Post on 18-Dec-2015
Ch.9-12Science of Forecasting Waves
GNM 1136
Chapter 9 – Surfing in the Storm
Challenge
• When does NJ typically get the most windswell events?
• How about groundswell events?• How can you tell the difference between the
two types of swell?
Buoy Models
• Buoyweather.com• Weatherunderground.com
SMB Theory
• H. Sverdrup, W. Munk, C. Bretschneider developed the SMB Model in the Mid 1940s.
• Calculates wave height from values of wind strength, fetch and duration.
• Currently not in practice, but used academically.
Life of a Windswell
• Gutless Upper Trestles, CA• Windswells are swells that are generated by local winds within a
few hundred miles of the coast. As a result, the swell periods are short (four to 10 seconds between crests) and the waves tend to stack up on each other with very consistent sets.
• Also, a windswell's energy doesn't extend very deep -- maybe only 100 feet or so -- so it can only travel a few hundred miles before dissipating because most of the wave energy is concentrated closer to the ocean surface.
• Swellinfo.com
Groundswell• Groundswells are swells that are usually generated by winds much
farther away. These swells are created by strong winds over long distances -- we refer to this distance as the "fetch" -- and over longer periods of time. As a result, more energy is transferred into the water during the generation of the waves, which create longer swell periods. Waves with longer swell periods can travel great distances without losing the swell energy like windswells do.
• Furthermore, swell energy in groundswells can also extend down to around 1,000 feet deep. This allows greater interaction with the ocean floor when the groundswells move into shallow water. For example, this process of refraction will allow groundswells to wrap into a spot -- up to 180 degrees or more -- while windswells wrap very little if at all.
Windswells of the Mediterranean• http://www.metacafe.com/watch/fu-1556072
136/on_surfari_italy/
• http://www.youtube.com/watch?v=7I95V7Df9CY&feature=related
What happens when the wind stops?
• Fig. 9.3• Gradual drop in size after storm dissipates.
Selective Refraction
• Different wavelengths are refracted to different degrees.
• Longer wavelengths bend more than shorter ones.
• What type of swell is better for flat bathymetric beaches? Peaky or long?
Chapter 10 - Local Winds on the Coast
Challenge
• What time of day does the Sea Breeze occur along our coast in the summer? Why?
• How does the Sea Breeze affect surfing conditions?
Local Winds
• How to read a weather map – NOAA• Surface Observations• Sailflow.com
Sea breeze Info
• Rutgers Data – Sea Breeze & Coastal Upwelling• Rutgers COOL Room
The “Sea breeze”
Opposing sea breezes meeting over Cuba forming a line of cumulus clouds.
Can this happen at night?
Ch.11 Water Temperatures
Challenge• What controls the ocean water temperatures?• What major currents control the ocean temperature off NJ?• What is the range of ocean water temp in NJ?• What causes coastal upwelling?
Ocean CurrentsWhat patterns do you notice?
Explain what happens to the water off of NJ.
• Labrador Current• Gulf Stream• NOAA – Water Temps
Layers of the Ocean
Thermocline
Coastal Upwelling
Coastal Upwelling
• Biogeochemical impact of summertime coastal upwelling on the New Jersey Shelf
• http://www.coolclassroom.org/cool_projects/tutorials/upwellingtutorial.html
Is there Upwelling?
Ch. 12 - Tides
Challenge
• What controls the tides?• How important are tides when surfing?• Are the tides the same all over the world?
Explain.
Tides
• http://www.srh.noaa.gov/jetstream/ocean/tides.htm
• NJ Tide Chart• Nor’East Tides• SaltwaterTides.com
Tide Apps for Smartphones
• Tide Graph• Shralp Tide• Mobile Geographics• tideApp
TIDES AND THE FORCES THAT GENERATE THEM
• Tides are periodic, short-term (6 hour) changes in the height of the ocean surface caused by the gravitational force of the moon and sun, and the motion of the earth
The Moon and Tractive Forces
• A balance between gravitational forces pulling earth/moon together and inertial forces that hold them apart – Inertia is sometimes referred to as centrifugal
force – Earth-moon system rotate around the earth-moon
center of mass – Earth is 81 times more massive than moon
The Moon and Tractive Forces
• High tides are the crests of really large 10,000 km wavelength stationary waves – Low tides are the trough of really large 10,000 km wavelength
stationary waves – The earth rotates beneath the wave which causes apparent motion – Lunar tides complete their cycle in a tidal day (24 hours 50 minutes) – Moon rises 50 minutes later each day – Moon doesn't orbit the earth in the plane of the equator – Causes bulges (tides) to follow the actual position of the moon into
northern and southern hemisphere
The Sun's Role
• Sun is 27 million times more massive than moon
• Sun is 387 times further way from earth than moon
• Net effect is that the sun's influence on tides is a bit over 1/3 that of the moon
• This is called solar tides
Sun and Moon Together • Spring tides (very high tides) are
when the alignment is either moon-earth-sun (full moon) or earth-moon-sun (new moon)
• Neap tides (hardly disturbed tides) are when there is a 90° angle between the moon-earth-sun system – Tides are pulled in four
directions
Tidal Patterns, Amphidromic Points
• Ocean depth isn't the only effect; continents also get in the way of tides
• Tides are somewhat similar to seiches in that water sloshes around in ocean basins
• Semidiurnal tides are twice a lunar day tides • Diurnal tides are once a day tides • Mixed tides if there is significantly different heights
between tides
Tidal Datum
• Tidal datum is the reference point that tidal height is compared – Zero point on tide graphs
• Not always mean sea level – Average ocean surface over several years time
Tides in Confined Basins
• Tidal range is the difference between high-water and low water height – Largest tidal ranges are in largest oceans – Especially in bays or inlets that concentrate tidal
energy
Tidal Currents
• Tidal currents form when water flow into or out of bays and harbors – Morro Bay – Golden Gate can reach 3 m/sec or 7 mph
• Flood Tide is a rise in sea level as a tide crest approaches
• Slack Tide is when there is no current • EBB Tide is when water is exiting the water
body (EBB = Exiting Barnegat Bay)
Predicting Tides
• About 140 tide-generating and tide-altering forces and factors
• Makes predicting tides extremely difficult • Most tide chars rely on past records and don't
really predict tides
Datum Plane
• MLLW most typical• MLW• MSL• MHW• MHHW
Tides Around the World• Semidiurnal Tides• Semidiurnal Mixed• Diurnal
• Predicting the Tides!
Why are there different tidal patterns?
Extreme tidal fluctuations
• The highest tides on planet Earth occur near Wolfville, in Nova Scotia's Minas Basin. The water level at high tide can be as much as 16 metres (45 feet) higher than at low tide!
Tidal Bore• A tidal bore (or just bore, or
eagre) is a tidal phenomenon in which the leading edge of the incoming tide forms a wave (or waves) of water that travel up a river or narrow bay against the direction of the current. As such, it is a true tidal wave (not to be confused with a tsunami