Measurements of hail size distributions (HSD) and trajectories are incredibly challenging to obtain owing to the infrequent and hostile nature of hailstorms; however, such observations are necessary to constrain microphysics parameterization schemes and hail detection and sizing algorithms from weather radar. Ground-based sensors are most frequently used to capture the HSD either using a time-integrated (e.g., hail pads) or time-recording (e.g., hail disdrometers) hardware; however, both techniques are limited by a small sample area and the difficulty in deploying them in the path of a hailstorm. These limitations result in poor sampling of the HSD when hail fall is sparse and/or it the hail swath has large spatial variability.

To overcome these issues, advances in (UAV) photogrammetry have be applied to capture the HSD over large areas (> 1000 m²) immediately following hail fall. We demonstrate this application with results from a hail survey conducted during the RELAMAPGO project in Argentina, using both traditional hail pads and a ‘HailPixel’ UAV survey that sampled >15,500 hailstones at a 2.5-mm pixel resolution. The significant increase in sample size provides more robust statistics for exploring trends in aspect ratio, diameter, and frequency. The results are compared to X-band, dual-polarization, dual-frequency Doppler on Wheels radar data. On radar, the storm displayed characteristic signatures of hailstorms, including weak anticyclonic shear indicative of the midlevel mesoanticyclone, three-body scattering signatures, and reduced co-polar correlation coefficient ρ_hv values in the hail growth region. The evolution of the storm as inferred from the radar data will be discussed in the context of hail production and the results of the UAV photogrammetric hail survey. In addition to the 'HailPixel' technique, a new sensor will be introduced that aims to provides the first in situ measurements of hailstone trajectories aloft. Planned applications and anticipated benefits of this technology will be discussed.