Waves in the summer ice in the winter

Nortek provides a combined wave and current profiling instrument in the form of the AWAC. This variant of the traditional ADCP has managed to circumvent the classic limitations of measuring short waves in deep waters by introducing a vertical beam that directly measures the height of the water-air interface (waves) above the instrument. This same vertical beam has also demonstrated that it is capable of measuring the distance to the water-ice interface, and as a result can be used as means to estimate ice draft or ice thickness. Measurement campaigns that have taken place in relatively shallow coastal waters (less than 50 meters) are finding themselves more often in deeper waters. These deployments are also occurring at more extreme latitudes where the presence of ice is more common. This means that common measurement requirements now include measuring ice thickness in addition to current profiles and directional wave observations. Nortek has addressed this need of measurements in deeper waters by building upon the success of the AW AC. A 400 kHz AWAC has been developed and intended for deployment depths of 100 meters. The 400 kHz AWAC is outfitted with a temperature compensated pressure sensor and firmware with a dual functioning surface tracking measurements for ice and waves. The wave burst measurement contains detection methods for both water-air interface and water-ice interface; this means the AWAC can transition seamlessly from wave measurements in the summer to ice measurements in the winter. Ice thickness data is presented for two AWACs deployed in the Beaufort Sea, Alaska. These data are compared to an ASL ice profiler. AWAC data was collected in a special diagnostic mode and allowed for the user to select the best water-ice interface detection method in post-processing. The results show that there is favorable agreement between the AWAC and the ASL ice profiler. This data set was useful in developing the now existing firmware used to detect the water-ice interface. These shallow water data did not illustrate errors associated with an unknown speed of sound profile which is identified as the primary source of error for the AST distance measurement. Additionally, wave results are presented for the 400 kHz AWAC and compared to a 600 kHz AWAC, which was deployed in the vicinity. The data show that non-directional estimates agree well, however directional estimates are complicated by the fact that the two AWACs were deployed at different locations and exposed to different wave directions due to local refraction. It is clear that another comparison test needs to be conducted with the reference instrument collocated with the 400 kHz AWAC.