Acoustic backscattering at a tidal intrusion front

Bassett, Christopher; Lavery, Andone C.; Ralston, David; Geyer, W. Rockwell; Jurisa, Joseph T.; Thomson, Jim; Honegger, David A.; Simpson, Alexandra; Scully, Malcom E.; Haller, Merrick C.

doi:10.1016/j.pocean.2023.103167

Cited by 7 publications

(7 citation statements)

References 97 publications

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“…While the locations of individual Kelvin‐Helmholtz billows cannot be related to specific occurrence of subcritical Ri, the echo sounding image demonstrates that instabilities are prevalent throughout the region of low Ri (Lavery et al., 2013). The conditions within 50 m of the front are harder to discern because bubbles entrained at the front dominate the acoustic backscatter signal (Bassett et al., 2023) but the vertical displacements of the isohalines (purple contours) are consistent with the presence of large‐amplitude instabilities near the front.…”

Section: Discussionmentioning

confidence: 99%

“…An upward‐looking, 325 kHz echo sounder was mounted on a streamlined tow‐body at the bottom of the towed array (Bassett et al., 2021), providing high‐resolution acoustic backscatter imagery of the subsurface frontal structure and associated shear instabilities (see Bassett et al. (2023) for a more detailed examination of the acoustic data from this experiment).…”

Section: Observationsmentioning

confidence: 99%

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The Structure and Dynamics of an Estuarine Tidal Intrusion Front

Geyer,

Ralston,

Haller

et al. 2024

JGR Oceans

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Tidal intrusion fronts are surface convergences that occur at constrictions in estuaries during the flood tide, separating incoming higher‐salinity water from lower‐salinity, stratified estuarine water. Previous observations of tidal intrusion fronts describe a V‐shaped planform, with the apex of the V pointing into the estuary, however the significance of this structure has not been previously explained. Observations near the mouth of the James River estuary during the flood tide reveal the development of a quasi‐steady, V‐shaped front. Considering a reference frame oriented normal to the front, the velocity and density structure are consistent with gravity‐current dynamics, but the oblique orientation of the front relative to the impinging flow indicates strong, along‐front shear, which results from vorticity produced by flow separation at the lateral boundaries as well as topographic torque from upstream. The combination of convergence and along‐front shear leads to enhanced mixing, as revealed by acoustic backscatter images of shear instability and persistent subcritical gradient Richardson number in the frontal zone. Oblique fronts such as this tidal intrusion front are common features of estuaries, and they play an important role in vertical exchange due to subduction and mixing of surface water.

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Section: Discussionmentioning

confidence: 99%

Section: Observationsmentioning

confidence: 99%

The Structure and Dynamics of an Estuarine Tidal Intrusion Front

Geyer,

Ralston,

Haller

et al. 2024

JGR Oceans

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“…An understanding of the bubble densi�es and size distribu�ons in estuarine fronts is cri�cal for constraining quan�ta�ve es�mates of their role in these processes. While oceanic bubbles have been studied for many years, few studies have examined the size distribu�ons of bubble popula�ons in �dal fronts or how they are influenced by the hydrodynamics of fronts (Basset et al, 2023;Marston et al, 2023;Reeder et al, 2022).…”

Section: List Of Tablesmentioning

confidence: 99%

“…Broadband acous�cal measurements over a wide range of frequencies permit high resolu�on es�mates of bubble size distribu�ons over a wide range of sizes. Ship-and vehicle-based backscatering over a wide bandwidth offer the necessary mobility for measuring frontal bubble clouds but involve the poten�al for excess transmission loss due to near-surface bubbles to impact measurements at certain frequencies (Basset et al, 2023;Basset and Lavery, 2021). The use of a towable instrument for measuring frontal bubbles in situ allows for con�nuous measurements with higher spa�al resolu�on than shipboard backscatering.…”

Section: List Of Tablesmentioning

confidence: 99%

“…However, it has been shown reasonable to assume that in field se�ngs there are generally mul�ple wavelengths between rela�vely large biological scaterers (e.g., fish) in an aggrega�on, so their individual target strengths sum linearly (Foote, 1982(Foote, , 1983. For dense popula�ons of other types of targets such as microbubbles, shoals of small fishes, and krill, mul�ple scatering may impact measurements (Andreeva and Belousov, 1996;Basset et al, 2023).…”

Section: Scatering From Mul�ple Objectsmentioning

confidence: 99%

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Broadband acoustical scattering in coastal environments: application to gelatinous organisms

Kahn

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Broadband acoustical technology revolutionized our ability to explore, monitor, and operate in the ocean. While strides have been made in numerous physical and biological applications, there remain many standing scientific questions well suited to broadband approaches. Physics-based sound scattering models allow us to interpret and draw quantitative observations from measurements. Such models have been developed and used to assess the biomass of many types of marine organisms of ecological significance, but we lack rigorous scattering models for gelatinous organisms despite their possibly accounting for a significant proportion of global marine biomass. Additionally, acoustical techniques for characterizing microbubble populations have been established for decades, yet little is known about the spectral characteristics of dense microbubble clouds associated with estuarine tidal fronts. These bubbles facilitate air-sea gas exchange and could interfere with acoustical operations in coastal environments; however, the density and size distribution of the bubbles must be known to assess their impacts. This dissertation addresses these deficiencies in our application of broadband techniques. In Chapter 2, a sound scattering model for gelatinous organisms is developed based on the Distorted Wave Born Approximation. The 3-D model is applied to a species of scyphomedusa and verified withlaboratory measurements of broadband backscattering from live individuals. The model predicts backscattering levels and broad spectral behavior within <2 dB. In Chapter 3, a towable instrument is developed for measuring broadband excess attenuation from bubbles from which the size distribution is inferred. The instrument is tested under breaking waves in a laboratory wave tank and then used to characterize the bubble size distribution in the Connecticut River tidal ebb plume front. In Chapter 4, broadband backscattering measurements from the Connecticut River front are used to infer the associated bubble size distribution. Spatial trends in the bubble size distribution are examined within the context of frontal kinematics. An observed disparity between the bubble size distribution measured with excess attenuation and volume backscattering is hypothesized to arise from a sampling bias caused by bubbles concentrated in the upper water column.

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Shear Instability and Turbulent Mixing by Kuroshio Intrusion Into the Changjiang River Plume

Tu,

Wu,

Fan

et al. 2024

Geophysical Research Letters

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Shear instability is a dominant mechanism for mixing in the stratified oceans and coastal seas. For the first time, we present fine‐scale, direct measurements of shear instabilities in the bottom front generated by the Kuroshio intrusion into the Changjiang (Yangtze) river plume. Shear instabilities were identified using a shipboard echo‐sounder and the resulting turbulent mixing was quantified using a turbulence microstructure profiler. The shear instabilities generate vigorous turbulent mixing with dissipation rate and vertical diffusivity up to O (10−4 m2 s−3) and O (10−1 m2 s−1), respectively, comparable to values associated with shear instabilities observed in river plumes and western boundary currents but several orders of magnitude larger than typical values in the open ocean. The enhanced turbulence may contribute significantly to mixing between the Kuroshio water and coastal water and thereby alter the coastal biogeochemistry cycles.

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Acoustic backscattering at a tidal intrusion front

Cited by 7 publications

References 97 publications

The Structure and Dynamics of an Estuarine Tidal Intrusion Front

The Structure and Dynamics of an Estuarine Tidal Intrusion Front

Broadband acoustical scattering in coastal environments: application to gelatinous organisms

Shear Instability and Turbulent Mixing by Kuroshio Intrusion Into the Changjiang River Plume

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