What 3D radiative transfer seems to be currently lacking is a clear conceptual framework. It is not unusual now to see practitioners use 3D concepts without stating it. For instance, when radiances or fluxes are used to determine cloud properties in a plane-parallel model that “fit the data,” these properties are the “effective” properties that account for the internal variability. This is not proof that 3D effects are negligible until independent estimates are provided for the inherent cloud properties and it is shown that they predict the same internal/boundary radiation field as observed, preferably across wavelengths.

Geophysical fluid dynamics developed the opposite way. Powerful concepts such as circulation, convection, turbulence, waves, instabilities, fronts, blocking, and so on, were defined even before the advent of computers and are still useful. Non-dimensional numbers can be estimated and used to predict what type of flow will occur. Radiative transferists are also dealing with flow, indeed that a rather strange fluid: a photon gas interacting with a complex optical medium. I will revisit our existing non-dimensional numbers (based on optical properties means) and propose a few more (based on their gradients). The classic example is optical depth and its utility as a demarcation between regimes dominated by photon streaming versus photon diffusion. As further illustration, I will use these numbers to predict

* when IP(C)A effects are maximal,

* when strong deviations from the IP(C)A --photon “channeling”-- can be expected,

* when photon diffusion goes from standard to anomalous.

The last item is in fact a statement about the fundamental transport kernel (when can it be assumed exponential, as in plane-parallel theory? and when is it effectively power-law?). In the two last entries, spatial correlations in the cloud- and extinction-fields are critically important. In all three diagnostics, the nonlinear (multiplicative) nature of matter-radiation interaction plays a key role.

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