Check and Correction of ADCP Measured Current Speed by the Comparison with the Geostrophic Calculation based on XCTD Observation

Abstract

A method of correction for misalignment errors in ship-mounted Acoustic Doppler Current Profiler (ADCP) measurement is proposed by comparing ADCP current data with geostrophic currents based on eXpendable Conductivity, Temperature and Depth (XCTD) profiler observations. Oceanographic observations using ADCP and XCTD were carried out in Toyama Bay and adjacent area on board the training vessel ‘Wakashio-maru’ of the National Institute of Technology, Toyama College in September 2016 and October 2017. Cross-track components of ADCP currents are averaged between two neighboring XCTD stations to compare them with the geostrophic currents calculated from respective XCTD data. For the first cruise in 2016, an offset velocity is commonly observed between the ADCP and geostrophic currents, while those show a coincident current profile in terms of vertical shear. The offset seems to be caused by a misalignment in ADCP observation and the misalignment angle estimated from the offset value is 1.7 degree, which is comparable with 1.5 degree estimated by the software ‘ADCP Tracker.’ For the second cruise in 2017, there is no offset in the same comparison and the vertical profiles of geostrophic currents well trace those of ADCP currents. The present study proposes a practical method of misalignment correction by using geostrophic calculation based on a small number of XCTD measurements, which is different from that proposed by Michida & Ishii (2000). The present one has an advantage that it will be applicable to non-research vessels such as training vessels, which will not always be able to share enough ship-time for scientific observations for operational and/or logistic reasons. It is also suggested that the proposed methodology allows the training vessel ‘Wakashio-maru’ to be more effectively utilized to obtain higher quality observation data toward better understanding of the oceanographic conditions in Toyama Bay and adjacent areas.