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HF radar observations in the Northern Adriatic: Surface current field in front of the Venetian Lagoon

Two HF radar stations looking seaward were installed along the littoral of the Venetian Lagoon (Northern Adriatic). They operated continuously for I year (November 2001-October 2002). The surface current data are obtained at a spatial resolution of about 750 in and are available every hour in a zone whose depths slope gently from the coast down to 20 in. The total area covered is about 120 km(2) reaching cca 15 km offshore. These observations give evidence of the different temporal and spatial scales that characterize the current field. Unlike for the flow in inlets of the Venetian Lagoon, the tidal forcing accounts for only up to 20% of the total variability. The tidal influence from the inlets is felt to about 4-5 kin away. Low-frequency signal, associated prevalently to meteorological forcing (through the action of winds and atmospheric pressure) at synoptic time scales, is superimposed on a mean southward flow of 10 cm/s. Formation of small-scale eddies (diameter of about 5 km) to the north and to the south of the Malamocco inlet is observed only during the periods of calm. During strong northeasterly bora and southeasterly sirocco winds, an increase of the along-shore southward current due to geostrophic adjustment is observed in a coastal band. A particularly strong bora event in December 2001 generated an almost uniform flow reaching 30-50 cm/s in the whole investigated area. During sirocco in June 2002, a current reversal toward north and a triggering of inertial motions were observed. Yearlong measurements give an insight into a seasonal variability through a sequence of mean monthly current maps. In November and December, the intensity was the highest (up to 18 cm/s), and the flow structure was almost uniform, with a weak shear in the offshore direction. The weakest mean flow was observed in May 2002, characterized however with a nonuniform distribution over the study zone and suggestions of anticyclonic eddies north and south of the Malamocco, inlet. On the other hand, possibly due to the highly variable wind, the eddy kinetic energy in May, associated with both- small-scale variability near the shore and inertial oscillations offshore, was maximum for the investigated period.

surface currents; HF radar; tides; residual currents; winds; Adriatic Sea; Venetian Lagoon

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