Quantitative evaluation of graupel shape observed by new particle imaging radiosonde, Rainscope – A case study of a convective cloud on 25 June, 2022

In this study, we quantitatively evaluated the shape and fall velocity of precipitation particles in convective clouds observed by Rainscope to better understand graupel formation processes. Rainscope is a newly developed particle imaging radiosonde that provides much clearer precipitation particle images than those obtained by a conventional videosonde. In addition, it can measure particle fall velocities in clouds. Rainscope was launched into a convective cloud with active lightning and gusts on 25 June, 2022. The particle images captured by Rainscope provide detailed information on particle shapes, surface conditions, and contours, facilitating the quantitative evaluation of particle shape. The observed circularity, defined as a function of the particle circumference, and aspect ratio* indicate that graupel just above the freezing level, which coexisted with frozen particles, differs from graupel with an ice crystal as an embryo. The particle fall velocity of graupel in the lower layer was smaller than that of frozen particles and larger than that of general graupel, which forms from an ice crystal. Therefore, graupel in the lower layer likely originated from a frozen particle, which was formed by freezing a raindrop lifted by updrafts and then rimed.