4 Studying the Baltic Sea Circulation with Eulerian Tracers
105
Fig. 4.4 Left: The tracer model is run on-line. Currents from the ocean circulation model are
directly used by the tracer model. Right: The tracer model is run off-line. Currents from the ocean
model are stored on disk and the tracer model reads the currents from disk. The tracer model can
be run several times with varying setups using the same currents
Lagrangian description. For the other special case when the observer is stationary
the relative velocity would be the same as the absolute velocity of the matter and we
would get the Eulerian description.
The Lagrangian method based upon trajectories of particles has the disadvantage that subgrid scale mixing used for the calculation of currents is not included. 3
For the results of the analysis, especially vertical turbulent mixing might be important on longer time scales. As the circulation models utilized so far for long runs,
like climate simulations, are only eddy-permitting (see below), the results might
also be affected by the horizontal mixing parameterizing mesoscale eddies. Analysis methods using Eulerian tracers do not have this limitation because the same
parameterizations are used for the calculation of currents and particle spreading.
Rather frequently one wants to utilize the simulated currents for the calculation of
drifting matter in the ocean. For instance, it can be the tracking of (1) a person who
has fallen overboard from a boat, (2) drifting algae, or (3) oil spills. Henceforth, the
simulated variable, which indicates drifting matter, is called tracer. A tracer model
may use stored currents from an ocean circulation model and it may be run at a later
occasion than the ocean circulation model. This is called to run the tracer model offline (Fig. 4.4). Alternatively, the tracer model may be built together with the ocean
circulation model and it may get the currents directly from the ocean model. This
is called coupling the models or running the tracer model on-line. One advantage
with the latter choice is that the currents can be updated with the same time step as
the ocean model is using, while in the off-line case the currents are typically stored
sparser in time compared to the time step of the ocean model. An advantage with the
off-line method, especially if the tracer model requires less computer power than the
ocean circulation model, is that it can more easily be repeated with small changes
3 The formal parameterizations of sub-grid processes described in Chaps. 3, 7 and 10 only improve
statistics of spreading of trajectories but do not improve the match of trajectories with reality.
105
Fig. 4.4 Left: The tracer model is run on-line. Currents from the ocean circulation model are
directly used by the tracer model. Right: The tracer model is run off-line. Currents from the ocean
model are stored on disk and the tracer model reads the currents from disk. The tracer model can
be run several times with varying setups using the same currents
Lagrangian description. For the other special case when the observer is stationary
the relative velocity would be the same as the absolute velocity of the matter and we
would get the Eulerian description.
The Lagrangian method based upon trajectories of particles has the disadvantage that subgrid scale mixing used for the calculation of currents is not included. 3
For the results of the analysis, especially vertical turbulent mixing might be important on longer time scales. As the circulation models utilized so far for long runs,
like climate simulations, are only eddy-permitting (see below), the results might
also be affected by the horizontal mixing parameterizing mesoscale eddies. Analysis methods using Eulerian tracers do not have this limitation because the same
parameterizations are used for the calculation of currents and particle spreading.
Rather frequently one wants to utilize the simulated currents for the calculation of
drifting matter in the ocean. For instance, it can be the tracking of (1) a person who
has fallen overboard from a boat, (2) drifting algae, or (3) oil spills. Henceforth, the
simulated variable, which indicates drifting matter, is called tracer. A tracer model
may use stored currents from an ocean circulation model and it may be run at a later
occasion than the ocean circulation model. This is called to run the tracer model offline (Fig. 4.4). Alternatively, the tracer model may be built together with the ocean
circulation model and it may get the currents directly from the ocean model. This
is called coupling the models or running the tracer model on-line. One advantage
with the latter choice is that the currents can be updated with the same time step as
the ocean model is using, while in the off-line case the currents are typically stored
sparser in time compared to the time step of the ocean model. An advantage with the
off-line method, especially if the tracer model requires less computer power than the
ocean circulation model, is that it can more easily be repeated with small changes
3 The formal parameterizations of sub-grid processes described in Chaps. 3, 7 and 10 only improve
statistics of spreading of trajectories but do not improve the match of trajectories with reality.