Radiation

2.1.1 Solar radiation

In Fig. 1.11 we saw that 58% of incident solar radiation reaches the Earth's surface; on average, about 15% of this is reflected back into space, the proportion varying according to the albedo of the surface. Over the oceans most of the incident solar radiation enters the water, although the albedo depends strongly on the angle at which the Sun's radiation hits the surface. Table 2.1 gives the reflectance of direct sunlight illuminating a calm water surface, over a range of angles of incidence.2 This should be contrasted with the albedo of some typical terrestrial surfaces, given in Table 7.2. Until acute angles are reached well over 90% of the radiation passes through the surface. Thus, over most of the globe, for most of the daylight hours, almost all direct solar radiation incident on the ocean surface enters the water.

There are several complications to this apparently simple picture. Solar radiation suffers significant scattering by air molecules on its passage through the atmosphere. This is known as Rayleigh scattering and occurs because the molecules are small in comparison to the wavelength of the radiation (10-10 m compared to 5 x 10-7 m). The degree of scatter is inversely proportional to the fourth power of the wavelength so lower, or blue, wavelengths (see Fig. A.1) are preferentially scattered. This scattered radiation is spread through the atmosphere, creating our perception of the sky being blue, and reaches the surface from a range of incident angles. Because of this range rather more scattered, compared with direct, light is reflected from the sea surface: 5-6%, compared to 2-3%, in most circumstances.

The presence of clouds does not significantly alter this proportion, although it does mean that a greater percentage of the radiation that reaches the surface is

2 Reflectance and albedo are not identical because some of the radiation entering the water can be forced, through scattering, to re-enter the atmosphere. Reflectance measures the immediately reflected radiation from the surface, while the albedo is the proportion of the incident radiation that is lost from all internal and external reflective processes.

scattered radiation. On cloudy days, therefore, perhaps twice as much radiation will be reflected from the sea surface as on clear days.

A further complication arises when the sea surface is not calm. The creation of waves means that the sea surface can present the full range of angles to the incident radiation. If the sun is high in the sky this tends to increase the mean angle of incidence, but the small dependence of reflectance on angle under such conditions means that the net effect on reflectance is negligible. However, at sunrise and dusk, and in high latitudes, rough seas tend to decrease the average angle of incidence. This can decrease reflectance by a factor of two and so allow significantly more radiant energy to enter the ocean.

A final consideration in determining the actual albedo of the sea surface is the fate of light that penetrates into the sea. A proportion of this will be scattered by the sea water back to the surface and will thus be able to pass into the atmosphere, supplementing the initial reflection. Countering this scattering loss, however, is the refractive index, n, of water. This gives the ratio of the speed of electromagnetic radiation in a vacuum to that in water. The ratio of n for two media through which light travels gauges the resistance to the passage of light from one medium to another, or the reflecting effect of the interface between the media. The refractive index for water is nw = 1.33 and that of air is na = 1.00, so much more light is reflected when passing from water to air (nw/na) than the converse (na/nw). In addition, Snell's law for refraction:

sin at na where angles a; and at are as shown in Fig. 2.1, has the consequence that there will be total internal reflection beneath the water surface when the angle of scattered under-water radiation returning to the sea-air interface (at, but upward light travel compared to Fig. 2.1) is greater than 48.5°. Therefore, only radiation incident on the sea surface from within the ocean at angles less than 48.5° will contribute to the back-scattered component of the albedo; 48% of the under-water incident radiation will be reflected back into the water.

This means, however, that about half of the radiation that enters the sea is potentially able to escape back into the atmosphere, if all of this radiation were scattered within the ocean back to the air-sea interface. In practice, almost all

Fig. 2.2. Energy spectrum, and its variation with depth, in a beam of solar radiation penetrating the ocean in (a) the western sub-tropical Atlantic, (b) the Baltic. Solid lines show the spectrum at the labelled depth. Note the rapid attenuation with depth in the sediment-laden waters of the Baltic. [Data from Jerlov, 1976.]

Fig. 2.2. Energy spectrum, and its variation with depth, in a beam of solar radiation penetrating the ocean in (a) the western sub-tropical Atlantic, (b) the Baltic. Solid lines show the spectrum at the labelled depth. Note the rapid attenuation with depth in the sediment-laden waters of the Baltic. [Data from Jerlov, 1976.]

of this radiation is absorbed within the sea, converting electromagnetic energy into heat. Most of this absorption is carried out by water molecules. Dissolved salts absorb weakly in the ultra-violet, while suspended sediment and plankton absorb variable amounts of solar radiation, depending on their type and concentration. We saw in §1.2.1 that water vapour is a powerful absorber of the infra-red part of the spectrum; liquid water has similar properties. Fig. 1.9 shows that significant absorption extends into the higher wavelengths of the visible portion of the spectrum. This means that a few metres below the surface of the sea the blue-green region of the spectrum has become more dominant because of selective removal of the higher, red, wavelengths. This is shown in Fig. 2.2. The sea therefore appears blue-green to an observer, as this is the light back-scattered out of the water (Fig. 2.3). The green, or murky grey, of coastal seas, such as the North Sea, is caused by high plankton population (except in winter, see Chapter 4), dissolved organic material and suspended sediment. Plankton photosynthesise, selectively absorbing wavelengths other than near the green segment of the visible spectrum to catalyse this reaction (see §4.1.1). Note that this is also why leaves are green! The high sediment concentrations in coastal waters tend to make absorption more uniform over the spectrum.

