Effects of Surface Roughness and Porosity on the Riming of Snowflakes

Abstract

Collision experiments were conducted with model snowflakes suspended on the end of a needle in a vertical wind tunnel to examine the effects of surface roughness and porosity of snowflakes on their collection efficiency for cloud droplets. The snowflake models were disks with small glass beads attached to their surface (roughness) or rectangular holes evenly spaced on their surface (porosity). The models were exposed to an airflow carrying saline water droplets or spherical glass particles. The effects of roughness and porosity were deduced from counting the numbers of captured particles on the model surface or from analyzing the motion of the particles around the models. The results showed that the collection efficiency was enhanced by these effects, especially in a small inertia region. Very small particles, which could not be scavenged on the basis of impaction theory, were captured by the models owing to these effects. Generally the effect of the porosity was larger than that of the roughness. These results suggest that actual falling snowflakes can gather more cloud droplets in the smaller sizes than might be expected from the traditional impaction theory.