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Vorticity and its dynamics: the stretching effect or ice-skater effect |
Stretching
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Here is a cylindrical atmospheric element with mass M and an axis parallel to the vorticity vector  0; it is called vorticity tube. If it is stretched along its axis by the ambient wind field, its mass is preserved and its length increases; it then turns faster and its vorticity increases: 1 > 0. The stretching creates vorticity. |
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This phenomenon is similar to the one used by a skater to turn faster: he throws himself with his arms apart into a rotation speed of 
0, then he brings his arms back alongside his body and his speed of rotation increases 1 > 0.
Contraction
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Conversely, if the cylindrical atmospheric element is contracted along its axis, its length decreases; it then turns more slowly and its vorticity decreases: 2 < 0. Contraction destroys vorticity. |
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To understand how small atmospheric elements can be deformed by the wind, let's return to the example of the vorticity tube.
Let's suppose that the velocity parallel to the axis of vorticity tube increases in the direction given by the axis of the tube. In this way, the right-hand section of the tube will be displaced more rapidly than the left-hand section. The vorticity tube will therefore be stretched and its vorticity will increase in accordance with the principle we saw previously.
Conversely, if the speed decreases in the direction given by the axis of the tube, the tube will contract, and its vorticity will decrease.
