Some experimental evidence hints at the co-existence of several modes of oscillations.
Equations which describe the oscillation frequencies of different modes are confirmed by laboratory measurements. 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. 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. On the theoretical side, raindrop deformation and oscillations are given by model computations. Experiments that are performed in fall shafts or vertical wind tunnels are complemented by ground-based and airborne field observations. 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.
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.
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