Characteristics Of Internal Waves Research Articles

Internal solitary waves in the central Andaman sea observed by combining mooring data and satellite remote sensing

The behavioral characteristics of internal solitary waves (ISWs) in the central Andaman Sea were revealed by analyzing multisource satellite remote sensing images combined with synchronous mooring data. The relationship between the generation time and tides of ISWs in the central Andaman Sea was analyzed and discussed. The ISWs in the central Andaman Sea exhibit a notable semidiurnal tidal cycle, primarily occurring during the spring tide. Additionally, it was observed that larger currents can generate ISWs with larger amplitudes. The propagation velocity of the ISWs initially increases and then decreases from the source, with no significant overall change in the deep areas. Based on the analysis of satellite remote sensing images that cover the source of ISWs in the central Andaman Sea, the generation time of ISWs was estimated. A comparison was made between the estimated generation time and the tidal flow predicted from the TPXO 9 global tidal model at the location of the ISW source. It was found that ISWs are generated when the westward tidal flow reaches its maximum. The ISWs in the central Andaman Sea are generated between the islands of Car Nicobar and Teresa Island. They propagate eastward into the Andaman Sea, grow in amplitude, split into multiple waves, dissipate, and vanish in the waters near the Danao Islands. The entire life cycle of these waves lasts for approximately 3∼4 days.

A Texture Enhancement Method for Oceanic Internal Wave Synthetic Aperture Radar Images Based on Non-Local Mean Filtering and Texture Layer Enhancement

Synthetic aperture radar (SAR) is an important tool for observing the oceanic internal wave phenomenon. However, owing to the unstable imaging quality of SAR on oceanic internal waves, the texture details of internal wave images are usually unclear, which is not conducive to the subsequent applications of the images. To cope with this problem, a texture enhancement method for oceanic internal wave SAR images is proposed in this paper, which is based on non-local mean (NLM) filtering and texture layer enhancement (TLE). Since the strong speckle noise commonly present in internal wave images is simultaneously enhanced during texture enhancement, resulting in degraded image quality, NLM filtering is first performed to suppress speckle noise. Then, the denoised image is decomposed into the structure layer and the texture layer, and a texture layer enhancement method oriented to the texture characteristics of oceanic internal waves is proposed and applied. Finally, the enhanced texture layer and the structure layer are combined to reconstruct the final enhanced image. Experiments are conducted based on the Gaofen-3 real SAR data, and the results demonstrate that the proposed method performs well in suppressing speckle noise, maintaining overall image brightness, and enhancing internal wave texture details.

Experimental investigation of propagation evolution characteristics of internal solitary waves passing through downslope topography

In contrast to the extensive research on the propagation evolution of internal solitary waves (ISWs) over continental shelf topography, few investigations have been conducted on the propagation evolution of ISWs over downslope topography. In this experiment, the characteristics of the ISWs passing over a downslope topography were investigated in an immiscible two-layer fluid system to clearly capture the waveform information. The main objective of this study was to analyze the propagation evolution of ISWs with different incident amplitudes passing through a downslope topography. Using the particle image velocimetry technique, the evolutionary characteristics of ISWs (such as waveform, wave height, energy budget, and flow field) are investigated. According to the flow field analysis results, the phase velocity near the downslope topography changes with the propagation of ISWs. Moreover, the gradient distribution of velocity progressively becomes perpendicular to the right surface of the topography. Furthermore, a counterclockwise vortex gradually forms at the right upper vertex of the topography. The propagation of ISWs is influenced by varying water depth and topography. The front waveform of ISWs is significantly flat, and the back is steeper than the original back waveform. Around the tail wave, an upward convex wave packet is generated. The trends of wave energy and wave height losses are broadly consistent. Furthermore, the energy loss is related to the incident wave height with a 20% higher loss rate for large-amplitude ISWs than small-amplitude ISWs at the inflection point of the topography.

Stripe Extraction of Oceanic Internal Waves Using PCGAN with Small-Data Training

Playing a crucial role in ocean activities, internal solitary waves (ISWs) are of significant importance. Currently, the use of deep learning for detecting ISWs in synthetic aperture radar (SAR) imagery is gaining growing attention. However, these approaches often demand a considerable number of labeled images, which can be challenging to acquire in practice. In this study, we propose an innovative method employing a pyramidal conditional generative adversarial network (PCGAN). At each scale, it employs the framework of a conditional generative adversarial network (CGAN), comprising a generator and a discriminator. The generator works to produce internal wave patterns as authentically as possible, while the discriminator is designed to differentiate between images generated by the generator and reference images. The architecture based on pyramids adeptly captures the encompassing as well as localized characteristics of internal waves. The incorporation of upsampling further bolsters the model’s ability to recognize fine-scale internal wave stripes. These attributes endow the PCGAN with the capacity to learn from a limited amount of internal wave observation data. Experimental results affirm that the PCGAN, trained with just four internal wave images, can accurately detect internal wave stripes in the test set. Through comparative experiments with other segmentation models, we demonstrate the effectiveness and robustness of PCGAN.

