Experimental and numerical investigation of collision induced and spontaneous raindrop breakup for forecasting and nowcasting of precipitation

点击翻译按钮获取中文摘要

The raindrop size distribution (RSD) is a fundamental parameter used for the description and analysis of precipitation. The evolution of the RSD is primarily affected by coalescence and breakup of colliding drops, as well as by spontaneous breakup of very large raindrops. The number of experimental studies on the fragment size distribution (FSD) after breakup of drops under well-controlled near-atmospheric conditions is so far very limited in literature. In the proposed study we will conduct dedicated laboratory experiments in the Mainz vertical wind tunnel on collision induced and spontaneous breakup of raindrops of sizes from 1 to 6 mm. The Mainz vertical wind tunnel is a world-wide unique facility in which single raindrops and cloud particles can be freely suspended in a vertical air stream without wall contacts or tethered. This physically represents the situation when drops are floating in or falling out from a cloud at their terminal velocities. The wind tunnel experiments are augmented by novel detection and imaging methods allowing the accurate determination of fragment sizes after raindrop breakup. The results from the breakup experiments will extend the existing data base; this will help to validate or falsify different existing FSD parameterizations utilized in cloud models. Therefore, an important objective of this study is to derive an improved parameterization for the FSD of collision induced and spontaneous breakup based on the results from the laboratory experiments. The novel parameterizations will be implemented in a spectral-bin cloud microphysical scheme and in a Monte Carlo super-droplet approach. Both methods guarantee mass conservation of the drops. In a first step they will be incorporated in a box and in a 1D rain shaft model to understand how breakup affects the behavior and evolution of the RSD. These process studies will also allow a first comparison of the shape of the simulated RSD with field observations. Finally, the new parameterizations will be applied in 3D cloud models. 3D simulations represent the interaction between dynamics and cloud microphysics in a realistic way and, thus, allow to assess the impact of breakup processes on precipitation and the rainfall rate. To improve the reliability of precipitation forecasting is of increasing importance as extreme precipitation events are propagating.