Ocean College, Zhejiang University, Zhoushan 316021, China
2.
State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou 310012, China
3.
School of Marine Sciences, Nanjing University of Information Science & Technology, Nanjing 210044, China
4.
National Engineering Research Center of Gas Hydrate Exploration and Development, Guangzhou Marine Geological Survey, Guangzhou 510760, China
Funds:
The National Natural Science Foundation of China under contract No. 42206033; the Marine Geological Survey Program of China Geological Survey under contract No. DD20221706; the Research Foundation of National Engineering Research Center for Gas Hydrate Exploration and Development, Innovation Team Project, under contract No. 2022GMGSCXYF41003; the Scientific Research Fund of the Second Institute of Oceanography, Ministry of Natural Resources, under contract No. JG2006.
The current velocity observation of LADCP (Lowered Acoustic Doppler Current Profiler) has the advantages of a large vertical range of observation and high operability compared with traditional current measurement methods, and is being widely used in the field of ocean observation. Shear and inverse methods are now commonly used by the international marine community to process LADCP data and calculate ocean current profiles. The two methods have their advantages and shortcomings. The shear method calculates the value of current shear more accurately, while the accuracy in an absolute value of the current is lower. The inverse method calculates the absolute value of the current velocity more accurately, but the current shear is less accurate. Based on the shear method, this paper proposes a layering shear method to calculate the current velocity profile by “layering averaging”, and proposes corresponding current calculation methods according to the different types of problems in several field observation data from the western Pacific, forming an independent LADCP data processing system. The comparison results have shown that the layering shear method can achieve the same effect as the inverse method in the calculation of the absolute value of current velocity, while retaining the advantages of the shear method in the calculation of a value of the current shear.
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Figure 1. Topography and station location in western Pacific Ocean. Red dots denote the six observation stations.
Figure 2. Schematic diagram of the LADCP overlapping profile.
Figure 3. Processing results of LADCP data. a. The results of the inversion of the eastern component of the current velocity, where yellow represents the calculated results of an instrument during a descent process, and blue represents the calculated results of the instrument during an ascent process; b. the results of the inversion of the northern component of the current velocity; c. the time series of the water depth of the instrument obtained by integrating the vertical velocity; d. the ship displacement during the observation process. Yellow dot: initial position; Blue dot: the maximum depth position.
Figure 4. Comparison diagram of simulation results between shear method and layering shear method.
Figure 5. Error comparison of real current velocity and current velocities calculated by shear method and layering shear method.
Figure 6. Comparison diagram of simulation results between inverse method and layering shear method.
Figure 7. Error comparison of real current velocity and current velocities calculated by inverse method and layering shear method.
Figure 8. Schematic diagram of layer thickness division in layering shear method.
Figure 9. Current velocity profiles obtained with different values Bk (a−d) and the difference in current velocity calculated during the lowering and lifting processes (e, f).
Figure 10. Current velocity profiles obtained with different values Bk (a–d) and the difference in current velocity calculated during the lowering and lifting processes (e, f)(0–500 m).
Figure 11. Schematic diagram of optimization method for anomalous strong current shear between two layers. A. Before optimization (the dashed line range is the abnormally strong current shear between two layers); B. after optimization.
Figure 12. Comparison of two depth acquisition methods.
Figure 13. Movement of the ship during observation period of MC001 and MC002. Yellow dot: initial position; blue dot: the maximum depth position.
Figure 14. Current velocity profiles obtained by two GPS processing methods.
Figure 15. Method improved to obtain water layer velocity profile with low echo rate.
Figure 16. Comparison diagram of current velocity profiles before (a) and after (b) vertical filtering (Sta. MC001), and the difference in current velocity calculated during the lowering and lifting processes (c).
Figure 17. Comparison of the current velocity profiles processed by layering shear method, inverse method and the current velocity profiles observed by the SADCP (a–d), and the difference between the results calculated by the two methods and the SADCP data (e–h).
Figure 18. Comparison of the current velocity profiles processed by layering shear method, inverse method and the CMEMS data (a–d), and the difference between the results calculated by the two methods and the CMEMS data (e–h).