Investigating the characteristics of internal discharge shock waves in gas‐insulated switchgear under varied gap configurations

AbstractGas‐insulated switchgears (GIS) are crucial components of high‐voltage power transmission and distribution systems. Internal discharges within GIS have garnered significant attention in the field of power engineering. This study investigates the characteristics of internal discharge shock waves in GIS under an air pressure of 0.3 MPa and three different discharge gap conditions: 1, 1.5, and 2 mm. High‐speed shadowing techniques are used to analyse the propagation speed, morphology, and post‐wave parameters. The study findings reveal that although the characteristics of internal discharge shock waves in GIS noticeably change with the gap size, they also exhibit a consistent trend: as the gap size increases, the initial shock wave accelerates, intensifying the discharge. Simultaneously, the rate of attenuation rises, with the shock wave becoming weaker after ≈30 µs. Furthermore, the post‐wave parameters follow a similar pattern, with an increase in gap size leading to higher parameter values but also a faster decay rate. The parameters decay more rapidly between 10 and 20 µs, slow down between 20 and 30 µs, and ultimately stabilize after around 30 µs. The results of this study hold significant theoretical and practical implications for the monitoring, diagnosis, and prevention of internal discharges in GIS.

Experimental study on the variation of optical remote sensing imaging characteristics of internal solitary waves with wind speed

Experimental study on the variation of optical remote sensing imaging characteristics of internal solitary waves with wind speed

Numerical study of submerged body tail linearization in a strongly stratified fluid on the characteristics of generated internal waves

Underwater motion generates internal waves in a density-stratified seawater environment. These waves can persist for several days in oceanic regions, making them detectable by synthetic aperture radar (SAR). To clarify the effect of tail linearization on the internal wave characteristics of an object, this paper builds a strongly stratified fluid flume based on the delayed detached eddy model (DDES), through which we numerically simulate the internal waves characteristics of two models, namely, a tail-planar type and a cylindrical rear spherical type. The numerical results show that the turbulent wake of the cylindrical rear spherical type diverges to both sides under the low and high Froude (Fr) number, leading to an increase in the amplitude of the generated volume effect internal wave and wake angle, but the wavelength of the volume effect internal wave is almost the same for both. Notably, at high Fr numbers, the influence of the trailing effect internal wave of the object with a linearized tail is significantly reduced. The tail-planar type of internal waves shows violent fluctuations over long distances and high-intensity violent changes in locally concentrated regions. In contrast, the internal waves of the object with tail linearization show slight fluctuations over long distances.

On the distinction of seasonal internal solitary waves characteristics in the Lombok Strait based on multi-satellite data

ABSTRACT This study presents a multi-satellite approach utilizing synthetic aperture radar (SAR) and optical sensors to distinguish the characteristics of internal solitary waves (ISWs) during different seasons in the Lombok Strait. SAR and optical sensors are employed to estimate the dynamic parameters (soliton number, wavelength, and phase speed) of ISWs during different seasons based on the detected ISW patterns. ISW characteristics in the Lombok Strait during two seasons were observed using Sentinel-1/SAR, GCOM-C/SGLI, and Terra/MODIS sensors. Results indicated that Sentinel-1/SAR detected a higher soliton number for the northward-propagating ISWs in the Lombok Strait during the north-west monsoon (NWM) than the south-east monsoon (SEM) period. The identified wavelengths of ISWs were wider during the SEM than during the NWM whenever two packets were detected in one image. Similar variations were observed by optical sensors (SGLI and MODIS). Estimation of ISW phase speed derived from multi-satellite images and the Korteweg-de Vries (KdV) equation showed that the phase speed was faster during the NWM than the SEM. These results correlated with the seasonal variation of thermocline depth and density differences between the two layers. Because of the bias caused by turbid water, a pattern of high chlorophyll-a was observed during the NWM period in the SGLI, MODIS, and VIIRS products. Propagation of turbid water to the north was assumed to be a result of the intense ISW activity in this area. Highly variable currents due to ISW activity in the Lombok Strait area were assumed to cause the anomalous sediment transport in this study. The multi-satellite observation data used in this study enhanced understanding of the influence of ISWs on coastal interactions in the Lombok Strait.

