Extended support for customisable kernels

[VeckoTheGecko]

I am wanting to run multiple simulations with different kernels resulting in different beaching probabilities for particles. Rather than specifying the kernels manually, I would like to specify them programmatically. I initially thought of doing a closure which returned the beaching kernel, however the resulting function is not compiling to C properly. I feel like my usecase isn't too niche; is there a solution where I could programmatically specify kernels like this?

My code is the following:

def get_beaching_kernel(proba_beach):
    """
    A closure to return the beaching kernel
    """
    def kernel(particle, fieldset, time):
        """
        Beaches the particle if it is within one gridcell of the land.

        """
        proba_dt = 1 - math.exp(- particle.dt / proba_beach) # Converting to probability to apply for each timestep
        if fieldset.land[particle] > 0: # ie. if particle is in beaching region
            particle.in_beaching_region = 1
            if ParcelsRandom.random() < proba_dt:
                particle.beach = 1
        else:
            particle.in_beaching_region = 0
            
    return kernel

pset.execute(AdvectionRK4 + pset.Kernel(get_beaching_kernel(proba_beach=0.5)),
            runtime=timedelta(days=24),  # runtime controls the interval of the plots
            dt=timedelta(minutes=5),
            ) 

which gives this error

...
C:\Users\nickh\AppData\Local\Temp\parcels-tmp\494021291945c5ced52ab1e1f36167a4_0.c:135:49: error: 'proba_beach' undeclared (first use in this function)
   proba_dt = (1 - exp((-(particles->dt[pnum]) / proba_beach)));
                                                 ^
C:\Users\nickh\AppData\Local\Temp\parcels-tmp\494021291945c5ced52ab1e1f36167a4_0.c:135:49: note: each undeclared identifier is reported only once for each function it appears in

[erikvansebille]

The reason for this error is that you can't assign the proba_beach variable like this; it does not work for the JIT-compiler.

Instead, you should use FieldSet.add_constant('proba_beach', value) and then use fieldset.proba_beach within the Kernel.

See also https://oceanparcels.org/gh-pages/html/#parcels.fieldset.FieldSet.add_constant