Fig. 2.2 also shows that less than 50 m below the surface the light intensity, even in the blue-green wavelength band, is reduced to a quarter, or less, of that just below the surface. Direct heating of the ocean from solar radiation is therefore confined to a band only a few tens of metres thick at most, helping to explain the thinness of the summer ocean mixed layer (see §2.4.2). In seas rich in sediment and plankton this decline in irradiance is even more pronounced (Fig. 2.2(b)). The euphotic zone is defined to be the depth at which light intensity is only 1% of the surface value. In the clear, biologically inactive, waters of the sub-tropics this depth can approach 100 m (Fig. 2.2(a)). In turbulent, muddy, and biologically rich coastal waters such as those of the North Sea, or Baltic, the euphotic zone may be less than 20 m deep (Fig. 2.2(b)). In Chapter 4 we will see how the concept of the euphotic zone dominates the biological productivity of the oceans.

Fig. 2.3. Energy spectrum of solar radiation at the ocean surface, but only of that scattered within the ocean back into the atmosphere, not the direct solar beam. The broken line shows a western sub-tropical North Atlantic spectrum, the solid line is from the Baltic Sea. Note the much reduced scattering in the sediment-laden waters of the Baltic. [Data from Jerlov 1976.]

Fig. 2.3. Energy spectrum of solar radiation at the ocean surface, but only of that scattered within the ocean back into the atmosphere, not the direct solar beam. The broken line shows a western sub-tropical North Atlantic spectrum, the solid line is from the Baltic Sea. Note the much reduced scattering in the sediment-laden waters of the Baltic. [Data from Jerlov 1976.]

2.1.2 Long-wave radiation

The sea receives solar radiation from the Sun. It also receives long-wave, infrared, radiation from the atmosphere. In addition, it emits long-wave radiation to the atmosphere. The balance between the energy gained by solar radiation and incoming infra-red, and the energy lost through outgoing infra-red determines the radiation budget of the sea surface.

In §1.2.1 we saw how the greenhouse effect results in the atmosphere absorbing significant amounts of infra-red radiation headed towards space, and the re-radiation of much of this energy back towards the Earth's surface. The amount of long-wave radiation lost by the ocean will therefore be strongly dependent on the water vapour concentration, this being the principal, and fastest varying, greenhouse gas. Clouds, as liquid water, will also absorb some of the infra-red radiation emitted by the ocean. The determining factor in the amount of long-wave radiation emitted by the sea into the atmosphere is, however, its surface temperature, TS. To the atmosphere the ocean appears as a body at temperature TS radiating according to the Stefan-Boltzmann Law (1.1); the thermal structure beneath the surface is only visible in its surface signature. The infrared radiation from the sea surface, the long-wave heat flux Q B (measured in Wm-2), is therefore given by (1.1), but reduced by the returning greenhouse emission from the atmosphere. This function can be empirically estimated:

where ea is the vapour pressure (measured in millibars) - the pressure exerted by the weight of water vapour molecules present in a column of air, one metre square, above the surface - of the atmosphere (taken above the standard observing height of 10 m) and nc is the proportion of cloud cover. The vapour pressure

(a) Longitude

Fig. 2.4. Net radiation (in Wm-2) at the ocean surface over the Atlantic in (a) January, and (b) July. The contour interval is 20 Wm-2. Dotted contours indicate negative, or net outgoing, radiation. [Data from Oberhuber, 1988.]

(a) Longitude can vary by a factor of ten, depending on the atmospheric conditions. Typical values are of the order of a few tens of millibars; a typical atmospheric pressure is a little over 1000 mb. The correction factors for the trapped, or return, longwave emission from clouds and greenhouse gases reveal the importance of water vapour for this process. A completely cloud covered sky reduces the loss of radiation by 60%. This is almost the same as is absorbed by all greenhouse gases, save near-surface water vapour, in the absence of clouds: 61%. Note also that in polar regions, where there is little water vapour in the air, the effect of water vapour is only an additional 10-15% on top of the other greenhouse gases, while in the tropics it can be as much as 25%,3 or almost a third of the total non-cloud return long-wave radiation.

The average energy of the net short-wave and infra-red radiation received by the Atlantic Ocean is shown for January and July in Fig. 2.4, calculated from such empirical formulae as (2.2) and a similar one for the incident short-wave radiation. In the winter hemisphere there is net loss of radiation poleward of 35-40° and the distribution is essentially zonal. In the summer hemisphere, and near the equator, variation in cloudiness has a strong impact, for instance it

Fig. 2.4. Net radiation (in Wm-2) at the ocean surface over the Atlantic in (a) January, and (b) July. The contour interval is 20 Wm-2. Dotted contours indicate negative, or net outgoing, radiation. [Data from Oberhuber, 1988.]

3 Note from (2.2) that the above proportions are multiplicative, not additive.

Longitude

Longitude leads to the minima in net radiation over the central North Atlantic off western Europe (July) and the eastern equatorial Atlantic (January and July).

Was this article helpful?

0 0
Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

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.

Get My Free Ebook


Post a comment