Establishing the Provenance of NDBC s Accuracy Statement for Directional Waves

Transcription

1 Establishing the Provenance of NDBC s Accuracy Statement for Directional Waves Richard H. Bouchard and Rodney E. Riley National Data Buoy Center (NDBC), Stennis Space Center, MS USA Ocean Waves Workshop, New Orleans, 15 January 2015

2 Provenance the place of origin or earliest known history of something. the beginning of something's existence; something's origin. a record of ownership of a work of art or an antique, used as a guide to authenticity or quality.

3 Are Ocean Wave Heights Increasing in the Eastern North Pacific? - Allan and Komar, Eos, Vol. 81, No. 47, November 21, Gemmrich, Thomas, and Bouchard (2011): Several recent studies reported long-term trends extracted from these records. However, significant modifications of the wave measurement hardware as well as the analysis procedures since the start of the observations result in inhomogeneities of the records.

4 Establishing the Provenance of NDBC s Accuracy Statement for Directional Waves Richard H. Bouchard and Rodney E. Riley National Data Buoy Center (NDBC), Stennis Space Center, MS USA Ocean Waves Workshop, New Orleans, 15 January 2015

5 Increase In Directional Wave Platforms

6

7 What Do We Mean by Wave Direction? NDBC (1996) for details NDBC uses the method of Longuet-Higgins, Cartwright and Smith (1963) for heave, pitch, and roll from a moored DISCUS buoy Pitch and Roll from: Datawell Hippy 40 Integration of three orthogonal angular rate sensors (ARS) Partitioning Magnetometer Measurements (MO) NDBC then makes slopes in the buoy frame of reference Determine the buoy heading with respect to Magnetic North from the earth s magnetic fluxes as measured by three orthogonal magnetometers and pitch and roll Magnetic declination then rotates to True North Slopes are passed through an FFT -> Co and quadrature spectra Use the first and second order Fourier Coefficients to determine directional wave spectra

8 HIPPY: 66,000 cm 3, 36 kg, US$17,500 3DMG: 145 cm 3, kg, US$ 1,300 Hippy 3DM-G

9 First and Second Order Fourier Coefficients: α ā 1(f), b 1(f), ā 2(f), b 2(f) First order are function s of the quad-spectra; Second order, co-spectra Mean Wave Direction (alpha1), WMO (1995): α 3 = π 2 ( ) b a 1 1 ( f ) tan 1( f ), 1( Principal Wave Direction (alpha2), WMO (1995): 3 = π 2 ( ) b a 1 2 ( f ) tan 2( f ), α follows the IAHR (1997) convention: As the angle between true north and the direction from where the waves are coming. Clockwise is positive 2( f f ) ) + 0 or π, closest to α 1

10 First and Second Normalized (wrt zeroth order Fourier Coefficient(spectral density) )Polar Coordinates, WMO (1995) r 1( f ) = 2 b1 ( f ) + a 0( a1( f ) f 2 ) r 2( f ) = 2 b2 ( f ) + a 0( a2( f ) f 2 ) From O Reilly, 2008

11 Fourier Coefficients: S( f, θ ) = a an + bn ) n= 1 [ cos( nθ ) sin( nθ ] Or in WMO terms: D( f S(f,α) = C 11 (f) D(f, α ), α) = ( α ) + cos( ( α )) r1cos α r 2 2 Note: Because of trig function properties and differences, this can yield negative S(f) See Earle, Steele, and Wang (1999). 1 π α 2

12 Longuet-Higgins (1963) angular distribution D(f,θ) = G(s) cos[(θ-θ)/2 2s Where: S = r 1 /(1 r 1 ) Directional Width = (2-2r 1 ) ½

13 Wave Direction Statistic Used in Certification or Commissioning is the Mean Wave Direction (at the Peak Frequency (Dominant Period): α1(f=max C11) SWD MWD & WWD MWD also is: Wind Wave Direction (WWD) & Swell Wave Direction (SWD) in Spectral Partitioning)

