7/25/2023 0 Comments Raindrop shape pathModels that describe the size dependence of the axis ratio of raindrops are based on the solution of the Laplace equation, presuming a force balance on a surface element of the drop between aerodynamic pressure, surface tension, and the internal hydrostatic pressure of the drop. Moreover, the distortion increases that is, the axis ratio (the ratio of the vertical and horizontal chords of the drop) decreases with drop size. Early observations (see, e.g., Pruppacher and Klett 1997) showed that the shape of the raindrops depends on the drop size: while drops with diameters less than 1 mm are almost spherical, larger drops are oblate spheroids. The accurate determination of the raindrop shape is of crucial importance in radar meteorology, where it is the key microphysical parameter involved in prediction of the rainfall rate with dual-polarization radars ( Seliga and Bringi 1976 Hagen and Meischner 2000). All of these characteristics are induced by the external airstream, they are dependent on the drop size, and they interact with each other ( Pruppacher and Klett 1997). Raindrops freely falling at their terminal velocity in air have three key characteristics that must be considered in the hydrodynamic description: the equilibrium drop shape, the oscillation behavior, and the internal circulation. Experiments are included in which the internal circulation associated with drop oscillation was investigated and compared to theory.Ī good understanding of the hydrodynamic behavior of water drops is essential for improving the remote measurement of rain fall rates and nowcasting of precipitation. The analysis of the oscillation frequency of the raindrops revealed that the drops undergo multimode oscillations and are oscillating in a transverse mode in addition to an axisymmetric oblate–prolate mode. The time-averaged axis ratio was found to be equal to the equilibrium axis ratio in the investigated raindrop size range. The drop size determination by means of the frequency method was found to be three times more precise than by volumetric methods. A new method was developed to determine the equivalent drop diameter with the help of the oscillation frequency. A very good agreement was found between the measured and the theoretically determined raindrop shape calculated by a force balance model. These parameters for individual water drops with equivalent diameter from 2.5 to 7.5 mm were investigated in a vertical wind tunnel using high-speed video imaging. Because the raindrops undergo oscillation, the most important shape parameters from the radar prediction point of view are the equilibrium drop shape, the time-averaged axis ratio, and the oscillation frequency. Precipitation prediction using weather radars requires detailed knowledge of the shape parameters of raindrops falling at their terminal velocities in air.
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