Stripe segmentation of oceanic internal waves in SAR images based on Gabor transform and K-means clustering

Oceanic internal waves are an active ocean phenomenon that can be observed, and their relevant characteristics can be acquired using synthetic aperture radar (SAR). The locations of oceanic internal waves must be determined first to obtain the important parameters of oceanic internal waves from SAR images. An oceanic internal wave segmentation method with integrated light and dark stripes was described in this study. To extract the SAR image characteristics of oceanic internal waves, the Gabor transform was initially used, and then the K-means clustering algorithm was used to separate the light (dark) stripes of oceanic internal waves from the background in the SAR images. The regions of the dark (light) stripes were automatically determined based on the differences between the three classes, that is, the dark stripes, light stripes, and background area. Finally, the locations of the dark (light) stripes were determined by shifting a given distance along the normal direction of the long side with the minimum bounding rectangle of the light (dark) stripes. The best segmentation results were obtained based on the intersection over the union of the images, and the accuracy of segmentation was verified. Furthermore, the effectiveness and practicability of the proposed method in the light and dark stripe segmentation of SAR images of oceanic internal waves were illustrated. The proposed method prepares the foundation for future inversion studies of oceanic internal waves.

Optical remote sensing image characteristics of large amplitude convex mode-2 internal solitary waves: an experimental study

Optical remote sensing image characteristics of large amplitude convex mode-2 internal solitary waves: an experimental study

Experimental and observational analysis of vertical characteristics of internal solitary waves

The basic elements of internal solitary waves (ISWs) were obtained by vertically placed sensors in the laboratory experiment and in-situ observation. The vertical characteristics of the amplitude and waveform are analysed experimentally and observationally. Then the experimental and observational results were compared with the theoretical model. The study shows that the experimental amplitude-frequency relationship is closer to that in the two-layer model while the observational amplitude-frequency relationship is more approaching to that in the continuously stratified model. Experimental and observational results reveal that the amplitude of ISWs increase first and then decrease with an increasing depth. The sech2 function of the KdV theory can describe the waveforms of ISWs at different depths well.

Seasonal variability of characteristics of nonlinear internal waves in the Kuril-Kamchatka region by Sentinel 1 data

The seasonal variability of the surface manifestations of nonlinear internal waves (NIW’s) in the Kuril-Kamchatka region from December 2018 to November 2019 was investigated based on the analysis of the Sentinel 1 satellite imagery archive. On 827 satellite images, 1876 manifestations of NIW’s were registered with a wavelength from 80 to 1900 m, containing from 2 to 18 waves in a packet. It was identified that the number of NIW’s registered in the water area varied significantly during the year. The minimum number of manifestations was recorded from January to March, and the maximum number of manifestations was observed from July to September. Areas of the frequent occurrence of NIW’s manifestations were identified: near the Kunashir Island, over the Vityaz ridge, near Cape Shipunsky and in Kamchatka Bay. Manifestations of NIW’s in these areas were recorded in all seasons of the year. These areas accounted for about 53% of all registered manifestations of NIW’s, and their area was about 20% of the study region area. The calculation of the tidal body force criterion for the main diurnal and semidiurnal tidal harmonics made it possible to identify potential generation sites of internal tidal waves. About 70% of NIW’s manifestations in August were detected in areas close to potential generation sites of diurnal internal tidal wave K1. This is explained by closeness of the generation and the disintegration sites of internal tidal waves under conditions of being beyond the critical latitude, where the tidal frequency coincides with the inertial one. Using the example of the area above the Vityaz ridge, the connection between the seasonal variability in the number of recorded manifestations and the annual variations in the tidal body force for harmonics K1, O1 and P1, mixed layer depth, the surface wind speed and the Brunt-Vaisala frequency was shown. This is consistent with the results of studies carried out in the adjacent regions of the Pacific Ocean. The existing archives of the Sentinel-1 radar image and the continuation of this satellite mission in the future will make it possible to estimate the long-term variability of NIW’s characteristic and to assess the impact of climate change on them.

Analysis of Internal Solitary Waves in the Gulf of Oman and Sources Responsible for Their Generation

A combination of multiple data sources has been used to study the characteristics of internal solitary waves (ISWs) in the Gulf of Oman (GoO). Water column stratification in the Gulf has been examined using field observations and World Ocean Atlas 2018 datasets. The spatiotemporal distribution of ISWs has been obtained from satellite images obtained by means of Synthetic Aperture Radar (SAR) and optical sensors taken from 2018 to 2020. The mechanisms of ISW generation in the GoO have been studied using the data revealed from different available sources. The results show that there are annually two major typical stratifications in the GoO throughout the year, strong stratification in May through September and weak stratification during other months. Dispersion relations corresponding to these types of stratification have been obtained with acceptable accuracy for both deep and shallow regions. The spatiotemporal distribution of ISWs demonstrates that the western and southern regions of the GoO are the hotspots for generation of ISWs in this basin. Several mechanisms of ISW generation in the GoO are discussed including tide, eddies, lee waves, and atmospheric perturbation; the latter one is, apparently, responsible for the appearance of large-amplitude ISWs.

