PIPERS Waves in Ice Observations
Description
The aim of this project was to observe wave propagation in the marginal ice zone (MIZ). A series of 14 wave buoys were deployed on ice floes during the 2017 National Science Foundation supported Polynyas, Ice Production & seasonal Evolution in the Ross Sea (PIPERS) voyage. There were two separate deployments. The first on the way into the ice (buoys 21, 22, 23 and 31) and the remaining on the way out. Each sensor performed on-board spectral analysis and data quality control. The data was returned via Iridium data packets. Ship based operations were funded through the US National Science Foundation. The instrumentation development and construction was funded through New Zealand's National Institute of Water and Atmospheric Research. The science development and analysis was funded through New Zealands Deep South National Science Challenge: Antarctic Sea Ice. The wave buoys were manufactured by P.A.S consultants, Melbourne, Australia.
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Here we describe the key components of the WIIOS, the onboard wave processing procedure and quality control measures. The main processor on the WIIOS is an Edison supported by a 32-gigabyte secure digital (SD) memory card. It receives information from the inertial measurement unit (IMU), the GPS receiver, temperature probe and an optional high-resolution Kistler accelerometer. The transceiver added to the system is a low-cost Iridium 9602 short burst data transceiver. We use the TDK Ivensense IMU MPU-9250. This is an affordable 9-axis motion tracking device that combines a 3-axis vibratory gyroscope, a 3-axis accelerometer, a 3-axis magnetometer and a digital motion processor. The MPU-9250 directly provides complete 9-axis motion fusion enabling reliable roll, pitch and yaw output. The WIIOS has the option of including a high specification accelerometer in the vertical axis, a Kistler microelectromechanical systems (MEMS) capacitive accelerometer. The GPS is an Adafruit Ultimate GPS Breakout. The batteries are Panasonic LR20 Alkaline 1.5 V mounted below the printed circuit board in eight cell packs giving a nominal voltage of 12 v. Using the version of the WIIOS without a high-resolution accelerometer, the estimated battery life is 30 d with continuous usage. This, however, will vary depending on temperature. The electronics are housed in a Pelican Case #1400. The case dimensions are 0.34 × 0.30 × 0.15 m. In total, 640 s (∼11 min) bursts of wave acceleration were sampled at 64 Hz and a low-pass, second order Butterworth filter was applied with a cut-off at 0.5 Hz and subsampled to 2 Hz. A high-pass filter was then applied and the acceleration integrated twice to provide the displacement. Welch’s method, using a 10% cosine window and de-trending on four segments (each 256 s long) with 50% overlap, was applied to estimate the power spectral density. Spectral moments are also calculated and the significant wave height (Hs) is obtained from the zeroth spectral moment, defining the total variance (or energy) of the wave system. The peak period (Tp) is calculated from the power spectral density. Quality control is maintained by returning data flags. If more than 20% of the Kistler data were unresponsive or the Kistler failed basic statistical tests, it is assumed the full-time series was corrupt and therefore flagged as a fail. If the IMU acceleration had an overall percentage of flagged data (the number of occasions of unchanged consecutive data and the number of spikes) >20% or fails the basic statistical tests, the IMU is flagged as a fail. Further details of the WIIOS (including testing procedures) can be found in Kohout and Williams (2019). Kohout A and Williams M (2019) Antarctic wave-ice observations during PIPERS. NIWA client report 2019060CH prepared for the Deep South Challenge. National Institute of Water and Atmospheric Research, NewZealand. Contact: library@niwa.co.nz.