High-frequency acoustic scattering from turbulent oceanic microstructure: The importance of density fluctuations

Abstract

Acoustic scattering techniques provide a unique and powerful tool to remotely investigate the physical properties of the ocean interior over large spatial and temporal scales. With high-frequency acoustic scattering it is possible to probe physical processes that occur at the microstructure scale, spanning submillimeter to centimeter scale processes. An acoustic scattering model for turbulent oceanic microstructure is presented in which the current theory, which only accounts for fluctuations in the sound speed, has been extended to include fluctuations in the density as well. The inclusion of density fluctuations results in an expression for the scattering cross section per unit volume, v , that is explicitly dependent on the scattering angle. By relating the variability in the density and sound speed to random fluctuations in oceanic temperature and salinity, v has been expressed in terms of the temperature and salinity wave number spectra, and the temperature-salinity co-spectrum. A Batchelor spectrum for temperature and salinity, which depends on parameters such as the dissipation rates of turbulent kinetic energy and temperature variance, has been used to evaluate v . Two models for the temperature-salinity co-spectrum have also been used. The predictions indicate that fluctuations in the density could be as important in determining backscattering as fluctuations in the sound speed. Using data obtained in the ocean with a high resolution vertical microstructure profiler, it is predicted that scattering from oceanic microstructure can be as strong as scattering from zooplankton.

On the turbulent co-spectrum of two scalars and its effect on acoustic scattering from oceanic turbulence

“…Φ T , Φ S , Φ T S (normalized such that, for example, 4π K 2 Φ T S (K) dK = T S ) are, respectively, the isotropic vector spectra for temperature and salinity, and the temperature-salinity co-spectrum evaluated at the Bragg scattering wavelength, 2k. Lavery et al (2003) showed that the coefficients are given by A = a µ − α and B = b µ + β, where a µ and b µ are, respectively, the fractional changes in sound speed from temperature and salinity changes, while α and β are the coefficients of thermal expansion and saline contraction. An examination of (2.1) reveals how the temperature-salinity co-spectrum has a large effect on the scattering cross-section.…”

“…Φ T and Φ S are always positive, but Φ T S can be either positive or negative, depending on the sign of the ratio of the ambient gradients δ = (dT /dz)/(dS/dz). Seim (1999) pointed out that when δ is negative, and the relative contributions to scattering from temperature and salinity are similar (he used the condition R η = a µ δ/b µ ≈ −1 as he neglected density fluctuations from his model; Lavery et al (2003) more completely propose using R ρc = Aδ/B ≈ −1), the turbulent scattering signal will be greatly reduced and may disappear. Thus, for R ρc ≈ −1, the turbulent sound scattering model is highly sensitive to the model used for the temperature-salinity co-spectrum.…”

“…This is important because acoustic methods are showing increasing promise as a relatively cheap and easy way to make quick synoptic measurements of turbulence (Seim, Gregg & Miyamoto 1995;Ross & Lueck 2003;Lavery, Schmitt & Stanton 2003).…”

Continued Analysis of High-Frequency Broadband Acoustic Scattering from Non-Linear Internal Waves during SW06

“…More recently, there has also been significant effort directed towards the quantitative use of narrowband acoustic scattering techniques for investigating small-scale physical processes, such as oceanic microstructure (e.g. Goodman, 1990;Seim et al, 1995;Lavery et al, 2003;Ross and Lueck, 2003;Warren et al, 2003). Acoustic scattering techniques provide a rapid, high-resolution, synoptic, remote sensing alternative to more traditional sampling strategies.…”

“…These data have been used with existing scattering models (Lavery et al, 2003) to predict scattering from microstructure, and to assess the importance of salinity versus temperature microstructure. These data helped guide the locations for which intensive acoustic analysis was performed.…”