Interpolation of the radial velocity data from coastal HF radars

In recent years, monitoring nearshore surface currents became an important application of the high-frequency radar (HFR) technology. The Doppler shifts of backscat- Tered radio signals from surface waves provide the surface velocity component in the direction of a HFR beam. These radial velocities observed by multiple (usually two) Radars have to be combined/interpolated to produce the gridded vector field, which can be used in applications. In view of a relatively high (5-10cm/s) HFR measurement Errors of the radial velocities, interpolation algorithms which take into account additional constraints on the velocity field (such as those imposed by the coastlines and Model dynamics) are of particular value. In this chapter, recent developments in the radial velocity processing methods are reviewed. The topics include advanced optimal interpolation techniques, kinemati- cally constrained Galerkin and the 2d variational interpolation methods, and the dynamically constrained assimilation of the HFR data using numerical models. Accurate monitoring of the velocity field may suffer from occasional malfunction of a radar which causes a substantial data loss on a relatively short (3-30hours) time interval.We examine performance of the gap-filling technique based on empirical orthogonal function analysis of the radial velocity observations and demonstrate its performance in tidally-driven coastal environments.