This paper reports a barotropic instability analysis of nonhydrostatic colu
mnar disturbances in a layer of homogeneous fluid over a flat bottom surfac
e. The extended form of Hamilton's principle of least action is used to obt
ain the canonical form of Green-Naghdi equations for this model. The basic
state has a monotonic shear flow and a variable depth. For values of the pa
rameters supposedly relevant to an environment of nonsupercell tornadoes (N
ST), the nonhydrostatic effect is found to have significant impacts. It is
capable of sufficiently slowing down some short gravity waves so that they
resonantly interact with vorticity waves of the same wavelengths in the she
ar zone to form strongly unstable gravity-vorticity hybrid modes. Their ins
tability properties are by and large compatible with the observed counterpa
rts of NST, such as growth rate, phase speed, length scale, aspect ratio, s
tructure, and energetics. Those results are robust for a reasonable range o
f each key parameter. Apart from a number of caveats, it is reasonable to s
uggest that nonhydrostatic barotropic instability is applicable to NST gene
sis.