def roughness_reynolds(wind_friction, kinematic_visc = 1.516e-5,z = 0.0002):
    """
    Return the roughness Reynolds number []
    source : ALOHA

    Parameters
    ----------
    wind_friction : Wind friction velocity [m/s]
    kinematic_visc :kinematic viscosity of air, default 1.516e-5 [m²/s] (20°C)
    z : Surface roughness lenght [m]. The default for open sea is 0.0002 m (wikipedia)

    """
    return (wind_friction*z)/kinematic_visc