A Wind Tunnel and Model Investigation into the Melting of Spherical Hailstones

Hail melting and shedding of surface water govern how hailstones evolve below the melting layer and modu-late precipitation, downdrafts, and cold pools. We present controlled wind tunnel experiments on pure ice spheres (10–24 mm) at 228C and varying humidity, combining high-speed video with digital holography to track melting, onset of shedding, shed water mass, and drop size spectra. Melting rates collapse onto a single normalized curve, enabling a humidity-independent parameterization of meltwater fraction versus relative time (ratio of instantaneous and total melting time). We provide Reynolds number relations as functions of ice core mass and a Sherwood number fit, implying slightly enhanced ventilation relative to smooth spheres. The retainable surface water mass develops nonlinear with ice core mass and peaks at a hailstone size of approximately 15 mm. The shed water mass per event follows power laws in Reynolds number and ice core mass, and the bimodal drop spectrum spans cloud drizzle to raindrop sizes. Implementing these parameterizations into a box model reproduces time series of terminal velocity, maximum dimension, volume, and meltwater fraction. Thus, they are applicable in cloud models to determine the critical meltwater mass before shedding, the amount of water re-leased per shedding event, and the resulting shed drop size distribution. Embedding them in a one-dimensional (1D) kinematic microphysics model shows that the recycling of liquid water by shedding and its impact on drop and hail spectra strongly depends on the incoming hail size distribution.