During its early stages of development, an atmospheric vortex can be destroyed by vertical shear in the environmental wind. However, some vortices survive, because they have a dominant tendency to stand upright.

We will review a recent theory for the vertical alignment of a tilted quasi-geostrophic (QG) vortex (Schecter, Montgomery and Reasor, J. Atmos. Sci., in press). In this theory, vertical alignment occurs by the damping of a discrete vortex Rossby mode. The damping rate, g, is proportional to the vorticity gradient at a critical radius, where the rotation frequency of the vortex is resonant with the mode. Furthermore, g varies with the internal Rossby deformation radius, l: g can either increase or decrease with l, depending on the radial structure of the vortex.

We will also present a more general theory that covers the vertical alignment of non-QG vortices, such as incipient tropical cyclones. This new theory is based on the asymmetric balance equations of Shapiro and Montgomery (1993: J. Atmos Sci., 50, 3322-3335). As in QG theory, vertical alignment is viewed as the damping of a discrete vortex Rossby mode. Unlike QG theory, the damping rate, g, does not merely increase linearly with the mean rotation frequency, W, of a vortex with a given radial structure. Rather, there are cases in which g/W increases dramatically as the Rossby number of the vortex increases from zero to order unity.