Comparison of Retracking Schemes for Pulse-Limited Coastal Altimetry
Passaro, Marcello¹; Cipollini, Paolo²; Quartly, Graham³; Snaith, Helen⁴; West, Luke^2
1University of Southampton, UNITED KINGDOM; ²National Oceanography Centre, UNITED KINGDOM; ³Plymouth Marine Laboratory, UNITED KINGDOM; ⁴British Oceanographic Data Centre, UNITED KINGDOM

Satellite altimetry has revolutionized our understanding of ocean dynamics thanks to high repetition rate and global coverage. Nevertheless, coastal data has been flagged as unreliable due to land and calm water interference in the altimeter and radiometer footprint and high frequency tidal and atmospheric forcing.

In recent years, a lively community has started to reprocess the altimetry dataset to retrieve valuable information in coastal seas by the development of new retracking algorithms and the improvement of geophysical corrections. Our study addresses the first issue, i.e. retracking, looking at some specialized algorithms for the coastal zone that have been proposed in recent years, and comparing their performance on a number of tracks in different regions (British coast, Mediterranean, Agulhas region)

Our aim is to find a retracking scheme that is potentially applicable to all the standard altimetry missions and is able to reprocess the whole 20 years of data currently available. Retrackers are tested and compared against each other, then retracked data are validated against tide gauges and geoid.

We have so far assessed, by comparison with the standard open-ocean Brown retracker, two retrackers specifically designed to deal with peaky waveforms such as those often found in the coastal zone: a) the "Brown plus Gaussian Peak" (BGP) proposed by Halimi et al . (2012), and b) the OceanCS retracker that optimises the fit of the leading edge of the waveform (Yang et al., 2012). We illustrate the results of this assessment with Envisat and Jason-2 passes. The main findings are that 1) the BGP retracker is particularly suited to recover the amplitude of the returned echo, which results in a better estimation of the wind parameter; and 2) the OceanCS retracker outperforms the other two in terms of noise levels on the height and significant wave height retrieval. In some cases the distance from coast at which height and swh can be retrieved with precision comparable to open ocean is halved with respect to standard altimetric products.

We conclude by presenting our research on how to optimally combine the information coming from the different retrackers, and by briefly discussing some of the applications of the new coastal product, like those being pioneered in the ESA DUE eSurge Project for the modelling and forecasting of storm surges through the increased use of advanced satellite products such as coastal altimetry.