{"id":5011,"date":"2010-07-26T14:21:52","date_gmt":"2010-07-26T14:21:52","guid":{"rendered":"http:\/\/www.labri.fr\/perso\/barla\/blog\/?p=5011"},"modified":"2012-07-09T16:04:15","modified_gmt":"2012-07-09T16:04:15","slug":"surf-science-an-introduction-to-waves-for-surfing","status":"publish","type":"post","link":"https:\/\/www.labri.fr\/perso\/barla\/blog\/?p=5011","title":{"rendered":"Surf Science, an introduction to waves for surfing"},"content":{"rendered":"<p id=\"top\" \/><em>Tony Butt, Paul Russell and Rick Grigg.<\/em><\/p>\n<h3>Large-scale weather patterns<\/h3>\n<ul>\n<li>The effect of the Coriollis force will have caused the large circulation cells mentioned above to be &#8216;short-circuited&#8217; &#8211; the result being six spiralling wind-bands, three in each hemisphere. The air around 60 deg north and south is continually rising, spiralling upwards. This constant sucking of air away from the surface will reduce the surface pressure locally, which is why the formation of low pressure systems is favourable between 40 deg and 70 deg latitude. We now have a rotating, water-covered planet with alternate low- and high-pressure belts around its surface, and a weird, three-dimensional spiralling circulation pattern. &#8211; P.17-18<\/li>\n<li>Remembering that air movement is due to uneven heating, we can see that this movement will be massively enhanced during the winter months, due to the extra temperature difference. Those spiralling bands will be workmg overtime to give us more wind, deeper lows, and bigger surf in winter. &#8211; p.18<\/li>\n<li>So summer means warm land, not-so-warm sea. And winter means cold land, not-so-cold sea. The east-west pressure variation, due to the presence of land, is much more noticeable in the Northern henisphere, simply because it contains more land. The Southern hemisphere suffers a lot less from seasonal variations, and its circulation patterns are much simpler. Typical characteristics of the Southern hemisphere are the roaring forties &#8211; a band of westerly winds that blow continuously around the globe between 40 deg and 60 deg south (associated with that 60 deg low-pressure band). &#8211; p.20<\/li>\n<li>Therefore, whichever way you push the air parcel, by a combination of two different mechanisms, it will swerve around to the right. It should now be clear that the Cariollis force is absolutely fundamental in the world of meteorology and oceanography. It not only makes highs and lows cascading effects, it is the reason why ocean currents bring cold, polar water to west-facing coasts. &#8211; p.22-23 <em>The two effects are variation of centrifugal force (for east-west)<\/em> and lag\/swerve <em>(for north-south)<\/em><\/li>\n<\/ul>\n<h3>The formation of a depression<\/h3>\n<ul>\n<li>The position of the polar front coincides with that general band of relatively low-surface pressure in the large-scale circulation patterns. It is the boundary between two circulation cells, where the surface air is flowing in opposite directions. Through a particular combination of circumstances, a disturbance may appear at some point along the front. For example, the north-south air temperature difference may be particularly intense at this point, or there might be some influence from an &#8216;external&#8217; factor like the sea surface tempereature. Such a disturbance is known to meteorologists as baroclinic instability. &#8211; p.25-26<\/li>\n<li>This section of the polar front can now be seen to split into a system of individual fronts &#8211; the warm front, behind a which is warm air, and the cold front, behind which is cold air. Between these two fronts is the warm sector &#8211; an area where the surface winds are strong and blowing in the same direction for some distance &#8211; the best conditions for rapid wave growth. As the depression propagates from west to east, the warm and cold fronts eventually catch up with each other, forming an occluded front. This is when the low starts to weaken and, ultimately, maybe after spawning a few peripheral systems, lose its identity altogether. &#8211; p.27<\/li>\n<li>The movement of a low pressure has a lot to do with the flow of air about 5,000 metres up &#8211; the jet stream. &#8211; p.28<\/li>\n<\/ul>\n<h3>The growth on waves on the Ocean<\/h3>\n<ul>\n<li>It is generally agreed that there are probably two mechanisms involved. The first starts by producing small waves from a completely flat sea, and then, once these small waves have established themselves, the second mechanism can take over. The second mechanism goes to work on the small waves to make them into bigger ones, and then makes the bigger ones into even bigger ones, until some kind of limit is reached whereby they cannot grow any more. &#8211; p.32-33<\/li>\n<li>As soon as some of these little capillary waves start to exist, the surface becomes ruffled. This modifies the air over the surface, producing larger vortices called turbulent eddies. These new vorices, instead of being random, are linked to the waves themselves and automatically follow them along. The waves will grow because the vortices increase the pressure over the troughs of the waves and decrease the pressure over the