However, if wind speed is slow, the resulting waves will be small, regardless of the fetch or duration. It takes all three factors acting together to create big waves.
Waves often result from storms, which tend to move across the ocean with the prevailing winds. So although a storm might only have nautical miles nm of fetch, the storm can travel greater distances, say 1, nm, creating a travelling fetch of more than 1, nm. Drag is a force that acts against the relative motion of one fluid with respect to another fluid. A "fluid" is anything that flows easily, such as gases air and liquids water.
These waves are created by storms far out to sea which create a large swell which eventually reaches the coast.
Where backwash is larger than swash more material is being eroded from the beach profile than is being accumulated. This carries material out to sea and makes for a steeper beach profile.
These waves are generated during large storms, such as the depressions which affect the British Isles. Wave refraction Wave refraction refers to what happens to waves when they approach an uneven coastline. Very few coastlines are perfectly straight, and few sea beds have uniform height and shape. This is known as the varying Geomorphology of our coastline and it will affect the waves travelling above it and towards it in differing ways.
As waves approach an uneven coastline they reach the headlands first, this focuses a lot of energy on those forelands and bends the waves into the bays where the energy expended is less. Tides are different from waves, but do influence WHERE on the beach or cliff profile a wave will break.
Therefore, they impact where sediment accumulates and where erosion occurs. Tides are caused by the gravitational pull of the moon on the water of our planet.
What determines how big a wave is? There are three factors wind that determine the size of the wave: 1 the speed of the wind, 2 the distance over which the wind has blown, and 3 the length of time that the wind has blown.
The greater each of these factors, the bigger the wave. What makes a wave powerful? For one thing, ocean waves are dense with energy. Although wind causes the powerful surface waves that we use for wave energy, the moon's gravitational pull is responsible for the tides, which is the rise and fall of the oceans twice a day. Tidal energy, distinct from wave energy, also holds energy potential. What is the wave period? Wave period is the distance between two waves passing through a stationary point, measured in seconds.
Understanding The Magic Number. At Coastalwatch we often receive emails with questions about our surf forecasts. What are the factors affecting the height of the waves? The factors affecting the height of waves are: The wind speed must be blowing faster than the transfer of energy from wave crest to wave crest. The amount of time the wind blows, or wind duration.
These winds are normally quite shallow, extending hundreds of feet to a few thousand feet above the surface. Large changes of wind, or wind shear, are found at their upper and lateral boundaries. One example of the effect of gap winds on wave growth occurs in the Gulf of Tehuantepec. Chivela Pass, a gap that has both important atmospheric and oceanographic effects, cuts through the Sierra Madre of Mexico.
The gap is approximately n mi long, 20 n mi wide, and has a maximum elevation of only ft. During the winter when cold, high pressure systems move southward along the eastern slopes of the Rockies and the Sierra Madre Mountains, a large pressure gradient can build across the gap.
This results in strong northerly winds, known as Tehuantepecers, immediately downstream of the Pass. Tehuantepecers can reach 20 to 40 kt, with gusts exceeding kt in extreme cases. The strong, persistent winds from Tehuantepecers can create waves that may propagate as swell as far south as the Galapagos Islands, nearly miles away. This and other strong, persistent atmospheric gap flows can have a significant influence on the nearby coastal waters. The strong winds that blow through the Chivela Pass result in substantial upper ocean mixing causing sea surface cooling of 10 to 13 degrees Celsius 8 to 14 degrees Fahrenheit.
Katabatic winds are caused by gravitational flow of cooler air from higher surrounding terrain. Their effect on wave growth occurs more often in northern latitude coastal areas where high terrain is near the coast. They are similar to gap winds in that winds flowing offshore will be mesoscale in nature. Katabatic winds are very fetch- and duration-limited. Therefore, even strong winds from a katabatic event may have only a small effect on wave growth.
Down slope winds will be strongest in a dry climate with high terrain when night skies are clear and winds aloft are weak. Click here to see the katabatic wind animation.
A type of katabatic wind that occurs in high latitude regions is referred to as a "glacier wind". These winds develop over glacier or snow fields located on high terrain. Unlike the katabatic wind just described, cold air is continually generated over these areas, and therefore affects winds both day and night.
The wind strength is determined by the temperature contrast between the glacially cooled air and the surrounding environment as well as the distance that air travels over the glacier. For mariners in Alaska or northern Canada, these winds may cause choppy seas near locations where glaciers are in close proximity to the coast. Significant differences between water and air temperature can cause instability in the marine boundary layer and hence contribute to the development of stronger winds at the surface.
