Observing water vapour and ozone in the tropical UTLS with the MIPAS instrument on ENVISAT

In this thesis, the potential to observe the distribution of water vapour (H2O) and ozone (O3) in the tropical upper troposphere and lower stratosphere (UTLS) from an atmospheric limb sounding spectrometer is assessed. Vertical profile data retrieved operationally from observations from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) are examined under "clear sky" and thin cloud conditions. In the tropical UTLS, the distributions of these two naturally occurring important climate gases are characterised by sharp gradients through a cold tropopause. This in combination with the high occurrence of cirrus clouds in this region, makes accurate "clear sky" satellite observations difficult to achieve.;The impact of clouds on MIPAS measurements is quantified by simulating the effects of tropical UTLS cirrus clouds into MIPAS H2O and O 3 retrievals. It is found that clouds in the MIPAS line of sight can lead to a systematic positive bias in MIPAS H2O measurements and an increase in random retrieval error for both H2O and O3 . It was also found that cloud errors can propagate into the "clear sky" profile above the cloud altitude. Recommendation for improvements to the operational cloud filtering methods of tropical MIPAS H2O and O3 are proposed.;Comparisons of MIPAS H2O and O3 screened with the proposed cloud filtering schemes, to correlative in situ and solar occultation data suggest that improved filtering improves the agreement. However, MIPAS H2O and O3 data quality remain limited by the increased retrieval uncertainty near the tropopause.;Finally, the seasonal variability of the tropical cloud filtered MIPAS H2O and O3 through the tropical UTLS is examined. Regional analysis reveals possible evidence of upper tropospheric O3 enhancements and monsoon effects. The results show that MIPAS can sufficiently sound the tropical UTLS region.