Observations from an aufeis windtunnel

Presented are observations from a special type of windtunnel, termed an “aufeis windtunnel”, used to investigate the initial spreading of aufeis forming on sloped planar surfaces subject to wind. The observations, which extend earlier work conducted on aufeis formation in a flat-sloped flume, are largely qualitative, though they include data on trends in aufeis morphology. The aufeis windtunnel facilitated investigation of the influences on initial aufeis formation of gravity and wind drag. Tilting the windtunnel and varying wind speed, respectively, produced the influences. Both gravity and wind drag may drive the spreading of shallow flows of freezing water introduced at the upstream end of the windtunnel test section. The combination of the two driving mechanisms results in a variety of aufeis morphologies (height and spacing of surface features) on the windtunnelʹs aluminum base. For such surfaces, the initial morphologies appear essentially wavelike (or terraced), and their spacing and height vary with slope and wind speed. On the other hand, aufeis initially forming on a smooth surface of ice develops a finer scale surface morphology. The differences in morphology reflect differences in the location and spacing of initial sites of ice-crystal growth that leads to aufeis formation. Whereas ice crystals may emerge and grow from any site on an ice surface, ice-crystal growth began at surface irregularities (scratches or burrs) on the aluminum base of the windtunnel.