  The Rayleigh-Taylor (RT) instability is ubiquitously observed, yet has
traditionally been studied using ideal fluid models. Collisionality can vary
strongly across the fluid interface, and previous work demonstrates the
necessity of kinetic models to completely capture dynamics in certain
collisional regimes. Where previous kinetic simulations used spatially- and
temporally-constant collision frequency, this work presents 5-dimensional (two
spatial, three velocity dimensions) continuum-kinetic simulations of the RT
instability using a more realistic spatially-varying collision frequency. Three
cases of collisional variation are explored for two Atwood numbers: low to
intermediate, intermediate to high, and low to high. The low to intermediate
case exhibits no RT instability growth, while the intermediate to high case is
similar to a fluid limit kinetic case with interface widening biased towards
the lower collisionality region. A novel contribution of this work is the low
to high collisionality case that shows significantly altered instability growth
through upward movement of the interface and damped spike growth due to
increased free-streaming particle diffusion in the lower region. Contributions
to the energy-flux from the non-Maxwellian portions of the distribution
function are not accessible to fluid models and are greatest in magnitude in
the spike and regions of low collisionality. Increasing the Atwood number
results in greater RT instability growth and reduced upward interface movement.
Deviation of the distribution function from Maxwellian is inversely
proportional to collision frequency and concentrated around the fluid
interface. The linear phase of RT instability growth is well-described by
theoretical linear growth rates accounting for viscosity and diffusion.
