A coastally improved global dataset of wet tropospheric corrections for satellite altimetry
Authors: Lázaro Clara , Fernandes Maria Joana , Vieira Telmo , Vieira Eliana 
Published: December 2020

Abstract. The accuracy of satellite radar altimetry (RA) 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 RA issue in the coastal zone, simultaneously responding to the growing
demand for data in these regions, motivated the development of the GNSS (Global Navigation Satellite System) derived Path Delay (GPD) algorithm. 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 not defined.
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. 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 SSM/I Sounder (SSMIS). The dataset is
available for the TOPEX/Poseidon (T/P); Jason-1 and Jason-2 (NASA and CNES); Jason-3
(NASA and EUMETSAT); ERS-1, ERS-2, Envisat and CryoSat-2 (ESA); SARAL/AltiKa
(ISRO and CNES); and GFO (US Navy) RA missions. The GPD+ WTC for Sentinel-3
(ESA and EUMETSAT) shall be released soon. The present paper describes the
GPD+ database and its assessment through statistical analyses of sea level
anomaly (SLA) datasets, calculated with GPD+, the ECMWF Reanalysis
Interim (ERA-Interim) model or MWR-derived WTCs. Global results, as well as
results for three regions (the North American and European coasts and the Indonesia
region), are presented for ESA's recent Envisat Full Mission
Reprocessing (FMR) V3.0. Global results show that the GPD+ WTC leads to a
reduction in the SLA variance of 1–2 cm2 in the coastal zones, when
used instead of the ERA WTC, which is one of the WTCs available in these
products and can be adopted when the MWR-derived WTC is absent or invalid. The
improvement of the GPD+ WTC over the ERA WTC is maximal over the tropical
oceans, particularly in the Pacific Ocean, showing that the model-derived
WTC is not able to capture the full variability in the WPD field yet. The
statistical assessment of GPD+ for the North American coast shows a
reduction in SLA variance, when compared to the use of the ERA-derived WTC,
of 1.2 cm2, on average, for the whole range of distances from the coast
considered (0–200 km). Similar results are obtained for the European coasts.
For the Indonesia region, the use of the GPD+ WTC instead of that from ERA
leads to an improvement, on average, on the order of 2.2 cm2 for
distances from the coast of up to 100 km. Similar results have been obtained for
the remaining missions, particularly for those from ESA. Additionally,
GPD+ recovers the WTC for a significant number of along-track altimeter
points with missing or invalid MWR-derived WTCs, due to land, rain and ice
contamination and instrument malfunctioning, which otherwise would be
rejected. Consequently, the GPD+ database has been chosen as the reference WTC
in the Sea Level Climate Change Initiative (CCI) products; GPD+ has
also been adopted as the 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 home page 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).