Satellite measurements of upper tropospheric water

Doctoral thesis, 2008

Water vapour and clouds in the upper troposphere have strong influence on the Earth's climate system. The present uncertainties regarding the distribution and feedback mechanisms of water to a changing climate are today among the greatest limitations for accurate climate predictions. The poor knowledge of upper tropospheric water results from the inability of earlier measurement systems to give a complete view of the upper troposphere. In-situ instruments are restricted in spatial coverage and satellite observations are needed to provide global coverage. Traditional infrared and optical satellite observations are mainly sensitive to the middle troposphere and applicable only in relatively cloud-free conditions. In this thesis satellite microwave limb-sounding is shown to be an advantageous technique to observe the upper troposphere with high vertical resolution and capability to penetrate thin cirrus clouds. Present measurement systems have the potential to give accurate measurements of both upper tropospheric humidity and ice water content. Retrieval methods have been developed for the sub-millimeter limb-sounding observations of Odin-SMR. The retrieved quantities are the all-weather upper tropospheric humidity at 12 and 15 km in altitude and a partial ice water path above 12 km. The humidity results agree within 10% with data from other microwave limb-sounders and high quality in-situ measurements. A comparison with infrared measurements emphasizes the necessity of microwave observations to measure in presence of clouds. The cloud ice retrieval shows a general agreement with other microwave data sets. The comparison highlights the difficulty of cloud ice retrieval since important assumptions on cloud microphysics and 3D inhomogeneities are presently unavoidable. Microwave measurements from Aura-MLS and CloudSat are used to investigate the relative amounts of water vapour and ice in the upper troposphere. Ice is found to dominate the total water budget at 14 km over regions of deep convection, but appears to give a small contribution to the moistening of the upper troposphere.