High-speed Video Observations for Precipitation Microphysics: Case of Raindrop Collisions

This study utilizes an optical-type disdrometer, called High-speed Optical Disdrometer (HOD), that we recently developed for precipitation microphysics observations and investigates raindrop collisions through HOD’s high-speed video observations during rainfall events.  HOD’s innovative technology enables capturing high-resolution sequential images of the same hydrometeor multiple times as it passes through the measuring volume.  Hydrometeor characteristics are then accurately measured via digital processing of the recorded images.  HOD offers unique observational capabilities such as, for the case of raindrops as hydrometeors, observations of raindrop oscillations and collisions as well as high-accuracy measurements of relevant characteristics.  This study focuses mostly on raindrop collisions observed during rainfall events using HOD.  Raindrop collision rates and outcomes are important quantities for applications such as raindrop size distribution (DSD) modeling in hydrological and meteorological models.  Direct field observations of these quantities have not been available due to the technological limitations of the existing disdrometer technologies, and HOD’s measurement capabilities provided us the opportunity to investigate these quantities through field observations.  Rainfall events considered in this study were observed during a 3-year long field campaign conducted at our outdoor rainfall laboratory located on the West campus of the University of Texas at San Antonio, Texas, USA.  This field campaign provided a dataset on collision observations that extended the small number of raindrop collision observations that we had previously reported as the first-time raindrop collision observations to visually demonstrate the presence of raindrop collisions in rainfall events.  We will provide an overview of this high-resolution disdrometer technology and present our observations on raindrop collisions with discussions on our findings and their applications.  This material is based upon work supported by the National Science Foundation under Grants No. AGS-1741250.