Numerical study of small-scale turbulent mixing in clouds

The influence of turbulent mixing of dry air with moist air on the water droplet ensemble is studied deep inside a warm cloud. A proposed simplified model of cloud mixing summarizes the balance equations of water vapor mixing ratio and temperature to one effective advection-diffusion equation for supersaturation field. Three-dimensional direct numerical simulations couple the scalar supersaturation field in the turbulent flow with cloud droplet dynamics, in particular to the droplet size distribution for different size of the computation domain, such that the Reynolds numbers of the turbulent flow increase correspondingly.

Finite-time Lyapunov exponents are monitored such that we can relate high compressive strain regions to those of high local supersaturation amplitudes. We find that the mixing process in terms of the droplet evaporation is always homogeneous in the bulk of the cloud, while being inhomogeneous in view to the relaxation of the supersaturation field.