Remote Sensing of Temperature and Salinity Microstructure in Rivers and Estuaries Using Broadband Acoustic Scattering Techniques

The long-term goals of this research are to 1) measure and understand high-frequency broadband acoustic scattering in rivers and estuaries characterized by strong temperature and salinity gradients and intermittent, high dissipation rates of turbulent kinetic energy, and 2) use these measurements and understanding to develop a remote sensing tool for quantifying the structure of stratified turbulence. The specific objectives are to: 1. Measure high-frequency broadband acoustic backscattering in highly stratified, energetic, estuarine environments, where there is significant salinity stratification, high shear, and high dissipation rates of turbulent kinetic energy. 2. Validate these measurements, and support their interpretation, by performing coincident, direct measurements of turbulence parameters at similar scales to the acoustic measurements. Estuaries provide an excellent environment to quantify stratified turbulence and its influence on acoustic backscattering, as these environments provide a broad range of stratification and turbulence intensities within a single tidal cycle. 3. Test the validity of existing microstructure scattering models, determining the range of conditions under which these scattering models are valid, and quantifying the contribution to scattering from salinity versus temperature variance over a broad frequency range. Background Significant improvements in both the visualization and quantification of acoustic scattering from stratified turbulence can be achieved through the use of broadband acoustic scattering techniques, spanning multiple octaves of bandwidth (Lavery et al., 2009; Lavery et al., submitted). The goal of these high-frequency broadband acoustic scattering techniques is to capitalize on the different characteristic frequency-dependent spectra associated with different scattering sources, which may in principle lead to decreases in the ambiguities associated with interpretation of acoustic scattering measurements. Furthermore, once a scattering source has been identified, broadband acoustic scattering measurements may lead to more robust inversion of the acoustic spectra for pertinent parameters, such as the dissipation rate of turbulent kinetic energy. The most robust method for inverting measurements of acoustic scattering from microstructure, as well as for classifying particular scattering features as being dominated by scalar microstructure, involves measuring acoustic spectra over sufficient bandwidth that both the temperature and salinity dissipative roll-offs are resolved (Lavery et al., 2009). To invert measurements of acoustic scattering from turbulence-