Improved wet path delays for all ESA and reference altimetric missions

Abstract

The wet tropospheric correction (WTC) is still a significant error source in most altimetric products. For studies such as sea level change, as those performed in the scope of the ESA Sea Level Climate Change Initiative (SL_cci) project, the use of uniform and consistent WTC datasets are of major importance. For this purpose, a set of improved WTC, using the Global Navigation Satellite System (GNSS) derived Path Delay (GPD) algorithm, was envisaged for the main six altimetric missions: the so-called reference missions (TOPEX/Poseidon, Jason-1 and Jason-2) and the three ESA missions (ERS-1, ERS-2 and Envisat). The GPD methodology is based on the combination of wet path delays derived from zenith total delays calculated at a network of coastal GNSS stations and valid microwave radiometer (MWR) measurements at altimeter nearby points. At each altimeter point with an invalid MWR value, the WTC is estimated from the set of observations, along with the associated mapping error, using a linear space-time objective analysis technique that takes into account the spatial and temporal variability of the WTC field and the accuracy of each data set used. In the absence of observations, tropospheric delays from the European Centre for Medium-range Weather Forecasts (ECMWF) ReAnalysis (ERA) Interim model are adopted. Originally designed to improve the WTC in the coastal zone, the GPD evolved to include the global ocean, correcting for land and ice contamination in the MWR footprint, or spurious measurements due to e.g. instrument malfunction. This paper presents an overview of the GPD implementation for the afore-mentioned six altimetric missions. The GPD products have been validated by comparison with the WTC adopted as the reference correction by the Archiving, Validation, and Interpretation of Satellite Data in Oceanography (AVISO): the so-called composite correction (Comp) for all missions except Jason-2, for which the version D of the Geophysical Data Records (GDR-D) Advanced Microwave Radiometer (AMR) WTC is adopted. Various sea level anomaly (SLA) statistical analyses have been performed and are summarised in this paper: differences in SLA variance calculated along satellite tracks and at crossovers; SLA variance difference function of distance from the coast or function of latitude. Results show that the GPD WTC evidence a very significant improvement with respect to the Comp correction, particularly at polar and coastal regions, for all ESA and TOPEX/Poseidon missions. For the last, the impact is particularly significant in the second part of the mission, since detected anomalies present in the TOPEX Microwave Radiometer products are corrected by the algorithm. For Jason-1 and Jason-2, some improvements are observed in the coastal regions, although globally not very significant, particularly for Jason-2. This is attributed to the good performance of the WTC present in the most recent Jason-1 and Jason-2 products. The GPD WTC constitutes a coherent dataset of global and continuous corrections, for most missions a major improvement with respect to the baseline MWR and the Comp wet tropospheric corrections.