A wind tunnel investigation of the hydrodynamic behaviour of growing, freely falling graupel

The hydrodynamic behaviour of riming frozen drops and riming simulated ice crystal plates was studied by meansofavertical wind tunnel.At all times theiceparticles werefreelysuspended in thevertical airstream of the wind tunnel where temperature and liquid water content could be accurately controlled. The ice particles studied had initial radii between 100 and 300,um and final radii between 400 and 600prn. The supercooled cloud in which riming of the ice particles was observed had a temperature near 15”C, a liquid water content of 2gm-3, and consisted of drops with a modal radius of 7pm. Spinning, helical, and various oscillating motions, or simple straight fall with an almost fixed orientation of the ice particle, but no tumbling motions, were observed. These fall modes appeared to be closely related to the instantaneous shape, surface roughness, and mass loading of the ice particle. Frozen drops as well as ice crystal plates were found to develop into conically shaped graupel. During the initial stages of the observed dry growth riming, the terminal velocity of rimed frozen drops decreased with increasing ice particle size as a result of a less rapid increase of the mass of the ice particle compared with its cross-sectional area perpendicular to the airstream, i.e. as a result of a less rapid increase in time of the force the ice particle experiences in the earth’s gravitational field compared with the hydrodynamic drag acting on it. This behaviour may have some important bearing on the lifetime of graupel and hailstones in atmospheric clouds.