NWS Southern Region Marine Workshop 2004

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NWS Southern Region Marine Workshop 2004

• Steve LyonsTropical Program ManagerThe Weather
  Channel

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OCEAN WAVE FORECASTING

• Steve Lyons
• The Weather Channel
• March 2004
• NWS Southern Region
• Marine Workshop

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What I will try to cover!

• 1) Briefly review fundamentals
• 2) Storm wave forecasting tips
• 3) Shallow water effects rip currents

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REVIEW FUNDAMENTALS

• WIND IS EVERYTHING!

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Fundamentals are the most
important!
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Have an overall understanding of the wave
forecast challenge

• Wave growth
• Wave spectra
• Swell propagation
• Swell decay
• Deep water waves
• Shallow water waves
• Shallow water wave modification

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Statistical Wave Spectrum
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WAVE SPECTRUM

• Measured sea state is typically H1/3 termed
  significant wave height
• H1/10 1.27H1/3
• H1/100 1.67H1/3
• H max 2.0H1/3

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• 'Rogue wave' theory for ship disaster
• The 44 crewmen perished when the bulk carrier
  sank
• Scientists have discovered that a rogue wave
  pattern helped cause one of the UK's biggest
  maritime disasters.

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Transient Wave-Wave Superposition
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Example of dispersive waves(just 2 components!)
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COMBINED SEAS

• Combined seas represent the sum total height of
  wind-wave and any number of swell components
  mixed in with it.
• Cssqrt(wwww s1s1s2s2s3s3
• Max Cs2.0Cs

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COMBINED SEAS CALCULATION

• 1) Combined seas square root of
  (WWS1S1S2S2)
• 2) Remember wave spectrum concept
• 3) This process explains wave sets observed by
  surfers and sailors
• 4) Oh, but what about SHALLOW WATER?

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COMBINED SEAS

• Two gnarly surf dudes are ready to surf, but will
  not go out unless waves exceed 15 feet
• The wind wave is 2 feet, and two swells one of 8
  ft and one of 9 ft are occurring togetherwill
  the dudes go out surfing?

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Wave Steepness

• H/L
• Affects boats ships differently
• Vessel size dependence
• Wave height dependent

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Angular Spreading and Dispersion
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WAVE GROUP VELOCITY

• OFTEN THE TERM USED TO INDICATE SPEED OF WAVE
  PACKET (Cg)
• FOR DEEP WATER Cg(1.56T)/2
• FOR SHALLOW WATER CgC

• CdSQRT(gL/2PIE)
• CsSQRT(gH)
• gGRAVITY
• LWAVE LENGTH
• HWATER DEPTH
• Pi3.14159

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STEVES WAVE TOOLS

• VALIDATE MODELS
• IMPROVE WAVE SWELL FCSTS
• FCST HURRICANE WAVES
• FCST WAVE SETUP
• MODIFY FCST USING LATEST OBSERVATIONS

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WAVE RELATIONSHIPS
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WAVE RELATIONSHIPS
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WAVE RELATIONSHIPS
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Summary
Fundamentals Review

• Understand basic wave characteristics and their
  behavior
• Some very simple concepts go a long way toward
  understanding waves and making good wave forecasts

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2) Storm wave forecasting
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Deep water storm wave
forecasting
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LIMITS TO WAVE GROWTH

• WIND SPEED
• WIND FETCH LENGTH
• WIND DURATION
• WIND FETCH WIDTH FOR SWELL
• These can be tricky!

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Maximizing wave growth in a gale,
storm, hurricane

• Wind speed is NOT the answer! Why?
• Wind duration is extremely important! Why?
• Wind fetch length is relatively important! Why?
• Effective fetch, dynamic fetchfancy names for
  combinations of the 2 elements above!
• Why must you know wave period?

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For Strong Winds

• Fetch length is usually limiting
• Fetch duration is nearly always limiting!
• If the wind field moves with the wave field it
  generates, then effective wind duration will
  increase.
• How doe we quantify this affect?
• How do we determine how fast a storm should
  move to maximize wave growth?

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Moving wind fetches

• Fetch speed MUST be linked to the group velocity
  of waves generated by that fetch
• Group velocity is ½ the wave celerity
• Wave celerity, C1.56T
• You are focusing on H1/3 periods even though an
  entire spectrum is being generated
• The ideal wave generation scenario is an
  accelerating wind fetch area. That acceleration
  matching the changing group velocity of waves the
  fetch is generating. Concentrate on H1/3 or you
  will go crazy!

