Abstract
The complex surface geometries of hailstones affect their fall behavior, fall speeds, and growth. Systematic experimental investigations on the influence of the number and length of lobes on the fall velocity and the drag coefficient of hailstones were performed in the Mainz vertical wind tunnel to provide relationships for use in numerical models. For this purpose, 3D prints of four artificial lobed hailstone models as well as spheres were used. The derived drag coefficients show no dependency in the Reynolds number in the range between 25,000 and 85,000. Further, the drag coefficients were found to increase with increasing length of lobes. All lobed hailstones show higher or similar drag coefficients than spheres. The terminal velocities of the the hailstones with short lobes are very close to each other and only reduced by about 6% from those of a sphere. The terminal velocities from the long lobed hailstones deviate up to 21% from a sphere. The results indicate that lobes on the surface of hailstones reduce their kinetic energy by a factor of up to 3 compared to a sphere. This has important consequences for the estimation of the destructive potential of hailstones.
Highlights
According to the fifth assessment report of the IPCC, and e.g., Lehmann et al [1] as well as Brimelow et al [2], extreme precipitation events are very likely to increase in rate and frequency in the decades due to climate change
A sphere with blockage ratio (BR) = 20 % freely floating in the center of a wind tunnel would experience a 25 % higher air velocity at the constriction plane compared to the mean tunnel velocity
Wind tunnel experiments were performed to investigate the effect of lobes on the aerodynamic behavior of hailstones
Summary
According to the fifth assessment report of the IPCC, and e.g., Lehmann et al [1] as well as Brimelow et al [2], extreme precipitation events are very likely to increase in rate and frequency in the decades due to climate change. Hailstorms are among the most destructive extreme weather events in several regions all over the world, causing substantial damage to crops, vehicles, buildings and other infrastructures. There is an urgency to accelerate the research on hail in order to reliably predict and thereby reduce the damage from hailstorms. According to the World Meteorological Organization International Cloud Atlas hailstones are heavily rimed ice particles which are larger than 5 mm in diameter. The diameters of hailstones range between 5 mm and 50 mm. While small hailstones have densities of about 0.5 g cm−3 [3] large hailstones have densities higher than 0.8 g cm−3 , because any internal hollows fill up with water as the hailstones are transported into temperature regimes above
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