Vorticity Patterns Offshore of the Venetian Lagoon from HF Radar Observations

Abstract

A network of high frequency (HF) radar systems was used to produce maps of surface velocity offshore of the Malamocco inlet during the 12-month period from November 2001 through October 2002. Inspection of the sub-tidal residual flow fields revealed frequent occurrences of small-scale (~10 km) eddy structures both north and south of the inlet. In addition, several monthly averaged velocity maps indicate the presence of a persistent meander in the mean current patterns. A more objective technique, based on vorticity, was developed to search the data set for the presence of eddies near the inlet and to separate those features from the larger-scale meander pattern. The vorticity at scales of 5-10 km was computed each hour using the low-passfiltered data set and year-long vorticity statistics were investigated at selected locations surrounding the inlet. The vorticity histograms and temporal variability changed significantly as a function of location relative to the Malamocco inlet. Both north and south of the inlet, the mean vorticity was weakly negative reflecting the larger-scale meander pattern whose crest is aligned, approximately, with the inlet. However, the range of vorticity north of the inlet was much greater than it was south of the inlet. A minor peak (or shoulder) in the year-long histogram of vorticity north of the inlet highlighted frequent occurrences of strong (~ 5 × 10-5 s-1) positive vorticity events in that area. A similar statistical behaviour was not seen south of the inlet. This observation was used to define a conditional sampling criteria that lead to a map of the typical flow field associated with strong eddy events. The vorticity time series were also used to investigate the relationship of eddy events to external forcing parameters. Strong winds, for instance, acted to destroy vorticity in the surface current field since they lead to a strong but horizontally uniform response in the surface currents. The roles of sea level variability and offshore current velocity were also investigated.