Research

Clouds are formed by tiny droplets (diameter around 10 μm) which move in a turbulent flow where quite regions coexist with much more violent ones. For you this means that you have to fasten the seat belt on a plane when going through a highly turbulent region. For the droplets this means to live in a roller coaster where they are occasionally driven by very high accelerations. For the scientists this is a real challenge because turbulent flows are very complex involving many time and length scales and a complete description of the system is out of reach of the present calculations.

However, a good understanding of the droplet dynamics is necessary because processes at the micro scale determine the cloud behaviour at much bigger scales. For example rain starts when droplets collisions are very frequent and the rate of collisions is determined by the droplets movement in the turbulent roller coaster.

Our aim is to understand the top cloud boundary in an unstable situation. There the local mixing of dry and wet regions (like in the picture above) increases the droplet population and stimulates their movement. This process start in a very thin region but it grows rapidly due its turbulent nature, having unpredictable consequences for the cloud. We use computer simulations (DNS) to investigate the process. Our objective is to determine under which conditions this instability occurs and when and how it has to be taken into account for climate and weather models.