Development of a Comma Pattern
The shape of the cyclonic comma pattern helps immediately to reveal the location of the vorticity maximum. This is illustrated in animations below. In the animations you can recognise the shape of two arches representing inflow (concave) and outflow (convex). The shape of these arches are related to both the intensity of the vorticity maximum and to the length of time the vorticity maximum has been acting on these arcs. The point of intersection of these two arches is called cusp and is the location of the cloud rotation and the vorticity maximum.
The vorticity maximum (X) is the addition of both horizontal wind shear (S) (simply referred to as shear hereafter) and rotational vorticity (R). The following graphics illustrate the concept. The size of the symbols indicates their relative intensity.
Given pure rotation, this conceptual cloud line will evolve in a symmetrical fashion. The position of the vorticity maximum (X) is at the center of the rotation, which is in this case also the cusp (the point where two curves intersect). The rotational vorticity (R) is the sole component of vorticity. There is no shear vorticity component. The shape of the comma is then determined by both the intensity of the vorticity maximum and its age. Please note that situations like this you will not often find in your daily operational work. Normally there is some increased shear component working on the rotation to act on the shapes of the in- and outflow arches. |
The second animation reveals some differences from the first. On the equatorial side of the rotation there is increased shear (as marked by the bigger S), the cusp and by this the total vorticity is shifted towards that region of greater shear. The arc of inflow and the location of the wind maximum are enhanced. The outflow arc is not enhanced and is created purely by the rotational component of the vorticity, which remains unchanged and located at the center of the circle. The vorticity maximum (X), resulting from the sum of the rotational (R) and shear vorticities (S), is hereby thus shifted to the south! This pattern is very common due to the prevailing westerly circulations around the globe. |
With even stronger winds the equatorial shear component of the vorticity increases and there is more displacement of the total vorticity and point of inflection toward the region of shear and away from the position of the rotational vorticity centre. The inflow arc in the location of the wind maximum is really enhanced. And again, with increased shear vorticity, the vorticity maximum will also be much more intensive. |
Another scenario takes place when we place the shear on the poleward side of the rotation, the same shifting of the cusp and the vorticity maximum takes place, again with the center of total vorticity shifting towards the region of shear. In contrast, however, the convex arc in the location of the wind maximum is enhanced. The concave arc is not enhanced and is created purely by the rotational component of the vorticity, which remains unchanged and located at the center of the circle. The vorticity maximum will be buried within the moisture area as opposed to the previous illustrations, where it is located in the dry portion of the circulation. This enhancing of the convex arc is "vexing" in that the point of inflection and vorticity maximum may be more difficult to locate than when the vorticity is dominated by its rotational component or when the shear is equatorward of the rotational center. Poleward shear comma shapes are common with the northeast trade winds in the tropics. |
The four scenarios presented above showed variable strengths of shear vorticity on equator and poleward side and how these act on the formation of comma shapes. It is important that when using satellite images you are able to recognise the position of the vorticity maximum. This maximum is situated at the cusp, which is the intersection point of both the outflow and the inflow arch! Using an animation of several satellite images will significantly help you to identify whether shear is found on the poleward or on the equatorward side of the maximum.