## Rotation

It is easy to show that the last terms on the right-hand side of (13.10.4) represent the horizontal components of a circulation as given by the Stokes's relation (13.8.1), in which Z represents the vertical component of vorticity and, therefore, a rotation about a vertical axis. Thus,

13.11 Types of Wind Fields - Graphical Representation

The component motions of a linear wind field which exhibit the above -mentioned differential properties of air flow individually are:

A, uniform translation;

B, x-component of deformation, or the dilatation;

C, the y-component of deformation, or the contraction;

Fig. 13.11 Illustrating the component motions of a linear wind field in the northern hemisphere

D, the total deformation;

E, divergence, or areal expansion;

F, convergence, or areal contraction;

G, positive rotation; and

H, negative rotation

These component motions are shown schematically in Fig. 13.11

In D, the point where the axes of dilatation and contraction intersect is called a 'Col'.

In the real atmosphere, however, wind fields are much more complicated and the simple linear wind fields of the type shown in Fig. 13.11 seldom occur by themselves. More often than not, the differential properties are superimposed on one another in a given wind field. Further, the flow patterns can be central, i.e., symmetrical with reference to a center, or may not be related to a center. Fig. 13.12 shows some central circulation patterns corresponding to pressure systems with closed isobars and how the patterns are transformed when deformation and divergence are superimposed on rotation.

Fig. 13.12 Central patterns without straight streamlines, corresponding to pressure patterns with closed isobars: A, B are cases of pure rotation. P shows a case in which deformation and convergence have been superimposed on rotational field, A. In Q, deformation and divergence have been superimposed on rotational field at B

Fig. 13.12 Central patterns without straight streamlines, corresponding to pressure patterns with closed isobars: A, B are cases of pure rotation. P shows a case in which deformation and convergence have been superimposed on rotational field, A. In Q, deformation and divergence have been superimposed on rotational field at B