In preparation for the Arctic Leads Experiment (LEADEX), two side scan Doppler sonars were deployed from the edge of Beaufort Sea leads in April 1991. The instruments, originally developed for ocean surface wave research, were used to investigate ice and water motions in the near-lead environment, as well as to collect preliminary acoustic data required in the design of a more capable leadside sonar. The sonars could determine lead geometry with great accuracy, identifying regions of open water and young ice. Opening and closing speeds of the lead ice cover could be measured to 0.01 cm s-1 using pulse-to-pulse coherent techniques. The flow velocity of the water just beneath the ice, of order 1--10 cm s-1, was estimated using pulse-to-pulse incoherent processing. Perhaps surprisingly, the strongest acoustic returns from the lead surface came from regions of open water, not ice cover. Frazil ice was a likely source of this scattering, although temperature microstructure might also play a role. On the basis of the 1991 measurements, a 28-beam sector scan Doppler sonar was developed for use in the LEADEX main field experiment (March--April 1992). With the fan of beams oriented in a vertical plane the sonar formed an image of the velocity field over a 45¿ sector, to ranges of 250 m, depths of 200 m.

The instrument was deployed at LEADEX lead 3 on April 8--9. At this point a significant ice cover had developed, limiting local convective forcing. The dominant motions observed beneath the lead were near-inertial internal waves. Significant variability in acoustic scattering was observed at the base of the mixed layer and in the upper thermocline. This presumably resulted from strong temperture and salinity fluctuations observed at these depths. The near-inertial modulation of the ice water relative velocity affects the transit time of water parcels under a lead. Probability density functions of parcel transit time have extremely high kurtosis, for mean transit times less than an inertial day. Resultant distributions of near-surface properties such as salt uptake or primary production will thus be extremely patchy. ¿ American Geophysical Union 1995