Exploitation of AIRWAVE for Retrieving the Wet Tropospheric Correction for Coastal Altimetry

AIRWAVE (Advanced Infra-Red WAter Vapour Estimator) Version 1 is an algorithm that has been expressly developed for the retrieval of Total Column Water Vapour (TCWV) from the measurements of the Along Track Scanning Radiometer (ATSR) on board ERS and Envisat missions. It is fast and independent from external constrains. The current version of the algorithm works on ocean/cloud-free scenes by combining advanced radiative transfer models and a sea surface spectral emissivity database. The simultaneous use of forward and nadir measurements minimise the impact of the knowledge of the sea surface temperature and of the atmospheric radiation on the quality of the retrieved TCWV. Exploiting only the TIR channels of the instrument, the algorithm enables the estimation of TCWV both in the day and night part of the orbits and the full exploitation of the ATSR instrument series, spanning from 1991 to 2012. One of the features of AIRWAVE V1 is that it enables the retrieval of cloud-free TCWV very close to the coastline, which is of paramount importance in Coastal Altimetry science. This study focus on the exploitation of AIRWAVE for retrieving the wet tropospheric correction (WTC) for coastal altimetry. Wet path delays (WPD) derived from the AIRWAVE TCWV dataset have been incorporated in the Global Navigation Satellite System (GNSS) derived Path Delay Plus (GPD+) algorithm aiming at the generation of a new WTC for each altimetry mission. They are combined by optimal interpolation with WPD observations derived from the microwave radiometer (MWR) on board each altimeter missions, except for CryoSat-2, from a set of scanning imaging MWR on board Remote Sensing missions (e.g. the Special Sensor Microwave Imager (SSM/I) and the SSM/I Sounder (SSM/IS)) and from a network of coastal and island GNSS stations, with the representation and quality of each dataset taken into account by the interpolation. In this way, the GPD+ WTC is expected to benefit from the high spatial resolution of AIRWAVE data, with an impact on the effectiveness of the retrieval of coastal altimetry parameters such as Sea Level Anomaly (SLA). The AIRWAVE-derived GPD+ WTC has been firstly released for 1-Hz ENVISAT data, taking advantage of AATSR and MWR data simultaneity. Results from the validation of this new GPD+ WTC by statistical analyses of SLA variance (along-track, at crossovers and function of distance from coast) and by direct comparison with independent sources such as GNSS data not used in the computations are shown. To fully exploit the high spatial resolution of AIRWAVE data and their availability closer to the coast, the GPD+ WTC for ENVISAT has also been computed for high-rate altimetry data from e.g. the Adaptive Leading Edge Subwaveform (ALES) rectracker. Results from the validation of this high-rate WTC are also shown for comparison.