A High-Speed, High-Resolution, Dual-Camera System to Image Hail and Other Large Hydrometeors

Despite decades of study, much remains unknown about the nature and fall behavior of natural hailstones. This incomplete understanding of the spatiotemporal distribution of hailfall and hail characteristics is limiting improvements in radar-based hail detection and quantification algorithms. Similarly, uncertainties about how hailstones fall in nature (e.g., the “tumbling” nature of falling hail) increase uncertainties in forward operators and numerical models. Herein, we introduce a new high-resolution (12.6 megapixel), high-speed (∼330 frames per second), stereoscopic imager designed to quantify the size, shape, and fall behavior of naturally occurring hailstones. Images collected in 2022 show that fine-scale features associated with falling hail are resolved, including the water torus around melting hailstones and lobes when present. Other microphysical processes such as drop shedding, breakup of large drops, and hydrometeor collisions are also observed. The stereoscopic nature of the imager allows us to estimate the size and, when tracked through time, the three-dimensional velocity of hydrometeors. The high frame rate allows estimation of rotation/gyration rates, and some sampled hailstones rotate at rates greater than 50 Hz. With this imager, we expect to (1) improve radar-based hail algorithms by providing a more complete record/sample of hydrometeor characteristics (e.g., hail sizes, start and stop times of hail fall, etc.), (2) quantify the canting angle distribution of and meltwater distribution on falling hailstones to improve polarimetric radar forward operators, and (3) provide direct observations of real hailstones and other large hydrometeors to improve numerical models.