crests. As long as the wind blows over any existing waves, the size of the turbulent eddies will increase, which makes the waves grow even more, which makes the eddies even bigger &#8211; and so on. The waves are no longer capillary waves; they are now called gravity waves so-called because their restoring force is gravity. The restoring force is that which restores the sea surface to its original position after it has been lifted by the air motion. With capillary waves, the restoring force is not gravity, but surface tension &#8211; the &#8216;skin&#8217; that is always present on any water surface. &#8211; p.34<\/li>\n<li>When the waves get to a certain height, a balance is reached between the generating force (the wind) and the restoring force (gravity). In other words, the wind cannot push them up any higher, because gravity keeps pulling them down. p.35\u00a0 <em>White caping, duration and fetch are other limiting factors.<\/em><\/li>\n<\/ul>\n<h3>Propagation of free-travelling swell<\/h3>\n<ul>\n<li>If you looked at any floating object from the side, you would see it do a complete circuit every time a wave passed through, ending up in more or less the same spot. Because wave\u00a0 speed depends on depth, the top of the wave goes faster than the bottom. Eventually, the wave topples over and It breaks, but before this happens, the orbital motions change slightly, so that\u00a0 the top part of the orbit is faster than the bottom part. The result is a slight forward displacement of water, known as Stokes drift. &#8211; p.41<\/li>\n<li>This is what happens with waves in a group. Waves are generated at the back of the group, move through the group to the front, and then disappear. The swell becomes less concentrated as it travels, so the waves get smaller. This is circumferential dispersion. In the Indian and Pacific oceans, fetches tend to be wider than in the Atlantic, therefore the reduction in waves&#8217; height with distance is even less. &#8211; P.42-43<\/li>\n<li>As the swell begins to propagate away from the storm centre, the various different wavelengths begin to sort themselves out, the longer, faster ones\u00a0 racing out in front, and the shorter, slower ones lagging behind. By the time the swell is a long way from the storm centre, the longer waves have made their way right out in front, and the shorter ones have been left way behind. &#8211; p.43 <em>The more distant, the more ordered and clean swell.<\/em><\/li>\n<li>The first waves to arrive would have more punch to them, because they would be faster, but the biggest ones normally arrive a short time later. After the swell has peaked in size, the waves do not have the same punch as they did before it peaked. Right at the end of the swell, when the very last waves are coming in, they are normally relatively weak. Sometimes the swell peters out quickly, not lingering too long, especially if something has happened along the propagation path to remove the shortest waves. Often these waves are eliminated, for example by opposing winds in the propagation path, or they might be affected by whitecapping (white-horses), Short waves tend to be affected much more by both these phenomena, because they are steeper and &#8216;stick up&#8217; more from the sea surface. This is especially noticeable near the storm centre. &#8211; p.45<\/li>\n<li>If conditions are small and crowded, then the ideal situation is as many waves in a set, and as little time between sets as possible. However, this is exactly what you do not want if it is big. Normally, the more dispersed the swell &#8211; in other words, the further you are from the storm centre &#8211; the more opportunity the group-forming mechanism has to work on the waves. Very close to the low pressure, sets are hardly noticeable. &#8211; p.46 <em>Mixed-up swell may be due to different storms or one moving storm (stable isolated storms with large fetches produce cleaner swells)<\/em>.<\/li>\n<li>Where the peaks or\u00a0 troughs coincide, a bigger peak or trough will result &#8211; called constructive interference; and where a peak and a trough coincide, the whole thing will cancel out altogether &#8211; called destructive interference. The resultant wave train will look like a primitive wave group. &#8211; p.47<\/li>\n<\/ul>\n<h3>Refraction<\/h3>\n<ul>\n<li>Refraction is the bending of a wave as it propagates over different depths. When one part of a wave travels more slowly than another, the wave bends towards the slower part. p.51<\/li>\n<li>The longer the period &#8211; the time between one wave and the next &#8211; the greater the refraction. In a new swell, where the long period waves arrive first, refraction has a more profound effect than it does later on in the life of the swell, when the shorter period waves arrive. This is very useful for detecting whether there really is a new swell just starting, or it is just a figment of your imagination. The reason why refraction is period-dependent is because longer waves produce larger water motions, which &#8216;reach down&#8217; further beneath the surface. This means that longer waves feel the bottom, and hence start to slow down, before shorter ones. p.57<\/li>\n<li>The most