Abstract
Abstract. 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. stepwise stationary. 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 fixed GCM time step. 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 time-dependent scheme. The trajectory schemes are tested offline, 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.
Highlights
The Lagrangian view of the ocean and atmospheric circulation describes fluid pathways and the connectivity of different regions, which are not readily obtained from a Eulerian perspective
A consequence of only updating the fields at the general circulation model (GCM) output times is that the velocities
The two trajectory schemes available in TRACMASS have are assumed to be in steady state for long periods and here been intercompared by calculating Lagrangian statis- changed abruptly with a discontinuity
Summary
The Lagrangian view of the ocean and atmospheric circulation describes fluid pathways and the connectivity of different regions, which are not readily obtained from a Eulerian perspective. The purpose of this work is to test the different schemes used in the TRACMASS trajectory model (version 6.0), here named the fixed GCM time step (Blanke and Raynaud, 1997; Döös, 1995), stepwise stationary (Döös et al, 2013) and time-dependent (de Vries and Döös, 2001) schemes. These schemes have previously only been tested using highly idealised velocity fields.
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have