocean wave measurements by the TOPEX, Jason-1, and Jason-2 satellites. Marine Geodesy. 35 (SI):238-257. Thomas, B. 2011. Environment Canada (EC) Marine Data Focus on the Pacific. NWS-MSC Pacific Marine Workshop, Seattle, WA,April 24-27, 2011. Thomas, B. 2011. Waveriders and MSC Buoys: Comparison of Hmax and Hs. Buoy Technical Working Group Meeting, Dec. 7, 2011, Halifax NS.