Ocean Waves & Sea-State Forecasting Flashcards
Define currents
- Movement of water in the main water bodies of the earth
- Affected by cold and warm sources
- Modified by Coriolis Force
- Conform to Continental boundaries
What is a key role of currents?
- Play a key role in warm water transport poleward, cold water transport equatorward
What are currents largely driven by?
Subtropical Highs
Describe the track of global surface winds
- Progress Equatorward => poleward E of the E coast
- Transported Equatorward W of W coast
=> cools by upwelling
Winds and wind stress ___ water initially in the ____ of the wind
push
direction
Coriolis force deflects the water to the ____ at the ____
right
surface
Net current is about ___ deg to the ____ of the wind
45 deg
right
PGF in the ocean is commonly dependent on ____
The depth of the ocean
What influence does PGF have in the ocean?
The balance between water pressure and Coriolis force aligns the current closer to the wind direction
Ocean currents ___ the ____ of global oceans
contort
topography
Where does a max of ocean dynamic topography exist?
Off the E coast of Asia (due to location of warmest water)
Sub-Surface Currents are driven by what two things?
- Continental boundary effects
- Thermohaline effects (sinking of cold and salty water; rising of fresh and warm water)
What are some examples of Continental boundary effects?
- large scale upwelling
- downwelling
What are some examples of Thermohaline effects?
- sinking of cold and salty water
- rising of fresh and warm water
- (is driven by topography)
What are 4 examples that demonstrate the complexity of currents?
1) Sharp gradients and current shear
2) Current acceleration by gap flow
3) Warm clockwise eddy
4) cool counterclockwise eddy
What is an important local current feature?
Gulf Stream Loop Current
What is the important role of the Gulf Stream Loop Current?
- Creates local maximum in water temperature
- is an important energy source for tropical storms
Name two effects of coastal winds
1) Upwelling
2) Downwelling
Name two effects of tides
1) lunar and solar effects
2) Ebb and flood
Upwelling is _____ escape
Mass
Name 4 impacts of currents
1) Heat transport and enhancement of ocean storms
2) Enhanced ocean waves when winds are opposed
3) Coastal water cooling/warming (low clouds, fog, convection)
4) Ship routing decisions
What are three factors in Wave Formation?
1) Wind strength
2) Fetch
3) Duration
Describe the 3-step process of ocean wave formation by WIND
1) Microscopic capillary waves
2) Slight ripples are pushed upward by wind
3) Wave height increases based on speed and duration
_____ energy of air creates _____ energy in the water
Kinetic
Potential
What is the equation for Kinetic energy?
1/2 m(air) * V^2
What is the equation for Potential energy?
m(water) * g * H
There is a ______ between wave height and velocity squared
proportionality
What is the equation for wave height?
H (ft) = 0.02 * V^2
What are other factors that affect energy transfer when creating potential energy in the water?
- wave shape
- steepness
- wind drag/shadowing
What is the equation for fetch to attain a fully developed sea (FDS)?
Fetch (nm) = 3.17 * Wind^(4/3) (kts)
What is the equation for fetch for a Significant Wave height (H) (ft)?
Fetch for H (nm) = 43 * H^(2/3)
The equations for fetch are _____ and assume a totally ______ sea
theoretical
unperturbed
______ have far more incipient wave activity
shorelines
Because shorelines have more incipient wave activity, the result is that waves grow _____, and require ____ distance to grow
grow faster
less distance
The equations for fetch are not useful for ___ or _____
lakes
bays
Wave breaking tends to occur in ____ water
shallow
With duration, the wind over a particular patch of water must blow for an _____ period to generate _____ waves
extended
largest
Biggest waves form in ____ moving storms
slow
____ moving or ____ windstorms (squalls) don’t linger long enough to generate significant wavese
fast-moving
brief windstorms
What is the equation for wind duration for a fully developed sea?
