Globalspectralobservationofocean waves from theCFOSAT

satellitemissionD.Hauserwith contributionsfrom

Withcontributionsfrom

Alraddawi D.,Aouf L.,Chapron B.,CollardF.,DalilaM.,Dalphinet A.,DufourC.,Gouillon F.,Grouazel A.,GuittonG.,Hermozo L.,HussonR.,Lachiver J-M,Lacrouts C.,Mironov A.,Nouguier F,OllivierA.,PerretB.,

PirasF.,RodriguezSuquet R.,Schippers P.,TisonC.,Tourain C.

ClimatchangeInitiative/Sea state,October 2019,Brest(France)

Outline

vIntroduction

vCFOSATobjectivesandmaincharacteristics

vWave parameters from SWIM

vFirstresults

üHs from nadir

üSpectralparameters from off-nadirobservations

vConclusion

1- Introduction

Itisrecognizedthatsea-stateisanessentialvariableforclimatesurveyandimpactstudies(asprovedbytheexistencethisCCIsea-stateprogram!!)

But,question:whatparameterscharacterizesea-state?- Hs alone- Hs+peak(mean)period(frequency)- Hs+peak(mean)period(frequency)+dominant(mean)direction- Wholedirectionalspectrumofoceanwaves- wave“trains”(partitions)andassociatedparameters

Allofthem!! Needs,forresearchand/ormonitoring:

ü coastalprocesses(directionanddirectionalspread,wavelength)

ü wave/current,orwave/iceinteractions(wavetrains,direction,directionalspread,wavelength,..)

ü extremeseas(detectionofmixedseacases,shapeofthedirectional spectruminfrequency/wavenumber)

ü validation/improvement/assimilationfornumericalmodels(evolutionoffrequency,directionalandfrequencyspreadfromyoungseatomaturecases,swellattenuation,wave/currentinteractions,..)

ü impactofclimatechange(changeinperiod/wavelengthanddirectionsinrelationwithstormtracksevolution)

Current observations

ü Insitudata:mainlyHs,sometimespeakperiod(orfrequency),rarely1Dspectra,extremelyrarely2Dinformation(usuallynotdirectlythespectra)

ü Satelliteobservations- Altimeter(HsandU)- 25yearsofobservations,global- SAR(2Dspectrum)– gooddataset(20years)butdifficultiesduetoazimuthcutoff,andnotalwaysglobal

- New :CFOSAT(China-FranceOceanographicSatellite):anewsatellitelaunchedon29thOctober2018

SWIMWave scatterometerKu-band

SCATWindscatterometerKu-band

OrbitSunsynchronousLocaltimeatdescending node

AM7:00Altitudeattheequator

519kmCycleduration

13days

TheCFOSATMission

CFOSAT:AChina/Francejointsatelliteoceanographicmission.

Jointmeasurementsofsurfacewindandwave

ü awindscatteromerer (SCAT)=>oceansurfacewindvector

ü awavescatteromer (SWIM)=>directionalspectrumofoceanwaves+windandHsfromnadir

2- CFOSAT

Funded andmanaged by3Agencies

PIship :D.Hauser(LATMOS/CNRS),LiuJianqiang (NSOAS)

polar,sun-synchronous,globalcoverage,13day

repeatcycles

SWIM SCAT

vA scientificmission:• Wavedynamics andevolution• Wind/waveinteractions,• Impactofwavesonair/seaexchanges,• Interactionofwaveswithcurrents,sea-ice• Contributiontowaveclimatestudy• Boundaryconditionsforcoastalstudies(andsecondaryobjectivesonsea-iceandcontinentalsurfaces)

vA demonstrationandpre-operationalmission:windandwavefieldanalysis,feedforecastsystems(assimilation),contributiontoglobaldatabases(CMEMS,..)