In the Great Lakes and other inland bodies of water, instability effects can be more important than pressure gradient forces in determining wind speed since water temperature variations can exceed those found over oceans. However, there are also many areas along the coast that experience instability in the marine boundary layer. If the air is cooler than the water, heat and moisture fluxes from water to air will destabilize the layer of air just above the water surface.
The larger the difference in air and water temperature, the deeper the unstable layer becomes. The instability lends itself to convective motions and hence mixing between adjacent layers. This causes a direct transfer of higher wind speeds from the marine boundary layer to the surface of the water.
These stronger, mixed winds will be more uniform in direction and speed and will increase the stress on the water surface.
These instability effects are most prevalent in the fall when water temperatures are still warm from summer heating but air temperatures have decreased. Notice that winds over land diminish to light or calm conditions as night progresses, but the winds over the water continue to persist with speeds of 10 to 20 kt due to the turbulent mixing of faster winds to the surface.
An ocean or lake current can have an effect on wind wave growth. This is not to be confused with the effect on swell from a current. For wind waves moving with the current, the waves will be less steep and slightly lower in height. This occurs because the current moves the water with the wave motion, increasing wave speed.
A water particle will not complete a circular motion but will be displaced in the direction of the current, effectively lengthening the wave. For wind waves running against the current the opposite is true. These waves are steeper with slightly increased heights and travel more slowly. This effectively lengthens the duration and fetch. Note that heights for long period swell can increase dramatically if they oppose a current.
While these concepts should be understood, they are difficult to incorporate into forecasts for two reasons: the exact location and speed of currents are not always known, and the angle of incidence between the wind and current is in constant flux. Scatterometry uses microwave remote sensing of the ocean surface to determine the wind speed and direction.
The microwave portion of the electromagnetic spectrum is used for scatterometry because much of the global ocean is frequently covered by clouds and these clouds are transparent in the microwave spectrum.
Small-scale roughness elements, such as capillary waves on the ocean surface, have about the same wavelength as microwave radiation. The fundamental assumption of scatterometry is that the amplitude and direction of small-scale roughness elements are an indicator of the local sea surface wind speed and direction. The fraction of energy returned to the satellite is known as backscatter. A model function that relates backscatter, surface wind speed and direction, and radar-viewing geometry has been created from a mass of empirical data.
Although there are observed aspects of scatterometry such as rain interference that have not been accounted for in this model, the wind speed can generally be determined from the strength of the backscatter signal.
Back to Top. For a fixed-viewing geometry, backscatter increases with increasing wind speed and decreases with decreasing wind speed. Multiple measurements of backscatter are made at nearly the same place on the ocean's surface at nearly the same time with different viewing geometries.
These data are plugged into the model function that derives wind speed and direction. Theoretically, wind speed and direction can be measured with the scatterometry technique. However there are observed aspects of scatterometry that are not well understood.
These are:. There are two kinds of instruments used in scatterometry. While we will not discuss them in detail here, it is important to point out that each has limitations. The fan beam approach can not measure wind speed and direction directly below the satellite. This blank area underneath the fan beam antenna is known as the nadir gap.
While this eliminates the problem of the nadir gap, the measurements within the nadir gap are not as accurate as those to the side of the satellite track. In addition, the measurements of backscatter near the very edge of the measuring swath are only sampled twice. At least four "looks" are needed to ensure an accurate interpretation of conical scatterometry output. Therefore, the winds measured at the edge of the swath of a rotating antenna are unreliable.
Rain contaminates the backscatter signal and causes QuikSCAT wind speeds to be higher than in situ measurements up to about 30 kt.
For wind speeds above 30 kt, the rain-caused error is less important and the QuikSCAT wind speed and direction products may be used operationally even if they are rain-flagged. Several Websites containing scatterometry data exist. A few of the more commonly referenced sites are listed here:.
All stations measure wind speed, direction, gusts, barometric pressure, and air temperature. In addition, all buoy stations and some C-MAN stations measure sea surface temperature, significant wave heights, and dominant wave periods. Wind speed and direction are averaged over an eight minute period for buoys and over a two minute period for C-MAN stations. Wind gusts are an average of the five or eight second speeds taken during the respective measurement period.
Sensors on board the buoys measure the heave acceleration or the vertical displacement of the buoy hull for approximately 20 minutes prior to the scheduled observation time. Wave energies, frequencies, significant heights, and dominant periods are mathematically derived from these data. An NDBC buoy is reporting a 25 knot wind, a wave height of 8 feet, and a wave period of 9 s. Will the wave height associated with the 9 second wave period continue to grow? Since the wave speed is greater than the wind speed of 25 knots the wave height will not grow.
