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Science of Forecasting Waves GNM 1136

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Challenge Think-Pair-Share What controls the weather on Earth? What type of climate is NJ? What are the prevailing winds in our latitude?

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Ch.1 The Paradox of Impossible Knowledge How accurate is a surf forecast or prediction? Each of our reports are different. i.e. Team Delusional While most surfers are scientists, few scientists are surfers. Increasing your knowledge will increase your wave count!

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The Beginning Where does the energy to build waves ultimately come from? How are we (human race) altering this energy? You need to understand climatology first! What factors will affect the surf? Quote from page 14

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Ch.2 Large-Scale Weather Patterns

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Amphidromic Points

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The highest tidal range in the world occurs in the Bay of Fundy, Nova Scotia, Canada. Tidal range = 15 m / 50 feet The highest tidal range in the world occurs in the Bay of Fundy, Nova Scotia, Canada. Tidal range = 15 m / 50 feet

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The Global Circulation

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Global Circulation Statistical highs and statistical lows uneven heating of the Earths surface What happens to the atmosphere after the Earths surface has been heated up initially by the suns rays?

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Earth A landless, stationary Earth would have lots of convection currents (fig 2.1) A landless, rotating Earth would have lots of spiraling wind bands(fig 2.2) A landless, rotating Earth with seasons would have lots of change due to varying temperatures (fig 2.3) Obliquity of the ecliptic = tilt of the Earth 23.5 degrees = seasons A rotating Earth with land, sea and seasons Heating differentials Specific Heat Capacity Variations in land vs. water temps in summer & winter

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Earth continued Which hemisphere has more land mass? Southern Hemi has Roaring Forties (fig 2.4) January vs. July When would you plan a surf trip to Indonesia?

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Earths Seasons (Fig 2.3)

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Coriolis Force Discovered by French physicist Gustave Gaspard Coriolis in the early 19 th century. This phenomenon dominates the entire physical oceanographic world. Forms the rotation of high and low pressures Which direction does Coriolis effect push particles in the Northern Hemi? Sothern Hemi?

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Challenge 1. What causes the swirling pattern of a low pressure system? 2. Which direction does a low pressure spin in the Northern Hemisphere? Formation of a Low Pressure (Fig. 2.7)

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High Pressure Systems (Anticyclones) Areas of sinking air which result in high pressure are called anticyclones. The changes in pressure are much smaller than those associated with low pressure. High pressure systems cover huge areas, the rate at which changes occur is slower and the weather patterns usually more stable. The weather conditions depend on the time of year In Summer Hot days with few or no clouds but there can be mist in the mornings. Light winds. Warm moist air rising from the ground may result in thunderstorms. Some cloud covercan form over Eastern England. This is caused by light winds blowing over the cooler North Sea. In Winter Cloudless skies but the low angle of the sun produces less heat days arecold and the nights even colder due to lack of cloud cover. Fog and frost often form at night. Cold air from Russia can bring snow to the east of the country. High Pressure (Fig. 2.8)

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In an effort to equalize the pressure, air tries to flow directly from a high pressure to a low pressure, but the Coriolis force causes it to turn to the right. As a result, the air circulates clockwise (anticyclonic) around a high pressure and anticlock 2009 - Dr. Tony Butt

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Coriolis Effect Coriolis Force Video Clip

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Centrifugal Force Univ. of Virginia Site Univ. of Virginia Site

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Ch.3 The Formation of a Depression NASA GRIP Hurricanes 101 Hurricanes Hurricane Irene

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Low Pressure Formation (fig 3.3) Fronts rotate around a Low pressure system in an anti-clockwise direction. Both are associated with a change in temperature, cloud, a lot of rain and a shift in wind direction. The diagrams below show the formation of an occluded front. When a depression or a low pressure system forms, it usually consists of a warm front and a faster moving cold front. This can be seen in diagram 1. To the north of the warm front is the cool air that was in the area before the depression developed. As the depression intensifies, the cold front catches the warm front. This can be seen in diagram 2. The line where the two fronts meet is called an occluded front. When an Occluded Front passed overhead, you would feel changes in temperature and wind speed Fig. 3.1-3.3

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Isobars

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Explosive Cyclogenesis (Fig. 3.4-3.6) What is cyclogenesis? Basically, the term cyclogenesis means the creation of cyclones (or depressions, as we better know them). This is in the same way that frontogensis is used to describe the creation of weather fronts. All Atlantic depressions have some form of cyclogenesis, ranging from wave depressions, to the modifications of ex- hurricanes. In the interests of this discussion, Im going to concentrate on those depressions which develop from frontal waves, as I reckon theyre the ones which are the most captivating. What is explosive cyclogenesis? This is basically the same as normal cyclogenesis, but it happens very rapidly, and very vigorously. That is, a depression can seemingly form from nothing in a very short space of time, often becoming a very vicious storm in a matter of hours. The general term used for these rapidly deepening depressions, is a bomb. The strict definitions of this, is a low pressure centre which deepens by 24mb in 24hrs, or less. These happen frequently throughout the Atlantic, and several times a year, we see this happening pretty close to the UK. Its only in recent years that such things have been better understood, and indeed, before the 1987 storm across southern England, meteorological models hardly had the signature of explosive cyclogenesis incorporated within them.

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Jet Stream Influence Above 5,000m (6-9 miles) very strong winds Influences the path and speed of storms Dr. Jeff Masters Blog on Hurricane Sandy

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Box 1 The geostrophic equation Windspeed = 1/Coriolis * Pressure Gradient Since coriolis forse increases with disctance from ecuator, for the same pressure gradient, windspeeds will be greater near the equator. Therefore, the same low will have stronger winds at lower latitudes.

