A Wind Tunnel Investigation on the Riming of Snowflakes. Part I: Porous Disks and Large Stellars

Abstract The effects of porosity on the accretional growth characteristics of ice crystal aggregates (snowflakes) are investigated by riming circular disks of ice in a cloud tunnel. Twelve disk models were used, sized 5 to 6 mm and 10 to 11 mm in diameter, with various hole sizes and numbers, resulting in porosities ranging between 15% and 50%. The onset for riming occurred much earlier for porous dish than for similarly sized nonporous disks as the tunnel airflow speed was increased. For porosities in excess of 15%, the rime growth rates were found to be relatively insensitive to the extent of porosity. However, these rates were an order of magnitude greater than those for nonporous disks of the same size and rimed at the same flow velocity. The appearance of the rime was similar to that observed in the atmosphere for similar conditions. A large stellar model was rimed using the same techniques, and its riming rate was found to be in fair agreement with previous experiments.