Experimental and numerical investigation of collision induced and spontaneous raindrop breakup for forecasting and nowcasting of precipitation
Final Report Abstract
To improve precipitation forecasting and the interpretation of satellite signals requires the correct description how drop sizes develop in a cloud. This process is significantly affected by the breakup of drops. We performed experimental studies at the Mainz vertical wind tunnel where individual drops were freely suspended in a vertical air stream as in a real cloud. In one series of experiments, we observed whether drops broke up spontaneously when their oscillations cause disintegration, either natural ones in a laminar air flow or those induced by turbulences in the air stream. The increasing amplitude of the horizontal oscillation mode played a key role under laminar conditions; about 5% of the observed drops with equivalent diameters between 8 and 13 mm broke up. The drops separated in the center and formed two new drops of equal sizes. Turbulences in the airflow changed the drop oscillation in the way that several oscillation types mainly in vertical and transverse directions caused significant deformations of the drops. However, this behavior seemed to promote the maintenance of the drops. Thus, we concluded that spontaneous breakup is negligible in comparison to collision-induced breakup. This was studied in another series of experiments, when two drops collided and broke up into several fragments. To fill the gaps of missing drop sizes in earlier measurements, a set of relevant drop pairs with diameters around 5 and 3 mm was investigated to determine the fragment size distribution for different breakup types. Due to the results, earlier parameterizations were modified. Besides the experiments, we performed cloud model simulations to determine in sensitivity studies the impact of breakup on drop sizes and precipitation. We compared a new parameterization based on recent and present experiments of collision-induced breakup and an earlier parameterization based on experiments of mixed spontaneous and collision-induced breakup. The results showed that important parameters are the breakup efficiency and the fragment number and size. Moderate breakup had the effect to even increase precipitation while breakup, as described close to real cases, reduced precipitation. Moreover, breakup changed the nature of precipitation in the way that it delayed its onset time and that it was more located in the cloud center.
Publications
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Experimental wind tunnel studies on spontaneous and collision-induced breakup of drops. Internation Conference on Clouds and Precipitation, South Korea, 2024, poster presentation.
Diehl, K., A. Theis, T. Jungblut, S.K. Mitra & M. Szakáll