basic concepts are of focusing and defocusing &#8211; concave or convex refraction &#8211; which either concentrates the wave energy towards one spot, or spreads it over a larger area. p.57<\/li>\n<\/ul>\n<h3>Wave-breaking<\/h3>\n<ul>\n<li>The action of the bed is felt more at the bottom of the wave than the top, and therefore slows down the bottom more than the top. This effect is progressively more pronounced as the water gets shallower, so a point is reached when the top of the wave overtakes the bottom, making it spill forward and break. The wave is &#8216;tripped up&#8217; &#8211; just as you might be tripped up by someone as you walk along &#8211; making the bottom half travel more slowly than the top, so that you fall over head first. p.61-62<\/li>\n<li>One major factor that the breaking depth &#8211; and hence the wave profile &#8211; depends upon, is the suddenness with which the bottom topography changes as the wave propagates towards the shore. As we have seen, the wave profile depends mainly upon the deep-to-shallow water transition and how radical this is. But it is also highly influenced by (a) the speed at which the wave approaches; (b) the steepness of the waves before they break, and (c) whether the wind is onshore or offshore. Short-period waves are closer together than long-period ones, so they are quite steep already, and almost ready to break. So waves from a local storm, or an old dying swell, tend to break further out, and to be quite slow and mellow, whereas waves from a distant storm, or the first waves in a growing swell, are further apart and less steep, and have a longer wave length. &#8211; p.62-64 <em>Good criterion to connect types of swell to breaking types!<\/em><\/li>\n<li>An offshore wind physically holds up the wave, and therefore delays breaking until the wave gets into shallower water. That is why there are more tubes when the wind is offshore. &#8211; p.64 <em>Good criterion for incorporating wind in breaking type!<\/em><\/li>\n<li><em>Four types of profiles: spilling, plunging, collapsing and surging. <\/em><\/li>\n<li>A spilling breaker has a profile very similar to what it had in deep water, the front of the wave being not much bigger than the back. This is what you might find at a very flat beach with small, onshore surf. The plunging breaker transforms its profile radically as it comes into shallow water, the front of the wave sucking out below sea level. This makes the front of the wave much bigger than the back. &#8211; p.65 <em>Use Iribaren number (p.67) to interpolate between different wave profile drawings? Should take wind in consideration as a second axis&#8230;<\/em><\/li>\n<\/ul>\n<h3>How waves move sediments<\/h3>\n<ul>\n<li>Velocity skewness is the key to sediment transport, and the starting point for the formation of sandbars. According to this simple rule, a sandbar will form more or less under where the waves are breaking. &#8211; p.71<\/li>\n<\/ul>\n<h3>Surfing in the storm<\/h3>\n<ul>\n<li>The major effect of an offshore wind is to &#8216;clean up&#8217; the surf. It does this by attacking the short waves, eliminating them like weeds in a garden, and leaving the cleaner, more desirable, long waves. &#8211; p.80<\/li>\n<li>The rules of wave mechanics tell us that, on the surface of the water, shorter wavelengths bend less, and longer wavelengths bend more. In effect, the reef or headland is acting like a filter, using refraction to separate out the long waves from the short. A typical beach-break with fairly flat bathymetry will be best on peaky swells and worst on long-lined groundswells. Long lines tend to close out, because there is no profound change in depth in the along-shore direction. But if the waves are peaky, they break from the middle outwards, producing good lefts and rights. p.82 <em>Good criterion to connect swell length to break patterns depending on the type of ground!<\/em><\/li>\n<\/ul>\n<h3>The temperature of the water<\/h3>\n<ul>\n<li>In areas where there is a strong surface current flowing past the coast, the surface water is continually replenished by water from somewhere else, giving it little chance to warm up or cool down with the seasons. The second reason for apparently illogical water temperature behaviour is the phenomenon of coastal upwelling, which affects western sides of continents such as Southern Africa and South America. Here the trade winds, aided by the Coriolis force, continually blow the surface water away from the coast, allowing cold water from underneath to rise up to the surface. p.96<\/li>\n<li>The gravitational pull of the Moon, and the Earth&#8217;s daily rotation, cause the tides to go in and out, and the gravitational pull of the Moon and Sun, combined with the relative positions of the Earth, Moon and Sun, cause the tidal range to vary throughout the month. The water has two bulges in it &#8211; one on either side. The bulge nearest the Moon is caused by the gravitational pull between the Earth and the Moon; the one on the other side is caused by the centrifugal force from the spinning of the system itself. The two forces must balance each other, both bulges are the same size. These bulges in the water on the Earth&#8217;s