"HailPixel" is a new technique for measuring the size and aspect ratio of hailstones simultaneously using imagery collected from a UAV (such as a DJI Phantom 4 Pro). The size measuring procedure applies a convolutional neural network that excels in detecting hailstones against complex backgrounds and an edge detection method for measuring the shape of identified hailstones. This semi-automated technique is capable of measuring many thousands of hailstones within a single survey, which is several orders of magnitude larger (e.g., 10,000 or more hailstones) than population sizes from existing sensors (e.g., a hail pad).

Comparison with a co-located hail pad for an Argentinan hailstorm event during the RELAMPAGO project demonstrates the larger population size of the HailPixel survey significantly improves the shape and tails of the observed hail size distribution. When hail fall is sparse, such as during large and giant hail events, the large survey area of this technique is especially advantageous for resolving the hail size distribution. For more information on this technique please check out the following paper - https://www.atmos-meas-tech-discuss.net/amt-2019-281/

Understanding of hailstone trajectories as they growth within thunderstorms clouds remains poorly understood owing to the complete absence of ground truth. Tracking the trajectories of individual hailstones inside convective storms remains impractical, however, the technology now exists to develop passive probes to act as pseudo hailstones. In collaboration with a hardware developer, a prototype remote sensing probe is under development. This ‘hail tracer’ probe will be launched using a balloon and released into the updraft region of hailstorms. Once the updraft reaches sufficient strength to lift the probe, the balloon tether is cut, allowing the probe to act as a passive tracer of the wind field inside the hailstorm.

 Multiple probes can be simultaneously launched, collecting positioning measurements for reconstructing trajectories (Figure 3(b)). Using the known physical properties of the probe, observations from the probe can be compared with simulations of trajectories to understand sources of error. Further, this technique does not require hail to be present and therefore can also be extended to understand the updrafts of non hail producing thunderstorms.

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The shape of hailstones determines how it melts, how it falls and what it looks like on radar. All of these processes have implications when attempting to model and predict hail production. Conventional hailstone measurements are typically taken using calipers along the major, minor and third axis of a hailstones, but these measurements cannot capture more complex geometries. As hailstones become larger, they typically become less spherical and exhibit more complex surfaces, and thus measuring this complexity becomes more important.

To correctly capture the shape of hailstones, 3D scanning is required. There are many ways to achieve this including using a hand held laser scanner or photogrammetry techniques. The collection of hailstones in the right panel were scanned using the photogrammetry technique and scaled using additional measurements. Once scanned, simulations can be performed to explore radar scatter, falling and melting. It's also possible to reproduce a scanned hailstone using a 3D printer for further laboratory work and education.

Collecting hailstones wouldn't be possible without the support of Higgins Storm Chasing and Victorian Storm Chasers Facebook groups. Social media groups like this allow researchers to quickly connect with citizen who collected hailstones.

Model library: https://sketchfab.com/joshua-wx/collections

Cross-sections provide a unique look into the internal composition of hailstones and an understanding of structural diversity within the same event or between different events. The contrasting opaque and transparent ice forms in response to the microphysical conditions during the hailstone's growth phase. Opaque ice contains a high concentration of tiny bubbles that are trapped when collected water droplets freezes near instantaneously. Transparent ice forms when collected water droplets takes longer to freeze, allowing trapped air bubbles to escape. It's also possible to determine the embryo particle from which the hailstone grew. For many hailstones this is an opaque embryo, often with a conical shape, indicating a grapuel embryo. For other stones a frozen droplet may be present or possibly other particles.

Measurements of the growth ring thickness and mode not only provide a better understanding of the growth conditions, but also provide validation for hail growth models. Many of the hailstones collected during the 2020 Melbourne hailstorm look different, but share similar internal layers, suggesting that common conditions occurred during the earlier stages of their growth.