Such a discrepancy will lead to systematic biases both in retrieving UTH as well as in direct assimilation of the water vapor radiances. The workshop also investigated the effects of model parameterizations, such as continuum absorption by water vapor and oxygen, on the simulated radiances. Thus, accurate treatment of continuum absorption is a necessity for UTH retrieval. Four of the models participated in a comparison of the gradient of their brightness temperatures with respect to water vapour for two extreme profiles. Preliminary results show that these calculations can be particularly sensitive to the vertical discretization of the forward models.
Further work is needed to explore the impact of such differences on data assimilation.
Any retrieval strategy depends heavily on the ability to quantitatively relate radiances to absorber concentrations. Therefore, systematic differences in transmittance models will inevitably introduce differences in the derived products even when using the same retrieval method. Since water vapor typically reduces the upwelling radiance in this spectral region, a model which overpredicts the radiance would require a larger amount of water vapor to yield the correct brightness temperature. Hence, a constant error in brightness temperature introduces a smaller absolute uncertainty at the dry end of the UTH spectrum and a larger absolute uncertainty at the moist end. It is emphasized, however, that consistency among the various radiative transfer models does not necessarily imply accuracy.