Shapes and oscillations of falling raindrops - A review

Since more than a hundred years ago the deformed shape of raindrops and their oscillations have been observed but investigations are still ongoing as the experimental and numerical techniques are being improved, and the demand from meteorological applications (e.g., weather radars) for more precise characterization of natural raindrops have increased. For laboratory measurements, the realistic simulation of atmospheric conditions is crucial so it is important that drops fall at their terminal velocities and appear as motionless in vertical wind tunnels. Experiments that are performed in fall shafts or vertical wind tunnels are complemented by ground-based and airborne field observations. On the theoretical side, raindrop deformation and oscillations are given by model computations. Comparing model axis ratios with different kinds of measurements allows one to conclude that the dynamic axis ratio of oscillating raindrops can be appropriately obtained from a force balance model. In the size range between 1 and 2.5 mm diameter, which is of great importance from the point of view of radar meteorology, it is still questionable whether the dynamic axis ratios are equal to the equilibrium axis ratios in all cases. Equations which describe the oscillation frequencies of different modes are confirmed by laboratory measurements. Some experimental evidence hints at the co-existence of several modes of oscillations. It is, however, still not completely clear which modes can be active and how the existence of higher modes depends on the raindrop size. In this paper a review of the current knowledge about the time average axis ratio, the oscillation frequencies and modes, and the shape of freely falling raindrops is presented: besides an overview of the adopted experimental techniques is provided, and the remaining open questions are highlighted. (C) 2010 Elsevier B.V. All rights reserved.