Coastal Sea Surface Heights from Improved Altimeter Data in the Mediterranean Sea
Standard and newly re-tracked altimeter data of the Topex/Poseidon and Envisat missions are analysed in the Mediterranean Sea in the proximity of selected coastlines.
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Coastal Sea Surface Heights from Improved Altimeter Data in the Mediterranean Sea L. Fenoglio-Marc, M. Fehlau, L. Ferri, M. Becker, Y. Gao, and S. Vignudelli
Abstract Standard and newly re-tracked altimeter data of the Topex/Poseidon and Envisat missions are analysed in the Mediterranean Sea in the proximity of selected coastlines. Over-conservative selection criteria in standard level 2 products cause rejection of many data in coastal regions. Among the standard criteria, most critical for the data rejection are the checks on microwave radiometer wet tropospheric correction and on standard deviation of the 18 Hz ranges. Using a model tropospheric correction, Envisat performs better than Topex and is approaching the coast up to 5 km at sea-land transitions. A further improvement in quality of coastal data is obtained using off-line retracked Topex-RGDR and Envisat data, these latter ones retracked with the β-5 and Improved Threshold empirical methods.
33.1 Introduction There has been considerable interest recently in addressing the retrieval of altimeter data in coastal regions to monitor more accurately sea level change. Data distributed by operational centers are not targeted to coastal areas. Standard altimeter products are therefore usually flagged as bad and removed. Moreover the signal to noise ratio is rapidly degraded as altimeter and radiometer are disturbed at 10 and 50 km to the coast respectively. In particular, due to
L. Fenoglio-Marc () Institute of Physical Geodesy, Technische Universität Darmstadt, Darmstadt, Germany e-mail: [email protected]
the contamination of land surface in the measurements of brightness temperature made by the on-board microwave radiometer (MWR), the MWR derived wet tropospheric correction cannot be used at distances smaller than 50 km to the coast (Desportes et al., 2007). New post-processing techniques recover additional coastal data and allow to detect smaller ocean dynamical processes (Roblou et al., 2007; Bouffard et al., 2008). In-situ sea level data at selected sites are used to assess the quality of the altimeter data (Fenoglio-Marc et al., 2007; Vignudelli et al., 2005). New pre-processing of the altimeter waveforms, using non-standard waveform models in the attempt to recover the ocean surface parameters, improves the accuracy of altimeter ranges (Anzenhofer et al., 1999; Deng et al., 2002; Deng and Featherstone, 2006). Purpose of this paper is to establish to what extent (1) standard criteria are applicable close to coast to both standard products and off-line re-tracked altimeter data, (2) non-uniform local conditions affect the altimeter retrievals on sea-land and land-sea alongtrack direction, (3) retracking methods improve the quality of data near coast. In Sect. 33.2 we describe data and methods. Results are presented in Sect. 33.3, conclusions in Sect. 33.4.
33.2 Data and Methods The Mediterranean Sea region is analyzed using Topex and Envisat data. Both standard products and newly retracked data are used. For Topex, standard Level 2 Geophysic
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