Total Column Water Vapour from Along Track Scanning Radiometer Series: Advanced Infra-Red Water Vapour Estimator (AIRWAVE) algorithm description and applications

A key issue in assessing climate change is the precise knowledge of distributions and variability of Total Column Water Vapour (TCWV). We developed a novel algorithm, named Advanced Infra-Red WAter Vapour Estimator (AIRWAVE), for the retrieval of TCWV form Along Track Scanning Radiometer (ATSR) missions measurements. The retrieval method does not require any tuning or adjustments to independent water vapour datasets and is based on: 1) calculations from a radiative transfer model that was specifically developed to simulate ATSR radiances, 2) informations on instrumental parameters, 3) sea surface spectral emissivity. AIRWAVE has been applied to ocean cloud-free ATSR radiances in the Thermal Infra-Red channels (11 and 12 microns) and exploits the simultaneous use of forward and nadir measurements. It allows for accurate and precise day-time and night-time TCWV retrievals as shown by preliminary inter-comparisons between ATSR and Special Sensor Microwave/Imager (SSM/I) coincident TCWVs. The algorithm was developed for ATSR-like instruments having channels in the TIR and exploiting dual viewing geometries. The Sea and Land Surface Temperature Radiometer (SLSTR), on board the European Copernicus Sentinel 3, fully satisfies these requirements. AIRWAVE can, therefore, be applied to SLSTR measurements to retrieve TCWV. Here we discuss the physical derivation of the AIRWAVE retrieval method and present preliminary calculations (based on available instrument characteristics) for the application of AIRWAVE to SLSTR measurements.