Friction Velocity Measurements using a Marine Radar

The friction velocity is retrieved from time series acquired by marine X-band radar. The radar operates at grazing incidence with horizontal polarization in transmit and receive and measures the backscatter of the ocean surface in space and time. The intensity of the measured radar backscatter is strongly related to the local small scale ocean surface roughness and therefore the radar backscatter gives a measure of the friction velocity.

The method for measuring the friction velocity consists of two parts: In the first part the wind directions are estimated from wind induced streaks, with a typical spacing below 500 m. These streaks are very well aligned with the mean surface wind direction. The algorithm defines wind direction as normal to the gradient of the amplitude image, which is approximated by finite differences over an appropriate length. The resulting wind direction is taken as normal to the most frequent values of the retrieved local gradients. In the second part the friction velocity is determined from the mean radar backscatter and the previously retrieved wind direction via a empirical model function (EMF). The parameterization of the EMF is performed by developing a Neural Network (NN). The main advantage of this new technique is that, the friction velocity can be determined directly without the knowledge of additional parameters, e.g., humidity, air and sea temperature. Furthermore, the system can be operated from offshore as well as onshore platforms as well as from moving platforms, e.g., ships.

To demonstrate the applicability of the methodology, nautical radar image sequences were collected on a continuous basis aboard the offshore research platform FINO-I and used to estimate the friction velocity, which was compared to results of friction velocities estimated from the in situ measurements recorded at the platform. FINO-I is located in the southern North Sea and provides since late 2003 various environmental parameters, e.g., wind, temperature and humidity measurements at several height levels, current and water temperature measurements at several depths as well as ocean wave measurements. The accuracy and limitations of the method is investigated and compared to different methodologies of estimating the friction velocity. Furthermore the probable error sources (radar cross section, estimation of wind direction, in-situ friction velocity) are discussed and the applicability of the method is shown for both tower based and ship borne (moving) radars.