The impact of differences between water vapor continuum models and measurements upon radiative transfer calculations

Recent measurements of the water vapour continuum have been combined to form an empirical continuum called the BPS continuum. This covers the 800-7500 cm^-1 spectral region for the self continuum and most of the major absorbing areas between 290 and 7500 cm^-1 for the foreign continuum. Longwave and shortwave line by line radiative transfer calculations have been performed for clearly-sky condition in three standard test atmospheres using line data from the HITRAN database. This has allowed the BPS continuum to be compared to the commonly used CKD and MT_CKD continuum models. Using experimental errors it has been possible to estimate the upper and lower limits of the continuum. In the longwave the outgoing and surface down-welling radiation calculated using all three continua agree within 0.5% in a MLS atmosphere. In the shortwave, even though the contribution of the continuum is smaller, it still accounts for up to 5% of shortwave water vapour absorption. Between 1000 and 17000 cm^-1 there is a more significant disagreement between the three formulations, with CKD predicting a 2% greater value of water vapour shortwave absorption than the BPS continuum. The role of the continuum in a warming atmosphere has been investigated. The results suggest that in the longwave, even though the continuum is responsible for a great deal of cooling, the magnitude is insensitive to the continuum formulation used. In the shortwave the role of the continuum is less significant, but there is a greater variation between the different formulations.