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Calculating effective wind
duration

• Compute wave group velocity
• Compute fetch speed in the direction of wave
  propagation
• Compute the duration of superposition of waves
  and wind fetch for the sector of interest to your
  forecast area
• Decay waves to final their destination as swell
  if they are not collocated with area of interest

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Hurricane Wave Generation
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The Stationary Hurricane
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Stationary Hurricane

• Only asymmetries in wind field
• Latitudinal change in Coriolis parameter
• Are responsible for wave asymmetries,
  otherwise waves of equal height period spread
  away equally from TC in all directions in deep
  water.
• What is deep water?

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The Moving Hurricane
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Moving Hurricane

• Wind field around hurricane modified by the
  forward motion of the hurricane
• Result is winds are strongest (weakest) on side
  with winds blowing in the same (opposite)
  direction from the TC motion
• But that wind asymmetry is NOT the primary reason
  for much higher waves in the TC righ-front
  quadrant in the Northern Hemisphere
• Increased wind duration over developing waves is
  the primary reason!!!!!!!!!!!

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The old right-front quadrant

• 1400 mph
• R34275
• R50125
• R64100
• Rmax25
• Duration72 hours
• Speed0 Speed12kt

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WAVE PROPAGATION

• MOST storms or strong wind areas are NOT
  collocated with your forecast area of interest,
  but can still cause swell there some time later!

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Dispersion of deep water waves
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Angular Spreading Dispersion modify wave
energy away from the generation
area
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IS IT POINTING AT YOU?

• WAVES/SWELLS MOVE ON GREAT CIRCLE PATHS
• ONE MUST KNOW GREAT CIRCLE PATH IN ORDER TO
  FORECAST SWELL AT YOUR LOCATION
• GREAT CIRCLES FROM WHICH YOU CAN MAKE A CHART FOR
  YOUR AREA http//www.indo.com/distance/

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GREAT CIRCLE EXAMPLE
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DRAW GREAT CIRCLE INITIAL TRAJECTORIES TO YOUR
FINAL DESTINATION
http
//

• www.indo.com/distance

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SWELL TRAVEL TIME TO FINAL DESTINATION

• A storm is 1000km from your coast
• H1/310 meters
• T1/310 seconds
• How many hours will it take for the H1/3 waves to
  reach your coast?

• 1) CgC/2
• 2) Cg(1.56T)/2
• 3) Calculate deep water swell group velocity
• 4) Divide travel distance by swell group velocity

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Swell Travel Time
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Summary

• Swell forecasts can be the best extended range
  forecast you can make
• NEVER forget the great-circle
• Make a great-circle chart for your office
• Try some unique graphs that help you and others
  in your office make wave forecasts easier!

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3) Shallow Water Effects Rip Currents

• HUGE CUSTOMER GROUP IN THIS ZONE
• SHOALING
• REFRACTION
• PERCOLATION/DAMPING
• REFLECTION

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Shallow water, where big transformations take
place!
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Vertical Distribution Of Water
Motion in Swell
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Shallow Water Affects

• The Coastal Impacts
• The Masses Affected
• Can be complex
• Can be extremely beach dependent
• You should know what happens
• Affects forecast timing

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WAVE SHOALING (100 MI W OF SAN
DIEGO)
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WAVE SHOALING

• Wave height change due to its interaction with
  the bottom
• Wave slows, but wave period is conserved
• Wave length shortens height increases, period is
  unchanged!
• Shoaling factor is between .5 and 2

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Shoaling Coefficient

• Ks sq root (Cgo/Cga)
• Cgodeep water group velocity
• Cgagroup velocity at depth a
• For breaking a is the depth of breaking
• (NOTE IF WAVES REMAIN IN SHALLOW WATER FOR
  SIGNIFICANT LENGTHS OF TIME WAVE DAMPING CAN
  BECOME VERY IMPORTANT)

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WAVE REFRACTION

• BENDING OF WAVE ENERGY DUE TO OCEAN CURRENTS, AND
  SHALLOW WATER
• BIGGEST EFFECT FOR LONGEST WAVE PERIODS. WHY?

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FIGURE IT OUT!

• DIRECTIONAL WAVE INFORMATION FROM A BUOY 100nm
  OFFSHORE INDICATES A WNW SWELL
• HUNTINGTON BEACH REPORTS A SW SWELL. WHY?

