Unveiling Raindrop Collision Outcomes and T09 Model Comparison Using Multi-Year High-Speed Optical Disdrometer Observations

Binary raindrop collisions are crucial for the evolution of raindrop size distributions (DSDs), a fundamental quantity with significant impacts on hydrological, meteorological, and related modeling.  This study presents multi-year field observations of these collisions, acquired using the High-Speed Optical Disdrometer (HOD), an instrument we developed for the accurate measurement of raindrop microphysical properties.  The HOD's high-speed image capture system, operating at 1000 frames per second in this study, yields a rich dataset of sequential raindrop images.  These high-frequency observations enable the precise measurement of raindrops' geometric (e.g., diameter) and dynamic (e.g., fall speed) properties before, during, and after collision events.  The rainfall events considered in this study were observed at our outdoor rainfall laboratories located on the West campus of the University of Texas at San Antonio, Texas, and at Clemson University, South Carolina, USA, as well as during the IPHEx field campaign conducted near Asheville, North Carolina, USA.  Our observations captured a diverse range of collision outcomes, including coalescence and various breakup types (neck, sheet, and crown).  This unique field dataset was used to evaluate the theoretical collision outcome regime diagram (T09) developed by Testik (2009) [Outcome regimes of binary raindrop collisions. Atmospheric Research, 94(3), 389-399], which predicts raindrop collision outcomes based on the Weber number and drop diameter ratio.  The strong agreement found between our field observations and the T09 predictions supports the model's effectiveness and its potential integration into warm rainfall microphysics schemes.  In this presentation, we will detail our observations of raindrop collisions and provide comprehensive comparisons with the T09 predictions.  This material is based upon work supported by the U.S. National Science Foundation under Grants No. AGS-1741250 to the first author (FYT).