High Resolution Measurements of Turbulence, Velocity and Stress Using a Pulse-to-Pulse Coherent Sonar

Abstract

Considered are the capabilities of a recently developed pulse-to-pulse coherent sonar called the High Resolution Current Profiler (HRCP). Special emphasis is placed on methods whereby reliable and accurate vertical profiles of turbulence parameters, such as turbulent kinetic energy and Reynolds stresses, may be extracted from such sonars. The prototype HRCP has been developed in order to obtain precise and reliable measurements of both the mean and the fluctuating components of the velocity vector profile in the bottom boundary layer (lowermost 10 m). The prototype HRCP developed through the project BSEX is a 307 KHz pulse-to-pulse coherent sonar with four beams mounted 30° off the vertical in 90° azimuthal increments. It has 50 range cells covering a vertical profiling distance of 10 meters. The data collected are radial speeds along the four acoustic beams. Thus, care has to be exercised in interpreting the measurements in terms of turbulence parameters. It is shown, based upon reasonable assumptions about oceanic turbulence and turbulence characteristics, that it is possible to construct methods whereby reliable estimates of Reynolds stresses and turbulent kinetic energy profiles may be obtained utilizing the HRCP technology. The HRCP was deployed during the summer of 1986 for 52 hours at a site in the northern North Sea. Reliable data with the HRCP were collected with 4 Hz sampling rate at 46 vertical levels 20 cm apart in the range 88–948 cm above the bottom. First, the observed velocities are decomposed into mean and fluctuating components based on spectral analysis. The mean velocities are then fitted to a logarithmic profile to provide estimates of bottom parameters, such as shear velocities and roughness length. Finally, the associated bottom boundary-layer turbulence is discussed in terms of Reynolds stress components and turbulent kinetic energy. The results are in agreement with similar studies of both bottom parameters and bottom turbulence. The estimates are also shown to be precise enough to estimate vertical mixing coefficients commonly applied in planetary boundary layer models, i.e., the parameterization of the Reynolds stresses. The success in estimating reliable turbulence parameters by use of the pulse-to-pulse coherent Doppler sonars is encouraging for the future applications and use of this technology.