Validation of ATSR Total Column Water Vapour (AIRWAVE v1 and v2) using sondes, GPS and independent satellite datasets

The ATSR water vapour dataset consists in Total Columns of atmospheric Water Vapour (TCWV) retrieved for cloud-free sea surface scenes using the two Thermal InfraRed (TIR) channels of the Along Track Scanning Radiometer (ATSR) instrument series. The retrieval scheme, named Advanced InfraRed WAter Vapor Estimator (AIRWAVE, version 1 and 2), uses the instrument physical characteristics in combination with advanced radiative transfer models and a sea surface spectral emissivity database. The native spatial resolution of the retrieved TCWV is 1x1 km2 with a water vapour column accuracy of about 15%; AIRWAVE products at lower spatial resolution (e.g. 0.25x0.25deg) are also available. The global validation of the AIRWAVE TCWV dataset, spanning from 1991 to 2012, is carried out using the WMO soundings located on islands or close to coasts. The validation exercise comprises the SUOMINET GPS dataset, freely available at http://www.suominet.ucar.edu/data.html#netcdf. Preliminary results show that the AIRWAVE version 1 dataset is bias-free and highly sensitive to the water vapour content in the atmospheric boundary layer, a part from the polar regions where version 1 is affected by dry biases for latitudes > 60 degrees. This problem has been solved in version 2 adopting a dynamic and self-consistent selection of the retrieval parameters. Inter-comparison with the Remote Sensing Systems Special Sensor Microwave Imager Level 2 products (RSS SSM/I, http://www.remss.com/missions/ssmi) has also been carried out, showing excellent agreement at global scale. Validation and inter-comparison methods and results for both AIRWAVE version 1 and 2 will be critically analysed and discussed.