A Numerical Study of Snow Aggregates

Accurate calculations of hydrometers' growth are necessary to the development of a scientific understanding of cloud and precipitation. Better knowledge in the hydrometeor shape and size evolution allows better estimate of precipitation and hence better forecast/nowcast of severe weather. Hydrometeors can grow by water vapor diffusion and by coalescence with other hydrometeors. Both mechanisms are influenced by the flow fields around hydrometeors. The flow fields for small hydrometeors are generally steady, whereas those for larger hydrometeors tend to be unsteady and even turbulent. For large asymmetrical hydrometeors, e.g., graupel or aggregated ice crystals, their horizontal translation and rotation complicate the flow fields even more. Most of previous studies were confined to steady flow past small hydrometeors. The flow fields around large hydrometeors and their impact on the hydrometeors’ growth are still not clear.
In this study, we numerically solved the unsteady Navier-Stoke equations to perform simulations of flow fields around aggregated falling ice crystals. To allow horizontal translation and rotation, the aggregate is allowed to move and rotate freely by solving the associated six degrees of freedom problem. Preliminary results of six aggregated ordinary dendritic ice crystals of different sizes show a pronounced horizontal translation motion. Rotations along the horizontal axes are larger than along the vertical axis. We will share the comparison of our preliminary results of the ventilation coefficients.