Time-stepping Schemes Review
Numerical time stepping uses a discrete approximation to:
where represents one of , , , or and represents all the right-hand-side terms. In ROMS, the goal is to find time-stepping schemes which are accurate where they are valid and damping on unresolved signals ( Shchepetkin and McWilliams (2005) and Shchepetkin and McWilliams (2009)). Also, the preference is for time-stepping schemes requiring only one set of the right-hand-side terms so that different time-stepping schemes can be used for different terms in the equations. Finally, as mentioned in Table Time Step, not all versions of ROMS use the same time-stepping algorithm. We list some time-stepping schemes here which are used or have been used by the ROMS/SCRUM family of models, plus a few to help explain some of the more esoteric ones.
The simplest approximation is the Euler time step:
where you predict the next value based only on the current fields. This method is accurate to first order in ; however, it is unconditionally unstable with respect to advection.
The leapfrog time step is accurate to O():
This time step is more accurate, but it is unconditionally unstable with respect to diffusion. Also, the even and odd time steps tend to diverge in a computational mode. This computational mode can be damped by taking correction steps. SCRUM's time step on the depth-integrated equations was a leapfrog step with a trapezoidal correction (LF-TR) on every step, which uses a leapfrog step to obtain an initial guess of . We will call the right-hand-side terms calculated from this initial guess :
This leapfrog-trapezoidal time step is stable with respect to diffusion and it strongly damps the computational mode. However, the right-hand-side terms are computed twice per time step.
Third-order Adams-Bashforth (AB3)
The time step on SCRUM's full 3-D fields is done with a third-order Adams-Bashforth step. It uses three time-levels of the right-hand-side terms:
where the coefficients , and are chosen to obtain a third-order estimate of . We use a Taylor series expansion:
We find that the coefficients are:
This requires one time level for the physical fields and three time levels of the right-hand-side information and requires special treatment on startup.
In equation (1) above, we assume that multiple equations for any number of variables are time stepped synchronously. For coupled equations, we can actually do better by time stepping asynchronously. Consider these equations:
If we time step them alternately, we can always be using the newest information:
This scheme is second-order accurate and is stable for longer time steps than many other schemes. It is however unstable for advection terms.
Forward-Backward Feedback (RK2-FB)
One option for solving equation (7) is a predictor-corrector with predictor step:
and corrector step:
Setting in the above, it becomes a standard second order Runge-Kutta scheme, which is unstable for a non-dissipative system. Adding the and terms adds Forward-Backward feedback to this algorithm, and allows us to improve both its accuracy and stability. The choice of and leads to a stable third-order scheme with .
LF-TR and LF-AM3 with FB Feedback
Another possibility is a leapfrog predictor:
and either a two-time trapezoidal or a three-time Adams-Moulton corrector:
where the parameters and introduce FB-feedback during both stages, while controls the type of corrector scheme. Without FB-feedback, we have:
Keeping maintains third-order accuracy. The most accurate choices for and are and , which yields a fourth-order scheme with low numerical dispersion and diffusion and .
Generalized FB with an AB3-AM4 Step
One drawback of the previous two schemes is the need to evaluate the right-hand-side (r.h.s.) terms twice per time step. The original forward-backward scheme manages to achieve while only evaluating each r.h.s. term once. It is possible to contruct a robust scheme using a combination of a three-time AB3-like step for with a four-time AM4-like step for :
This is second-order accurate for any set of values for , , and . It is third-order accurate if . However, it has a wider stability range for . Shchepetkin and McWilliams (2009) choose to use this scheme for the barotropic mode in their model with , , and , to obtain , close to the value of 2 for pure FB, but with better stability properties for the combination of waves, advection, and Coriolis terms.