FORECASTING DRIFT
509
scientific basis for the forecast numerical models used and the information infrastructure required for operational service. Since the theme of
the Summer School is ocean forecasting, we will only briefly describe the
modeled processes and concentrate more on the practical implementation and operational aspects. Indeed, models for drifting objects, ship
drift and (especially) oil fate are quite complex and a fuller understanding of the modeled processes is beyond the scope of this summer school.
However, some insight into the workings of these models is necessary to
understand the operational data input and dissemination requirements.
The sections 2 and 3 will introduce modeling of floating objects and oil
spills, including typical methods for implementing them. It will become
apparent that the geophysical data needed to drive these models in an
operational service are quite similar. Therefore, section 4 will deal with
the structure of operational services for both floating objects and oil.
2.
Drift of floating objects
The motion of a drifting object on the sea surface is the net result of
several forces acting upon its surface (water currents, atmospheric wind,
wave motion, and wave induced currents), and its center of mass (the
gravitational force and the buoyancy force). It is possible to estimate
the trajectory given information on the local wind, surface current, and
the shape and buoyancy of the object.
The position of a floating object is computed by numerically integrating the total drift velocity V drif t of the object, given by
V drif t = V curr + V rel ,
(1)
where V curr denotes the ocean current velocity relative to the earth, and
V rel denotes the object drift velocity relative to the ambient water. The
ocean current is made up of two components: the surface current, which
includes the Ekman drift, baroclinic motion, tidal and inertial currents,
and the Stokes drift induced by waves. V curr is assumed to influence
all floating objects in the same manner. It is typically equated with the
(near-)surface current obtained from a numerical ocean model, a parameterization on the wind velocity and/or local observations. V rel results
from the wind and wave forces acting on the object, and is strongly
dependent on the characteristics of the object.
The basic model in eq. 1 may be separated into two modules, based on
the forces that determine V rel . A well-known result from hydrodynamics
is that wave effects are small when the length scale of the object is smaller
than the wave length and increase dramatically when the lengths are
about the same (Grue and Biberg, 1993, Hodgins and Hodgins, 1998).
Thus, one module is for relatively small objects (in practise, less than
509
scientific basis for the forecast numerical models used and the information infrastructure required for operational service. Since the theme of
the Summer School is ocean forecasting, we will only briefly describe the
modeled processes and concentrate more on the practical implementation and operational aspects. Indeed, models for drifting objects, ship
drift and (especially) oil fate are quite complex and a fuller understanding of the modeled processes is beyond the scope of this summer school.
However, some insight into the workings of these models is necessary to
understand the operational data input and dissemination requirements.
The sections 2 and 3 will introduce modeling of floating objects and oil
spills, including typical methods for implementing them. It will become
apparent that the geophysical data needed to drive these models in an
operational service are quite similar. Therefore, section 4 will deal with
the structure of operational services for both floating objects and oil.
2.
Drift of floating objects
The motion of a drifting object on the sea surface is the net result of
several forces acting upon its surface (water currents, atmospheric wind,
wave motion, and wave induced currents), and its center of mass (the
gravitational force and the buoyancy force). It is possible to estimate
the trajectory given information on the local wind, surface current, and
the shape and buoyancy of the object.
The position of a floating object is computed by numerically integrating the total drift velocity V drif t of the object, given by
V drif t = V curr + V rel ,
(1)
where V curr denotes the ocean current velocity relative to the earth, and
V rel denotes the object drift velocity relative to the ambient water. The
ocean current is made up of two components: the surface current, which
includes the Ekman drift, baroclinic motion, tidal and inertial currents,
and the Stokes drift induced by waves. V curr is assumed to influence
all floating objects in the same manner. It is typically equated with the
(near-)surface current obtained from a numerical ocean model, a parameterization on the wind velocity and/or local observations. V rel results
from the wind and wave forces acting on the object, and is strongly
dependent on the characteristics of the object.
The basic model in eq. 1 may be separated into two modules, based on
the forces that determine V rel . A well-known result from hydrodynamics
is that wave effects are small when the length scale of the object is smaller
than the wave length and increase dramatically when the lengths are
about the same (Grue and Biberg, 1993, Hodgins and Hodgins, 1998).
Thus, one module is for relatively small objects (in practise, less than