Cloud dynamics

Clouds play an essential role in the water and energy cycles on earth. Our understanding of clouds is so limited that it is the cause of major uncertainty in atmospheric circulation models and in predicting climate change. Our research focuses on processes that occur in warm, turbulent clouds, where liquid water drops interact with each other and the surrounding turbulent air, which causes droplet collisions and droplet evaporation. As is well known, the dynamics are difficult to capture, because they occur on a wide range of scales, which is the nature of of high Reynolds number turbulence. The Reynolds number in a cloud may be as high as 108, which implies inhomogeneities on spatial scales between 102 m and 10-3 m, and motions with frequencies between 10 Hz and 10-2 Hz. At present, the full dynamics are impossible to capture in the laboratory or by direct numerical simulation. Yet it is also impossible to make field measurements with adequate resolution, or to control the conditions in a systematic way. For this reason, we complement our laboratory experimental expertise with the field experience and the numerical investigations, of our collaborators.

The experiments we perform fall in three categories.

We study the dynamics of particles in turbulence, in the dilute limit, to know how their behavior differs from that of fluid particles as a function of their size and density. The particles are dilute in the sense that they are far apart and do not interact with each other or influence the statistics of the fluid motion. For this, we study the statistics of particle trajectories in three dimensions (3D) captured in bench-top scale apparatuses.

In a separate line of experiments in similar apparatuses, we explore directly the mechanism of particle collisions in turbulence. Such collisions may occur more frequently in more vigorous turbulence, and so may be responsible for the generation of droplets large enough to fall out of clouds as rain. Here, we employ not only 3D imaging, but phase doppler particle anemometry to measure particle diameters.

Finally, the new wind tunnel in our facility will study the mixing between clear air and cloudy air. A cloud constantly evolves through its boundary with surrounding air, and this interaction influences rain formation, the extent and lifetime of clouds, and may cause feedback on the large-scale flows in the cloud through buoyancy. We expect that we can match the dissipation rate and particle characteristics present in real clouds, while attaining a Reynolds numbers of about 105.

Contact: Eberhard Bodenschatz