Improvement of the Topex/Poseidon altimetric data processing for hydrological purposes (CASH Project)
Franck MERCIER(1) and Ouan-Zan ZANIFE(1)
8-10 rue Hermes,
31520 Ramonville Saint Agne,
Satellite altimetry technique and products are primarily designed for open ocean studies. The same technique has been progressively applied to inner seas, lakes and large rivers. Early results have highlighted the potential contribution of this technique to the monitoring of continental water bodies levels, and the gains than can be expected in measurement quality (accuracy, frequency) through better definition of ground track location, improved algorithms for waveform retracking, improved methods to quantify tropospheric propagation delays.
Actually, over non-ocean surfaces (wet or dry), the accuracy of the altimetric measurements is degraded to several cm or tens of cm, mainly because of the heterogeneity of the reflecting surface (a mix of water and emerged lands). Another important source of error lies in the propagation of the signal through the atmosphere. This study focuses on these 2 issues.
In the framework of the CASH project (Contribution de l’Altimétrie Satellitaire à l’Hydrologie) founded by the French Ministry of Research, it has been decided to initiate a global re-processing of the Topex/Poseidon data (1992-2005) that is dedicated to the constitution of an hydrology-oriented altimetric data base.
Over the open ocean, only water is present within the radar footprint. Over continental water bodies, emerged lands within the footprint generate complex radar echoes (waveforms) over which the height retrieval process is not as accurate as it is for oceanic echoes. As a first step, we applied to the Topex waveforms the same 4 retracking algorithms that are routinely applied to the ENVISAT measurements. Consequently, Topex/Poseidon products become coherent with ENVISAT Geophysical Data Records. These retracking algorithms are known as “Ocean”, “Ice1”, “Ice2” and “SeaIce” in the ENVISAT processing. Although not specifically dedicated to the large variety of waveforms that can be found over continental waters and therefore not fully optimized for hydrological purposes, these algorithms nevertheless provide, over water bodies, promising results in terms of accuracy improvement and recovering of data that are missing in the Topex/Poseidon MGDRs. Several examples showing the resulting gain are presented.
Simultaneously, we investigated the correction of the propagation delay induced by the water vapour within the troposphere. This correction can amount 50 cm, with an annual cycle amplitude of up to 20 cm, and is usually computed over oceans with simultaneous radiometric measurements. Such measurements generally default over non-ocean surfaces and may be superseded by a correction computed from meteorological model outputs (usually the ECMWF model).
We first show that the model correction included in the widespread altimetric data sets is not reliable over non-ocean areas because the changes in the altitude of the reflecting surface (and thus the thickness of the atmosphere column) are not taken into account. Then, we demonstrate that a computation based on the use of a gridded Digital Elevation Model is not adequate. We finally propose a new method where the altitude of the reflecting surface is deduced from the altimetric measurement itself. This method is applied (with the NCEP Reanalysis model outputs) and is evaluated via comparisons with radiometric measurements acquired over a selection of large inland water bodies.