BFX Flag: Use Planck Fn v. Optical Depth within layer
Calculate Radiance R = ∫ B.dτ assuming
B (Planck Function) varies
linearly with altitude and Optical Depth χ = ln(τ) varies
path within each layer (effectively integrate B(χ)dτ
for each layer assuming
B is linear in χ for Zenith/Nadir paths,
and quadratic in χ for limb paths).
Calculate radiance R = ∫ B.dτ assuming B is
constant within each
layer, with weighted value determined by the Curtis-Godson temperatures for
each absorbing species.
At least one flag generating a radiance-type output must be enabled:
(otherwise this option would have no effect).
This flag activates the 'linear in tau' algorithm described, for the
nadir case, in Clough et al
(1992). An equivalent approximation is used for limb-viewing.
This different method of integrating radiance
should be more accurate for thick layers since it allows for the fact
that the effective temperature 'seen' for a particular layer will vary
according to the temperature gradient through the layer and the optical
thickness, eg in optically thick regions of the spectrum only the 'near' side
of the layer is seen.
In the limit of optically thin layers the two methods of calculating
radiance converge and the Flag has only a small effect.