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REFRACTION

• WAVES ALWAYS BEND TOWARD SHALLOW WATER
• WAVES ALWAYS BEND AWAY FROM DEEP WATER
• WAVES ALWAYS BEND AWAY FROM MAX. CURRENT

• REFRACTION MAGNITDUES VARY GREATLY FROM ABOUT
  10 DEEP WATER WAVE HEIGHT TO 300 DEEP WATER
  WAVE HEIGHT

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Wave bends toward shallow water, energy is
concentrated
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WAVE REFRACTION
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Calculating Wave Refraction

• From Snells Law
• sin Aa /Ca sin Ao/Co
• Aodeep water wave angle to coast
• Codeep water wave celerity
• Aawave angle to coast at desired depth
• Ccwave celerity at desired depth

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Refraction Coefficient

• Depends on the magnitude of convergence or
  divergence of wave paths!
• Kr((1-sin2 Ao)/(1-sin2Aa))1/4

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WAVE REFRACTION!
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Shoaling, Refraction Breaking Waves
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BREAKING WAVE TYPE

• SPILLING BREAKERS
• PLUNGING BREAKERS
• SURGING BREAKERS
• COLLAPSING BREAKERS

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Modeled wave breaking by Liu
Pengzhi (Cornell U.)

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Breaker Types
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Shallow Water Wave Breaking
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Spilling Breaker
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Waves break in many shapes. Why?
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BREAKER DEPTH

• Breaker depth for a spilling breaker is defined
  as 5/3H (Hheight at breaking)
• The end of the pier is at a depth of 30 feet
• Significant wave heights are 15 feet
• Will they break before the end of the pier?

• First calculate breaker depth of H1/3
• Then for H1/10
• Then for H1/100
• Then for H max
• What is your answer?

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WAVE SETUP

• WATER RISE AT THE COAST DUE TO BREAKING WAVES ON
  THE BEACH
• PROPORTIONAL TO BREAKING WAVE DEPTH, THUS HEIGHT
  FOR GENTLE SLOPING BOTTOM

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WAVE SETUP

• KINETIC ENERGY OF ONSHORE WAVE ENERGY TO
  POTENTIAL ENERGY OF WATER SURFACE
• WAVE HEIGHT AND BREAKER DEPTH DEPENDENT

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WAVE SETUP

• RANGE IS FROM INCHES TO 10 FEET FOR SOME
  HURRICANES!
• COEFFICIENT IS APPROXIMATELY
• 0.12 BREAKER DEPTH

• A SPILLING BREAKER IS 30 FEET HIGH
• WHAT IS THE BREAKER DEPTH?
• WHAT IS THE MAX WAVE SETUP ELEVATION ON THE BEACH?

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Wave Setup Approximation

• S (g1/2 Hs2 T) / (64piBD3/2)
• where g gravity 9.8 (units in meters)
• Hs deep water significant wave height
• T wave period
• BD depth of breaking
• pi3.14159

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Wave Run-up

• R1/31.38HE (.77) Significant run up
• Rmax2.32HE (.77) Maximum run up
• Hbreaker height
• EB/sqrt(H/L)
• Bbeach slope (any units)
• Run up is less for fluffy, rapidly absorbing
  sande.g. rising tide

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WAVE REFLECTION

• RARE EVENTS
• MUST HAVE A WALL TO REFLECT FROM
• TYPICALLY ENGINEERED STRUCTURES AND BREAK WATERS

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Wave Reflection!

• Newport Beach
• Wedge gt35

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What do you see in this photo?
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RIP CURRENTS

• MUST HAVE
• BREAKING WAVES
• BEACH CURRENTS
• TIDE CHANGES
• TO GET RIP CURRENTS

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RIP CURRENTS

• Characteristics
• Danger
• Cause
• Locations
• Forecasts

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RIP CURRENT RESCUES
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Rip Current Process
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Rip Current Processes
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Rip Current Examples
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RIP CURRENTS
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RIP CURRENTS
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Rip Current Video
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RIP CURRENTS

• More likely as breaking wave height increases
• More numerous with shorter period breakers
• Stronger with longer period breakers
• Anchored by natural and man-made barriers

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Simplifying to extremes!
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Longshore Current

• V(mid breaker zone)1.17sqrt(gHb)
• sin (breaker angle)cos(breaker angle)
• Hbbreaker height, ggravity
• Try Hb5 meters, breaker angle15 degrees
• V1.17sqrt(105)sin(15)cos(15)
• V 2 m/sec

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Rip Current Index?

• Scale from 0-10 or 0-5
• Related to risk to swimmers/surfers/jet
  skiers/boaters/ fishermen

• Rip current frequency and strength depends on
• Wave height
• Wave period
• Wave angle to coast
• Coastal barriers
• Width of breaker zone

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Rip Currents

• Rip current spacing 3 times the distance of the
  surf zone?
• Surf zone width is directly related to wave
  height for any given beach beach slope
• Coastal irregularities account for more than 60
  of rip current longshore variability in frequency
  and about 30 variability in intensity

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Rip Currents

• Rip current strength is directly proportional to
  breaking wave height. Large waves cause strong
  rip currents.

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RIP CURRENT FORECAST?

