Enhanced GPD+ wet tropospheric corrections for the Copernicus Sentinel-3 missions

Amongst its instruments, the Copernicus Sentinel-3 (S3) missions carry the Synthetic Aperture Radar Altimeter (SRAL) for precise sea surface height (SSH) observations and its companion Microwave Radiometer (MWR) for the retrieval of the path delay in the altimeter range caused by the presence of water vapour and cloud liquid water in the atmosphere. In the scope of an activity sponsored by the European Organization for the Exploitation of Meteorological Satellites (EUMETSAT), the University of Porto (UPorto) has been developing an improved WTC for the S3 missions. This improvement is accomplished by means of GPD+, Global Navigation Satellite Systems (GNSS)-derived Path Delay Plus, an algorithm developed at UPorto to retrieve enhanced WTC for radar altimeter missions. The requested improvement is particularly relevant over surfaces where the WTC from the on-board MWR is not available or is invalid due to e.g. the presence of land or ice in the large MWR footprint, such as coastal zones, inland waters and high latitudes. GPD+ is a data combination procedure (Objective Analysis), where valid observations from the on-board radiometer are combined with external measurements from GNSS and imaging radiometers (SI-MWR), to obtain new WTC estimates for all altimeter points for which the Baseline MWR WTC is invalid. In this way, GPD+ preserves the properties of the best available WTC, from the on-board MWR, and further extends this correction to regions and epochs where and when the former is not valid. To ensure the continuity and consistency of the corrections, all input data used in GPD+ undergo various quality checks and calibration procedures. The various MWR (both the on-board and external SI-MWR) are calibrated against the Special Sensor Microwave Imager/Sounder(SSMI/S), considered a stable radiometric reference. The corrections have been assessed by comparisons with other WTC sources: radiometers, GNSS and atmospheric models from the European Centre for Medium range Weather Forecasts (ECMWF). The GPD+ WTC validation includes analysis of sea level anomaly (SLA) along-track variance differences and SLA Root Mean Square (RMS) differences at crossovers (mean cycle values, function of latitude and function of distance from coast). This paper presents the most recent results of the GPD+ WTC for Sentinel-3A and Sentinel-3B, since the start of each mission until April 2021. The main features of the S3 GPD+ WTC include: are continuous and consistent corrections, valid over all surface types; are calibrated against the SSMI/S dataset, taken as reference; improve data coverage, mostly in coastal and polar regions, and fill existing gaps in the valid MWR observations; are improved WTC both with respect to the Baseline MWR WTC for each satellite and to the ECMWF model-derived WTC. The operational provision of the corrections is being implemented, with predicted inclusion in the S3 SRAL/MWR Level 2 products starting in mid-2022.