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An operational An operational wavemodel for the wavemodel for the

Faroe ShelfFaroe ShelfThesis defence Thesis defence

2/4-20072/4-2007

BárðurBárður A. Niclasen A. Niclasen

Statoil, Phillips,

Enterprise og Veba

Náttúruvísindadeildin

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Tidal currents and their Tidal currents and their influence on sailing influence on sailing

conditionesconditiones

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Effect of stationary currents on Effect of stationary currents on waves waves (size<20 km)(size<20 km)

0& 0U U

x t

((quasi-stationar approxquasi-stationar approximation)imation)

Wave height and steepness increase while absolute period remains constantWave height and steepness increase while absolute period remains constant

Wave

Current

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Large breaking wave hit Large breaking wave hit SjóberinSjóberin

at ~18:30 on the 8/3-2007at ~18:30 on the 8/3-2007

Reason : Reason : current treveled in opposite direction of the windcurrent treveled in opposite direction of the wind

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Buoy Buoy  Time HTime Hm0 m0 Dir Dir North WV-3North WV-3 12:04 9.7 33812:04 9.7 338West WV-2 West WV-2 12:22 5.4 35912:22 5.4 359East WV-1East WV-1 11:51 5.6 1711:51 5.6 17South WVD-4South WVD-4 12:39 7.4 34212:39 7.4 342

Existing operational wave Existing operational wave forecastforecasthttp://vs.en.sigling.is – based on ECMWF forecast

http://ocean.dmi.dk

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Motivation for this Motivation for this projectproject

The impact of tidal currents on the sea state in theThe impact of tidal currents on the sea state in thecoastal sone of Faroe Islands are very importantcoastal sone of Faroe Islands are very important...but these effests are not included in present ...but these effests are not included in present operational wave models for the areaoperational wave models for the area

Need local wave model with increased resolutionNeed local wave model with increased resolution that includes the effect from the tidal currents that includes the effect from the tidal currents

on the waves. on the waves. As this can improve sea safety in the long runAs this can improve sea safety in the long run

OutlineOutline

General informationGeneral information Validation of wave forecasts from Validation of wave forecasts from

ECMWFECMWF Optimal settings of a local wave Optimal settings of a local wave

model (SWAN)model (SWAN) SWAN runs including the effect from SWAN runs including the effect from

tidal currents on the sea state.tidal currents on the sea state. Conclusions/OutlookConclusions/Outlook

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Waves according to Waves according to linear theorylinear theory

2 2( , ) sinx t a x t

T

Steepness of the wave is H/λ

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Wave parameters:

Hm0 = 2.0m wave height

Tp = 6.3s peak-period

Tm02 = 4.9s average-period

...

Wave measurement Wave measurement exampleexample

T-1

Hm0 =2.0 m

Tp =6.3 s

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How linear wave models How linear wave models represent real sea statesrepresent real sea states

( , ) sin( )n n n nn

x t A k x t

( , )E

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Governing equations Governing equations in wave modelin wave model

Propagation:Propagation:

yxc Nc N c N c NN

Sourcest x y

( , )( , )

EN

4 , 3 , ,( )in nl ds w nl ds b ds brSources S S S S S S

( , )E

where, where,

Dynamics:Dynamics:

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ECMWF validationECMWF validation

4 hindcast periods spanning one month4 hindcast periods spanning one month All include one major storm All include one major storm (the 4 largest in 1999-2004 at (the 4 largest in 1999-2004 at

WVD-4)WVD-4)

2 summer events and 2 winter events2 summer events and 2 winter events

EveEventnt

PeriodPeriod

11 20/8 - 20/9 20/8 - 20/9 20002000

22 15/1 - 15/2 15/1 - 15/2 20022002

33 1/6 - 1/7 1/6 - 1/7 20022002

44 1/1 - 1/2 1/1 - 1/2 20032003

WV-1, WV-2, WV-3 & F-8: Landsverk. WVD-4: DataQuality. K7: MetOffice.

