A coastally improved global dataset of wet tropospheric corrections for satellite altimetry
Authors: Lázaro Clara , Fernandes Maria Joana , Vieira Telmo , Vieira Eliana 
Published: October 2019

Abstract. Global mean sea level is a valuable proxy to understand climate change and how it operates, since it includes the response from various components of the climate system. Global sea level rise is accelerating, which is a concern for coastal areas management from medium to long-term time scales. Satellite radar altimetry (RA) has been providing us information regarding the sea level anomaly (SLA) field and its space-time variability since the early 90s. As satellite orbit determination, reference surfaces (e.g., mean sea surface) and instrumental, range and geophysical corrections improved over the decades, the data from past missions were reprocessed subsequently, leading to an SLA dataset over open ocean accurate to the centimetre-level. The accuracy of satellite altimetry is known to deteriorate towards the coastal regions due to several reasons, amongst which the improper account for the wet path delay (WPD) can be pointed out. The most accurate WPDs for RA are derived from the on-board microwave radiometer (MWR) radiance measurements, acquired simultaneously as the altimeter ranges. In the coastal zone, however, the signal coming from the surrounding land contaminates these measurements and the water vapour retrieval from the MWR fails. As meteorological models do not handle coastal atmospheric variability correctly yet, the altimeter measurements are rejected whenever MWR observations are absent or invalid. The need to solve this altimetry issue in the coastal zone, simultaneously responding to the growing demand of data in these regions, motivated the development of the GNSS-derived Path Delay (GPD) algorithm. The GPD combines WPD from several sources through objective analysis (OA) to estimate the WPD or the corresponding RA correction accounting for this effect, the wet tropospheric correction (WTC), for all along-track altimeter points for which this correction has been set as invalid or is absent. The current GPD version (GPD Plus, GPD+) uses as data sources WPD from coastal and island GNSS stations, from satellites carrying microwave radiometers, and from valid on-board MWR measurements. The GPD+ has been tuned to be applied to all, past and operational, RA missions, with or without an on-board MWR. The long-term stability of the WTC dataset is ensured by its inter-calibration with respect to the Special Sensor Microwave Imager (SSM/I) and SSMI/I Sounder (SSM/IS). The dataset is available for TOPEX/Poseidon (T/P), Jason-1 and Jason-2 (NASA/CNES), Jason-3 (NASA/EUMETSAT), ERS 1, ERS-2, Envisat and CryoSat-2 (ESA), SARAL/AltiKa (ISRO/CNES) and GFO (U.S. Navy) RA missions. The GPD+ WTC for Sentinel-3 shall be released soon. The present paper describes the GPD+ database and its independent validation through statistical analyses of SLA. Overall, results show that the GPD+ WTC is the most effective in reducing the SLA variance in the coastal regions, in particular for the ESA missions. Moreover, GPD+ recovers a significant number of measurements, which otherwise would be rejected due to land, rain and ice contamination and instrument malfunctioning. Consequently, GPD+ database has been chosen as reference WTC in the Sea Level Climate Change Initiative (CCI) products; the GPD+ has also been adopted as reference in CryoSat-2 Level 2 Geophysical Ocean Products (GOP). Strategies to further improve the methodology, therefore enhancing the quality of the database, are also discussed. The GPD+ dataset is archived on the homepage of the Satellite Altimetry Group, University of Porto, publicly available at the repository https://doi.org/10.23831/FCUP_UPORTO_GPDPlus_v1.0 (Fernandes et al., 2019).