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 tem perature of the middle atmosphere and the concentration of various trace sp ecies in it. The spectral resolution of the instruments used is such that t he measurements are usually made at a number of frequencies across the widt h of a single spectral line. It is therefore important to model the line sh ape and the instrument response accurately as inaccurate modelling will lea d to a poor match between the measured radiances and those re-calculated fr om the retrieved atmosphere. In this paper, we consider the 183.3 GHz rotat ional transition of the water molecule and the 184.4 GHz transition of ozon e. These transitions have been used to measure water vapour and ozone in th e middle atmosphere by two orbiting instruments: the Microwave Limb Sounder (MLS) on Upper Atmosphere Research Satellite (UARS) and the Millimetre-wav e Atmospheric Sounder (MAS) which was flown on the ATLAS platform on board the space shuttle. Both instruments have had similar problems matching meas ured and re-calculated 183 GHz radiances. We show that by allowing the retr ieval algorithm to fit certain spectral and instrumental parameters in addi tion to the mixing ratio profiles, we can improve both the quality of the r etrieved profiles and our knowledge of certain spectral parameters, in part icular the pressure shift of the line. (C) 1999 Elsevier Science Ltd. All r ights reserved.