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Time series from WVD-4 vs. Time series from WVD-4 vs. ECMWF in Event 1ECMWF in Event 1

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Wave spectra at WVD-4. in Wave spectra at WVD-4. in Event 1Event 1

Mod

elM

easu

red

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Statistical average from Statistical average from all eventsall events

SiteSite ParameParameterter

BiasBiassummer/summer/winterwinter

Sc.IndeSc.Indexx

WVD-4WVD-4 HHm0m0 0.0/-0.70.0/-0.7 0.180.18

- - TTm02m02 0.50.5 0.130.13

-- TTpp 0.10.1 0.160.16

WV-2WV-2 HHm0m0 0.0/-0.30.0/-0.3 0.200.20

-- TTm02m02 0.60.6 0.190.19

K7K7 HHm0m0 -0.1/--0.1/-0.70.7

0.180.18

-- TTm02m02 -0.2-0.2 0.090.09

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Effect of high ferquency Effect of high ferquency cut offcut off

Cut-off frequency fhigh [Hz]

0.58 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10

Hm0

BiSc

0.000.00

-0.010.00

-0.010.00

-0.010.01

-0.020.01

-0.030.01

-0.050.02

-0.090.03

-0.170.07

-0.390.15

-1.160.44

Tm0-1

BiSc

0.030.00

0.040.01

0.060.01

0.080.01

0.120.02

0.170.03

0.260.04

0.420.06

0.720.10

1.390.18

3.490.44

Tm01

BiSc

0.130.02

0.150.02

0.200.03

0.260.04

0.350.05

0.470.07

0.640.10

0.920.14

1.370.20

2.240.32

4.500.64

Tm02

BiSc

0.300.05

0.340.06

0.430.07

0.530.09

0.660.11

0.840.14

1.080.18

1.430.24

1.980.32

2.960.48

5.310.85

WVD-4 fhigh = 0.58 Hz, WV-2 fhigh = 0.50 Hz, K7 fhigh = 0.25 Hz

padded f -5 tail

measurements

Tm02 bias

WVD-4: 0.5 s WV-2: 0.6s K7: -0.2s

After correcting for different measurement range

(padding f-5 tail)

WVD-4: 0.8 s WV-2: 1.0s K7: 1.2s

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Conclusions – on ECMWF Conclusions – on ECMWF validationvalidation

Overall it is found that ECMWF Overall it is found that ECMWF analysis wind and wave data are analysis wind and wave data are suited to force local wave modelsuited to force local wave model WV-1 and WV-3 not suited for validationWV-1 and WV-3 not suited for validation Seasonal variation with less negative Seasonal variation with less negative HHm0m0 bias bias

in summer periods compared to winter periodsin summer periods compared to winter periods Smooth predicted spectra with some missing Smooth predicted spectra with some missing

swell eventsswell events Slightly less energy in spectral tail compared Slightly less energy in spectral tail compared

to measurements (positive bias in to measurements (positive bias in TTm02m02))

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Choosing a local modelChoosing a local model

ModelModel NumericNumericss

CurrentsCurrents TuningTuning

WAM4WAM4 expl.expl. stat.stat. ””none”none”

WW3WW3 expl.expl. stat./stat./unstatunstat

””requiererequiered”d”

SWANSWAN impl.impl. stat./stat./unstatunstat

””some”some” SWAN was choosen because:SWAN was choosen because: High resolution runs favour implicit High resolution runs favour implicit

propagationpropagation Want to include unstationary currentsWant to include unstationary currents Less site specific tuningLess site specific tuning

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Nested Nested SWAN SWAN model model

domainsdomains

NestingNesting 3 31/60 1/60 ° x 1/120 ° x 1/120

°°

72 directions72 directions

dt=15 mindt=15 min

NestingNesting 1 11/8 1/8 ° x 1/16 °° x 1/16 °

24 directions24 directions

dt=30 mindt=30 min

NestingNesting 2 21/60 1/60 ° x 1/120 ° x 1/120

°°

24 directions24 directions

dt=30/15 mindt=30/15 min

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Different model Different model physics/numericsphysics/numerics

DynamicsDynamics4 ,in nl ds wSources S S S

Run typeRun type CommentsComments

1G1G 1. generation physics1. generation physics

2G2G 2. generation physics2. generation physics

KomenKomen 3G WAM3 physics3G WAM3 physics

Komen n=...Komen n=... 3G WAM3 physics with changed 3G WAM3 physics with changed ’n’’n’ in S in Sdsds

JanssenJanssen 3G WAM4 physics3G WAM4 physics

Janssen Janssen CCdsds=...=...

