Physical forcing of the shelf seas: what drives the motion of ocean? - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 29

(a)

4

3 Q s

2

Q b 0

1

0

(b)

100

80
60
40
20
0

0.1 0.3 1 3 10 30 100

l ( m m)

Figure 2.4 Long wave radiation and atmospheric transmission. (a) Spectral distribution of
black body radiation from the sea surface, Q b0 ,atT K ¼ 283 K ( ∼ 10 C) and for radiation from
the Sun, Q s ,atT K ¼ 6000 K arriving at the top of the atmosphere; (b) An estimate of the
fraction of energy absorbed during transmission through a cloud-free atmosphere showing
the window for energy at the radiation peak for Q b allowing energy to exit into space.

with the larger values being found in strongly mixed regions where there are high levels
of suspended sediments.

2.2.2 Back radiation from the sea surface, Q b

The sea surface acts as an emitter of radiation and closely approximates a black body
with emissivity e s ¼ 270-310 K. The total amount of

energy radiated upwards from the sea surface is given by Stefan's law: Q b 0 ¼

0.985 and a temperature

∼

e s s T K
where T K is the temperature in Kelvin and s s is Stefan's constant. Because of
the fourth power law dependence on temperature, Q b0 from the sea surface (T K
270-310K) is much smaller than Q s (T K 6000 K) as can be seen in Fig. 2.4a .

Moreover, when conditions in the atmosphere are overcast, much of Q b0 is inter-
cepted by clouds and re-radiated back downwards to the sea surface, so the net loss
by long wave radiation Q b is generally considerably less than Q b0 .Undercloud-free
conditions, however, the situation is quite different. According to Wien's displacement
law, peak emission in the Planck radiation spectrum at temperature T K should occur at
a wavelength of l

10 3 /T K metres so that, for T K ¼ ¼ 2.897 290K, the spectral peak

will be at l m. Figure 2.4b shows that this peak in emission coincides with

a minimum in atmospheric absorption by a cloud-free atmosphere which acts as a
'window' through which a high proportion of long wave energy can escape into space.
As well as being a significant contributor to the sea's heat budget, the back
radiation is also important in that its intensity is dependent on sea surface tempera-
ture (SST). In cloud-free skies, satellite radiometers measure the long wave radiation
in the 10-12

10 m

m band. From the results the temperature may be found by inverting
the Planck radiation law. Refinements of this procedure use a split spectral window

m