State of the sea

The state of the sea is the description of the sea surface subjected to the influence of the wind (which generates the system of Vague S) and of the Houle. The associated terminology (strong sea, calm sea, etc…) was standardized by the services of maritime Météorologie to provide to the ships and the installations located at sea information which can be usable. Indeed the state of the sea can cause danger, in a way independent of the force of the wind, in particular in the not very major water zones (with the accesses of the coasts, on high funds,…) or in the passages tightened or traversed by important currents (strong current,…).

The action of the Vent on the Mer gives rise to the waves. The sea of the wind is the system of vaguenesses created at the place even where breath the wind. By leaving the place where they were generated, the waves are regularized and become the swell, which can be propagated very far same in the absence of wind. When the swell follows another direction that the vaguenesses generated by the wind, one speaks about cross sea . The reflection of the trains of waves on an abrupt coast, can also generate a sea crossed with the broad one. In Europe, the phenomenon is known and dreaded in the the Bay of Biscay and in the Mediterranean.

Description of the state of the sea

The height of the waves measures hollow between two waves with the peak of vagueness. Because of random nature of the waves, the state of the sea is described by statistical parameters, like the average heights, periods or directions. Such measurements are difficult to make with the naked eye. Many systems of measurement were developed, since the pole with swell to the radars embarked on satellites.

The state of the sea is often composed of swell without bond with the local wind, and of a sea of the wind . The camber of the sea of the wind being generally stronger, it is the significant height of the sea of the wind which is often the most important parameter. The sailors usually use the scale of Douglas, which gives 9 classes of value for this height of the sea of the wind , of sea 0 with sea 9:

Numerical forecast of the state of sea

The numerical forecast of the state of sea exists in its current form since the years 1950, and it was initially developed for the propagation of the swells at long distances. Thus the team of the Center of meteorology of Casablanca, at Morocco, developed then improved in France the spectral method (Gelci and coll 1957). It is a question of calculating the characteristics of the state of the sea starting from the forecasts of the wind. The principle more used with large scales (of the oceanic basin to coastal bay) is that of spectral modeling in realized phase. Into each point of the sphere, the state of sea is broken up into a superposition of waves of periods and different directions. The current models use approximately 600 different waves, covering all the directions by step of 10 with 30° and periods 2 to 30 S. This type of model calculates the transport of the energy of each one of these components because of propagation, and its evolution under the effect of several processes. By deep seas one is interested especially in the wind, the formation of sheep which dissipate energy, and in the energy exchanges between the various components. By small funds he is necessary to add to it friction effects on the bottom and of reflection by underwater topography.

Thus in each point, the whole of this information makes it possible to calculate the variance E of the rise in the surface, from which one draws the significant height Hm0=4 $\ sqrt E$ . Energy per oceanic unit of area is about the product of E by the density of water and the acceleration of gravity G. One can also draw from it the period from the peak Tp, period of the maximum of energy of the spectrum, or the average direction of the waves.

The quality of the forecasts depends primarily on the quality of the forecasts of the wind, and the quality of the parameter settings of the various processes (generation, dissipation…). The use of measurements can also be used in a system of forecast to readjust the initial state of the state of sea compared to observations (altimetric measurements of the satellites, imagery radar). The best current models give the Hm0 height with an average error from 10 to 30% for the immediate forecast, according to the geographical areas. The best results are generally obtained with the averages and high latitudes, in particular in the northern hemisphere because the winds are well envisaged there. The errors important are especially localized in coastal area, in closed seas surrounded by mountains, like the Mediterranean, and in the tropics, dominated by swells coming by far. These errors increases with the expiry of forecast, because of increase in the errors in the winds envisaged.

State of the sea and maritime activity

The state of the sea provided by the weather services is exploited to define the road followed by the ships. The presence of a strong sea, of a significant swell can drive the ships to modify their trajectory or to even make half-turn.

The state of the sea takes an particular importance in certain configurations:

zones of high funds or places characterized by a brutal raising of the funds (accesses of the continental shelf,…) can put in danger the ships by strong sea
at the accesses of a coast, a strong sea can prohibit the access to certain ports because the waves can be raised and break as the depth decreases (depends on the profile of sea-beds to the accesses of the coast)
the access to an estuary or with an almost closed basin when there is presence at the entry of a not very major threshold, is made dangerous sometimes by a simple moderate swell which can create waves enormous (for example Bassin of Arcachon or Rivière of Etel in France)
in the strong currents traversed by high tension currents, swell or the sea of the wind if she is opposed to the current can raise a sea chaotic and dangerous (in France Raz of Center, Raz Blanchard)
in the same context the zones of high tension currents located at broad can generate by strong sea of the waves abnormally high (for example Running of the Needles to broad of the South Africa)

See too

Vague
Swell
Scale of maritime Beaufort
Meteorology
Storm

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