Under atmospheric conditions, cloud droplets form when air becomes (super-) saturated with respect to water. However, the excess water is not forming pure water drops but is deposited on so-called CCN (Cloud Condensation Nuclei). Whether a particle serves as a CCN in a certain situation depends on the particle properties (mainly size and chemical composition) as well as on atmospheric dynamics (mainly updraft velocity). Almost all air-borne particles above a certain size (50-80 nm) are potential CCN. Generally, drop numbers become larger when
- CCN number increases
- Particles become larger
- Particles are water-soluble
- Updraft velocity increases
Changes in cloud droplet formation influence cloud properties such as radiation properties and precipitation formation by coalescence. With the help of microphysical models (e.g., SPECS) it can be studied in which way a cloud (in terms of drop number, water mixing ratio, precipitation, etc.) reacts on changes in the aerosol population.
The detailed description of cloud microphysics is realised with a spectral approach (Simmel et al., 2002; Simmel und Wurzler, 2006). This means that the spectra of hydrometeors (aerosol particles, droplets, ice particles) are resolved according to their mass-size distribution (e.g. 66 size classes for the size range between 1 nm up to several mm). In this way the relevant microphysical processes of the liquid (droplet nucleation, condensation, coalescence, droplet disintegration) and solid (ice nucleation, freezing processes, riming,& ) phases can be computed explicitly without the need for parameterizations.
A particular focus is on the description of ice nucleation. Pure water drops freeze in the atmosphere typically at temperatures of -35°C to -40°C or lower. Nevertheless, ice particles are observed at much higher temperatures (sometimes only few degrees below 0°C, often at -15°C or below). These ice particles are formed by heterogeneous ice nucleation processes, involving mostly insoluble particles, so-called ice nuclei (IN). Within the atmosphere four freezing processes are relevant: Deposition-, condensation-, immersion- and contact freezing. Typically, immersion and contact freezing are the most effective of these processes. During immersion freezing an IN within an undercooled drop initiates the freezing process, while during contact freezing an undercooled drop collides with an IN, such that the freezing process is initiated from the outside. Freezing temperatures and -efficiencies strongly depend on the type of the IN. Biological particles are much more efficient IN for immersion freezing than mineral particles or soot. Contact freezing typically starts at higher temperatures, and differences between biological and mineral IN particles are lower (Diehl und Wurzler, 2005; Diehl et al., 2006).
These processes are investigated with sensitivity studies and simulations of realistic situations. Cloud microphysics is mostly computed within air parcel models (partly due to the high computing times caused by the large number of variables and complex process descriptions). Nevertheless the spectral microphysical model as well as the cylinder-symmetric Asai-Kasahara model was also implemented into the COSMO model of the German weather service (DWD).
For both, CCN and IN, models allow to compare the importance of aerosol influence on cloud microphysics to other driving forces such as the underlying general dynamics, turbulence, or other parameters.