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We use direct numerical simulation to study the temporal evolution of aperturbation localized on the turbulent layer that typically separates a cloudfrom the surrounding clear air. Across this shearless layer, a turbulentkinetic energy gradient naturally forms. Here, a finite perturbation in theform of a local initial temperature fluctuation is applied to simulate ahydrodynamic instability inside the background turbulent air flow. A numericalinitial value problem for two diametrically opposite types of drop populationdistributions is then solved. Specifically, we consider a mono-dispersepopulation of droplets of 15 $\mu$m of radius and a poly-disperse distributionwith radii in the range 0.6 - 30 $\mu$m. For both distributions, it is observedthat the evaporation and condensation have a dramatically different weightinside the homogeneous cloudy region and the interfacial anisotropic mixingregion. It is observed that the dynamics of drop collisions is highly effectedby the turbulence structure of the host region. The two populations show acommon aspect during their energy decay transient. That is the increasedprobability of collisions in the interfacial layer hat houses intenseanisotropic velocity fluctuations. This layer, in fact, induces an enhanceddifferentiation on droplets kinetic energy and sizes. Both polydisperse andmonodisperse initial particle distributions contain $10^7$ droplets, matchingan initial liquid water content of $0.8 g/m^3$. An estimate of the turbulentcollision kernel for geometric collisions used in the population balanceequations is given. A preliminary discussion is presented on the structure ofthe two unsteady non ergodic collision kernels obtained inside the cloudinterface region.

Intermittency acceleration of water droplet population dynamics inside the interfacial layer between cloudy and clear air environments