Assessing the contribution of the subgrid scales in large eddy simulations of cloud turbulence

Large eddy simulations (LES) are a powerful and widely used tool in atmospheric flows, especially in studies of cloud turbulence. In LES, subgrid scale (SGS) models which represent the effects of the small scales on the resolved scales are required in order to close the equation set. To date, a significant amount of research has focused on developing accurate models for the SGS momentum and scalar fluxes. However, the role of SGS interactions between turbulence and microphysical processes in LES of cloud turbulence has received relatively little attention in comparison. Additionally, relatively few research efforts have focused on the development and testing of SGS models for turbulence-microphysics interactions.

As a first step in developing accurate SGS models for cloud turbulence, we present results from DNS of a cloudy, turbulent Rayleigh-Bernard convection at Ra ~ 1.5 x 10^9 including O(10^6-10^7) Lagrangian droplets that are fully two-way coupled with the fluid. Simulations encompassing three different aerosol injection rates ( ṅ = 1, 10, 100 cm⁻³ min⁻¹) are compared to an LES with bin microphysics of the same flow at various spatial resolutions. Simulations are used to assess the extent to which SGS representation of turbulence-microphysics interactions are necessary to recover results in good agreement with benchmark DNS.