J. Henrique Alves WAVEWATCH III: NCEP's operational wave models NOAA WAVEWATCH III Overview and...

J. Henrique Alves WAVEWATCH III: NCEP's operational wave models

NOAA WAVEWATCH III

Overview and outlook of NCEP's operational wave models

J. Henrique Alves & Hendrik L. Tolman

SAIC-GSO at NOAA/NWS/NCEP

Marine Modeling and Analysis Branch, EMC

[email protected]


Outline

Short story of wind waves From birth to breaking, what are they? Representing the sea-state: significant wave

and the wave spectrum Forecasting waves using numerical models

The NCEP ocean wave guidance Performance evaluations Recent Changes and Future directions Maximum wave in hurricane advisories

Tutorial: NCEP operational wave products What are they? Grib & grab guide


Wind Waves

What are they? Always born windseas, sometimes die as swells How are they generated?

Wind turbulence: resonance Pressure differences, disturbed field

Many scales are excited From a cm to a whole km in length As fast as a crawl, near 100mph From a mm to 10 stories high Multiple directions

If you’re out at sea, can’t ignore them

L = 1.56 T2

C = L / T

C = 1.56 T


Wind Waves Important parameters describing one wave

Water movements associated with wind waves Need to be described, and predicted. But how? How many parameters to describe sea state?

1ary: Height – Length – Period – Direction 2ary: Speed - Steepness

Many waves at same time: impossible to have a set of descriptive parameters for each wave individually

Solution: describe sea-state with average measures for wave fields: representative wave, wave spectrum


Wind Waves

1st approach: single representative wave (1940’s) Most popular: Significant wave height Hs

Average height of the highest 33% of all waves. Hs is closely what we see <= smaller waves are

dwarfed against the background of larger ones Theory, empirical data: heights follow a Rayleigh

distribution. For Hs = 10m (33ft), one can expect: 1 in 10 waves to be larger than 10.7m (36ft) 1 in 100 waves to be larger than 15.1m (51ft) 1 in 1000 waves to be larger than 18.6m (62ft).

Largest individual wave may be roughly 2 x Hs

Associated parameters: Tp, Tz, p, m Rayleigh breaks down in growing and mixed seas In mixed seas, single wave parameter means little


Wave Spectrum

Goal: represent main properties of multiple wave systems

What defines fully a wave? L or T (dispersion relation) Direction Height (wave energy)

Wave spectrum bundles such information for multiple wave systems

Displays distribution of wave energy in “classes” of direction and frequency (period)

Easily represented graphically (sample spectrum at buoy 51004)


Wave Spectrum

WAVEWATCH III Output spectra Blue: nearly no energy Pink: maximum energy Each contour has twice the

energy of the previous Directions show where wave is

going to Frequency range:

Center: f=0.04Hz (T=25s) Outer: f=0.25Hz (T=4s)

Resolution: 24 directions 25 frequencies 600 “waves” (components)


Wave Spectrum

| 26 12 | 2.2 4 | 1.4 15.9 16 | 0.7 6.6 306 | 1.4 7.0 239 | 0.3 9.9 136 |

Wave model spectral data Red box: wave field traveling

toward SE, with Tp~10s. Energy concentrated in f and : regular, organized wave field

Blue box: wave field travels toward SW at lower periods. But more chaotic as energy spreads over wider range in at higher f

Main fields: information in wave spectrum can be summarized

Hs = 0.7m, Tp = 6.6s Hs = 1.4m, Tp = 15.9s Hs = 1.4m, Tp = 7.0s Hs = 0.3m, Tp = 9.9s

Sea state on 06/26/2000 Buoy 51004 Single wave scenario

Hs = 2.2m Tp = 15.9s Tm = 10s p = 16o m = 180o


Such summary, in tabular format, is the basis of bulletins provided in our web site and in AWIPS To a forecaster, spectrum provides

qualitative information Bulletins give quantitative info on up to six

largest wave fields at a given time in the fcst horizon

Wave Field Bulletin

| 26 12 | 2.2 4 | 1.4 15.9 16 | 0.7 6.6 306 | 1.4 7.0 239 | 0.3 9.9 136 |


Wave Field Bulletin

Sample bulletin1st column: date and hour2nd column: overall Hs and number of identified individual wave fieldsNext six columns (only two shown): height, period and direction of individual wave fields

