The effect of turbulence, gravity, and non-continuum hydrodynamic interactions on the drop size distribution in clouds

TL;DR

This paper investigates how turbulence, gravity, and non-continuum effects influence the growth of cloud droplets, especially crossing the size-gap where traditional models are inaccurate, by analyzing collision mechanisms and hydrodynamic interactions.

Contribution

It provides a detailed analysis of turbulent collision processes, water vapor fluctuations, and non-continuum hydrodynamics affecting droplet growth in clouds, using a Monte Carlo simulation approach.

Findings

Turbulent shear and differential sedimentation drive growth to drizzle-sized droplets.

Water vapor fluctuations maintain polydispersity and promote growth beyond condensation.

Non-continuum hydrodynamics significantly affect droplet collision rates.

Abstract

The evolution of micron-sized droplets in clouds is studied with focus on the 'size-gap' regime of 15-40 $\mu m$ radius, where condensation and differential sedimentation are least effective in promoting growth. This bottleneck leads to inaccurate growth models and turbulence can potentially rectify disagreement with in-situ cloud measurements. The role of turbulent collisions, mixing of droplets, and water vapour fluctuations in crossing the 'size-gap' has been analysed in detail. Collisions driven by the coupled effects of turbulent shear and differential sedimentation are shown to grow drizzle sized droplets. Growth is also promoted by turbulence-induced water vapour fluctuations, which maintain polydispersity during the initial condensation driven growth and facilitate subsequent growth by differential sedimentation driven coalescence. The collision rate of droplets is strongly influenced by non-continuum hydrodynamics and so the size evolution beyond the condensation regime is found to be very sensitive to the mean free path of air. Turbulence-induced inertial clustering leads to a moderate enhancement in the growth rate but the intermittency of the turbulent shear rate does not change the coalescence rate significantly. The coupled influence of all these phenomena is evaluated by evolving a large number of droplets within an adiabatically rising parcel of air using a Monte Carlo scheme that captures turbulent intermittency and mixing.