Calculate the "enhanced extraterrestrial irradiance".

def _calc_i0p(i0, w, aod700, p):
    """Calculate the "enhanced extraterrestrial irradiance"."""
    p0 = 101325.
    io0 = 1.08 * w**0.0051
    i01 = 0.97 * w**0.032
    i02 = 0.12 * w**0.56
    i0p = i0 * (i02*aod700**2 + i01*aod700 + io0 + 0.071*np.log(p/p0))

    return i0p