Water vapour line and continuum absorption in the thermal infrared-reconciling models and observations

High-spectral-resolution thermal infrared radiance observations made with the Airborne Research Interferometer Evaluation System instrument on the Met OfŽ ce C130 aircraft in tropical and sub-arctic atmospheres are used to evaluate the water vapour continuum and line absorption. Coincident microwave radiometer measurements at 183 GHz are used to help constrain the water vapour proŽ le. Through careful selection of wavelengths where the water vapour continuum has varying impacts on the observed radiances, analysis shows that the continuum is too strong; this is using the General Line-by-line Atmospheric Transmittance and Radiance Model with the CKD2.4 water vapour continuum and High Resolution Transmission Molecular Absorption 2000 spectral database. Zenith observations from low altitude indicate that the continuum requires reducing by between 6% and 15% depending on the frequency, which is in good agreement with recent laboratory data. A modiŽ ed continuum is presented and used in modelling to compare against upper-troposphere nadir measurements obtained in tropical and sub-arctic atmospheres.