14 Accuracy criteria used to certify/commission systems for operational use Other parameters are evaluated, and errors on the same order Field evaluations over a limited time Comparison with operational standard Generally: Eliminate failed QC reports Limited to cases when Peak Frequencies match Statistic is either Root Mean Square Deviation (RMSD) or Functional Precision (Hoehns, 1977 & Gilhousen, 1987) FP = BIAS 2 + STDDiff 2

15 Nomenclature Short Name First Deployed Last Deployed Reference Experimental Environmental Research Buoy Directional Wave Data Analyzer Directional Wave Analyzer Wave Processing Module Directional Wave Processing Module Wave and Marine Data Acquisition System Digital Directional Wave Module NDBC s Directional Wave Family Tree XERB ~1977? Steele et al., 1985 DWDA ~1980? Steele et al., 1985 DWA ~1986 MO, Dec 2010, Great Lakes WPM ~1989 Oct 2009, Great Lakes DWPM ~1999 Analog, still used with Hippy 40 WAMDAS March 2006 ~2010 NDBC 1.8m & still on WHOI ASI NDBC, 1996 NDBC, 1996 NDBC, 1996 Teng et al., 2007; Crout et al., 2008 DDWM April 2007 Operational Teng et al., 2009; Riley et al., 2011

16 Documenting Basis for Accuracy Statements Candidate System Reference Station PoR # Samples Statistic Type Result ( ) 46042, Hippy, DDWM 2.03 co-hosted Jan - Jun MWD, RMSD , DWPM DWPM minute, Cohost May- Sep MWD, FP minute 7.9 DDWM?????~10 DWPM, ARS Gulf of Mexico ~ 2003???~ , GoM, DWPM, Hippy WPM-MO Sep-Oct MWD, FP 8 WPM? ~ 1996??? DWA, MO (Wang, 1995) DWA, Hippy (Gilhousen, 1990) Monterey Bay, DWA Hippy & (Mobile) & (Duck) Oct 91-Mar ? MWD, Bias and STDDiff alpha1 at high frequencies; consistency between buoys and wind ~

17 Other Comparisons Pre-SWADE (Anctil et al., 1993) 3-m discus with wind vane, DWA using Hippy 40 sensor Deep-water evaluation in Gulf of Mexico with Bullwinkle platform Mean Wave Direction, directional width, skewness and kurtosis May June 1989, 68 measurements Poor correlation with skewness and kurtosis, better in higher seas Direction and width: ~45/68 within 90% confidence limits!

18 Harvest Platform (O Reilly et al., 1996) 3-m discus -DWA Hippy 40 & Datawell Waverider & pressure array at the platform Focused on Pacific swell; MWD integrated over the swell band ( Hz)

19 BUOY DIRECTIONAL WAVE OBSERVATIONS IN HIGH SEAS Wang and Teng (1989) December 1987, Monterey Bay, H s ~ 9 meters Hippy 40 DWA MWD and S Wave response to wind As a result, we have increased confidence in the utility and quality of the data produced by NDBC's directional wave measurement buoys.

20 10 & 12-m Discus Hulls affected by currents Based on Steele, 1997: Ocean Current Kinematic Effects on Pitch Roll Buoy Observations of Mean Wave Direction and Nondirectional Spectra NDBC does not release in realtime directional wave measurements for frequencies above 0.20 Hz (< 5 sec) NDBC does archive (NODC) and preserve directional spectra at all frequencies 10-m anemometer height indicates hull in pre-2011 NODC records

21 Yaw induced by swell opposing wind increased errors Generally restricted to Gulf of Mexico and Great Lakes, a couple on the West Coast Notorious deviation occurred during Hurricane Ivan, 2004

22 How Stable is the Accuracy Claim over Deployment Before leaving SSC, buoy heading accuracy < 4 Brought back four undamaged DDWM systems intact Reran buoy heading check Small sample, but found error ~ 6, with largest longest deployed, oldest battery and instrument ages, and hottest recal Buoy Hull Number 3D87 3DV13 3D33 3DV11 Pre-Cal Age (years) Wave Instrument Age (years) Battery Age (years) Air Temperature at Pre-Cal (C) Air Temperature at Post-Cal (C) Maximum Post-Cal Heading Error ( )

