High frequency radar measurements of friction velocity in the marine boundary layer

This work uses the hydrodynamic theory of the turbulent boundary layer to interpret High Frequency (HF) radar data and to address the potential of HF radar for determination of the friction velocity in the water over a broad spatial region. Recent advances in HF radar instrumentation have resulted in the development of a radar capable of detecting small changes in surface wave phase velocities as a result of an underlying current. The development of a multi-frequency radar system extends this capability to the determination of the vertical distribution of the flow in the water column, or current shear. In this work, the Levenberg-Marquardt method of nonlinear least squares is used to determine the near surface current profile based on a theoretical model of the effect of the current on the phase velocity of a surface gravity wave. The results of this analysis are compared to in situ measurements of wind velocity and measurement-based calculations of friction velocity. The results show that estimates of the friction velocity compare well with in situ measurements under moderate wind conditions.