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Ch.4 The Growth of Waves On the Ocean

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Challenge Think-Pair-Share What creates waves? Who benefits from wave models?

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The Miles-Phillips Theory J.W. Miles and O.M. Phillips were legendary oceanographers that helped shed light on wave formation in 1957. Two parts waves created on a flat sea then build on each other.

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Wave Anatomy

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Wave Formation Linear growth, Exponential growth, saturation

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Capillary Waves Tiny bumps form on a completely flat sea from wind blowing on or over the surface. Hairdryer on the water (fig 4.1)

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Exponential Growth Turbulent Eddies follow wave and help them grow (fig 4.2) As Capillary waves get larger there is more friction for the wind to grab the water and make them bigger. No longer capillary waves, they are gravity waves because GRAVITY is the restoring force. Restoring force for capillary waves = surface tension.

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Limiting Factors Struggle between wave height and gravity limits overall size (height). White-capping (white horses) steals energy Duration-limited sea = the wind stops (storms fades) before waves reach max size. Smaller water bodies experience fetch-limited seas. If water body is large enough and winds are strong enough then a FAS (fully arisen sea) occurs = Saturation (fig 4.3).

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Models Oceanographer Walter Munk help develop first mathematical models during WW2 (1940). Empirical Models = relating wind strength, fetch & duration, directly to wave height After Miles and Phillip developed ideas in 1957, the radiative transfer equation (active balance equation) was created to determine how big waves would be factoring in energy loss and gain = 1G wave models. 1973 Joint North Sea Wave Experiment (JONSWAP) waves grow similar in different conditions (2G model). WAM Model was created in 1988 which revolutionized swell modeling and predictions 3G Model). Fig. 4.4 shows a WAM Model called Wavewatch III from the Fleet Numerical & Oceanographic Center (FNMOC).

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WAM Model

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Ch.5 Propagation of Free-Travelling Swell (Fig 5.1 & 5.2) Waves propagate from the storm center There are several factors that determine the size and duration of the swell: When waves travel they can experience: Circumferential Dispersion get smaller & spread out. Radial Dispersion Some speed up faster than others. Grouping form into sets of waves.

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Movement of Wave Energy *Watch the water droplet move in a vertical circle as the wave passes. The droplet moves forward with the wave's crest and backward with the trough.*

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Swell Energy (Fig 5.3) Swell travels in deep water differently than shallow water.

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Wave base is the depth to which a surface wave can move water. If the water is deeper than wave base: orbits are circular no interaction between the bottom and the wave. If the water is shallower than wave base orbits are elliptical orbits become increasingly flattened towards the bottom. 7-2 Wave Motions

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Deep- and Shallow-Water Motion

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There are three types of waves defined by water depth Deep-water wave (d>or=1/2 of L) Intermediate-water wave (d>1/20 and

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The shallower the water, the greater the interaction between the wave and the bottom alters the wave properties, eventually causing the wave to collapse. Wave speed decreases as depth decreases. Wavelength decreases as depth decreases. Wave height increases as depth decreases. Troughs become flattened and the wave profile becomes extremely asymmetrical. Period remains unchanged. Period is a fundamental property of a wave. Celerity equation of shallow water wave. 7-3 Life History of Ocean Waves

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The Strange Characteristics of Swell Stokes Drift - The slight forward displacement of water because the top of the wave moves faster than the bottom (fig. 5.4) Group Speed Carries a message from one point to another. Each individual wave in the group travels at twice the speed of the group = phase speed Waves move from the back to the front and disappear (fig. 5.5)

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Dispersion Circumferential Dispersion Waves get smaller as they spread out from storm center. Fetch and storm size plays a very larger role Storms are not point sources i.e. If a storm sits just south of Iceland in N.Atlantic swell will reach Ireland (1,000km) at 5m (15ft.) and become breaking waves giving up their energy to the reefs and beaches. Lots of swell will pass Ireland and head 2,000km away towards Spain, with a wave height 30% less (3.5m). Other waves reach the Canary Islands (another 2,000km away) and lose more size (30%) to 2.3m. (fig.5.6 & 5.7) Note that the Pacific storms may have considerable differences in swell decay. Why? http://magicseaweed.com/North-Atlantic-Winter-Exposed- Content/2343/ http://magicseaweed.com/North-Atlantic-Winter-Exposed- Content/2343/

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Dispersion Radial Dispersion waves speed is controlled by wavelength. i.e. longer wavelength = faster (tsunami) Waves all start from one point (storm center) and the faster waves move to the front = Radial Dispersion North Atlantic example - waves will appear confused closer to the storm center and settle out the further away they get (fig. 5.8)

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Grouping Eventually the waves organize into sets = grouping Why do surfers want to know how many waves in a set? How far apart are the sets = settiness? http://www.surfline.com/community/whoknows/who knows.cfm?id=1120 http://www.surfline.com/community/whoknows/who knows.cfm?id=1120

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Interference When wave trains interfere with each other two things can happen Constructive or Destructive Interference (fig 5.10) Constructive Intereference = bigger peaks Destructive Interference = no peaks Overall, the mechanism of wave trains/sets is still somewhat of a mystery. It is hard to tell exactly what is happening in every part of the storm at all times.

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Wave interference is the momentary interaction between waves as they pass through each other. Wave interference can be constructive or destructive.

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7-3 Life History of Ocean Waves A Rogue wave occurs when there is a momentary appearance of an unusually large wave formed by constructive interference of many smaller waves.