Development of a Rapidly Deployable Network of Hail Impact Disdrometers

Disdrometers have been used routinely for several decades to obtain size spectra for hydrometeors. With increasing losses due to severe hail events, there is an increasing need to develop a greater understanding of hail size spectra and the spatial and temporal characteristics of severe hail events. A large portion of the historical literature which describes hail size distributions resulted from data collected by hailpads and photogrammetric systems. Conventional hailpad networks are difficult to maintain, can become saturated with impacts during high concentration events, and do not provide any temporal information. Piezoelectric ceramic disks have been coupled with impact plates of various materials to capture both droplet and hail spectra (Joss and Waldvogel 1967; Lane et al. 2006; Salmi et al. 2011).

Six rapidly deployable hail impact disdrometers were developed and deployed in 2015. The system uses a four-sided, shallow pyramid impact plate following the general characteristics of the device developed by Lane et al. (2006). Acquisition of the voltage signal from the piezoelectric disk produced by hail impacts is handled by a low-cost microcontroller unit capable of 5 kHz timed sampling. To develop voltage to kinetic energy relationships, probes were impacted with laboratory ice spheres at known kinetic energies. The diameter of individual hailstones are then estimated by applying the diameter-kinetic energy relationships described in Heymsfield et al. (2014). Each platform also integrates an onboard GPS module for timing and position information. A small engineering tripod provides the base and support structure for the impact plate and data acquisition system when deployed.

The network of six probes was first deployed during a field campaign in 2015 applying the adaptive network concept of Weiss and Schroeder (2008). Probes were able to be deployed in one minute or less. Data were collected from four different parent thunderstorms, two of which were supercells and two multicellular clusters. Field teams were able to make measurements of hailstones shortly after each had passed over the probes. Hail size distributions from the disdrometer probes were in good agreement with those measured by the field teams for hail sizes greater than 1.5 cm. While the probe resolved smaller impacts, these stones likely melted before they could be measured.