Continuum Model
The MT_CKD continuum model includes continuum absorption due to water
vapor, nitrogen, oxygen, carbon dioxide, and ozone. With respect
to water vapor, the contributions of the foreign and self continuum are
considered separately. The optical depths associated with Rayleigh
scattering may also be computed. The MT_CKD model (contnm.f) is
available as part of a separate standalone code, and is present in the models LBLRTM and MonoRTM.
The perspective we have adopted in developing the MT_CKD water vapor
continuum model is to constrain the model so that it is consistent with
quality analyses of spectral atmospheric measurements. Only
cases for which the characterization of the atmospheric state has
been highly scrutinized have been used. Laboratory measurements
are also considered. Keeping the MT_CKD continuum consistent with
current observational studies necessitates periodic (typically once per
year) updates to the water vapor continuum coefficients. Details
of the most recent revisions to the MT_CKD continuum can be found at
the “What’s New” tab on the left-hand
frame.
Water vapor continuum coefficients in spectral regions that have not
been subject to compelling analyses are determined by a mathematical
formulation of the spectral shape associated with each water vapor
monomer line. This formulation is applied consistently to all
water vapor lines from the microwave to the visible, and the results
summed (separately for the self and foreign) to obtain continuum
coefficients from 0-20,000 cm-1. This formulation has been derived
based on the continuum in spectral regions in which the coefficients
are known with low uncertainties.
The current continuum line shape formulation was developed for
MT_CKD_1.0 and represented the first recomputation of the entire self
and foreign broadened continua since the original CKD model was
developed in the 1980s. The self and foreign continuum models are
each based on the contributions from two components: a
'weak interaction' component and an 'allowed' line shape component. The
change in perspective resulted from the difficulty in developing a line
shape model based on sound physics that could explain the magnitude of
the increased absorption in the intermediate wing over that provided by
the impact approximation. In this formulation, eight and seven
parameters are needed to sufficiently specify the self and foreign
continua, respectively, over the full spectral domain. The ratio
of the self continuum coefficients at 296 K to those at 260 K have been
kept the same as in the CKD model. The only temperature
dependence for the foreign continuum arises from the radiation term, as
with CKD. Both the MT_CKD and CKD model should be regarded as
semi-empirical models with strong constraints provided by the known
physics.
The MT_CKD water vapor continuum model was first implemented in the
line-by-line model LBLRTM v7.0 and has been utilized in all subsequent
AER Radiative Transfer models. This new model was developed by E.J.
Mlawer, D.C. Tobin and S.A. Clough, using the original CKD
formulation as its foundation; hence the name MT_CKD.
Atmospheric and Environmental Research