Settling velocity of cloud droplets in homogenous isotropic turbulence from high-resolution simulations

In recent years, increasing efforts have been made to understand the role of turbulence in a number of atmospheric processes such as cloud microphysics and dynamics. For example, turbulent collision-coalescence of cloud droplets substantially accelerates formation of warm rain. Small-scale turbulent motion augments the radial relative velocity between droplets and induces droplet accumulation in the low vorticity region. This leads to enhancement of collision-coalescence and consequently accelerates the formation of the raindrops. Turbulent collision-coalescence plays an important role for droplets of radius from 10 to 60 μm. For larger droplets gravity dominates the motion but turbulence still affects collision rate through altering the settling velocity. Relative differences in settling velocity have a direct effect on the collision rate. In this study, we examine numerically several mechanisms that may enhance or reduce the settling velocity of the cloud droplets. One of those is preferential sweeping, which occurs for strong turbulence and large droplet inertia. On the other hand, vortex trapping or loitering (droplets spend more time in upflow than in downflow regions of the flow) could lead to reduction of the settling velocity. In both cases, the distribution of droplets relative to vortical structure is mainly controlled by their inertial interactions with the small-scale turbulence. However, the level of increase or decrease in the average settling velocity depends strongly on the large energetic eddies.

Recently we developed a highly scalable implementation of direct numerical simulations of turbulent transport and collision of inertial particles which allow simulations at high flow Reynolds numbers. Using this implementation we simulate the dynamics of cloud droplets in isotropic homogenous turbulence, to study the conditions for the different mechanisms affecting the settling rate. The effect of large-scale flow will be studied by systematically increasing the scale range or domain size relative to the Kolmogorov scale. Since the relative importance of gravity and particle inertia changes with the flow dissipation rate, we will also present results for a variety of flow dissipation rates.