Calibration of salinity data of a domestically-produced HM4000 deep profiling float

doi:10.3969/j.issn.1001-909X.2025.01.002

Abstract

Abstract:

In December 2023, the project “Construction of Regional Deep-Argo Observation Network” sponsored by Laoshan Laboratory deployed a domestically-produced HM4000 profiling float with the maximum profiling depth of 4 000 m (the World Meteorological Organization number is 2902895) in the Philippine Sea, which was equipped with an RBRargo³ deep 6k Temperature-Conductivity-Depth (CTD) sensor produced by RBR, Canada. It was found that the salinity observation data reported by the float exhibited a systematic deviation compared to the shipboard CTD and climatological salinity. In order to correct the salinity data of the float, the conductivity slope of the RBR CTD was calculated by using bottle salinity measured by the Autosal 8400B salinometer and salinity measurements from the shipboard CTD cast. Salinity profiles of the float were then calibrated, and the calibrated salinity was found to be basically consistent with the nearby float and the climatological data. With the implementation of the “Construction of Regional Deep-Argo Observation Network” project, an increasing number of domestically-produced deep Argo floats will be deployed. Compared to the Core Argo floats that measure temperature and salinity profiles in the upper 2 000 m of the ocean, Deep-Argo requires higher accuracy to resolve smaller variations in deep waters. Currently, technical problems are still found in deep CTD sensors, and improper handling and operation during storage, transportation, and usage of some floats and sensors are inevitable, resulting in large errors in the observations, especially the salinity data. Therefore, this study proposes a method of calibrating Deep-Argo floats’ observation data using in-situ shipboard CTD cast, which can provide essential technical support for quality control of the Deep-Argo floats.

Key words: Deep-Argo, HM4000 profiling float, RBR CTD sensor, Autosal 8400B salinometer, shipboard SBE 911 CTD, conductivity drift, salinity calibration

CLC Number:

ZHANG Xuan, LIU Zenghong, CHEN Zhaohui, REN Chong, XIONG Haixia, GAO Zhiyuan, YAN Xiaoluan, ZHANG Linlin. Calibration of salinity data of a domestically-produced HM4000 deep profiling float[J]. Journal of Marine Sciences, 2025, 43(1): 14-21.

Figures/Tables 9

Fig.1 Trajectories of the float 2902895 and its nearby float 2902881 (In the trajectory of the float 2902881, the green points represent the positions of profiles 83 to 91. In the trajectory of the float 2902895, the red dot represents the last profile position; the green crosses represent the first and second profile positions, and the other profile positions are indicated by blue dots.)

Fig.2 The maximum diving depth of the float varied with time

Fig.3 Temperature-salinity curves observed by shipboard CTD

Fig.4 Vertical distributions of temperature (a), salinity (b), and temperature-salinity curves(c) from the first three profiles of the float and the shipboard CTD

Tab.1 Analysis results from the salinometer and shipboard CTD measurements at the float deployment station

序号	采样深度/m	盐度计分析电导率 /(S·m^-1)	船载CTD测量电导率 /(S·m^-1)
1	600	3.465 5	3.464 9
2	700	3.360 6	3.359 4
3	800	3.291 2	3.291 2
4	1 000	3.225 7	3.225 7
5	1 200	3.186 6	3.189 7
6	1 600	3.155 3	3.154 9
7	2 000	3.140 3	3.140 0
8	3 000	3.148 0	3.147 7
9	4 000	3.179 0	3.178 7
10	5 000	3.219 1	3.218 9
11	5 560	3.243 0	3.242 7

Fig.5 Comparison of float-measured salinity (before and after calibration) with shipboard CTD data and climatological salinity

Fig.6 Comparison of the temperature-salinity diagram between the floats 2902895 and 2902881

Fig.7 Differences between the float salinity (after calibration) and ISAS13(a) and WOA2018(b) climatological salinity at different isotherms (The gray shading denotes the target salinity accuracy of ±0.01 proposed by the international Argo program.)

Fig.8 The damaged coating locations of the RBR CTD sensor

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