Relative Vorticity

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Relative Vorticity

As we have just seen in our example, relative vorticity is observable. You can examine any point on an upper-air chart and say whether an air parcel at that point does or does not have vorticity. It’s simply a matter of checking the wind on either side of the point to determine the shear and noting the parcel’s position in the air stream for the effect of curvature. Relative vorticity is a measure of the spin created by shear and by curvature.

SHEAR.— Let’s examine the shear effect by looking at small air parcels in an upper-air pattern of straight contours. The wind shear in our

Figure 8-5-1.—Illustration of vorticity due to the shear effect.

example causes two of the three parcels to rotate. See figure 8-5-1.

1. Parcel No. 1 has stronger wind speeds to its right. As the parcel moves along, it is rotated in a counterclockwise direction and thus has positive vorticity.

2. Parcel No. 2 has the stronger speed to its left; therefore, it rotates in a clockwise direction as it moves along. Its vorticity is negative.

3. Parcel No. 3 has speeds evenly distributed. There’s no shear. The parcel moves, but it does not rotate. It has zero vorticity.

Remember, air parcels have vorticity (rotation) when the wind speed is stronger on one side of the parcel than on the other.

CURVATURE.— Vorticity also results from curvature of the air flow or path. Examine figure 8-5-2 and try to imagine the progress (and direction of spin) of an air parcel traveling in the stream. In the trough and ridge, the diameter of the parcel is rotated from the solid line to the dotted position (because of the northerly and southerly wind components on either side of the trough and ridge lines). Parcels have

Figure 8-5-2.—Illustration of vorticity due to curvature effect.

counterclockwise rotation in troughs (positive vorticity) and clockwise rotation (negative vorticity) in ridges. At a point between a trough and a ridge where there is no curvature, the inflection point, there is no spin imparted. This is shown at point P in figure 8-5-2.

COMBINED EFFECTS.— To find the rela-tive vorticity of a given parcel, you must consider both shear and curvature. It is quite possible that the two effects will counteract each other. That is, shear may indicate positive vorticity, and curvature indicate negative vorticity, or vice versa. See figure 8-5-3.

When shear and curvature counteract each other, the amount of vorticity in each must be measured. The two figures are then added together algebraically to determine if vorticity is positive or negative. Measurements also indicate whether the vorticity is increasing or decreasing. When vorticity is increasing, the rotation is becoming more cyclonic. When it decreases, the converse is true.

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