'Cloud Arc' Cloud Structure

Why didst thou promise such a beauteous day,
And make me travel forth without my cloak,
To let base clouds o'ertake me in my way,
Hiding thy bravery in their rotten smoke?
William Shakespeare, Sonnet XXXIV
Structure Description
Global Occurrence
Observation Diagrams

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"Cloud Arc" Structure (CAS)

The structure called "Cloud Arc" looks from above with optimal conditions (there is hardly cross-wind) circular or oval.
There is more highly lying cloud mass in the centre of the very narrow circle belts of cumulus and stratocumulus. It consists of stratus and altostratus radial moved clouds, which are bordered from stratocumulus clouds.
The luff side very often disappears (or is squeezed along the wind direction) and the cloud form of the centre shifts to the lee side.
It can be also "Cloud Arc" structures observed, where there are no clouds insides; then only empty circles is seen.

These structures are not rare observed as a group of two, three and more circles.
Also there are cases, where circles (often empties) fuse and form a chain from some cells (it looks as a chain) along the wind direction.

The size of this structure is from 15 km (the smaller one was not noticed because of the resolution of satellite images) up to 250 km and more.
The Cloud top height of the circular belts is 1.5-2.5 km;
The Cloud top height the interior clouds is 3-3,5 (sometimes to 4,5) km.
The albedo of the centre section is 0.5-0.9.
The structure emerges normally within +-30 latitude

  • above central and west Pacific,
  • above west Atlantic,
  • above Indian ocean usually in the south of equator.

This structure is to be observed not seldom.
It can be assumed, that it develops still more frequently, but the narrow belt releases fast and only the interior clouds remain.

The interior clouds are thicker than the others; they are located within a vertical convective flow and rise close to the centre of the circle upward.
While air rises to inversion layer, it spreads horizontal in all directions, until it finally cools and sinks down back to the sea surface.
This process pushes all existing small cumulus clouds away of the central region of convection. In addition if air sinks, it warms and prevents the forming of other small clouds. Therefore the area within the circle is kept cloudless.
The horizontal flow of sinking air becomes rather weaker on the circle border. It will be influenced of central rising flow. Air moves towards the centre now, where it can rise again, perhaps, supporting of the new quantity of the small cumulus clouds.

It is interesting to observe the form of the sea surface under the "Cloud Arc" structure.
If this structure lies in a sun glint area for the satellite point of view, the brighter lighting of the sea surface from the sun side is seen, so it looks that the water surface which is under the structure would raise a little.
The phenomenon could be explained as follows. The deeply flowing wind is stronger near to the centre of "Cloud Arc" cell. It produces the waves, which reflect the sun light less.
From the other side the water surface could be really sucked upward with the air, which rises in the centre of the cell. The sea surfase rises by a very small amount so, that it can be noticed.

Global Occurrence Diagram
Global occurrence of 'Cloud Arc' Structure



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