Critical comments to results of investigations of drop collisions in turbulent clouds

During the last decade numerous studies considered collisions of inertial particles in turbulent flows. A magnitude of the turbulence-induced collision rate enhancement factor reported in these studies ranges from a few percent to several hundred. The authors of the majority of the studies apply their results to explanation of rain formation in atmospheric clouds. At the same time many of these investigations were performed under the conditions quite different from those encountered in real clouds. For instance, in most analytical and direct numerical simulations (DNS) the effect of gravity-induced differential drop sedimentation was neglected. Using the collision enhancement factors obtained in these studies for cloud modeling may lead to unrealistic cloud evolution and impair research in cloud physics. In this study we present an analysis of the applicability of the results obtained in different recent studies (mainly DNS simulations) to actual clouds. We discuss the progress reached in the topic as well as unsolved problems.