8

SWIM

Wave scatterometer

vKu band real aperture radarvSequential illumination with 6 incidence

angles : Ø Beams 0°, 2°, 4°, 6°, 8°, 10°

vRotating antenna (all azimuth direction acquisition ): 5,6 rpm

vProducts :Ø Directional wave spectraØ Significant wave height and wind speedØ 𝝈𝟎mean profiles, 0 to 10º

MainSWIMvariablesintheoperationalproducts

vSignificant wave height and wind speed (along-track)- similar to altimeter mission

Ø2D wave spectra for wavelengths in the range [70-500] m- with 180°ambiguity in direction

vIn continuous wave cells (70 km x 90 km) on each side of the track

ØBackscattering coefficient (sigma0) profile

v Relativewaveenergyintheoriginalradialgeometryoftheinstrument

v Space-timeanalysisoflongswellsystemsfollowingtheirpropagationpathsafterrefocusingtotheirorigin

AlternativeSWIMproducts from theIfremerdatacenter(IWWOC)

Noiseduetoechoescorrelation(Dopplereffectalong-track)

NicematchbetweenWW3andmeasuresforbothpartitions

SlopespectraalongSWIMfootprintsat10° withcollocatedWW3model

Noiseduetoechoescorrelation(Dopplereffectalong-track)

NicematchbetweenWW3andmeasuresforbothpartitions

SlopespectraalongSWIMfootprintsat10° withcollocatedWW3model

exampleofradialdensityspectra(colorcodes)

alongtheSWIMsampling(here8° incidencebeam)

vNonspectraldata(Hs)from SWIMnadir(see alsotalkbyR.Husson)

vSpectralinformationfrom SWIMoff-nadirobservations

3- Firstresults (CAL-VALstudies)

NadirSWIM(Hs)

Excellentperformancesthankstoanew« retracking »altimeteralgorithm(“adaptive”)whichcompensatestherelativelylowrepetitionrateofthenadirsequences(5Hz)duetomulti-incidencegeometry

Hs_nadir versHs MFWAM

Hs_nadir_SWIM versusHsJasonHs_nadir_SWIM vers HsECMWF

mean biais 6 cm w.r.t std 34cm

Spectraldata(from offNadirSWIMobservations)Examplesof2Dwavespectra

inprogressbetternoiseelimination

2Dspectra,temporarymasked(±15° oneachsideofthesatellitetrack)inthedataproducts

(upgradeexpectedinafewmonths)

comparisontoMFWAMmodelcurrentproduct

14

Buoy (black)MFWAMSWIM (10°, 8°, 6°)

omni-directionel

Buoy SWIM-10° MFWAM

Comparison tobuoy (here Brittany-Atlantic)

Assessment ofthemean spectrum shape

Mean 1Dslope spectra :SWIMcompared toWW3fordifferent wind speedclasses

Very goodagreementforwaves greater than70minwavelength

Preliminary assessments: Main parameters of the 1st partition(SWIM and MFWAM partitioned independently

limits: no cross-assignment of partitions, ±15° azimuth sector maskedon SWIM spectra)

16

SWIM

MFWAM

Hs direction wavelength

IllustratedherewithSWIMbeam10° results

SWIMversusMFWAMparametersofpartitions

17

wavelength=>wellretrievedexcepthere:partlyduetotothemask?parasitic

peaksinSWIMspectra?

direction=>wellretrievedexceptforwavespropagatinginthealongtrackdirection

(becauseofmaskonSWIMspectra)

waveheightÞ goodcorrelation

overestimationatHs<3m,underestimationatHs>3m

Wave evolution attheregional scale inafetchlimited case(North Mediterranean sea)

peak wavenumber versusfetch bycombiningSWIM(from 4to10° incidences),S1B,compared toJonswap law (andWW3)

4- ConclusionAlreadyagreatsuccessforCFOSAT(launchedonly1yearago)

v Veryinnovativemission,instrumentandproducts

v Wave(Hs)andwind(U)productsfromnadir:excellentquality,

v Spectraldatafromoff-nadir:verypromising(inversionmethodstillinprogressfornoisemitigation)

ü consistentshapeof1Dheightorslopespectra,2Dspectra(inspiteoftemporarymasking)

ü verypromisingforcasestudiesatregionalscale(fetch-limited,wavesincurrent,wavesunderseaice(notshown),wavesgeneratedbystorms,hurricanes,..

ü workinprogresstoimprovedetailedperformances(partitionparameters)-currentlyperturbedbythenonperfectcorrectionofspecklenoise(andmasking)

ü =>Dataaccess:alreadyavailableforscienceteam,accessenlargedthroughAVISO+startingthisfall(TBC)https://www.aviso.altimetry.fr/en/data/

NRTdeliverytooperationalcentersviaEumetcast (end2019-beginning2020)