| Qu Ke,Zhu Fengqin,Song Wenhua. 2019. A novel method for internal wave monitoring based on expansion of the sound speed profile. Acta Oceanologica Sinica, 38(4):183－189 |
| A novel method for internal wave monitoring based on expansion of the sound speed profile |
| 一种基于声速剖面展开的内波监测新方法 |
| Received:May 12, 2018 |
| DOI：10.1007/s13131-019-1422-6 |
| Key words:internal waves hydrodynamic normal modes internal solitary waves South China Sea |
| 中文关键词: 内波 水动力简正模态 内孤立波 南海 |
| 基金项目:The National Natural Science Foundation of China under contract No. 41406041; the Natural Science Foundation of Guangdong Province under contract No. 2014A030310256; the Project of Enhancing School with Innovation of Guangdong Ocean University under contract No. GDOU2016050246; the Excellent Young Teachers Program of GDOU under contract No. HDYQ2015010. |
| Author Name | Affiliation | E-mail | | Qu Ke | Guangdong Province Key Laboratory for Coastal Ocean Variation and Disaster Prediction, Guangdong Ocean University, Zhanjiang 524088, China
College of Electronics and Information Engineering, Guangdong Ocean University, Zhanjiang 524088, China | | | Zhu Fengqin | Guangdong Province Key Laboratory for Coastal Ocean Variation and Disaster Prediction, Guangdong Ocean University, Zhanjiang 524088, China
College of Electronics and Information Engineering, Guangdong Ocean University, Zhanjiang 524088, China | fqzhu_07@163.com | | Song Wenhua | College of Information Science and Engineering, Ocean University of China, Qingdao 266100, China | |
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| Abstract: |
| For acoustic detection of internal waves, the core issue is to obtain the temporal and spatial distribution of the sound speed profile (SSP). In the inversion process, the SSP is usually expressed by a few parameters through expansion. However, information about internal waves may sometimes be hard to read directly from the inversion results. The aim of this paper is to characterize the internal waves directly though expansion coefficients. By deducing the dynamic equations of the internal waves, an orthogonal basis called the hydrodynamic normal modes (HNMs) can be extracted from a certain number of SSP samples. Unlike the existing widely used empirical orthogonal functions (EOFs), the HNMs have a more explicit physical meaning that is directly related to internal wave activity. The HNMs are then used to expand the SSP time series, and the expansion coefficients are derived. Eventually, information about internal waves can be read directly from the time derivative of the expansion coefficients of the first two modes. In this study, this method is applied to thermistor string profiles from the northern shelf of the South China Sea, where the SSP shows evident spatial and temporal variations due to internal waves. The results show that the SSP can be described approximately by the first two modes with adequate precision. The special oscillation structure of the time derivative of the expansion coefficients can be used to detect internal solitary waves. The expansion coefficients can also give information on internal solitary wave amplitude and width. According to theoretical and experimental analysis, it can be concluded that the internal waves monitoring method introduced in this paper is effective. The HNMs method is simple to apply and depends less on sample data than EOFs. It could be used as an efficient alternative to EOFs to expand the use of the SSP in highly variable areas, where internal waves are intensive. |
| 中文摘要: |
| 声速剖面时空分布的获取是利用声学方法监测内波的核心问题。在反演算法中，声速剖面通常是采用展开的方式用若干个参数来表示的。这就导致了有时很难从反演结果中直接获得内波的相关信息。本文的目标是找到一种通过展开系数直接获取内波特性的方法。通过推导内波水动力方程，可以从较少的声速剖面样本中提取出水动力简正模态（Hydrodynamic Normal Modes，HNMs）作为声速剖面展开的正交基。较之广泛采用的正交经验函数（Empirical Orthogonal Functions, EOFs），HNMs直接与内波活动相关，具有更明确的物理含义。然后，基于HNMs对声速剖面的时间序列进行展开，获得展开系数。最后，从前两阶展开系数的时间导数中可以获取内波活动的信息。将方法应用于受内波影响而具有明显时空扰动的南海北陆架区温度链数据，结果表明：只用前两节模态就可以在较好的精度范围内重构声速剖面。前两阶系数的时间导数具有独特的双震荡结构可以用于探测内孤立波。从展开系数也可以获得幅度以及波长信息。理论推导和实验分析证明了本文方法在内波监测中的有效性。HNMs方法使用便利且对样本的依赖性较小，可以在内波活跃海域作为EOFs的有效补充用于声速剖面的展开。 |
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