surface are, in fact, high tides. Between them, there is a low tide. &#8211; p.102-103<\/li>\n<\/ul>\n<h3>The tides<\/h3>\n<ul>\n<li>First, high tide does not occur at exactly the same time every day. Every day it gets a bit later. While the Earth takes 24 hours to go around on its own axis, the Moon takes about 28 days to go around the Earth. Another complication is that the Earth&#8217;s axis, and the axis or the Earth-Moon system, are not always parallel to each other. &#8211; p.103<\/li>\n<li>This is the spring-neap cycle, which causes the tidal range to grow and shrink alternately twice during every 28 days that the Moon goes around the Earth. In other words, one week the tide goes in and out a lot, and the next week it goes in and out only a little. So when the Sun and Moon are lined up (opposition or conjunction, full Moon or new), their gravitational influence adds together to increase the size of the bulges. The bigger the bulges, the bigger the tides &#8211; which means spring tides. Conversely, when the Sun and Moon are at right angles to each other (quadrature, or half Moon), the gravitation of the Sun acts against that of the Moon, and each tries to create bulges in opposite directions. This tends to &#8216;even out&#8217; the water on the Earth, and make the bulges smaller &#8211; meaning neap tides. &#8211; p.104<\/li>\n<li>The Corriolis force makes the tide swirl around imaginary centres of rotation, called amphidromic points, which are located in all sorts of places, depending on the configuration of the land and the sea-floor topography. &#8211; p.105 <em>The effect of tides depends on location.<\/em><\/li>\n<\/ul>\n<h3>Forecasting the waves<\/h3>\n<ul>\n<li>To make a surf prediction, there are three parameters that the surfer wants to extract from these resources. These are (i) the arrival time of the surf; (ii) the size of the waves, and (iii) the local wind conditions. &#8211; p.122<\/li>\n<li>On some wave-period charts there is a thick black line called the swell front, which represents a region of abrupt change in period. Since waves from a new swell have a longer period than those from an old one, the swell front can be used to work out the arrival time of a new swell, which is extremely useful to surfers. &#8211; p.124\u00a0<em><\/em><\/li>\n<li>Directional spectra are combined plots of direction, at a single direction, at a single point on the surface of the ocean. The period is the radial distance out from the centre &#8211; long periods in the middle and short periods on the outside. &#8211; p.125<\/li>\n<li>The atmospheric model differs from the wave model in that it is three-dimensional, the atmosphere being divided into a number of layers. &#8211; p.127 <em>The same approach could be used to predict scenarios about the formation of clouds, in 3D or 2.5D!<\/em><\/li>\n<li>Therefore, the wave height charts should always be looked at in combination with either the period charts, isobaric charts or wind prediction charts, to obtain an idea of the local wind conditions and swell quality as well as the height. &#8211; p.129 <em>Need combination of predictions to separate spectra in layers and adapt them to a specific coast.<\/em><\/li>\n<li>Vague statements about the future are more reliable than specific ones, because they have a wider scope for error. In the North Atlantic, the &#8216;stability&#8217; of the situation, and therefore the reliability of the forecasts, is linked to a phenomenon known as the North Atlantic Oscillation (NAO). The NAO is based on the pressure difference between the Azores and lceland, which oscillates up and down on various different time-scales, from a few days to tens of years. &#8211; p.131 <em>Visualization should incorporate reliability.<\/em><\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"<p>Tony Butt, Paul Russell and Rick Grigg. Large-scale weather patterns The effect of the Coriollis force will have caused the large circulation cells mentioned above to be &#8216;short-circuited&#8217; &#8211; the result being six spiralling wind-bands, three in each hemisphere. The air around 60 deg north and south is continually rising, spiralling upwards. This constant sucking &#8230; <a title=\"Surf Science, an introduction to waves for surfing\" class=\"read-more\" href=\"https:\/\/www.labri.fr\/perso\/barla\/blog\/?p=5011\" aria-label=\"Read more about Surf Science, an introduction to waves for surfing\">Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[621],"tags":[],"class_list":["post-5011","post","type-post","status-publish","format-standard","hentry","category-books"],"_links":{"self":[{"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5011","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=5011"}],"version-history":[{"count":11,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5011\/revisions"}],"predecessor-version":[{"id":5031,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5011\/revisions\/5031"}],"wp:attachment":[{"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=5011"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=5011"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=5011"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}