  We use direct numerical simulation to study the temporal evolution of a
perturbation localized on the turbulent layer that typically separates a cloud
from the surrounding clear air. Across this shearless layer, a turbulent
kinetic energy gradient naturally forms. Here, a finite perturbation in the
form of a local initial temperature fluctuation is applied to simulate a
hydrodynamic instability inside the background turbulent air flow. A numerical
initial value problem for two diametrically opposite types of drop population
distributions is then solved. Specifically, we consider a mono-disperse
population of droplets of 15 $\mu$m of radius and a poly-disperse distribution
with radii in the range 0.6 - 30 $\mu$m. For both distributions, it is observed
that the evaporation and condensation have a dramatically different weight
inside the homogeneous cloudy region and the interfacial anisotropic mixing
region. It is observed that the dynamics of drop collisions is highly effected
by the turbulence structure of the host region. The two populations show a
common aspect during their energy decay transient. That is the increased
probability of collisions in the interfacial layer hat houses intense
anisotropic velocity fluctuations. This layer, in fact, induces an enhanced
differentiation on droplets kinetic energy and sizes. Both polydisperse and
monodisperse initial particle distributions contain $10^7$ droplets, matching
an initial liquid water content of $0.8 g/m^3$. An estimate of the turbulent
collision kernel for geometric collisions used in the population balance
equations is given. A preliminary discussion is presented on the structure of
the two unsteady non ergodic collision kernels obtained inside the cloud
interface region.