AWIPS :

feet, dir. from

http://polar.ncep.noaa.gov/waves/pres/primer


Wave Propagation

i. great circleii. refractioniii. dispersion

NOAA WAVEWATCH III (NWW3) Model

Model Physics

i. growth (wind input)ii. decay (whitecapping)iii.wave-wave interaction

10m winds, SST, Tair, [ice]

WAVEWATCH III MODEL

Full spectral wave field

FCST

bathymetry & coast line

Reduced Output

i. local spectraii. mean/integral

field parameters


Operational NWW3 Model Suite (NCEP Guidance)

NOAA WAVEWATCH III: Version 1.18 replaced all previous operational models at NCEP, March 2000. NWW3: Global, 1.25x1o, 168h, 3-hourly GFS winds WNA: W North Atlantic, 0.25x0.25o, 168h, 3h GFS

winds NAH: Hurricane version of WNA, 0.25x0.25o, 72h, 1h

GFS+GFDL winds ENP: E North Pacific, 0.25x0.25o, 168h, 3h GFS winds NPH: Hurricane version of ENP, 0.25x0.25o, 72h, 1h

GFS+GFDL winds AKW: Alaskan Waters, 0.5x0.25o, 168h, 3h GFS winds

All models: 24 directions, 25 frequencies, 4 daily cycles (00z, 06z, 12z, 18z) with 6h hindcasts for continuity

NOAA WAVEWATCH III: Version 2.22: operational since Aug. 2002

180h180h

180h

180h

GFDL

March 2004


NCEP Guidance: The Global Model Domain

Global Model Domain

March 2000


NCEP Guidance: The WNA Model Domain

Operational WNA Model Domain implemented since March 2000

Also serving NAH Model since July 2001

Previous ECG WAM Model Domain (Operational at NCEP till March 2000, red square)


NCEP Guidance: The ENP and AKW Model Domains

ENP Model Domain implemented June 2002

Also serving NPH Model since Aug 2003

AKW Model Domain implemented on March 2000


http://polar.ncep.noaa.gov/waves/validation.html

NCEP Guidance: General Quality Evaluation

Validation statistics per month/season vs. buoy or satellite data. Since February 1997 for Global model. Since August 2000 for Regional models.

Six-month comparison with old operational global wave model (WAM). Strong points: Good overall performance (rms errors ~10-

20% of mean measured Hs). Improved max and min storm Hs relative to

previous global model (WAM). Comprehensive coverage of areas of interest

by regional models


NCEP Guidance: General Quality Evaluation (Winds)

Weak points of external forcing affecting wave models: Wave models, only be as good as its driving forces. For our models (deep water): Wind!

Most important problems, GFS and GFDL resolutions: Temporal output resolution, of GFS GFDL is much

coarser than internal model time step: Interpolation smears small systems with strong

winds => underestimation of extremes. Solutions: shorter output step for surface fields

Spatial resolution in GFS and GFDL, constrained by economical considerations (more difficult):

Problems with prediction of small-scale systems (frontal system moving offshore GMex and EC).

Poor land-sea (boundary layer) transition: problem for wave growth within systems moving offshore.


NCEP Guidance: General Quality Evaluation (Winds)

Weak points of the present wave models. Resolution:

Spectral resolution is “borderline” enough, lacking for swell dispersion

Spatial resolution could locally be better for islands (partially solved) and hurricanes when GFDL winds are available

Shelf bathymetry not well resolved nearshore: problem for propagation of waves generated offshore

Physics (secondary, fine tuning): Slow initial growth, but good predictions of peak

and waning stages of storm. Errors in spectral shape near peak make swell front

arrival gradual and slightly late (DIA + numerical).


Why do we need a special Hurricane version (NAH, NPH) of regional models (WNA, ENP). Wave model can only be as good as the

winds that drive it. Hurricane winds are not done particularly well

by the GFS due to resolution problems and due to limitations of the model physics.

Better results expected when higher resolution models are used such as the GFDL model.

Need for blended GFS/GFDL winds.

NCEP Guidance: Why Have Special Hurricane Models?


AVN AVN+GDFL

What got us started: Floyd and Gert Sept. 1999



Debby peak wave period (s)2000/08/23 12z ( + 36 h)

AVN winds AVN + GFDL winds



AVN winds AVN + GFDL winds



NCEP Guidance:NAH vs. WNA during Lili and Isidore (2002)


Isabel’s track and locations with buoy observations

NCEP Guidance:NAH vs. WNA during Hurricane Isabel (2003)


WNA and NAH hindcasts of Isabel collocated with Jason-1 data.

