Hailstone Inertial Adjustment to Storm Wind Fields and Implications for Growth

Hailstone growth trajectory models are increasingly being used to understand complex processes in hailstorms and to elucidate environmental controls on hail production. These trajectory models often are microphysically detailed, explicitly representing all relevant hail growth processes. Less attention has been paid to hailstone aerodynamic behaviors. Typically, hailstone motion throughout the storm is modeled by a combination of hailstone fall speed and advection: A hailstone’s vertical motion is the difference between the storm’s vertical velocity and the hailstone’s terminal velocity, and its horizontal motion is given by the storm’s horizontal winds. The hailstone is assumed to instantaneously adjust to the storm’s wind fields at each time step.

However, relative to other hydrometeors, hailstones have substantial mass, and thus inertia, which prevents an instantaneous adjustment to the storm’s wind fields. Especially for larger (more massive) hailstones, the adjustment timescale can be several seconds. This means the hailstone can move through the storm with a horizontal velocity vector that is different than the storm’s horizontal wind vector. This has implications for the vector at which hailstones impact surfaces (and thus damage potential), as well as for hailstone growth. Typically, hailstone growth by collection is assumed to occur only in the vertical; hailstone inertia can lead to collection in three dimensions, which can increase growth rates.

In this study, we examine the impact of including hailstone momentum/inertia on their trajectories and growth. The theory is developed, and implemented in a series of simple, idealized flows. Then, hailstone inertia is implemented in a full-physics trajectory model. The impact of explicitly considering hailstone inertia is quantified and compared to other sources of uncertainty in trajectory models (including uncertainty in fall speeds).