Settling and Rotation of Frozen Hydrometeors in Turbulent Air

Numerical model predictions of precipitation rates rely heavily on representations of how fast hydrometeors fall, assuming settling is governed by the opposing balance of gravity and drag. Radar returns of snowfall depend strongly on particle orientation. We have used a novel suite of ground-based winter measurements to evaluate how air turbulence affects particle settling speed and orientation. Depending on the ratio of the particle terminal fall speed to the characteristic velocity of turbulence (the Settling Parameter Sv), we find evidence for multi-minute averaged sub-terminal settling speeds by up to a facfor of five, and super-terminal settling by up to a factor of three. Curiously, super-terminal settling is seen when the particle orientation is associated with maximum drag, that is broadside with respect to the downward direction, and sub-terminal for a "sky-diver" position. Moreover the particle orientation consistently maintains a 90 degree with respect to the particle angle of attack in strong winds. The results point to simplified parameterizations for how particles settle in turbulent air that deviate substantially from the more common assumption that frozen hydrometeors fall broadside downward at their terminal velocities.