Vorticity Patterns Offshore of the Venetian Lagoon from HF Radar Observations
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Authors
Paduan, J.D.
Gacic, M.
Kovacevic, V.
Mosquera, I. Mancero
Mazzoldi, A.
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Date of Issue
2003
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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.
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Oceanography
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This publication is a work of the U.S. Government as defined in Title 17, United States Code, Section 101. Copyright protection is not available for this work in the United States.