Evaluation of feature extraction algorithms for oceanic internal waves based on nighttime detection data of spaceborne low light imager

The day/night band channel on the JPSS series of satellites can detect the light and dark fringes of oceanic internal waves due to the reflectivity difference caused by the roughness of the sea surface under moon flare conditions. After optical imaging of oceanic internal waves, three image processing algorithms, i.e., the two-dimensional S transform, windowed Fourier transform, and wavelet packet transform methods, can be used to extract the parameter features of horizontal wavelength and propagation direction. The wave domain with known parameters is established through data simulation, and both image quality and image resolution are analyzed to assess algorithm performance in terms of relative errors. Finally, the experimental conclusions are verified in two examples of satellite observations in the South China Sea in 2020. We found that the windowed Fourier transform and wavelet packet transform methods exhibit better noise immunity, and the two-dimensional S transform method exhibits less calculation error and is more applicable to cases with small wavelengths. For large wavelengths, the windowed Fourier transform method is more suitable for calculating the horizontal wavelength, and the wavelet packet transform method is more suitable for calculating the propagation direction. By evaluating the applicability of these algorithms, this study provides a theoretical basis to support the analysis and processing of internal wave characteristics in future.

Features of the Field of Internal Waves on the Abkhazian Shelf of the Black Sea according to Remote Sensing Data and In Situ Measurements

The field of internal waves in the Black Sea is quite significant. The Black Sea shelf is of particular interest, but it has not been studied enough in some specific regions. For example, a narrow and steep shelf of Abkhazia has been poorly explored. Particularly unexplored are the actual parameters and causes of the generation of internal waves in this area. In this article, we have attempted to fill this gap by analyzing remote sensing data and in situ data. An analysis of a set of optical multispectral satellite images (Sentinel-2, Landsat-8) and a collection of sea-truth data of the shelf zone of Abkhazia was carried out to identify features of internal wave fields of this region. In situ data were acquired over 9 years using ADCP, CTD, and SVP probes and moored stations with point and line temperature sensors. It is shown that internal waves are widespread on the Abkhazian shelf. They appear as trains of short-period waves (as a rule, soliton-like). The quantitative parameters and features of internal waves are shown and analyzed. The form of manifestation and direction of internal wave trains’ travel depend on the mechanisms of their origin, among which are the transformations of inertial internal waves, generation by river plumes, and submesoscale structures. In general, the article is the most complete and relevant study of the field of internal waves on the shelf of Abkhazia.

Characteristics of internal solitary waves in the Maluku Sea, Indonesia

The appearance of internal solitary waves (ISWs) in the Maluku Sea is often captured by satellite imagery. However, no study has revealed details on this phenomenon to date. Here, the characteristics of such ISWs were investigated based on their appearance in synthetic aperture radar (SAR) imagery on 20 February 2015. Two different sources of ISW packets were observed: one packet propagating from the Lifamatola Passage and another from the Sangihe Passage. The vertical structure of the waves was constructed using the Korteweg-de Vries (KdV) model, which suggests an average phase speed of ∼2.8 and 2.7 m s−1 for the first and the second sources, respectively. ISWs originating from the first source had a typical amplitude of O(80 m), while those from the second source were characterized by a lower amplitude of O(40 m). The waves generated horizontal and vertical currents with typical magnitudes of O(1 m s−1) and O(10 cm s−1) for the first source and O(0.6 m s−1) and O(4 cm s−1) for the second source, respectively. The mean energy densities of the first and second sources reached 461 MJ m−1 and 185 MJ m−1, respectively. Single leading solitary wave contained a fraction of approximately 20% and 15% of the baroclinic tidal energy generated in the Lifamatola Passage and Sangihe Passage, respectively.

