Climate: Direct numerical simulation of cloud turbulence and its interaction with cloud drops

The role played by the clouds is fundamental for the atmosphere and the water of the earth. Nowadays our knowledge in cloud dynamics is still so poor that it represents the cause of an amount of uncertainty in climate predictions and in atmospheric circulation models.

Our project investigates the dynamics of liquid water drops in warm clouds interacting with the surrounding turbulent flow. These processes are difficult to capture because the multi scale nature of the turbulent flows given by the high Reynolds number inside a cloud. Although the characteristic length scale of the droplets (microns) it is smaller than the smallest active turbulence scale (Kolmogorov scale: millimeters), it is known that the droplet size distribution is strongly affected by turbulent flows. In particular, turbulence is responsible of the droplet size spectra broadening that promotes collisions between droplets of different sizes and consequently rain formation. Clearly at the moment, the complete dynamics are impossible to capture both in laboratory experiments and direct numerical simulation (DNS). Moreover, it is also impossible to measure directly these properties in a real cloud because of the limited resolution and control of the experimental devices. For this reasons, our aim is to quantify the water droplet dynamics in a small portion of a real cloud (order 10 meters) by means of direct numerical simulation where the complete and full droplet-turbulence interactions will be analyzed in details. This simulations will be the basis to extend the results to a real cloud size by means of upscale models. The DNS simulations will be realized by means of a pseudo-spectral Navier-Stokes solver coupled with an equation for the supersaturation field and a Lagrangian solver for the droplet dynamics. The code is fully parallelized to be efficiently used in supercomputing infrastructures.