3G WAM4 physics with changed 3G WAM4 physics with changed ’C’Cdsds’ in Sds’ in Sds

Janssen-RoopJanssen-Roop 3G WAM4 physics with WAM4.5 3G WAM4 physics with WAM4.5 lmiterlmiter

WesthuysenWesthuysen 3G New 3G New

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Different SWAN runs, Event 1, Different SWAN runs, Event 1, Nesting 1Nesting 1

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Intermediate results from Intermediate results from Nesting 1Nesting 1

SWAN-Komen: SWAN-Komen: HHm0m0 and and TTpp OK, but OK, but serious negative bias in serious negative bias in TTm02m02

SWAN-Janssen: SWAN-Janssen: TTpp and T and Tm02m02 OKOK,, but but negative bias in negative bias in HHm0m0

SWAN-1G and SWAN-2G: OK, but SWAN-1G and SWAN-2G: OK, but too much scatter in resultstoo much scatter in results

A need to ’retune’ source terms A need to ’retune’ source terms in SWANin SWAN

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Whitecapping dissipation in Whitecapping dissipation in SWANSWAN

, , ,nm

ds w ds mPM m

s kS C E

s k

, , 1 ,m

ds w ds mm PM m

k s kS C E

k s k

SWAN-Komen (WAM3) :

SWAN-Janssen (WAM4):

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SSds,wds,w ’n’’n’-tuning-tuningno no f f -5-5 tail tail with with f f -5-5

tailtail

Least error if

n=2.0-2.1

Least error if

n=1.9

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Time Time averageaverage

d d spectra spectra

from from modellemodelle

d d spectra spectra vs. data vs. data

from from WVD-4 WVD-4

in Event in Event 11

ECMWF-averaged spectra close to the measured, but slight undershoot in the high frequenciesDefault SWAN poor fit to the measured spectra, but good fit with new ’n’

Default SWAN-Janssen poor fit to the measured spectra, but good fit with new limiter

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Compatability at the Compatability at the boundaryboundary

Waves

Do the nested models accept the incoming sea state?

If the nested models are compatible with the forcing they should have Hm0 and Tm02 close to the forcing model (black)

Default SWAN is not compatible with the forcing model, but retuned SWAN-n=2 and SWAN-Janssen with new limiter are compatible with the ECMWF-WAM4 forcing !!

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Conclusions from 1.-Conclusions from 1.-nestingsnestings

Optimal nested-model performance with Optimal nested-model performance with SWAN-Komen with SWAN-Komen with n =1.9 (2.0-2.1)n =1.9 (2.0-2.1)

SWAN-Komen with retuned ’SWAN-Komen with retuned ’n’n’ much much

better compatibility with ECMWF-WAM4 better compatibility with ECMWF-WAM4 than default setupthan default setup

Hersbach-Janssen limiter greatly Hersbach-Janssen limiter greatly improves the SWAN-Janssen runs, but improves the SWAN-Janssen runs, but runs are unstable in high resolutionruns are unstable in high resolution

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2. and 3.-nestings with tidal 2. and 3.-nestings with tidal currentscurrents

Direct validation of the numerical tidal model not possible, but it can be compared against independent tidal predictions based on measurements

One such example from a location close to the south buoy...

U-component (East) of the tidal current ellipse in [m/s] for Event 1

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Influence from tidal currents on Influence from tidal currents on wave height in idealized test casewave height in idealized test case

(difference plot)(difference plot)

Waves

Hm0=2.5m

Wind 10 m/s

m

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Influence from tidal currents on Influence from tidal currents on wave steepness in idealized test wave steepness in idealized test

casecase (difference plot)(difference plot)

Offshore wave steepness ~ 0.058

Waves

Hm0=2.5m

Wind 10 m/s

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Influence from tidal Influence from tidal currents on wave currents on wave

dissepation in idealized test dissepation in idealized test casecase

Waves

Hm0=2.5m

Wind 10 m/s

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3.nesting with and without 3.nesting with and without currentscurrents

example where the tidal effect is example where the tidal effect is clear at WVD-4clear at WVD-4