Experimental Investigation on the Vertical Structure Characteristics of Internal Solitary Waves

An experimental investigation of the vertical structure characteristics of internal solitary waves (ISWs) was systematically carried out in a large gravitationally stratified fluid flume. Four different stratifications were established, and basic elements of ISWs were measured by a conductivity probe array. The vertical distributions of the amplitude, characteristic frequency and waveform of two types of ISWs under different stratifications were obtained, and the experimental results were compared with the theoretical model. The study shows that most vertical structures of the amplitude under different stratifications agree with those of the theoretical model, while there are some deviations for ISWs with large amplitudes. Neither the two-layer model nor the continuously stratified model can effectively describe the variation in the characteristic frequency at different depths with amplitude. For a single small-amplitude ISW, the characteristic frequency first increases and then decreases with increasing depth. The characteristic frequency is largest at the depth of the maximum buoyancy frequency. For an ISW with a relatively large amplitude, there is likely to be a local minimum of the characteristic frequency near the depth where the maximum buoyancy frequency lies. In different stratifications, the sech2 function of KdV theory can describe the waveforms of ISWs at different depths well.

Research on Internal Solitary Wave Detection and Analysis Based on Interferometric Imaging Radar Altimeter Onboard the Tiangong-2 Space Laboratory

The Tiangong-2 space laboratory was launched by China on 15 September 2016, carrying the Interferometric Imaging Radar Altimeter (InIRA), the first of the latest generation of imaging altimeters that can perform imaging and acquire elevation information simultaneously. This paper analyzes the feasibility of using InIRA images to obtain two-dimensional characteristics of oceanic internal solitary waves (ISWs) and information about vertical sea surface fluctuations caused by the propagation of ISWs. The results show that InIRA demonstrates a relatively reliable ability to observe ISWs with high resolution and can identify the fine-scale features of ISWs of different forms. Furthermore, InIRA can observe centimeter-level changes in the Sea Surface Height Anomaly (SSHA) caused by ISWs. The geometric relationship between the sensor’s flight direction and the propagation direction of ISWs does not affect its detection effect. However, the swath width of InIRA is too narrow to fully capture ISW information, and the height accuracy of InIRA height product images is not insufficient to detect the height information of small-scale ISWs. These shortcomings need to be considered in the future development of imaging altimeters to increase their potential for detecting mesoscale phenomena in the ocean.

Stratification effects on shoaling internal solitary waves

This combined numerical/laboratory study investigates the effect of stratification form on the shoaling characteristics of internal solitary waves propagating over a smooth, linear topographic slope. Three stratification types are investigated, namely (i) thin tanh (homogeneous upper and lower layers separated by a thin pycnocline), (ii) surface stratification (linearly stratified layer overlaying a homogeneous lower layer) and (iii) broad tanh (continuous density gradient throughout the water column). It is found that the form of stratification affects the breaking type associated with the shoaling wave. In the thin tanh stratification, good agreement is seen with past studies. Waves over the shallowest slopes undergo fission. Over steeper slopes, the breaking type changes from surging, through collapsing to plunging with increasing wave steepness $A_w/L_w$ for a given topographic slope, where $A_w$ and $L_w$ are incident wave amplitude and wavelength, respectively. In the surface stratification regime, the breaking classification differs from the thin tanh stratification. Plunging dynamics is inhibited by the density gradient throughout the upper layer, instead collapsing-type breakers form for the equivalent location in parameter space in the thin tanh stratification. In the broad tanh profile regime, plunging dynamics is likewise inhibited and the near-bottom density gradient prevents the collapsing dynamics. Instead, all waves either fission or form surging breakers. As wave steepness in the broad tanh stratification increases, the bolus formed by surging exhibits evidence of Kelvin–Helmholtz instabilities on its upper boundary. In both two- and three-dimensional simulations, billow size grows with increasing wave steepness, dynamics not previously observed in the literature.

Numerical Simulation for the Evolution of Internal Solitary Waves Propagating over Slope Topography

In this study, the propagation and evolution characteristics of internal solitary waves on slope topography in stratified fluids were investigated. A numerical model of internal solitary wave propagation based on the nonlinear potential flow theory using the multi-domain boundary element method was developed and validated. The numerical model was used to calculate the propagation process of internal solitary waves on the topography with different slope parameters, including height and angle, and the influence of slope parameters, initial amplitude, and densities jump of two-layer fluid on the evolution of internal solitary waves is discussed. It was found that the wave amplitude first increased while climbing the slope and then decreased after passing over the slope shoulder based on the calculation results, and the wave amplitude reached a maximum at the shoulder of the slope. A larger height and angle of the slope can induce larger maximum wave amplitude and more obvious tail wave characteristics. The wave amplitude gradually decreased, and a periodic tail wave was generated when propagating on the plateau after passing the slope. Both frequency and height of the tail wave were affected by the geometric parameters of the slope bottom; however, the initial amplitude of the internal solitary wave only affects the tail wave height, but not the frequency of the tail wave.