  The mechanism describing the recently developed notion of kernel gravity
waves (KGWs) is reviewed and such structures are employed to interpret the
unstable dynamics of an example stratified plane parallel shear flow. This flow
has constant vertical shear, is infinite in the vertical extent, and
characterized by two density jumps of equal magnitude each decreasing
successively with height, in which the jumps are located symmetrically away
from the midplane of the system. We find that for a suitably defined
bulk-Richardson number there exists a band of horizontal wavenumbers which
exhibits normal-mode instability. The instability mechanism closely parallels
the mechanism responsible for the instability seen in the problem of
counter-propagating Rossby waves. In this problem the instability arises out of
the interaction of counter-propagating gravity waves. We argue that the
instability meets the Hayashi-Young criterion for wave instability. We also
argue that the instability is the simplest one that can arise in a stratified
atmosphere with constant shear flow. The counter propagating gravity waves
mechanism detailed here explains why the Rayleigh criteria for shear flow
instability in the unstratified case does not need to be satisfied in the
stratified case. This illustrates how the Miles-Howard theorem may support
destabilization through stratification. A normal mode analysis of a foamy layer
consisting of two density jumps of unequal magnitude is also analyzed. The
results are considered in terms of observations made of sea-hurricane
interfaces.
