def zr_b(mu):
    """Assuming a rainfall parameter relationship of Z=AR^b,
    Compute the b exponent using gamma distribution.  

    Ulbrich and Atlas (JAMC 2007), Eqn T5
    Parameters
    ----------
    mu: float
        Shape parameter of gamma DSD model [unitless]

    Returns
    -------
    b: float
        Z-R prefactor (see description)
    """
    b = (7 + mu) / (4.67 + mu)
    return b
