Instrumental and spectral parameters: Their effect on and measurement by microwave limb sounding of the atmosphere

Microwave limb sounding is an important new technique for measuring the temperature of the middle atmosphere and the concentration of various trace species in it. The spectral resolution of the instruments used is such that the measurements are usually made at a number of frequencies across the width of a single spectral line. It is therefore important to model the line shape and the instrument response accurately as inaccurate modelling will lead to a poor match between the measured radiances and those re-calculated from the retrieved atmosphere. In this paper, we consider the 183.3 GHz rotational transition of the water molecule and the 184.4 GHz transition of ozone. These transitions have been used to measure water vapour and ozone in the middle atmosphere by two orbiting instruments: the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite (UARS) and the Millimetre-wave Atmospheric Sounder (MAS) which was flown on the ATLAS platform on board the space shuttle. Both instruments have had similar problems matching measured and re-calculated 183 GHz radiances. We show that by allowing the retrieval algorithm to fit certain spectral and instrumental parameters in addition to the mixing ratio profiles, we can improve both the quality of the retrieved profiles and our knowledge of certain spectral parameters, in particular the pressure shift of the line.