def METHOD_NAME():
    """
    tests the top layer
    elements should end up fairly evenly distributed by the end of along run
    """
    D_mixed = 10.0 # cm^2/s
    mixed_layer_depth = 10.0 # m
    time_step = 60
    total_time = 6000
    num_elements = 1000
    num_timesteps = total_time // time_step
    mv = RandomMover3D(vertical_diffusion_coef_above_ml=D_mixed,
                             vertical_diffusion_coef_below_ml=0.0,
                             mixed_layer_depth=mixed_layer_depth,
                             ) # m
    sc = sample_sc_release(num_elements=num_elements,
                           start_pos=(0.0, 0.0, 0.0),
                           release_time=model_time,
                           )
    # re-set z positions:
    sc['positions'][:, 2] =  5.0 # middle of the layer
    # call get_move a bunch of times
    for i in range(num_timesteps):
        # print "positions:\n", sc['positions']
        delta = mv.get_move(sc,
                            time_step,
                            model_time,
                            )