A New Warm Cloud Microphysics Parameterization on the Droplet Collision Statistics

English Abstract:

Warm, shallow convective clouds are important components of Earth’s weather and climate systems. The parameterization on the microphysics processes of these clouds is a major source of uncertainty in weather and climate models. Accordingly, more quantitative analysis is required to seek a realistic parameterization in cloud processes.

In this study, we investigate the effects of turbulence on the cloud droplet collision-coalescence process. An MPI-based direct numerical simulation (DNS) model modified from the work of Vaillancourt et al. (2001 and 2002) and Franklin et al. (2005) is performed to simulate the turbulent flow. Different turbulence intensities (eddy dissipation rates from 50 cm²/s³ to 1500 cm²/s³, which covers most of the observations in cumulus clouds) and a relatively broad Reynolds number range (Taylor-based Re from 63 to 589) are investigated to quantify the influence of turbulence intensity and the computaional Reynolds number on droplet collision statistics. Mono-disperse and bi-disperse collisions between droplets from 5 micron to 25 micron are considered since those sizes are critical to the formation of larger droplets to trigger gravitational collisions.

Based on the statistics of the DNS results and previous theoretical and empirical studies, we develop an empirical parameterization that accurately describes the collision statistics (i.e. the radial distribution function, radial relative velocity, and finally the collision kernel) in terms of several known non-dimensional parameters involving the radii of the droplets and the eddy dissipation rate.