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| Titel |
Surface transport in coastal and open-sea areas of the Adriatic Sea from Finite-Scale Lyapunov Exponents |
| VerfasserIn |
Maristella Berta, Annalisa Griffa, Angelique C. Haza, Laura Ursella |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250052529
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| Zusammenfassung |
| In two-dimensional flows, a simple time dependence in the velocity field is enough to induce
turbulent structures and consequently chaotic advection in particles trajectories.
Analysing these fields in the context of dynamical systems, an Eulerian approach
would provide the configuration of the eddies at a given scale and time, while the
application of a Lagrangian technique will show the spatio-temporal variability of these
eddies.
The fundamental assumption of chaotic advection is the sensibility of trajectories to
initial conditions, meaning that fluid elements initially very close would follow diverging
trajectories.
The exponential rate of separation between two particles is defined by the Lyapunov
exponent λ.
The classical definition of Lyapunov exponent refers to the exponential rate of divergence,
over infinite time, of infinitesimally closed initial points. The FSLE technique keeps the
original idea of capturing the rate of divergence between trajectories, but overcomes the limit
operations dealing with real data.
λ (x ,δ ,t,r) = 1-ln r
r i i Ï
Since the choice of δi andr depends mainly on currents characteristics and length scale of
the structures of interest, there is not a parameters choice suitable for any velocity
field.
FSLE technique shows how portions of fluid initially compact can be stretched and/or
folded as time goes by, producing a “cascade” of inhomogeneities from large to smaller
scales. This allows to identify dynamical structures within scales far below the finest
resolution achievable by the classical Eulerian analyses, without any assumption on the
sub-grid velocity field.
In the previous years, the FSLE technique was applied in the Adriatic Sea dealing with
dispersion studies based on drifters experiments or on velocity data produced by NCOM
model. These analyses, involving different areas and times, led to the identification of
the whole basin mixing scales (Lacorata et al., 2001; Haza et al., 2008) and to the
characterization of an hyperbolic point located in the South Adriatic (Haza et al.,
2007).
The new analysis proposed involves the northernmost part of the Adriatic Sea and it is
based on high resolution measurements of the velocity field coming from a radars system
placed along Italian and Croatian coasts. Data, belonging to NASCUM project - North
Adriatic Surface CUrrent Mapping, are available for the period 2007-2008 and cover a
regular grid with the following dimensions and resolution: 50km x 50km with 2km spatial
resolution and 1h temporal resolution.
Since Adriatic Sea is strongly affected by Bora and Scirocco winds the aim is to find a
correlation between wind forcing over the basin and surface dispersion.
Typical wind events have been already identified in the time series, coming from a
meteo-oceanographic buoy in the Gulf of Trieste, measuring wind intensity and
direction.
During these specific episodes, a preliminary analysis has been done identifying a clear
correspondence between Bora blowing direction and the main direction along which
filamental structures develop.
Further results, including the comparison between FSLE maps and wind maps (from
ALADIN Numerical Weather Prediction Project), will be shown in this work.
REFERENCES
Lacorata et al. (2001), “Drifters dispersion in the Adriatic Sea: Lagrangian data and
chaotic model”, Annales Geophysicae, vol.19.
Haza et al. (2007), “Model-based directed drifter launches in the Adriatic Sea: Results
from DART experiment”, Geophysical Research Letters, vol.34.
Haza et al. (2008), “Relative dispersion from a high-resolution coastal model of the
Adriatic Sea”, Ocean Modelling, vol.22. |
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