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

TL;DR

This study uses direct numerical simulation to analyze how turbulence and interfacial dynamics accelerate droplet collisions and size differentiation within cloud-clear air boundaries, revealing enhanced collision probabilities and anisotropic effects.

Contribution

It introduces a detailed numerical analysis of droplet population dynamics at cloud interfaces, highlighting the impact of turbulence structure on collision rates and droplet differentiation.

Findings

Increased collision probability in the interfacial layer due to anisotropic turbulence.

Enhanced droplet size differentiation caused by turbulence-induced velocity fluctuations.

Distinct behaviors observed between mono-disperse and poly-disperse droplet populations.

Abstract

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.