Since many years it is assumed and in scientific journals intensively discussed that turbulence, that is ubiquituous in all types of clouds more or less in clouds, has an impact on collisions between cloud droplets. For investigation of this effect scientists of the Aerodynamisches Institut (AIA) of RWTH Aachen 
, the Institut für Meteorologie und Klimatologie (IMUK), Hannover, and IMK-TRO have gathered as part of the priority program 1276 ‘Metstroem’ (meteorology and fluid dynamics) of the German Research Foundation. In detail it is investigated which influence of the local turbulence (in clouds) exists on the collision efficiency of small cloud droplets and thus on the coagulation function the latter of which is important to the physical effect of precipitation evolution. In particular it should be clarified how the precipitation process is altered if turbulence effects are taken into account. Applications of the new results will be an improved (or adapted) version of a parameterization of the coagulation function to be used in 2D cloud model (IMUK) and in the 3D simulation of deep convective clouds by the operational NWP model COSMO of the German Weather Service. Starting point of the investigations which are performed mainly at AIA is a program package that has been developed by AIA since many years and which has successfully used for situations with technical background. This package works with a (fluid) solver of the RANS equations considering Cartesian hierarchically refined grids with the help of a finite volume method. Large a number of droplets (more than 107) are introduced in an isotropic-turbulent fluid field as passive tracers. The turbulent flow (with certain predefined characteristics at the entrance plane) decays on its further way. The trajectories of the droplets and their mutual collisions are then calculated. The number of resolved numerical cells is approximately 50 million, the time step is about less than 10-4 s. The drop radii considered vary between 5 and 95 microns. First results show, relative to other independent calculations as from Franklin et al. (2007) and Ayala et al. (2008), mostly a good agreement for the polydisperse case. For monodisperse particles which are not important for cloud microphysical problems a larger discrepancy appears relative to the other investigations mentioned previously.
Literature:
Kunnen, R.P.J., Siewert, C., Meinke, M., Schröder, W., Beheng, K.D., 2012:
Numerically determined geometric collision kernels in spatially evolving isotropic turbulence relevant for droplets in clouds.
Atmos. Res. (submitted)