  Accretion disks in close binary systems are subject to a tidally driven
parametric instability which leads to the growth of internal waves near the
outer edges of such disks (Goodman 1993). These waves are important in
understanding the torque exerted on a disk by tidal forces and may play a role
in the structure of the disk at small radii. Here we calculate the growth rate
of this instability, including the effects of vertical structure and fluid
compressibility. We find growth rates which are only slightly different from
Goodman's original results, except that near the vertical resonance radius the
growth rate can have an extremely broad and strong peak when the disk is stably
stratified in the vertical direction. Higher order modes, in the sense of
increasing number of vertical nodes, have similar growth rates. Our results
differ from a previous calculation along these lines by Lubow et al. (1993).
The difference is mostly due to their neglect of radial gradients in the
tidally distorted streamlines.
