Timestepping Parameters

Overview

A TimesteppingParams structure is used to pass information relating to timestepping to the simulation. The following parameters as specified by passing appropriate keyword arguments the TimesteppingParams constructor:

• niter0: the iteration number at which the simulation. Set niter0 to 0 to initialize a 'clean' simulation, or niter0 to a positive integer to (attempt to) pickup from the a corresponding checkpoint (see Checkpoints and Pickups below)
• dt: the simulation timestep
• end_time: the clock time at which the simulation should terminate
• n_iter_total: the total number of timesteps to be performed. NB: you must specify at least one of end_time and n_iter_total (the simulation must know when it is going to finish!); specifying both is possible, but they must be compatible (i.e the end_time must equal n_iter_total * dt).
• chkpt_freq and pchkpt_freq: the model time interval between outputting temporary and permanent checkpoints, respectively (see Checkpoints and Pickups)
• step_thickness: a flag which, when turned off (stepthickness = false) turns of thickness updates when timestepping. This is an experimental feature, necessary for coupling WAVI to the MITgcm (see the MITgcm coupling for more info)

Constructor

An instance of a TimesteppingParams is constucted using the TimesteppingParams(;<kwargs>) constructor:

        TimesteppingParams(;
                niter0 = 0,
                dt = 1.0,
                end_time = 1.0,
                t0 = nothing,
                chkpt_freq = Inf,
                pchkpt_freq = Inf,
                chkpt_path = './',
                step_thickness = true)

        - 'niter0': Iteration number of the first timestep. niter0 = 0 corresponds to a new simulation, while niter0 > 0 (positive integer) corresponds to a pickup.
        - 'dt': Model timestep
        - 'end_time': Simulation termination time
        - 't0': Starting time of the simulation
        - 'chkpt_freq': Frequency of outputting temporary checkpoints
        - 'pchkpt_freq': Frequecy with which permanent checkpoints are pass
        - 'chkpt_path' : Path to location checkpoint output
        - 'step_thickness': Toggle whether to update the ice thickness (true) or not (false) at each timestep

Checkpoints and Pickups

Large simulations are computationally expensive, and may take a long time to run. To permit simulations to run for longer than maximum runtime limits which are imposed on many machines, WAVI.jl is equipped with a checkpoint-pickup system that allows the state to be outputted frequently, and the simulation to be picked again from that point.

Checkpoints

Checkpoints contain a snapshot of all of the information on the current state of the simulation. As a result, they may be large and thus outputting checkpoints very frequently is discouraged, except for when debugging simulations which are ran for only a few timesteps (see Simulation Tips)

Both temporary and permanent checkpoints are output in jld2 format. The path of both of these is specified by setting the chkpt_path argument in the TimesteppingParams constructor.

Temporary checkpoints are outputted every chkpt_freq years (model time). These checkpoints are rolling: first checkpoint 'A' (named chkptA.jld) is written at time chkpt_freq, then checkpoint 'B' (named chkptB.jld) is output at a time 2 * chkpt_freq. Checkpoint 'A' is overwritten at time 3 * chkpt_freq, and so on.

Similarly, permanent checkpoints are outputted every pchkpt_freq model years. These are output sequentially (i.e. not overwritten) as pchkpt_xxxxxxxxx.jld2 where xxxxxxxxxx is the iteration number, prepended by zeros to be ten digits long (e.g. the filename of a checkpoint outputted at timestep number one thousand is chkpt_0000001000.jld).

Note that the number of timesteps, rather than the model time, is used to determine when to output the solution: permanent checkpoints are output every floor(pchkpt_freq / dt) timesteps (and similarly for chkpt_freq), so the actual output frequency may not exaclty match that specified by the parameter.

Pickups

Continuining from the state stored in a checkpoint is initiatied by setting the niter0 parameter to correspond to the number of the timestep to be picked up from. For example, to continue the simulation from a checkpoint outputted at timestep number one thousand, set niter0 = 1000 in the TimesteppingParams constructor.

WAVI.jl only currently supports pickups from permanent checkpoints. The permanent checkpoint from which a pickup is initiated must be in the local directory.