9 Statistics of Lagrangian Transport Reveals Hidden Features of Velocity Fields
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identification of areas of high risk in terms of pollution of selected coastal segments. Their presence may substantially impact the necessary length of trajectory
calculations in order to obtain a sufficient amount of coastal hits. This length obviously depends on both the distance from the release site of an adverse impact to the
vulnerable area and on the cross-shore component of the net Lagrangian transport.
Different transport patterns and pathways contribute to an early evaluation of the
potential benefit of the entire approach via a demonstration how strong the concealed features of current-driven transport are. Such features may become evident,
for example, in the field of the average net transport rate in different directions
and the ratio of average net and bulk transport (equivalently, the ratio of the final
displacement and the length of the trajectories). The viable options for their identification are an important ingredient of a corresponding decision support system.
The associated implicit possibility for roughly estimating whether or how properly the underlying circulation model reproduces the mesoscale dynamics can be
used in various contexts. It is natural that some of the results of such a check are
intuitively obvious or match the known picture of the dynamics of the surface layer
in the given sea area. The performed analysis, however, is a step forward towards a
deeper understanding of the role of different dynamic and transport phenomena in
the context of environmental management of vulnerable sea and coastal areas.
The established extensive (mostly seasonal) variability of many revealed patterns
shows on the one hand that unambiguous conclusions about their spatial distribution, persistency, relative strength or time scales of their formation are not always
possible. On the other hand it suggests that a proper way forward is to develop different solutions for different seasons (or more irregularly occurring flow regimes
such as the inflow or outflow conditions in the SW Baltic Sea) (Lu et al. 2012).
Several highlighted aspects may serve as starting points of future research. This
first concerns sea areas with very small baroclinic Rossy radius and short turnover
time of synoptic eddies where several properties of the Lagrangian transport may
have time scales on the order of a few weeks. This time scale considerably exceeds
the synoptic time scale (which is about a week in the Gulf of Finland) but is substantially shorter than the length of typical seasonal variations (2–4 months). Such
a separation of the synoptic and seasonal time scales encourages the search for phenomena that persist over ‘intermediate’ time scales of a few weeks. This range is
the most promising for the detection of yet unknown features in the dynamics of
the Gulf of Finland. A similar separation is hardly possible in the open ocean where
the synoptic time scale is about a month and the lifetime of many synoptic features
overlaps with the duration of seasonal variations.
In conclusion, the presented results demonstrate the feasibility of the approach
of Lagrangian trajectories for the identification of semi-persistent transport patterns
in the surface layer. The next step consists in merging the detected patterns with the
probability analysis of vulnerable regions being hit by adverse impacts stemming
from different sea areas. The techniques for the calculation of Lagrangian trajectories and the developed technology can be generalized in a straightforward manner
to the full 3D analysis of the propagation of adverse impacts.
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identification of areas of high risk in terms of pollution of selected coastal segments. Their presence may substantially impact the necessary length of trajectory
calculations in order to obtain a sufficient amount of coastal hits. This length obviously depends on both the distance from the release site of an adverse impact to the
vulnerable area and on the cross-shore component of the net Lagrangian transport.
Different transport patterns and pathways contribute to an early evaluation of the
potential benefit of the entire approach via a demonstration how strong the concealed features of current-driven transport are. Such features may become evident,
for example, in the field of the average net transport rate in different directions
and the ratio of average net and bulk transport (equivalently, the ratio of the final
displacement and the length of the trajectories). The viable options for their identification are an important ingredient of a corresponding decision support system.
The associated implicit possibility for roughly estimating whether or how properly the underlying circulation model reproduces the mesoscale dynamics can be
used in various contexts. It is natural that some of the results of such a check are
intuitively obvious or match the known picture of the dynamics of the surface layer
in the given sea area. The performed analysis, however, is a step forward towards a
deeper understanding of the role of different dynamic and transport phenomena in
the context of environmental management of vulnerable sea and coastal areas.
The established extensive (mostly seasonal) variability of many revealed patterns
shows on the one hand that unambiguous conclusions about their spatial distribution, persistency, relative strength or time scales of their formation are not always
possible. On the other hand it suggests that a proper way forward is to develop different solutions for different seasons (or more irregularly occurring flow regimes
such as the inflow or outflow conditions in the SW Baltic Sea) (Lu et al. 2012).
Several highlighted aspects may serve as starting points of future research. This
first concerns sea areas with very small baroclinic Rossy radius and short turnover
time of synoptic eddies where several properties of the Lagrangian transport may
have time scales on the order of a few weeks. This time scale considerably exceeds
the synoptic time scale (which is about a week in the Gulf of Finland) but is substantially shorter than the length of typical seasonal variations (2–4 months). Such
a separation of the synoptic and seasonal time scales encourages the search for phenomena that persist over ‘intermediate’ time scales of a few weeks. This range is
the most promising for the detection of yet unknown features in the dynamics of
the Gulf of Finland. A similar separation is hardly possible in the open ocean where
the synoptic time scale is about a month and the lifetime of many synoptic features
overlaps with the duration of seasonal variations.
In conclusion, the presented results demonstrate the feasibility of the approach
of Lagrangian trajectories for the identification of semi-persistent transport patterns
in the surface layer. The next step consists in merging the detected patterns with the
probability analysis of vulnerable regions being hit by adverse impacts stemming
from different sea areas. The techniques for the calculation of Lagrangian trajectories and the developed technology can be generalized in a straightforward manner
to the full 3D analysis of the propagation of adverse impacts.