The role of turbulence in drop coagulation
An obvious reason for drop motion is the fall (sedimentation) under the influence of gravity and air friction. The resulting drop velocity increases with drop size, so most drops fall at different speed than the neighbouring drops. This differential sedimentation is responsible for many collisions and has been studied since many decades. However, the gravitational coagulation alone cannot explain the fast growth of large cloud drops (15-40 micrometres radius) as observed in nature.
Numerical models of coagulation are often based on the bin approach. For pure gravitational coagulation, the formulation of the kernel is mathematically simple and is based on the different fall speeds of the two colliding drops. For turbulence, however, matters are much more complicated, and research on the formulation of the kernel is still ongoing and involves detailed numerical simulations and wind tunnel experiments.
Compared to gravitational coagulation, the inclusion of turbulence makes it more likely that very small drops as well as drops of similar size do collide and merge.
This can be seen in the development of precipitation below a cloud in 2 km height, simulated with no turbulence (black), realistic turbulence typical for clouds (blue) and very strong turbulence (red). The stronger the turbulence is, the earlier is the arrival of rain on the ground, because more larger drops have been formed.
The impact of turbulence on drop growth by collision and merging (coagulation) has been the focus of the collaborative bundle project “Cloud drop dynamics”: six research groups from Germany worked together within the framework of the priority program SPP 1276 MetStröm sponsored by the German research foundation DFG.