Abstract

Precise rainfall measurements at sea play a crucial role in the development of accurate weather and climate models. However, the current acoustic methods used to estimate rainfall at see fail to consider that some raindrops produce a secondary sound. When large drops hit the ocean surface, a secondary sound is generated as the Worthington jet from the splash collapses. Current models therefore overestimate the rainfall rate during heavy rain. The aim of this project is to integrate these secondary sounds into existing models to improve the accuracy of acoustic rainfall measurements at sea. Through a novel dimensional analysis, the three dimensionless parameters that influence the splash of a drop and the height of the Worthington jet in the splash have been identified. These three parameters are: the Eötvös number, the Morton number, and the ratio between the droplet’s characteristic length and the height of the Worthington jet. To validate the identified relationship in the dimensional analysis, a series of experiments and computational fluid dynamics simulations were conducted. Experiments and simulations were employed to precisely characterize this relationship. In the experiments, the height of the Worthington jet were identified from slow motion videos of drops impacting a liquid pool at terminal velocity. Adjustments to existing acoustic models for rainfall measurements have also been discussed to account for the secondary sounds generated by some droplet splashes. In connection with the experiments, the height of the Worthington jet has been systematically described through empirical methods. This contistutes the first step towards imporving acoustic rainfall measurements.