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

雨滴大小分布(RSD)是描述和分析降水的基本参数。相对标准偏差的演变主要受碰撞雨滴的合并和破裂以及极大雨滴的自发破裂的影响。到目前为止，关于近大气条件下液滴破碎后碎片大小分布(FSD)的实验研究还非常有限。在拟议的研究中，我们将在美因茨垂直风洞中进行专门的实验室实验，研究1至6毫米大小的雨滴的碰撞诱导和自发破碎。美因茨垂直风洞是世界上独一无二的设施，在该风洞中，单个雨滴和云粒可以自由悬浮在垂直气流中，而不需要与墙壁接触或捆绑。这在物理上代表了水滴以其终端速度漂浮在云中或从云中掉落的情况。风洞实验增加了新的探测和成像方法，允许准确确定雨滴破裂后的碎片大小。破裂实验的结果将扩展现有的数据库；这将有助于验证或证伪云模型中使用的不同现有FSD参数化。因此，本研究的一个重要目标是在实验室实验结果的基础上，推导出一种改进的碰撞诱导和自发解体的FSD参数化方法。新的参数化方案将在谱柱云微物理方案和蒙特卡罗超液滴方法中实现。这两种方法都保证了液滴的质量守恒。在第一步中，他们将被合并在一个盒子和一个一维雨井模型中，以了解分手如何影响RSD的行为和演变。这些过程研究还将首次将模拟的相对标准偏差的形状与现场观测进行比较。最后，将新的参数化法应用于三维云模型。三维模拟以真实的方式描述了动力学和云微物理之间的相互作用，从而能够评估分解过程对降雨量和降雨率的影响。随着极端降水事件的传播，提高降水预报的可靠性变得越来越重要。

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.