Direct Observation of Extreme Particle Fall Velocities near the Cloud Top using a High-Resolution Zenith-Pointing X-band Doppler Radar

A high-resolution zenith-pointing X-band Doppler radar was used to directly observe extreme downward particle velocities during the overhead passage of a convective storm on 12 July 2023 over the Kanto Plain, Japan. The portable radar operated with a temporal resolution of 0.1 s and a vertical range resolution of approximately 30 m, enabling time-height cross sections of short-lived, localized velocity structures. Near the cloud top, at approximately 12-14 km altitude, coherent streak-like regions of Doppler velocity reached -25 to -30 m s-1 for durations of 10-30 s, corresponding to horizontal scales of a few hundred meters given the storm translation speed. Data quality checks suggest that these extreme velocity streaks were coherent signals rather than isolated folding artifacts. Under the assumption of weak vertical air motion in the anvil region, the observed Doppler velocities primarily represent particle fall speeds and are consistent with hail-sized ice particles. The results demonstrate the meteorological usefulness of high-temporal-resolution zenith-pointing Doppler observations for capturing sparse, rapidly falling large particles and cloud-top fallout processes that are difficult to infer from reflectivity observations or conventional scanning radars.