Ocean–Atmosphere Interactions - Ocean in the Earth System - page 107

- Rnet : net radiative balance;
- Fturb : turbulent heat fluxes.
3.2.1. Radiative fluxes
The Sun's spectrum covers the domain of electromagnetic waves,
ranging from ultraviolet waves to microwaves, including visible and
infrared bands.
Like all bodies with a physical temperature, the wavelength
distribution of radiation emitted by the Sun is a function of its
temperature. Planck's law expresses this relation for a black body: a
body whose surface entirely absorbs the incident radiation.
Because of Planck's law, the maximum energy radiated by the Sun
is situated in the visible domain, approximately 0.5 µm, whereas the
Earth, whose surface temperature is in the order of 290 K, has a
maximum emission in thermal infrared (10-12 µm).
The atmosphere is not totally transparent to solar radiation: the gas
molecules, the droplets, cloud crystals and dust particles (aerosols)
have a significant absorption in infrared bands. They also reflect and
scatter the received radiation (mainly in the visible spectrum, as well
as in infrared and microwaves in specific frequency bands). Thus, the
atmosphere absorbs part of the solar radiation, and it emits in turn a
thermal infrared radiation downward and upward; the clouds reflect
part of the radiation upward, and so into space; they also absorb,
re-emit and scatter the remaining in all directions.
On the surface, the radiation received is the sum of all these
components: solar radiation attenuated and diffused by the
atmosphere, and infrared radiation emitted by the atmosphere. Strictly
speaking, it would be necessary to consider the transmissions and
emissions in the entire spectrum. However, the balance of energy
received by the Earth's surface is largely dominated by short waves
(visible spectral domain, from 0.4 µm up to near infrared,
approximately 3 µm) and infrared (from 3 to 12 µm).