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Marine Meteorology > Chapter3 summery > Flashcards

Chapter3 summery Flashcards

1
Q

Waves form due to: (apart from waves generated by tsunamis)

A
  • The action of the wind blowing over the sea surface. which generate from
    • wind sea: local wind, in real time
    • swell or groundswell: wind that blew over the ocean surface thousands of km away up to several days before
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2
Q

Swell:

A
  • AKA surface gravity waves
  • Series of mechanical waves that propagate along the interface between water and air
  • Long wavelength (rarely more than 150m in Mediterranean)
    wavelength varies due to
    • Weather systems
      • Size,
      • Strength and
      • Duration
    • Size of water body
  • Wavelength varies from event to event (longer than 700m are a result of severe storms like tropical cyclones)
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3
Q

Swells and locally generated wind

A
  • Narrower range of frequency and directions than locally generated wind because
    • Swell waves have dispersed from their generation area.
      they have a more defined
      • Shape and direction
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4
Q

Swell direction

A

The direction from which the swell is coming. It is measured in degrees (as on a compass), and often referred to in general directions, such as a NNW or SW swell.

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5
Q

Causes of swell formation

A
  • Wind speed or strength relative to wave speed — the wind must be moving faster than the wave crest for energy transfer, stronger prolonged winds create larger waves
  • The uninterrupted distance of open water over which the wind blows without significant change in direction (called the fetch)
  • Width of area affected by fetch
  • Wind duration — the time over which the wind has blown over a given area
  • Water depth
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6
Q

The causes of swell formation determine

A
  • The size of the wind waves
    • Wave height (trough to crest)
    • Wave length (crest to crest)
    • Wave period (time interval between arrival of consecutive crests at a stationary point)
    • Wave propagating direction
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7
Q

Fully developed sea has the maximum wave size possible for a wind of a

A
  • Specific strength
  • Duration
  • Fetch
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8
Q

Exposure to that wind can cause

A
  • Loss of energy due to the breaking of wave tops
  • Formation of whitecaps
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9
Q

Wave in specific area have a

A

range of heights

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10
Q

Sea water waves is generated by

A

disturbances such as

  • Seismic events
  • Gravity
  • Crossing wind
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11
Q

Generation of wind wave is initiated by

A
  • Disturbances of cross wind field on the surface of the sea water
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12
Q

Mechanisms that explain how wind generate waves and perturbation on the surface ocean

A
  • Kelvin-Helmholtz instability (KHI)
  • Miles-phillips mechanism
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13
Q

Kelvin-Helmholtz instability (KHI)

A
  • Relevant to the generation of large wavelength purturbations
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14
Q

Kelvin-Helmholtz instability (KHI)

  • Relevant to the generation of large wavelength purturbations

Can occur when

A
  • There is velocity shear in a single continuous fluid or
  • There is a velocity difference across the interface between two fluids
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15
Q

Kelvin-Helmholtz instability (KHI)

  • Relevant to the generation of large wavelength purturbations

Can occur when

  • There is velocity shear in a single continuous fluid or
  • There is a velocity difference across the interface between two fluids

Example is:

A

Wind blowing over the water

  • The instability manifests in waves on the water surface. More generally
    • Clouds
    • Ocean
    • Saturn’s bands
    • Jupiter’s red spot
    • Sun corona
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16
Q

KHI require

A
  • Minimum wind speed of 6 m/s to make waves grow against the competing effect of
    • Gravity and
    • Surface tension
17
Q

Miles phillips mechanism

A
  • Describe how waves are generated from a flat sea using two mechanisms
    • First: produce small ripples “capillary waves”
    • Second: produce bigger waves “gravity waves”
  • Relevant to low wind speeds and short wave length perturbations
  • Involved a resonant interaction between the surface of the water and turbulent fluctuations in the air
18
Q

How waves are produced in general:

A

Air moving over the surface of the water transmits its energy to the water.

19
Q

Capillary and gravity waves:

A
  • Capillary grow from flat sea
  • Gravity wave form from sea containing capillary waves
20
Q

Capillary and gravity waves:

  • Capillary grow from flat sea
  • Gravity wave form from sea containing capillary waves

Reason behind the naming:

A

Because of the restoring force

21
Q

Restoring forces of capillary and gravity waves:

A
  • Gravity in case of gravity and capillary
  • Surface tension in case of capillary
22
Q

Swell dissipation for short waves

A
  • Stronger than long waves, this is why swells from distant storms are only long waves
  • Dissipation of waves with periods >13s is weak but significant at the pacific ocean scale
  • Long swells lose half of their energy over 20 thousand km to over 2 thousand km
    • This variation is a systematic function of swell steepness (due to friction at the air-sea interface)
      • Ratio of the swell height to the wavelength
23
Q

Swells are often created by

A

Storms thousands of nautical miles away from the beach where they break

24
Q

The propagation of the longest swells is limited by

A

The shoreline

25
Q

The propagation of the longest swells is limited by

The shoreline

Example:

A

Swells generated in the Indian Ocean have been recorded in California after more than half a round-the-world trip. This distance allows the waves comprising the swells to be

  • Better sorted
  • Free of chop as they travel toward the coast
26
Q

Characteristic of waves that are generated together by storm winds

A
  • Have the same speed
  • Group together and travel with each other
    • Others with slower speed will lag behind arriving late due to distance covered
27
Q

The time of propagation from the source t is proportional to

A

the distance X divided by the wave period T. In deep water it is t=4Pix/(gT)

where g is the acceleration of gravity

28
Q

How can you tell the distance at which swells were generated

A
  • This dispersive arrivals of swells,
  • long periods first with a reduction in the peak wave period over time
29
Q

Sea state

A

The effect that the local winds have on sea conditions

30
Q

Sea state is independent of

A

Travelling swell waves generated by winds outside of the local area

31
Q

Sea state is related to

A

The Beaufort scale which describes the state of the sea. The Beaufort wind force scale is an empirical measure that relates wind speed to observed conditions at sea

32
Q

Wind wave heights are significantly affected by

A

Local conditions, such as whether the wind is

  • onshore
  • offshore
  • fetch and its duration

Marine Meteorology (11 decks)

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