VERBI VEnice Research BIbliography

L’istogramma della vorticità e la sua variabilità temporale sono significativamente diverse in funzione della posizione relativamente alla bocca di Malamocco. Su entrambi i lati, nord e sud della bocca, i valori di vorticità sono deboli ma negativi, indicando la presenza di un meandro a grande scala la cui cresta si trova allineata con il canale. Inoltre, la variabilità più ampia si nota a nord, dove si osservano eventi con valori positivi significativi (~ 5 × 10-5 s-1), indicando una rotazione in senso anti-orario. A sud, invece, non si rivelano eventi analoghi. In base ad un criterio stabilito sul valore e sul senso di rotazione, sono state costruite le mappe della circolazione tipica associata agli episodi caratterizzati dalla presenza di vortici relativamente forti. L’indagine sulla possibile influenza dei parametri forzanti esterni ha stabilitio che un forte vento tende a distruggere questi moti rotatori, e crea un campo di corrente piuttosto omogeneo ed uniforme.

Sono state studiate le eventuali influenze del livello del mare e della intensià della corrente al largo. 

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.

All details about the installation, configuration and determination of the surface current vectors have been reported in [Kovacevic et al., 2002].

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.