Constructing surface current maps from HF radars with different operating frequencies

HF radars operating at decameter wavelengths (3 to 30 MHz) in ground wave mode are capable of mapping near-surface currents. Several different HF radar systems are often in operation to observe an area, using different radar operating frequencies. Users of the data products of HF radars need to know the likely uncertainties or ´errors´ 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 difference vectors that are between 1 and 7 cm/s when averaged over a ten day period. We also present a physical model that predicts the difference between current vectors observed at different frequencies in the 3 to 25 MHz range. The predicted difference in current speed between currents observed at 4.8 and 21.8 MHz is ≈5 cm/s for a 10 m/s wind speed and ≈9 cm/s for a 15 m/s wind speed. Typical maximum wind speeds on Monterey Bay are 10 to 15 m/s. We discuss briefly how HF radar observations at different frequencies can be used to produce a current map corresponding to a single reference frequency of 13.5 MHz.