Evaluation of oceanic and atmospheric trajectory schemes in the TRACMASS trajectory model v6.0
Three different trajectory schemes for oceanic and atmospheric general circulation models are compared in two different experiments. The theories of the trajectory schemes are presented showing the differential equations they solve and why they are mass conserving. One scheme assumes that the veloci...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-04-01
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| Series: | Geoscientific Model Development |
| Online Access: | http://www.geosci-model-dev.net/10/1733/2017/gmd-10-1733-2017.pdf |
| Summary: | Three different trajectory schemes for oceanic and atmospheric general
circulation models are compared in two different experiments. The theories of
the trajectory schemes are presented showing the differential equations they
solve and why they are mass conserving. One scheme assumes that the velocity
fields are stationary for set intervals of time between saved model outputs
and solves the trajectory path from a differential equation only as a
function of space, i.e. <q>stepwise stationary</q>. The second scheme is a
special case of the stepwise-stationary scheme, where velocities are
assumed constant between general circulation model (GCM) outputs; it uses hence a <q>fixed GCM time
step</q>. The third scheme uses a continuous linear interpolation of the fields
in time and solves the trajectory path from a differential equation as a
function of both space and time, i.e. a <q>time-dependent</q> scheme. The trajectory
schemes are tested <q>offline</q>, i.e. using the already integrated and stored
velocity fields from a GCM. The first comparison of the schemes uses
trajectories calculated using the velocity fields from a high-resolution
ocean general circulation model in the Agulhas region. The second comparison
uses trajectories calculated using the wind fields from an atmospheric
reanalysis. The study shows that using the time-dependent scheme over the
stepwise-stationary scheme greatly improves accuracy with only a small
increase in computational time. It is also found that with decreasing time
steps the stepwise-stationary scheme becomes increasingly more accurate
but at increased computational cost. The time-dependent scheme is
therefore preferred over the stepwise-stationary scheme. However, when
averaging over large ensembles of trajectories, the two schemes are
comparable, as intrinsic variability dominates over numerical errors. The
fixed GCM time step scheme is found to be less accurate than the
stepwise-stationary scheme, even when considering averages over large
ensembles. |
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| ISSN: | 1991-959X 1991-9603 |