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Conclusions from 2. and 3. Conclusions from 2. and 3. nestingsnestings

SWAN capable of recreating most of the SWAN capable of recreating most of the tidally induced variations seen in the tidally induced variations seen in the measurementsmeasurements

SWAN-Komen with SWAN-Komen with n=2n=2 is a better option is a better option than default SWAN for deepwater areas than default SWAN for deepwater areas with unstationar currentswith unstationar currents

DIA (SDIA (Snlnl) counteracts variations in spectral ) counteracts variations in spectral shape (shape (TTm02m02) forced by unstationar currents) forced by unstationar currents

Lag in measured Lag in measured HHm0m0 variations compared to variations compared to relative current due caused by dynamical relative current due caused by dynamical effects (up-wind slowing down of wave energy)effects (up-wind slowing down of wave energy)

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Suggested operational Suggested operational setup setup

Reducing computational costReducing computational cost 2.nesting only2.nesting only 1.-order propagarion 1.-order propagarion Larger time stepLarger time step

Comp. time v.s realtimeComp. time v.s realtimeNest 3 setup extended to the entiereNest 3 setup extended to the entierenest-2 domain: comp. ratio ~ 1.4 nest-2 domain: comp. ratio ~ 1.4 (i.e. 2-day forecast finnished after 2.8 days)(i.e. 2-day forecast finnished after 2.8 days)

Operational setup of nest-2 area:Operational setup of nest-2 area:comp. ratio ~ 0.1comp. ratio ~ 0.1i.e. 2-day forecast in 5 hours if serial comp. or approx. 1.0-1.5 hours i.e. 2-day forecast in 5 hours if serial comp. or approx. 1.0-1.5 hours

if paralell comp.)if paralell comp.)

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SummarySummary Regional wave/wind model is vaildated Regional wave/wind model is vaildated

for the Faroese area (ECMWF)for the Faroese area (ECMWF)

Local wave model is implemented, Local wave model is implemented, adjusted and verifyedadjusted and verifyed

Operational setup for a local wave Operational setup for a local wave model including the effects from tidal model including the effects from tidal currents is suggetsed. Is possible to currents is suggetsed. Is possible to run on the local linux-closter.run on the local linux-closter.

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OutlookOutlook

Operational waveforecasts that Operational waveforecasts that include the effect of unstationary include the effect of unstationary tidal currents are possible for the tidal currents are possible for the Faroe Shelf ... if $$!Faroe Shelf ... if $$!

Implementation of high-resolution in Implementation of high-resolution in wave- and wind-models, inclusion of wave- and wind-models, inclusion of wave reflections, dynamic currrents wave reflections, dynamic currrents (oceanic, wind, pressure)(oceanic, wind, pressure)

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Thank you

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Don't be cruel ....pity me!!

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Wave spectra at WVD-4, event 1, Wave spectra at WVD-4, event 1, 3.-nesting3.-nesting

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3.nesting with and without 3.nesting with and without currentscurrents

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Intermediate conclusions Intermediate conclusions from 3.nestingsfrom 3.nestings

Default SWAN-Komen better predictiones Default SWAN-Komen better predictiones without including the effects of the currentwithout including the effects of the current

SWAN-Komen with SWAN-Komen with n=2n=2 better predictiones better predictiones when including the effects from the currentwhen including the effects from the current

Variations in wave height and direction Variations in wave height and direction recreated to resonable extent when including recreated to resonable extent when including the influence from the currentthe influence from the current

Modelled variations in Modelled variations in TTm02m02 are missing ... ? are missing ... ? Modelled variations in Modelled variations in HHm0m0 are lagging ... ? are lagging ... ?

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3.nestings with different 3.nestings with different source terms turned offsource terms turned off

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Numerical tidal model used Numerical tidal model used in the 2. and 3.-nestings in the 2. and 3.-nestings model vs. tidal ( prediction from measurements at WV-4)model vs. tidal ( prediction from measurements at WV-4)

Estimated maxmum current strength

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Effect of stationary currents on Effect of stationary currents on waves waves (size<20 km)(size<20 km)

0& 0U U

x t

((quasi-stationar approxquasi-stationar approximation)imation)

Wave height and steepness change while absolute Wave height and steepness change while absolute period remains constantperiod remains constant