Condensational and collisional growth of cloud droplets in a turbulent environment

Abstract

We investigate the effect of turbulence on the combined condensational and collisional growth of cloud droplets by means of high resolution direct numerical simulations of turbulence and a superparticle approximation for droplet dynamics and collisions. The droplets are subject to turbulence as well as gravity, and their collision and coalescence efficiencies are taken to be unity. We solve the thermodynamic equations governing temperature, water-vapor mixing ratio, and the resulting supersaturation fields together with the Navier-Stokes equation. We find that the droplet-size distribution broadens with increasing Reynolds number and/or mean energy dissipation rate. Turbulence affects the condensational growth directly through supersaturation fluctuations, and it influences collisional growth indirectly through condensation. Our simulations show for the first time that, in the absence of the mean updraft cooling, supersaturation fluctuation-induced broadening of droplet-size distributions enhances the collisional growth. This is contrary to classical (non-turbulent) condensational growth, which leads to a growing mean droplet size, but a narrower droplet-size distribution. Our findings, instead, show that condensational growth facilitates collisional growth by broadening the size distribution in the tails at an early stage of rain formation. With increasing Reynolds numbers, evaporation becomes stronger. This counteracts the broadening effect due to condensation at late stages of rain formation. Our conclusions are consistent with results of laboratory experiments and field observations, and show that supersaturation fluctuations are important for precipitation.

arXiv commentrevised version, 17 pages, 1 table, 13 figures

References in corpus (19)

• Simulations of nonhelical hydromagnetic turbulence
• Super-Droplet Method for the Numerical Simulation of Clouds and Precipitation: a Particle-Based Microphysics Model Coupled with Non-hydrostatic Model
• Caustics in turbulent aerosols
• Adding particle collisions to the formation of asteroids and Kuiper belt objects via streaming instabilities
• Caustic activation of rain showers
• Statistical models for spatial patterns of heavy particles in turbulence
• A representative particle approach to coagulation and fragmentation of dust aggregates and fluid droplets
• Distribution of relative velocities in turbulent aerosols
• Clustering of particles falling in a turbulent flow
• Relative velocities of inertial particles in turbulent aerosols
• Eulerian and modified Lagrangian approaches to multi-dimensional condensation and collection
• Large deviations and rain showers
• Statistical steady state in turbulent droplet condensation
• Inertial drag on a sphere settling in a stratified fluid
• Effect of turbulence on collisional growth of cloud droplets
• Statistics of the relative velocity of particles in turbulent flows : monodisperse particles
• Cloud-droplet growth due to supersaturation fluctuations in stratiform clouds
• Relative velocities in bidisperse turbulent suspensions
• Relative velocities in bi-disperse turbulent aerosols: simulations and theory

Cited by in corpus (7)

• Large-eddy simulations of marine boundary-layer clouds associated with cold air outbreaks during the ACTIVATE campaign-part 1: Case setup and sensitivities to large-scale forcings
• Key parameters for droplet evaporation and mixing at the cloud edge
• Quantitative prediction of sling events in turbulence at high Reynolds numbers
• Intermittency acceleration of water droplet population dynamics inside the interfacial layer between cloudy and clear air environments
• Coagulation of inertial particles in supersonic turbulence
• Small-Scale air turbulence structure, microphysical time scales and local supersaturation balance at a warm Cloud Top Boundary
• Diffusion of turbulence following both stable and unstable step stratification perturbations