HailPixel - Measuring Hail From Above!
"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/
Procedure for measuring hailstone size from drone imagery
Drone used to capture aerial imagery for project RELAMPAGO
Zoomed imagery swath showing concentration of large hailstones. Captures in the Mendoza region on Argentina on 25-11-2018
Hail Tracer Probe
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.
Check back later for more info!
Conceptual design of Hail Tracer Probe