Recent Progress in the Numerical Simulation of Ice Hydrometeor Free Fall Behavior

This paper will summarize our recent results on the numerical simulation of ice hydrometeors in clouds that play a crucial role in precipitation physics. We believe this will be the most comprehensive report on the theoretical study of hydrodynamic behavior of large ice particles in the atmosphere to date.

The fall behavior of hydrometeors is an important factor influencing the cloud microphysical processes. Due to the falling motion, a flow field is created in the air around the hydrometeor. This flow field influences how water vapor is distributed around the hydrometeor and hence the diffusion growth rate of this hydrometeor. It also influences the motion of other cloud particles around the hydrometeor and hence the collision growth rate. In principle, these flow fields can be determined experimentally but this requires highly sophisticated equipment such as vertical wind tunnels which are few and so far only the fall behavior of a limited set of hydrometeors are determined this way. But even with successful experiments, we only get the general picture of the fall and the gross mechanical properties (such as drag) and the detailed properties of the flow field are still hard to obtain.

Another approach is to perform numerical modeling of the fall motion which can yield the fluid dynamical details of the flow field and use the calculated field to determine diffusion and collision growth of the hydrometeor. This is the approach taken by this paper. We will focus on the case of ice hydrometeors. Previous studies were either confined to steady state motion of small ice crystals or unsteady motion of small to medium ice crystals with fixed orientation. Unsteady motion of the totally free fall of ice hydrometeors was not studied.

In this paper, we will summarize our recent success in numerically modeling the total free fall of ice hydrometeors including large ice crystals, snowflakes, graupel and hailstones. We use the CFD software ANSYS to perform the numerical solution of the unsteady Navier-Stokes equation. Because the hydrometeor may change its posture at each time step during the fall, we need to utilize the 6-dimension option and the dynamic mesh technique to simulate its realistic motions. We have performed such simulations for large hexagonal columns, plates, dendrites, broad branches, aggregates, conical graupel, spherical and lobbed hailstones. The covered Reynolds numbers range from ~ 40 -~1000 for ice crystals to ~ 100 – ~ 200,000 for graupel and hailstones. We will also report the ventilation coefficients calculated for these falling particles.

In addition to the descriptions of hydrodynamic characteristics, we will show animations of the simulated motions of these ice hydrometeors including zigzag motions, rotation, pendulum vibration, circular sailing and tumbling.