The Saturation Chronicles - 19

Fig. 1 DictionaryDotCom

The words "saturation" and "saturate" have various meanings in various settings. 

For example Fig. 1 shows a dictionary page which has "American, British, Scientific" etc. as some of those realms where the meaning can morph.

In the setting of the country the word is used in (e.g.) American or British, or in the scientific discipline (e.g.) chemistry or physics the word meaning  can have an interesting impact on the conversation.

But, when we come to oceanography and the concept of ocean heat those words have yet another sense, which ultimately boils down to "potential enthalpy" in the broader sense (In Search Of Ocean Heat - 20).

Ultimately though, when we get down to photon saturation, the quantum physics meaning comes into play, which makes the aforesaid meanings mere child's play:

"This page presents a heuristic or qualitative derivation of the the scalar radiative transfer equation (SRTE). The end results (Eqs. (9), (10), or (12)) are correct, but the steps of the derivation are physically incorrect because polarization is ignored. In addition, the derivation of this page gives no way to estimate the errors that result from ignoring polarization. Nevertheless, this form of the derivation of the SRTE is found is almost every textbook and it does serve as a useful way to remember the various physical processes that contribute to light propagation in an absorbing and scattering medium like the ocean or atmosphere. This derivation is also of historical interest because it shows how the founding fathers of radiative transfer theory proceeded in order to obtain a governing equation (e.g., Preisendorfer (1965), page 65) before the link between fundamental physics and radiative transfer theory was firmly established."

"The Level 2 pages beginning at The General Vector Radiative Transfer Equation outline the steps of a physically rigorous derivation of various levels of RTEs, ending with the SRTE seen below."

Radiative Processes

"To the extent that polarization can be ignored, the SRTE expresses conservation of energy written for a collimated beam of radiance traveling through an absorbing, scattering and emitting medium. We thus begin by considering the various processes that can occur when light interacts with an atom or molecule."

"The light (electromagnetic radiation) may be annihilated, leaving the atom or molecule in an excited state with higher internal (electronic, vibrational, or rotational) energy. All or part of the absorbed radiant energy may be subsequently converted into thermal (kinetic) or chemical energy (manifested, for example, in the formation of new chemical compounds during photosynthesis). The annihilation of the light and conversion of its energy into a nonradiant form is called absorption. (See The Physics of Absorption page for further discussion of the quantum mechanics of absorption processes.) If the molecule almost immediately ... on a femtosecond ... or shorter time scale) returns to its original internal energy state by re-emitting radiation of the same energy as the absorbed radiation (but probably traveling in a different direction from the original radiation), the process is called elastic scattering. Because of the extremely short time required for these events, elastic scattering can reasonably be thought of as the light interacting with the molecule and simply 'changing direction' without an exchange of energy with the scattering molecule."

"The excited molecule also may emit radiation of lower energy (longer wavelength) than the incident radiation. The molecule thus remains in an intermediate excited state and may at a later time emit new radiation and return to its original state, or the retained energy may be converted to thermal or chemical energy. Indeed, if the molecule is initially in an excited state, it may absorb the incident light and then emit light of greater energy (shorter wavelength) than the absorbed light, thereby returning to a lower energy state. In either case the scattered (emitted) radiation has a different wavelength than the incident (absorbed) radiation, and the processes is called inelastic scattering. One important example of this process in the ocean is Raman scattering by water molecules. Fluorescence is an absorption and re-emission process that occurs on a time scale of to . If the re-emission requires longer than about, the process is usually called phosphorescence. The physical and chemical processes that lead to the vastly different times scales of Raman scattering vs. fluorescence vs. phosphorescence are much different. The distinctions between the very short time scale of Raman 'scattering' versus the longer time scale of fluorescence 'absorption and re-emission' do not concern us in the derivation of the time-independent RTE. However, the terminology has evolved somewhat differently, e.g., Raman scattering usually refers to 'incident' and 'scattered' wavelengths, whereas fluorescence usually refers to 'excitation' and 'emission' wavelengths."

"The reverse process to absorption is also possible, as when chemical energy is converted into light; this process is called emission. An example of this is bioluminescence, in which an organism converts part of the energy from a chemical reaction into light."

In order to formulate the RTE, it is convenient to imagine the total light field as many beams of electromagnetic radiation of various wavelengths coursing in all directions through each point of a water body. We then consider a single one of these beams, which is traveling in some direction and has wavelength. "

(The SRTE: Heuristic Development, emphasis added; cf. Ocean OpticsBook-scattering, Absorption, Radiative Transfer Equations). I call it the photon current (The Photon Current, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 , 17, 18, 19, 20, 21) which is easier to grasp if we remember that The Second Law of Thermodynamics clarifies it even more ("hot flows to cold" or "warm flows to cooler").

The previous post in this series contains HTML tables that detail hundreds of temperatures at various depths (up to thousands of meters) in hundreds of WOD zones in 18 Layers (latitude bands).

They show that the ocean temperatures are sometimes warmer at deeper depths than at shallower depths.

They also show that the ocean zone depth layers change in temperature from time to time.

This is what is expected when photons from the Sun enter the ocean, then are absorbed, scattered, and emitted to cooler water molecules nearby.

Photon Saturation, then, depends on how much photon energy atoms and/or molecules can absorb before they emit a photon.

But, photon Saturation also means warmer water atoms or molecules emit a photon spontaneously from a warmer water area into a cooler water area nearby.

Both photon emitting and photon scattering are "heat transfer" events which will cause different temperatures to take place at both locations (i.e. the emitter location and the absorb-er location).

When those temperatures are detected afterwards by the thermometers of oceanographers who record the in situ temperature measurements, then store them in the WOD and other databases.

Then I come along and make HTML tables out of those recorded measurements.

Then you read them until you get saturated.

The previous post in this series is here.