Correct noise cascade initialisation

[mpvginde]

Correct noise cascade initialisation

[mpvginde]

Created a new helper function in the pysteps.blending.steps.py file which initializes each level of the noise cascade with noise having the correct spatial correlation.
All ar_order steps have identical noise to ensure that noise cascade immediately has the correct power in the subsequent timesteps (no spinup needed).

def _init_noise_cascade(
    shape,
    n_ens_members,
    n_cascade_levels,
    generate_noise,
    decompositor,
    pp,
    randgen_prec,
    fft_objs,
    bp_filter,
    domain,
    noise_method,
    noise_std_coeffs,
    ar_order
):
    noise_cascade = np.zeros(shape)
    if noise_method:
        for j in range(n_ens_members):        
            EPS = generate_noise(
                pp,
                randstate=randgen_prec[j],
                fft_method=fft_objs[j],
                domain=domain
                )
            EPS = decompositor(
                EPS,
                bp_filter,
                fft_method=fft_objs[j],
                input_domain=domain,
                output_domain=domain,
                compute_stats=True,
                normalize=True,
                compact_output=True
                )
            for i in range(n_cascade_levels):
                EPS_ = EPS["cascade_levels"][i]
                EPS_ *= noise_std_coeffs[i]
                for n in range(ar_order):
                    noise_cascade[j][i][n] = EPS_
            EPS = None
            EPS_ = None
    return(noise_cascade)

[mpvginde]

The noise was previously initialized before the initialization of the random generator. But since you now need them to initialize the noise.
The noise_cascade = _init_noise_cascade(...) should be called after the random generator initializations.
Also check with developments in issue #1

[mpvginde]

Fixed with commit 2eb125b

[RubenImhoff]

This is excellent!!