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 determined only by the opposing force balance of gravity and drag. Here, we use a novel suite of ground-based winter measurements to show large departures of the mean snowflake settling speed from the terminal fall speed $v_t$ of a particle falling broadside. Where $v_t$ is lower than the air root-mean-square turbulent velocity fluctuation $u^{\prime }$, settling is sub-terminal by up to a factor of five, and if it is higher, then settling is super-terminal by up to a factor of three. Mean winds and aerodynamic lift appear to play an unexpectedly important role, by tilting snowflake orientations edge-on while slowing their mean rate of descent. New parameterizations are provided for relating winds and small-scale turbulence to hydrometeor orientations, drift angles, and precipitation rate reductions and enhancements.