Atmospheric water vapor continuum model for the sub-THz range

Empirical and semi-empirical models of the continua absorption are still ubiquitously used in atmospheric science and applications despite almost a hundred-years-long persistent theoretical and experimental investigation of the continuum' nature. Based on the empirical knowledge accumulated to-date about the water vapor continuum we propose a physically sound continuum model for practical applications in the subterahertz frequency range (0-1 THz). Our model interpret the water vapor continuum in terms of a combination of various contributions owed to bimolecular absorption. The self-continuum component is presented in the model as a sum of the contributions from absorption by bound and quasibound dimers, which are evaluated with the help of the water vapor second virial coefficient and existing ab initio simulation of the water dimer absorption. The contribution from the far wings of the water monomer resonant lines is taken into account by virtue of a simple analytical function approximating available empirical data. The foreign-continuum component of absorption is taken in a conventional empirical form. The values of its numerical coefficients are updated to achieve better agreement with results of laboratory measurements in the sub-THz range. We demonstrate that our new model is in good agreement with modern versions of atmospheric propagation models. However, the atmospheric brightness temperature calculated using our new model systematically deviates from the results obtained with its empirical version. The deviation amounts up to several Kelvins in the microwindows between resonant water lines.