Time (hrs) = 6.4 * wind^(1/3) (kts)
= 12.3 * H^(1/6) (ft)
Waves are seldom _____ and are best understood ______
uniform
statistically
Any water perturbation is made up of several ____ waves (i.e.: _____ series)
periodic waves
Fourier series
To calculate energy/unit area, we really need a ____ term
rho-g term
What is the significant wave height?
The average waves in the upper one-third of waves to be encountered
Hs (ft) = 0.02 * Wind^2 (kts)
A wave system is also a group of waves of various ____ or _____
periods or lengths
______ is the most often-used wave forecasting parameter
significant wave height
Hs is useful for mariners because it favors the _____ waves that might be encountered allowing for better preparation
larger waves
Average wave height is ____% of significant wave height
62%
There is a ____% chance of getting a wave twice as high as the Hs
1%
A typical ocean wave is made up of a series of individual ____ ____
wave lengths
Why are distant storms evidenced by long period swells?
Once formed the wave starts to disperse with the longer wave lengths traveling faster
______ interaction occurs between waves
Nonlinear
constructive and destructive interference
____ affects the speed of waves
gravity
______ _______ is how waves dissipate energy
wave dispersion
Real ocean waves come from _____ swells
superimposed
What are two characteristics of superimposed swells?
1) they are rarely regular and orderly
2) they are more like oscillating hillocks
Define a rogue wave
height estimate > twice the Hs
When will a wave break?
If its height is approximately 10% of its length
What is the equation for wave length?
L = 0.15 * wind^2 (kts)
Near shore, waves break if H > ___% depth (h)
80%
What are 4 dangerous wave situations?
1) strong slow-moving lows
2) lows moving at certain speeds
3) currents with wave motion opposed
4) waves near land - breaking
With a slow-moving low, waves _____ for maximum threat
superimpose
Define dynamic fetch and group (energy) velocity
Two or more wave lengths combine, each moving at a different velocity
=> With constructive and destructive interference
Dynamic fetch and group (energy) velocity forms a wave packet that moves at ______ the average speed of the waves, or ____ times the wind speed.
This forms wave ___ for surfing
one-half
0.6 times
wave “sets”
If a storm moves at a speed, then energy maximum will stay in wave generation zone for many hours. This is called ____
Dynamic Fetch
Dynamic Fetch requires what?
relatively slow moving lows
Describe the process of ocean waves with currents
- ) wind opposing current; growth faster
2. ) water piles up much faster, wave height builds quickly, and the waves are steep
On a lake or bay, waves happen ____ and grow ____
sooner
grow faster
What is the equation for wave height on a lake or bay?
H = 0.020 * wind^2 * (fetch/Ffds)^(1/2)
= .011 * wind^(4/3) * fetch^(1/2) (nm)
What is the equation for wind duration for fully developed waves?
Time (hrs) = H (ft)
_____ waves form on ____ ____ density discontinuities
Internal
sub-surface
Internal waves ____ wave drag on surface ships
Increase
What is a source of Ocean Wave Climatology?
European Center for Medium Range Forecasting
Describe the European Center for Medium Range Forecasting
- Mean and standard deviation of significant wave height, and also wave period
- Based on 45 years of ship reports and wave model
- Is Global
- and by month
Describe Wave Watch III
- used by both FNMOC and NCEP
- Driven by Global and Regional forecast models
- solves spectral balance equations for wave frequencies for a number of propagation directions
- takes into account currents, refraction, breaking, and ice coverage
Wave Watch III has no _____ ____ _____, but uses 12-hr wave forecast from previous run
wave data assimilation
How often doe the Wave Watch III run?
Every 6 hours out to 6 days
What effect does friction have on the geostrophic wind?
- Forces in Balance
- Slows the wind
- Reduces Coriolis Force
- Wind “pulled” toward lower pressure
Wind is reduced by ___% and backed ___ degrees due to friction
reduced 30%
backed 30 degrees
