Although not a new technique, the more accessible line absorption data-bases mentioned above have brought the application of the correlated k--distribution (CKD) to a broader audience. This technique has gained favour with modelers over the past few years and it is now becoming more universal in its use in atmospheric radiative transfer. The approach is well described in Lacis and Oinas [1991] and limited tests against rigorous line-by-line calculations have been carried out [ e.g Lacis and Oinas, 1991; Fu and Liou, 1992, 1993; Chou et al., 1993]. It is expected that this approach will be applied to a more diverse range of topics in the future. For instance the CKD approach, and its close cousin the exponential sum fit (ESFT), now find their way into radiation schemes used in weather forecast and climate models [ Edwards and Slingo, 1994; Ritter and Geleyn 1992].
The essence of both the CKD and ESFT approaches is that both reduce
the expression for the transmission of a spectral band to a sum of a
number of exponential-in-path transmission functions. The real
advantage of this technique is that it not only can be incorporated
with scattering in a formal, rigorous way but also the computational
burden of radiative transfer solvers that use the technique (such as
two-stream models or stacked-layer models) vary linearly with the
number of model layers as opposed to a squared dependence inherent
with other approaches. The disadvantage is that the number of
exponential transmission functions in the summation tend to be large
for transmissions that change by orders of magnitude typical of
paths associated with troposphere--stratosphere radiative transfer.
Overlapping absorptions by other trace gases. As an example, Table 1,
taken from Fu and Liou [1992], indicates the spectral band
structure of their model (6 bands in the solar region, 
m and 12 bands for the longwave spectrum 
m).
The total number of radiative transfer calculations for a given water
vapor only atmospheric column is the sum of the second column in the
table (50 for the infrared spectrum and 42 for the solar spectrum).
Further complexity is introduced when absorptions by other gases
overlap within the same spectral interval.