The potential impact of turbulent velocity fluctuations on drizzle formation in Cumulus clouds in an idealized 2D setup

TL;DR

This study investigates how turbulent velocity fluctuations influence drizzle formation in Cumulus clouds using idealized 2D simulations, revealing turbulence accelerates drizzle development and affects droplet spectrum evolution.

Contribution

It introduces and compares two turbulence representations in a Lagrangian microphysics model, highlighting their effects on droplet collision and drizzle formation in clouds.

Findings

Turbulence significantly enhances droplet collision rates.

Drizzle forms faster with turbulence inclusion in simulations.

Aerosol spectrum changes are minimal during 20 minutes of cloud evolution.

Abstract

This article discusses a potential impact of turbulent velocity fluctuations of the air on a drizzle formation in Cumulus clouds. Two different representations of turbulent velocity fluctuations for a microphysics formulated in a Lagrangian framework are discussed - random walk model and the interpolation, and its effect on microphysical properties of the cloud investigated. Turbulent velocity fluctuations significantly enhances velocity differences between colliding droplets, especially those having small sizes. As a result drizzle forms faster in simulations including a representation of turbulence. Both representations of turbulent velocity fluctuations, random walk and interpolation, have similar effect on droplet spectrum evolution, but interpolation of the velocity does account for a possible anisotropy in the air velocity. All discussed simulations show relatively large standard deviation ($\sim$1${\mu}m$) of the cloud droplet distribution from the onset of cloud formation is observed. Because coalesence processes aerosol inside cloud droplets, detail information about aerosol is available. Results from numerical simulations show that changes in aerosol spectrum due to aerosol processing during droplet coalescence are relatively small during $\sim$20 min. of the cloud evolution simulated with numerical model. Drizzle forms initially near the cloud edge, either near the cloud top, where the mass of water is the largest, or near the entrainment eddies.