Global swell wave observation and application for NRT...

SEASAR 2008 Symposium, 21-25 January 2008, ESA-ESRIN, Frascati, Italy

Global swell wave observation and application for NRT storm swell tracking and swell attenuation estimation.

Fabrice Collard : BOOST TechnologiesBertrand Chapron : IFREMER

Fabrice Ardhuin : SHOM


Swell partition extraction and validation

• Extraction of swell systems integratedparameters from ENVISAT WAVE MODE level2 products. (based on improved wavespectra algorithm now operational in PFASAR).

• One set of parameters (E,Dir,K) per partition


1) Static validation against directional BUOYS

• Collection of 50 image mode and 35 wide swath single look complex image.

• Collection of collocated buoy directional spectra over deep and shallow water.

• NDBC buoys : courtesy of NOAA• Datawell buoys in iroise sea and golfe Normand Breton

: courtesy SHOM


Buoy data preparation

• Maximum Entropy Method estimation• SAR spectrum BUOY spectrum


Deep water buoy configuration


Swell height comparison


Swell wavelength comparison


Swell direction comparison


Validation summary

-0.070N/A0.2 0.2 N/A Φ, rad0.760.270.010.940.87 1.1 Tp, s 0.090.190.250.880.280.7Hs, m

Bias WSBias IMBias WMRMSe WSRMSe IMRMSe WM

• Comparison of integrated swell parameters extracted from SAR and BUOY wave spectra for different ASAR modes. RMS error and BIAS are provided for the three usual parameters that are the significant wave height (Hs) the dominant wave period (Tp) and the dominant wave direction (Φ)


“Dynamic” validation

• basic idea is to verify the coherence of swell observation when propagated in time along great circle rays at group velocity.


Observed swell at a given location versus time (virtual buoy)

• Time-frequency diagram for one month at position longitude 100 °W and 0°N

color indicates propagation direction.


Ridge validation

DAYS OF JULY 2004

Wav

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Wav

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fireworks

• now implemented in near real time. 3 hourly animated gifs produced every morning for Altantic, Pacific and Indian oceans.

• available at following address :

• www.boost-technologies.com/esa/images/nrt_pac.gif• www.boost-technologies.com/esa/images/nrt_atl.gif• www.boost-technologies.com/esa/images/nrt_ind.gif


Observed propagation of 17s swell from July 8 to July 20, 2004


Observed propagation of 13s to 17s swell from July 8 to July 20, 2004

• 6 hour time step• Wavelength from

300 to 450m • Wave period from

13 to 17 seconds


Observed propagation of 17s swell from Aug.10 to Aug. 21, 2004


Observed propagation of 13s to 17s swell from Aug. 3 to Aug. 21, 2004

• 6 hour time step• Wavelength from

300 to 450m • Wave period from

13 to 17 seconds


• Focusing of 17s swell observed from July 8 to July 20, 2004

Identification of swell origin


Application : Analysis of swell attenuation

Bias up to 30% of obs. !


Data mining . .. 13Sec

17Sec


Swell attenuation estimation

• Comparison with theoretical direction spreading and dispersion related attenuation -> dissipation ?

• Color dots and black line SAR data

• Red : model with dissipation• Purple model without

dissipation• Green : theoretical model

(geo+dispersion)



• 16 Sec



• 15 Sec



• 14 Sec



• 13 Sec



• 12 Sec


Dissipation coefficient estimation

• Estimation of the vTcoefficient

1( ) sinT

TE e ν ϕϕ ϕ ϕ−

,


Conclusion

• Coherence of SAR swell observation to build a level 3 product

• Swell attenuation compared well with early Snodgrass et al. estimates but previous scatter from event to event can now be explained by the initial wave steepness condition.

• Swell attenuation is at first order governed by simple dispersion and geometrical spreading and at second order non linear in orbital velocity driven dissipation.

• Next step will be to combine with in situ directional buoy network to derive global scale swell analysis for ocean nowcasting.

Global swell wave observation and application for NRT...

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