Merging surface current data from HF radars operating at different frequencies

HF radars operating at decameter wavelengths (3 to 30 MHz) in ground wave mode are capable of mapping near-surface, coastal ocean currents. Several different HF radar systems are often in operation to observe a given area, using different radar operating frequencies. Users of the data products of HF radars need to know the likely uncertainties and biases caused by the use of data from radars with different operating frequencies and how to merge the data from these different systems into a single current map. During the year 2000 two MCR´s operating at 4.8, 6.8, 13.4 and 21.8 MHz were sited at Moss Landing and Santa Cruz, California, collecting current maps at hourly intervals. We show that the current maps over Monterey Bay constructed from 4.8 MHz and from 21.8 MHz data have rms current speed differences of about 10 cm/s. We also present a physical model that predicts the difference between current speeds observed at different frequencies in the 3 to 25 MHz range as a result of wind stress induced, vertical current shear. We demonstrate theoretical and empirical methods for mitigating what we think is wind-stress-related bias between current observations at different HF radar operating frequencies.