Personal attack noted.

Dfrank, you are trying to reduce the rules of fluid behavior to some imaginary simple case where you can say "definitely yes" or "definitely no." It was your choice to try to argue that fluid dynamics doesn't allow for this observation, and therefore it is your burden of proof to show that fluid dynamics can't produce the effect seen. Simply wondering how it can, and assuming it can't, doesn't cut it; nor does accusing others of religious dogmatism when all you have is handwaving and wishful thinking. You seem to have little understanding of what proof entails in a scientific sense.

Fluids interact in complex ways with complex surfaces. I live in Utah. I fly frequently over the deserts of southern Utah and Nevada. I have seen endless depictions of the interactions between air and particulates, including highly characteristic duning behavior, and I am highly disinclined to fit any one or two simple rules to it. My scientific understanding of fluid flow and my observation of its effect at many scales is the basis of my opinion that your claim is delusional simply because it tries to oversimplify.

Duning in this case suggests prevailing laminar fluid flow in a single direction with moderate chaotic diversions. However, passing over the depression there will create a highly turbulent flow under the laminar layer, but without necessarily disturbing much of it above, so it can continue to be laminar and produce dunes downstream.

In enclosed turbulent flow such as that, the lowest velocities are found in the center, which is then where entrained particles will tend to precipitate. These will be much finer particles than the surrounding pre-existing material (because they were light enough to be entrained). And so an aggregation of them will appear lighter in value when photographed.

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