1. To provide validation data for laboratory impact testing of building materials
2. To provide ground-truth validation data for developing radar-based hail detection algorithms
3. To provide ground-truth validation data for modeling applications
Hailstone characteristics data have been used by standard testing organizations to develop impact resistance test standards for roofing products. The current impact test methods match the theoretical impact kinetic energy of steel ball or laboratory ice ball projectiles to the kinetic energies that similarly-sized hailstones would have when falling at theoretical terminal velocities. The theoretical kinetic energy calculations assume no wind, fixed drag coefficient, spherical shape, and hailstone density equal to that of solid ice. It is hypothesized the hardness property of individual hailstones may also affect resulting building damage, because of the duration of impact and transfer of energy to the material. Only qualitative descriptions of hailstone hardness are provided in the existing literature. Therefore, in 2012, Brown et al. developed a coordinated field research program focused on measuring characteristics of hailstones from Great Plains storms, including their compressive force to evaluate the hardness. A unique, rugged and portable, custom-designed instrument was developed for this effort. The instrument package utilizes a load cell to determine the compressive force required to fracture a hailstone, where it is understood that harder stones require more compressive force to fracture. This is similar to methods used to determine the compressive strength of building materials such as concrete. In addition to the compressive force data, multiple dimensions of size and the mass of each hailstone are collected, along with a photograph for use in examining characteristics such as shape and layering structure.
In 2013-2014, the project expanded to facilitate collecting a larger number of hailstones at multiple points across a hail swath, to increase the size of the database and examine spatial variability in size, shape, distribution, and compressive force as a result of storm-relative location. With the nationwide upgrade of the WSR-88D network to dual-polarization radar in 2013, the ground-based spatial hail data across swaths have proved to be useful in providing validation data for improving radar-based hail detection algorithms. The polarimetric measurements can be used to extract information about the size, and orientation of hail. The data captured by the field program can address some of the factors that may affect radar detection and classification, such as ranges of distributions of hailstone axis ratios and densities, as well as the lobe structure and irregularities present on hailstone surfaces. Collecting geolocated data across the hail swath allows the ground-truth data to be matched up to specific radar data bins. Future improved hail detection algorithms can improve the current reflectivity-derived hail swaths created by third-parties, which are provided to the insurance industry, building product manufacturers and suppliers, roofing contractors, and emergency management groups for the purpose of determining potential hail damage and staging resources.
In the three-year history of the project, 2,557 stones have been measured from 34 storms on 21 storm days. The sizes of stones measured ranged from 0.11-10.70 cm and 0.5-163.3 g. This paper presents a summary of the developed compressive force instrument, deployment and measurement methodology, and a database of cases and hailstones collected by the project. Additional applications, research, and continued field measurement objectives will be described.