• RIP CURRENT STRENGTH BREAKER HEIGHT (SCALE 0 -
  30)
• RIP CURRENT SPACING 3.0 BREAKER ZONE DISTANCE
• RIP CURRENT LOCATION RANDOM FOR STRAIGHT BEACH
  ANCHORED BY COASTAL OBJECTS
• RIP CURRENT FREQUENCY PULSES WITH BREAKER BEAT.
  CONSIDER IT CONTINOUSLY OCCURING FROM A FORECAST
  VIEWPOINT

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Long-term longshore (sand) transport rate

• Dependent upon mean breaker height squared!
• Q(cubic yards/year) 2 x 105 x Hb2
• Hb breaker height in feet

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Summary

• WOW shallow water is important
• WOW 99 of my customers are in shallow water
• WOW I better understand these simple shallow
  water concepts
• WOW I can make some real value-add to the model
  forecast near shore
• WOW no one should ever die from a rip current on
  my forecast shift!

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THE END

• Remember forecast for your customers, not for
  yourself
• Understand the basics and you will make superior
  forecasts

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Coastal Beach Erosion
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January 3, 2002 3.5 tide!
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Coastal Erosion Rate

• Shoreline change (meters/year)
• -.27(sea level rise mm/yr) 0.41
• Global sea level rise estimated to be 2mm / year
  over the past 100 years

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Coastal Subsidence Rates
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Summary

• I now have some sound bites for the media when
  they come around after a big beach erosion event!
• Gee, I can actually make some generic forecasts
  of the magnitude of the erosion potential for
  each wave event!
• Wow, I should practice these applications in my
  spare time

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Summary

• WOW I can think of 1,000s of wave/weather
  scenarios that this simple information can be
  applied to
• Gee, I better practice applying this and training
  the rest of the office or we are going to blow it
  during some big events!

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EVERYONE LIKES WAVES!
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What do you see in this photo?
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What do you see in this photo?
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What do you see in this photo?
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What do you see in this photo?
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What do you see in this photo?
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What do you see in this photo?
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Surfing a Tidal Bore
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Summary

• Man I feel like a pro surfer, I can now see
  things in wave photos I never knew about.and I
  have never even gone swimming in the ocean!
• Gee, maybe I should head down to the beach this
  weekend and see how much of all this I can apply
  to what I see! Ill take (a copy of) Lyons wave
  cheat sheet with me!
• THE END!

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3) How to analyze marine data
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Marine Wind Analysis

• Use STREAMLINES AND ISOTACHS!
• Pressure is rather worthless!
• Get good at analysis
• Get fast at analysis
• Synthesize data
• Be better than the model initial condition!!!

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Satellite Cloud Drift Winds

• Lo/mid/high levels
• Bias to cloud
• Uncertain height
• Must correct to
• surface wind speed
• From GOES, hence
• continuous coverage

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Use ALL tools available to you!

• Observations
• Numerical models
• Analyses
• Satellite imagery
• Satellite remote sensing tools
• Wave relationships
• Simple models

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Satellite Cloud Winds

• Correction to 10 meters seasonal!
• Correction dependent on air/sea temp difference
• Correction must include footprint size
• General correction factor 80
• Range or correction factors 65 - 100
  excluding strong sfc water current affects

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Interpreting Buoy Observations

• Significant Wave Height (H0) 7.9 ft
• Swell Height 7.9 ft
• Swell Period 14.3 sec
• Wave Steepness SWELL
• Average Wave Period 8.3 sec

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Interpreting Buoy Observations

• Wind NNW ( 330 deg ) 15.5 kts G19.4
• Wave Height 7.9 ft
• Dominant Wave Period 14 sec
• Air Temp 55.0 F SST 53.8

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88D Winds?

• Must be very close to radar due to beam elevation
• Must include modification for wind direction
  angle to inbound/outbound
• Know beam elevation angle as a function of
  distance for 1.5 degree elevation angle

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Wind Adjustment!

• Correction for height (10 meters)
• Correction for duration
• Correction for direction
• Gusts?
• Currents?
• Air-sea temperature contrasts?

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Wind Height Adjustment

• Log-wind profile with height
• Uz U/k x ln (z/z0)
• Kvon Karmans constant .4

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Interpreting Buoy Observations

• 2 am 1.9 kts
• 6 am 1.9 kts
• 10 am 3.9 kts
• 2 pm 5.8 kts
• 6 pm 11.7 kts
• 10pm 15.5 kts
• 2 am 15.5 kts

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Interpreting Buoy Observations

• What does this tell us about local conditions?
• What does this tell us about swell?
• What would you expect to see at the beach?

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Summary

• Define the wind and forecast it accurately, if
  not you cannot make good wave forecasts.
• Use ALL available data to define the wind field!