Research
Physics and chemistry of clouds: Visualization, laboratory, and field experiments
Clouds are complex systems within the atmosphere which interconnect suspended objects of very different nature through highly delicate microphysical and chemical processes. Detailed knowledge and -in particular- the capability for adequate mathematical representation are essential for the numerical simulation of clouds, and realistic forecasts of precipitation and weather, as well as for studies of cloud effects on climate. To obtain useful parameterizations for the various processes laboratory experiments still are key tools. Examples are the characterization of the collision-coalescence process underlying warm-rain formation, and experiments on the freezing of cloud/rain drops. Other experiments are dedicated to the investigation of the chemical “retention process” of soluble trace gases during riming on small ice spheres and onto snowflakes. “Retention” here means the “carry over” of these substances from the initial liquid solution of the polluted, riming, supercooled droplets onto the frozen hydrometeors. Some of the materials are returned back to the gas phase –and lost from the hydrometeors- during the freezing process. The amounts of this retention influence the vertical distribution of the trace gases inside the large convective clouds as well as their “export” through the anvil outflows at high altitudes. This is of relevance for example in the context of the Asian Monsoon Anticyclone where large amounts of trace materials are transported from the boundary layer to high altitudes by deep convective clouds.

Since 30 years the Mainz Vertical Wind Tunnel facility is operational for experiments in this research field. After the extending the facility for performing experiments down to -30 C processes relevant for so-called "mixed phase clouds" (where glaciated cloud particles coexist with supercooled liquid droplets) can be simulated. At first in the presentation several cloud physical processes (like rain formation, freezing, and riming) demonstrated by means of high speed camera video clips. Then an overview of several experimental results is provided together with a discussion of their significance.