9/13/2003:

Isabel is small and poorly resolved by GFS

9/17/2003:

Isabel is larger and well resolved by GFS



Maximum wave heights very well represented.

Initial swell arrivals underestimated. Swells generated when Isabel was cat. 5 hurricane (wind errors).

Do we believe last observation of 41025. Buoy in surf zone !


Hindcast at buoys near track


Hindcast at buoys south of track


For buoys at the Atlantic seaboard in shallow water, swells are overdone, most likely because shallow water processes are not well resolved yet.

Some tidal influence to be seen in data (not in model yet).


Hindcast at buoys north of track Many features as in previous time series.

Swells from NAH systematically higher.

Highest wave heights from WNA larger due to weaker winds over larger areas

Last two points also clear in previous slides.



WNA and NAH similar


WNA misses Isabel, swells


• This is an hourly animation of Isabel wave heights from hindcasts.

• Note the pulsating nature of the wave height fields, which has to do with the hourly wind fields.

• Possible reasons

• Nature

• GFDL peculiarities

• Garden Sprinkler Effect for swells.



Ranges of wave heights in 72h forecasts

NAH forecast envelope

Buoy 95% confidence

interval



nowcast

Max Hs > 50 ft

12 h forecast

Max Hs > 45 ft

24 h forecast

Max Hs > 55 ft

48 h forecast

Max Hs > 50 ft

Forecast wave heights very consistent and verify well against buoy and altimeter data

Timing is the biggest issue in the forecast, with NAH (GFDL) being a little fast.

WNA (GFS) with similar patterns, being a little slow in bringing Isabel to the shore.

NAH, 18/9/2003 12:00 UTC



Parametric model of Young (1988) [modified by Wu] basis of estimation of maximum Hs for TPC advisory

Relates maximum Hs and an equivalent fetch Basis of Young’s approach:

2G wave model data using idealized hurricane winds Relates max(Hs) and equivalent fetch (XEQ) using

JONSWAP parametric formula XEQ is assumed a function of Vf and Umax O(2) polynomial in Vf & Umax is fit to JONSWAP

formula, given model maximum Hs, providing XEQ Given Vf, Umax and Rmax (nonlinear scaling factor),

model estimates maximum Hs EMC: Testing sensitivity to tuning data (wave model Hs)

and to assumptions behind XEQ

Maximum Hurricane Wave Height:Parametric Model for Advisory


Effect of changing model only Same parameterization of XEQ

New tests: WAVEWATCH III Over 100 model runs, several

Vf, Umax and Rmax New XEQ, f(Rmax), adjusted by

Vf, scaled by Umax Scatter reduced O(100)

Highly sensitive to base wave model

New form of XEQ: higher accuracy (model independent?)

WAVEWATCH III, widely tested in real cases: better?

Maximum Hurricane Wave Height:Sensitivity of Parametric Model


Recent upgrades

The following changes and of the model suite were made in the past two years: Upgrade of blending scheme for NAH winds and

upgrade time resolution of NAH wind fields to 1 hour (start of 2002 Hurricane season).

Second release of WAVEWATCH III code (Islands)... Four cycles, eight-day forecast for all non-hurricane

models. North Pacific Hurricane (NPH) wave model. Continuous upgrades to output points. New buoys

and new requests added. West coast temporary output points removed from global model.

http://polar.ncep.noaa.gov/waves/changes.html


Recent upgrades:Hourly Winds in Hurricane Models

Michelle at 11/04/2001 15zwind speeds

every 6 hours every 1 hourwave heights

every 6 hours every 1 hour

Two eyes of hurricane at the off hour due to rapid movement of

Michelle, no consistency in wave fields.

Michelle with single eye at off hour, much more consistent and

higher waves.


Recent upgrades:Island Blocking

Unresolved islands have been included in version 2.22 of WAVEWATCH III as obstructions in the grid. Even in the high resolution (25 km) WNA model this can have a major impact on local biases, as is shown here with model biases against

the ERS-2 altimeter for the year 2001.

bias without obstructions bias with obstructions


Recent upgrades:Data Assimilation

Data assimilation is not essential in wave modeling, due to the forced and damped nature of the problem.

There is grossly insufficient data for making a data-dominated analysis

Benefit in using data in hindcast and short term fcst shown at NCEP (WAM), as well as in other centers.