Therefore, looking at the buoy observations, one can tell if the waves are growing, or if they have reached their maximum height, and are just propagating through the area. The United States program is the largest in the world, with over vessels. Observations are taken by deck officers, coded in a special format known as ship's synoptic code, and transmitted into the NOAA data stream every six hours. It is important to note that instrumentation aboard VOS ships sits higher than instrumentation on buoys; therefore, a ship may report a higher wind speed than a buoy in the same area.
Because VOS observations are made subjectively without instruments, caution should be taken when using them. The date and time of the observation should be noted since the frequency of VOS reports is much less than land-based observations. On 8 October at UTC a long-wave trough with several embedded disturbances was over the northeast Pacific ocean. A strong polar jet had moved into the long wave with the left exit region in a favorable position to enhance cyclogenesis in the southern embedded disturbance.
The surface low that formed with this disturbance was located about miles west of Vancouver Island at UTC and was moving toward the coast of west central British Columbia. The extratropical system would eventually reach an observed minimum pressure of hPa before making landfall. Water vapor imagery depicted an expanding dry slot subsidence region indicative of strong cyclogenesis. At this time NDBC buoy , which is just south of the cyclone, was experiencing 40 kt sustained winds gusting to 70 kt and 30 ft seas while some of the Canadian buoys closer to the storm had even larger values.
While strong winds and precipitation affected the coastal areas during landfall, cold air advection continued well upstream, as evidenced by the large area of stratocumulus clouds shown in the infrared satellite imagery and by the magnitude of baroclinicity shown in the hPa model wind and temperature fields. This helped maintain a broad area of kt winds that extended back to the Aleutian Island chain. The correct answer is e. Areas A and B indicate fetch due to the homogeneous characteristics of wind direction and speed within the area.
Area C does not indicate homogeneous wind features. Direction and speed are both changing within this region. Given forecasted wind speed and a fetch length of approximately n mi, what approximate wave height is possible in fetch area A? An analysis of wind barbs shows that the lowest wind speed is 20 kt. However, the average wind speed over much of the fetch area is approximately 25 kt.
Using the wave nomogram, and following across the 25 kt wind speed line to the vertical fetch length line of n mi shows that approximately a 15 ft wave is possible. Do you expect the model wave heights calculated in Question 2 to be under or overestimated in fetch area A compared to satellite wind estimates? The correct answer is c. Thus, corresponding wave heights are 15 to 25 ft, indicating that the model wave heights will be underestimated.
Given the wind speed of 25 kt calculated in Question 2 and a fetch length of n mi, calculate the wave height based on duration of generation shown by model forecasts for area A.
The next time period of 12 UTC on 09 October shows that the winds have shifted counter clockwise and the wind speed has decreased due to a decreasing pressure gradient. This change in the wind field indicates that a separate set of waves will develop based on the new wind characteristics and hence a different fetch region.
The waves from fetch area A will no longer grow at this point, and will propagate away from their source region. Following the 25 kt line horizontally until it intersects the blue dashed line of 18 hours reveals that wave growth of 10 feet will occur. Note that this value of wave height is less than that found in question 2 when only wind speed and fetch were considered. Given the duration of 18 hours, wind speed of 25 knots, and fetch length of nautical miles determined in the previous questions, what is the limiting factor for wave growth in area A?
Using the wave nomogram and 25 kt wind speed, notice that wave height will continue to grow for 48 hours then minimally after that. In this case, fetch length does not restrict wave growth because fetch area A is longer than the minimum required fetch length of n mi for a duration of 18 hours.
As seen in the previous question, the wave height for a 25 kt wind changes from 15 ft to 10 ft when the duration is considered. If duration is not taken into consideration, the forecast of wave height will be too large. In this case, there is ample fetch length and wind speed, but the amount of time needed to generate the maximum wave height is not sufficient. Therefore, our maximum wave height will be limited by duration. Given the plot of wind speeds and a fetch length of n mi, what approximate wave height is possible in fetch area B?
Analysis of the wind barbs shows that the average wind is approximately 15 kt throughout the fetch area. Carefully follow the 15 kt wind line on the Wave Nomogram over to n mi to find that an approximate wave height of 6 ft will occur in fetch area B. Given the 15 kt wind speed calculated in the previous question and a fetch length of n mi, calculate the wave height based on duration of generation shown by the model forecast for area B.
For fetch B the entire period has consistent 15 knot winds. Therefore we use a duration period of 36 hours to calculate a significant wave height of approximately 5 feet. Do you expect the model wave heights calculated in Question 6 to be under or over estimated in fetch area B compared to satellite wind estimates? Thus, corresponding significant wave heights would be about 9 feet indicating that the model wave heights will be underestimated.
In this case the wind speeds are relatively low.
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