A Wind Tunnel Investigation into the Aerodynamics of Natural Hailstones

Experiments at the Mainz vertical wind tunnel were carried out to deepen our present knowledge on the aerodynamics of natural hailstones and to improve current fall velocity parameterizations. Thus, the present study provides parameterizations accounting for shape effects and thus variability when calculating the fall velocity of ice particles growing in the accretional growth mode. For this purpose, two sets of 89 3D<printed replicas of natural hailstones were freely levi-tated in the Mainz vertical wind tunnel from which information on the fall speed, Reynolds number, and drag coefficient was obtained. Sphericity as a shape descriptor was found to be an important parameter to account for the variability of C_D. Thus, by taking their shapes into account, the variations in C_D of natural hailstones could be reduced by a factor of 2. The comparison of the present results with previous C_D parameterizations for nonspherical particles revealed that they are not adequate to represent C_D for natural hailstones. The drag coefficients from the present study were overestimated by 26%– 142% in those studies. Based on sphericity, new parameterizations were derived describing the variations in C_D and Re for natural hailstones. Furthermore, a parameterization was developed applicable for a lump graupel growing into a hailstone of about 8 cm in maximum dimension, which is valid for a wide range of Reynolds numbers, i.e., from 1 3 10¹ to 1 3 10⁵ .