Objective: assimilate wave data into global wave model: Buoy data from fixed platforms. Altimeter wave data from ERS2 satellite.

Data expected in the near future: GFO and Jason-1 Observations assimilated using variational method as in,

for instance, SST analysis. Observations provide Hs, model works with E(f,). Model spectra adjusted for corrections in Hs by rescaling.

Operational,

Mar 2, 2004


Data assimilation of buoy and ERS2 data: systematically positive impact on the quality of the hindcasts and the short term wave forecast in the global NWW3 wave model.

Implementing this data assimilation scheme will put us in an excellent position to rapidly test and add other altimeter wave data source, with expected additional incremental improvement to hindcast & short term forecast



The future The next step in expanding the functionality of

WAVEWATCH III is to generate a multi-scale version of this model : Regional models have a spatial resolution of 25km. In

the next decade, we expect to be able to increase the near-coast resolution to 5-10km, using multi-scale technology

Two-way nesting of models with different scales that run simultaneously.

Moving nests follow features of interest, particularly hurricanes.

Hurricane nests plus coastal nests remove the need for running separate large regional models.

Selective application of highest resolution nests makes ensemble wave forecasting more feasible.

coupling with WRF


The Future

Deep ocean model resolution dictated by

GFS model

Higher coastal model resolution dictated by

model economy

Highest model resolution in areas of

special intest

Hurricane nests moving with storm(s) like GFDL and WRF


The future

At the moment we are not looking at adding regional models, or at increasing the spatial resolution of the existing model. Instead, the focus will be on new products: Adding one or more steepness measures to the

model output Safety issue for (small) ships Input from field appreciated (NCEP/OMB)

Doing separation of individual wave systems for all grid points, not just for output points.

Identify each individual swell field in space and time. Ideal for IFPS

Collabotation with Jeff Hanson (JHU). Looking for funding Bulletins in spreadsheets (Jeff Lorens, WR)

(USACE)


The future

Further efforts have been focused on physics of the model, to improve general behavior: Improved nonlinear interactions for more

realistic directional spreading in wind seas, and crisper swell arrival.

Improve particularly dissipation to prepare for later coupling with ocean models.

Improve stress calculations and hence growth rates for extreme (hurricane) conditions. Collaboration with Univ. of Rhode Island and Univ. of Miami.

These are all long term (multi-year) projects.


Products

Mean wave parameters in GRIB format Overall significant wave height. Mean direction and period. Peak direction and period. Wind sea direction and period. NOT AVAILABLE : mean swell height and

direction (see next slides). Text bulletins with different wave systems for

output locations


Products

There is rarely just one swell field.

What is the meaning of "the" swell height and the mean swell period and direction?

Up to nine swell fields can be identified in the spectral plots on the right!


Products

significant wave height (m)

Wind seas

Swell ?


Products

mean wave period (s) and direction

Wind seas ?

Swell ?

Swell !


Products

peak wave period (s) and direction

Swell !

Swell !


Products

wind sea wave period (s) and direction

Wind seas

Wind sea !


Products

wind speed (kn) and direction


Products

marginal wind sea + swell

T = 8s

marginal wind sea H = 1.6m

T = 8s

Swell H = 0.3m T = 15s

H = 0.6m T = 12s




different!

Numerical data and time evolution from bulletin (buoy 51004)


Products

Available on AWIPS : Most model fields. Errors in AWIPS graphics near

coast. It takes some time to catch up with added cycles and extended forecast ranges.

Text bulletins in version modified for the use of WFOs (wave heights in feet, meteorological convention for directions).

Not available on AWIPS : Spectral plots as used in this presentation. There are

no plans for including these in in AWIPS. See also future plans slides below.


Products

Available on web site:. ALL model data, usually within 1 hour of the model

run. Historical hindcast data. Validation data. Background information, including a primer and

presentations like this one. We will work with any WFO or region to get products out

as needed. New output points (2-3 week turn around time). Suggestions for new products welcome. NAWIPS … (OPC, Hawaii …)

http://polar.ncep.noaa.gov/waves


For fast response to questions, remarks, requests etc., contact us at

[email protected]

This E-mail will be distributed automatically among our entire wave staff, and therefore will give you the fastest response. To get us personally, try

[email protected]@NOAA.gov

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J. Henrique Alves WAVEWATCH III: NCEP's operational wave models NOAA WAVEWATCH III Overview and...

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