23 Summary

24 Thanks Title slide picture courtesy of Weather Forecast Office Portland, OR Journal articles & Conference Proceedings: NOAA Library, Boulder, CO Maury Oceanographic Library, Naval Oceanographic Office, Stennis Space Center Chung-Chu Teng, Ken Steele, Dave Gilhousen, David Wang, Rex Hervey, Ted Mettlach, Bob Jensen

25 References Allen, J and P Komar (2000). Are Ocean Wave Heights Increasing in the Eastern North Pacific?, Eos, 81 (47), pp. 561, Anctil, F, MA Donelan, GZ Forrestall, KE Steele, and Y Oullet (1993). Deep-Water Field Evaluation of the NDBC-SWADE 3-m Discus Directional Buoy, J. Atmos. Oceanic Technol., 10, Crout, RL, RV Hervey, and RH Bouchard (2008). Operational field test and evaluation of NDBC's compact ocean observing system configured for ocean wave, meteorological, and ocean current profiling measurements, Proc. 12th Conference on IOAS-AOLS, AMS. Earl, MD, KE Steele, and DW Wang (1999). Use of advanced directional wave spectra analysis methods, Ocean Engineering, 26, (12), Gemmrich, J, B Thomas, and R Bouchard (2011). Observational changes and trends in northeast Pacific wave records, Geophys. Res. Lett., 38, L22601, doi: /2011gl Gilhousen, DB (1987). A Field Evaluation of NDBC Moored Buoy Winds, J. Atmos. Oceanic Technol., 4, Gilhousen, DB (1990), NDBC Directional Wave Measurements, Proc. Marine Instrumentation 90, San Diego, CA, Hoehns, WE (1977). Progress and results of functional testing, NOAA Tech Memo T&EL-15, NOAA, 11 pp. IAHR, IAHR List of Sea Parameters : an update for multidirectional waves, Proc. of the 27th IAHR Congress, San Francisco, August 1997: IAHR Seminar : Multidirectional Waves and their Interaction with Structure, Canadian Government Publishing, pp Longuet-Higgins, MS, DE Cartwright, and ND Smith (1963), "Observations of the directional spectrum of sea waves using the motions of a floating buoy, in Ocean Wave Spectra, Prentice-Hall, NDBC, NDBC Technical Document 96-01, Nondirectional and Directional Wave Data Analysis Procedure, O Reilly, W, THC. Herbers, RJ Seymour, and RT Guza (1996): A Comparison of Directional Buoy and Fixed Platform Measurements Of Pacific Swell, J. Atmos. Oceanic Technol., 13, O Reilly, W., The IOOS 'First 5 Standard' for Wave Measurements, Proc. JCOMM Technical Workshop on Wave Measurements from Buoys, 2-03OCT (2008), New York City, USA, JCOMM Technical Report No. 47 (ftp://ftp.wmo.int/documents/publicweb/amp/mmop/documents/jcomm-tr/j-tr-47-waveobs/presentations/4.1-oreilly-jcomm.pdf) Riley, R, C-C Teng, R Bouchard, R Dinoso, and T Mettlach (2011). Enhancements to NDBC s Digital Directional Wave Module, Proc. OCEANS 11, IEEE, Steele, KE (1997). Ocean Current Kinematic Effects on Pitch Roll Buoy Observations of Mean Wave Direction and Nondirectional Spectra, J. Atmos. Oceanic Technol., 14, Teng, C-C, T Mettlach, J Chaffin, R Bass, C Bond, C Carpenter, R Dinoso, M Hellenschmidt, and L Bernard, National Data Buoy Center 1.8- meter Discus Buoy, Directional Wave System, Proc. MTS/IEEE Oceans 2007 Conference, Vancouver, Canada Teng, C-C, R Bouchard, R Riley, T Mettlach, R Dinoso, and J Chaffin (2009). NDBC s Digital Directional Wave Module, Proc. OCEANS 2009, IEEE, 1-8. Wang, DW and C-C. Teng (1989). Buoy Directional Wave Observations in High Seas., Proc. OCEANS 89 IEEE, 89, Wang, DW (1995). Field Evaluation of the Magnetometer-Only Directional Wave Measurement System, NDBC WMO, FM65-XI WAVEOB, WMO No. 306, Manual on Codes, Part A, I.1,