Waves the production of aerosols and condensation nuclei

The moving atmosphere produces a stress on the sea surface. The ocean is a fluid and exertion of a stress upon it will result in motion within the fluid. This takes many forms. To the atmospheric observer the most obvious of these is the production of surface waves. The type of waves, or the sea state, is a clear function of the wind speed; light zephyrs give tiny ripples, great storms produce towering mountains of foaming water. The sea state is of vital importance for the air-sea exchange of atmospheric gases that play important roles in oceanic chemistry and biology of climatic relevance, such as CO2 or O2 (see Chapters 3 and 4). A violent sea full of breakers strongly enhances this exchange through bubble generation. The exchange process, and the physical break-up of waves, is also important for the atmosphere as oceanic particles and gases are significant sources of cloud condensation nuclei. Thus an understanding of surface wave processes is a necessary prerequisite to an understanding of ocean-atmosphere climatic interaction.

2.9.1 Wave formation and characteristics

Surface waves occur due to subtle interactions between small-scale pressure fields in the boundary layers of the atmosphere and ocean. The full, dynamic, wave field that we see on the ocean surface springs from these interactions, but is also affected by interactions within the sea between waves of different frequency.

If surface waves build up in size they can reach a peaked shape which becomes gravitationally unstable because the leading edge of the wave crest leans over the front of the wave itself. The wave then breaks as this crest collapses. Momentum is released, taken by the waves from the wind, to the body of the ocean.

Breaking can be caused in several ways. Strong winds may provide the energy to amplify waves to this extent. Combinations of waves of smaller size from different source regions may locally reinforce to produce sufficiently large waves for breaking to occur; this is known as constructive interference. Both of these modes of breaking will be observed at sea; the latter tends to produce localized patches of wave breaking. The major source of breaking waves observed from land is the amplification caused by the waves travelling over a shoaling beach. This can be extremely dramatic, as shown in Fig. 2.24. Coastal wave breaking occurs because the shoaling sea floor is squeezing the wave energy into a smaller depth range. To conserve energy (though, of course some is being lost due to friction at the bottom) the wave height grows. As a rough guide, wave breaking occurs as the height of a wave approaches one seventh of its wavelength.

The spectrum, or combination of sets, of waves depends on the wind conditions, both locally and at distance. There tends to be a dominant frequency, with little energy in waves with longer period (or smaller frequency) but considerable energy at shorter periods. Stronger winds produce a sharper peak in energy, at a lower frequency, as shown in Fig. 2.25. Each set of waves of differing frequency moves at a different speed (see (2.15) below), radiating energy in all directions from its source. This has the effect of dispersing energy from the generation area. Such dispersion is particularly noticeable some distance from a storm, as the waves radiated by the storm-induced wave field arrive at a coast at different times depending on their frequency. The swell produced by the storm will therefore appear very regular, in contrast to the continually changing wave field produced locally.

We noted earlier that surface waves attain greater amplitude on approaching the shore. This illustrates two important facts about surface waves: they carry energy, and their influence extends some distance beneath the surface. Surface waves transmit energy but not matter. The particles which appear to be carried along with the wave actually move in circular orbits. The appearance of the surface wave is due to the phase of the orbit of the surface particles. The direction of travel is prescribed by the orbital orientation. As the wave approaches the coast, and the sea floor rises, the bottom begins to interact with the wave, normally where the water depth is less than half the wave's wavelength. The

Fig. 2.24. Photograph of a wave breaking along the Dorset coast near Golden Cap on 11 August 1985. A strong onshore wind, combined with a rapidly shoaling beach at a stream inlet, has led to vigorous surf. Note the size of the breaking waves in relation to the adults standing on the beach.

Fig. 2.24. Photograph of a wave breaking along the Dorset coast near Golden Cap on 11 August 1985. A strong onshore wind, combined with a rapidly shoaling beach at a stream inlet, has led to vigorous surf. Note the size of the breaking waves in relation to the adults standing on the beach.

Fig. 2.25. Variation of the spectrum of normalized wave energy with frequency for three different wind speeds (1 knot = 0.51 ms-1). [Reproduced, with permission, from J. G. Harvey, Atmosphere and Ocean: Our Fluid Environment (London: Artemis Press, 1985), p. 77, Fig. 9.4.]

orbits become elliptical in shape as this occurs. In deep water the phase speed, c, of the waves is c = kv

where v is the frequency and X is the wavelength. In shallow water, where the wave is affected by the bottom (and where depth is typically less than a twentieth of a wave's wavelength), the waves become independent of frequency, and lose their open ocean dispersive character, with

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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