An evaluation of atmospheric absorption models at millimetre and sub-millimetre wavelengths using airborne observations

Accurate gas absorption models at millimetre and sub-millimetre wavelengths are required to make best use of observations from instruments on board the next generation of EUMETSAT polar-orbiting weather satellites, including the Ice Cloud Imager (ICI), which measures at frequencies up to 664 GHz. In this study, airborne observations of clear-sky scenes between 89 and 664 GHz are used to evaluate two state-of-the-art absorption models by performing radiative closure calculations. Observed brightness temperatures are compared to simulated values from the Atmospheric Radiative Transfer Simulator (ARTS) for both upward and downward-looking viewing directions. It is shown that uncertainties in the atmospheric water vapour profile can have a significant impact on individual comparisons, but these errors can be reduced by averaging across multiple flights. For upward looking views, which have the greatest sensitivity to the absorption model, the mean differences between observed and simulated brightness temperatures are generally close to, or within, the estimated spectroscopic uncertainty. For downward-looking views, which more closely match the satellite viewing geometry, the mean differences were generally less than 1.5 K, with the exception of window channels at 89 and 157 GHz, which are significantly influenced by surface properties. These results suggest that both of the absorption models considered are sufficiently accurate for use with ICI.