Climate and variability of water vapour in the troposphere

Atmospheric water vapour is an important (natural) greenhouse gas, infuencing both directly and indirectly the global radiative balance of the Earth as well as heat and moisture fluxes at the surface. Since water vapour concentrations may change as a result of surface warming and other environmental/meteorological factors, there is a need to understand the long-term behaviour. Changes are small, so it is critical to use high quality data with well-defined uncertainties and biases, particularly for climate research. Hyper-spectral infrared (IR) sounders such as the Infrared Atmospheric Sounding Interferometer (IASI) and the Atmospheric Infrared Sounder (AIRS) allow for higher vertical resolution profile measurements of water vapour to be obtained compared to their predecessors. First results of global comparisons of AIRS profiles show that for tropospheric layers between 925-374 hPa biases are within +-10% and a significant dry bias of 20% in regions of the upper troposphere, consistent with recent comparisons to reanalysis. A consistent approach to the retrieval of temperature and water vapour profiles from all hyper-spectral IR sounders is outlined and an initial set of retrievals from IASI at global climate sites is performed. Results from the University of Leicester Water Vapour Processor (UoL-WVP) show errors of ≈1 K and Atmospheric water vapour is an important (natural) greenhouse gas, infuencing both directly and indirectly the global radiative balance of the Earth as well as heat and moisture fluxes at the surface. Since water vapour concentrations may change as a result of surface warming and other environmental/meteorological factors, there is a need to understand the long-term behaviour. Changes are small, so it is critical to use high quality data with well-defined uncertainties and biases, particularly for climate research. Hyper-spectral infrared (IR) sounders such as the Infrared Atmospheric Sounding Interferometer (IASI) and the Atmospheric Infrared Sounder (AIRS) allow for higher vertical resolution profile measurements of water vapour to be obtained compared to their predecessors. First results of global comparisons of AIRS profiles show that for tropospheric layers between 925-374 hPa biases are within +-10% and a significant dry bias of 20% in regions of the upper troposphere, consistent with recent comparisons to reanalysis. A consistent approach to the retrieval of temperature and water vapour profiles from all hyper-spectral IR sounders is outlined and an initial set of retrievals from IASI at global climate sites is performed. Results from the University of Leicester Water Vapour Processor (UoL-WVP) show errors of ≈1 K and ≤20% within the troposphere for temperature and humidity respectfully. The significance of surface characterisation at a sub-pixel level is also demonstrated. Retrievals from IASI at selected climate sites for 2012 are inter-compared in a three-way analysis with high resolution radiosonde profiles and AIRS data. Results show that the mean standard deviation of observational error for IASI is 2.2% while AIRS is 3.6%. This thesis work represents a step forward in the use of AIRS and IASI data for climate research. However, proven long-term stability is needed for the water vapour data question to be answered. and humidity respectfully. The significance of surface characterisation at a sub-pixel level is also demonstrated. Retrievals from IASI at selected climate sites for 2012 are inter-compared in a three-way analysis with high resolution radiosonde profiles and AIRS data. Results show that the mean standard deviation of observational error for IASI is 2.2% while AIRS is 3.6%. This thesis work represents a step forward in the use of AIRS and IASI data for climate research. However, proven long-term stability is needed for the water vapour data question to be answered.