Observing the impossible – in situ observations of hail trajectories using the HailSonde

From the early stages of hailstone growth to the ground-impact finale, a trajectory is taken by each hailstone through the parent hailstorm. Larger hailstones form as their trajectory takes them into regions of the storm that are more favorable for growth, while others may miss out entirely. Simulation-based studies have shown that interactions between the hailstone fall speed, aerodynamics, storm winds (which continue to change along the trajectory and with new growth) can take hailstones on a myriad of different trajectories. Despite improvements in radar technology over the last 20 years, operational hail analysis techniques have changed little, and do not consider trajectories, leaving a high degree of uncertainty when estimating ground impact.

Case studies have demonstrated that trajectory information provides significant improvements to hail impact mapping and nowcasting services, but the lack of robust
observational datasets to leverage new radar technology and verify trajectories prevents the transition of this new science into operations. The follow proposal presents an innovative approach to measuring trajectories within a hailstorm using hailstone-shaped probes called “HailSondes”. Recent advances in low-energy telemetry, battery technology and electronics miniaturization are combined to make this new sensor possible, which, until recently, was the realm of fantasy for meteorologists (e.g., the 1996 Hollywood classic “Twister” imagined a similar sensors for observing tornadoes). The design challenges, simulations, prototype development and deployment of HailSondes are discussed.

HailSonde measurements will provide critical validation for the practical application radarderived trajectories for hailstorm analysis and nowcasting, supporting the transition to future hail services and benefiting a wide range of sectors from aviation, risk management, transport and public safety. This transition from science fiction into real science signifies extraordinary potential for further remote micro-sensor applications in the future.