  This letter investigates the effect of shear on Rayleigh-Taylor instability
(RTI). Even simple uniform shear strongly influences the instability; longer
waves are completely stabilized when density stratification is large (higher
Atwood numbers). This apparently counter-intuitive result is due to the
presence of Atwood number in the shear term. When the unstable density
interface is embedded in a shear layer, shear is again found to stabilize the
RTI. However, this configuration introduces additional unstable shear
instability modes in the lower wavenumber regime. A new type of shear
instability, whose growth rate increases with Atwood number, plays a dominant
role, while Kelvin-Helmholtz instability (KHI), which was previously understood
to be the only possible shear instability in this context, has little
significance. Hence the billows observed in the nonlinear stages of RTI, which
are usually attributed to KHI, may actually be the nonlinear manifestation of
this new instability.
