Comparison between VHF radar observations and data from drifter clusters in the Gulf of La Spezia (Mediterranean Sea)
Résumé
Results from a WERA radar system in very high frequency (VHF) mode in a small coastal area (range of 7 km. resolution of 250 m) are compared with surface drifter data. The measurements are performed in the Gulf of La Spezia, a highly populated area characterized by a complex use of the territory, where monitoring of surface currents can play an important role in correct management. The drifters have been launched in clusters of 3-6 units, aimed at investigating the significant time and space variability of the flow. In particular, two clusters launched at the same location 2 days apart show strikingly different patterns of evolution. The first cluster is swiftly advected with little particle spreading, while the second cluster divides into two branches with significant drifter spreading. Radial velocities from radar and drifter data are compared. The difference is characterized by a RMS similar to 5 cm/s, on the lowest side of the RMS values reported in the literature. In-situ drifter trajectories are then compared with synthetic trajectories computed from VHF radar fields, and a separation rate of similar to 1 km/12 h between individual trajectories is found. Finally, the spreading patterns of the in-situ clusters are qualitatively compared with the synthetic clusters and in all cases the synthetic trajectories are found to correctly reproduce the patterns, including the drastic difference between the two launches from the same location. The in-situ trajectories appear to fall in the envelope of the synthetic ones. The positive results are likely to be due to a combination of factors and primarily to the high frequency and space resolution of the radar fields, as well as to the flow regime characteristic of protected areas. The results confirm that radar data are well suited for the study of coastal flows in limited areas with complex patterns of velocity and transport. (C) 2009 Elsevier B.V. All rights reserved.