Abstract's details
Technical aspects of coastal altimetry data processing
CoAuthors
Event: 2017 Ocean Surface Topography Science Team Meeting
Session: Advances in coastal altimetry: measurement techniques, science applications and synergy with in situ and models
Presentation type: Type Oral
Contribution: PDF file
Abstract:
The main limitations of conventional (Low Resolution Mode – LRM) ocean altimetry products in the coastal regions are related to the large footprint size of both the radar altimeter and the microwave radiometer, i.e., the difficulty of ocean algorithms to retrieve accurate surface parameters (surface height, significant wave height and backscatter) from coastal waveforms as well as errors in geophysical and range corrections associated to these regions.
In the last ten years, largely fostered by the set of coastal altimetry and OSTST workshops, great progress has been achieved in these topics, namely due to the advent of new instruments—Ka band and Synthetic Aperture Radar (SAR) altimetry—, new retracking algorithms, handling of the coastal contamination of radiometers, and improved corrections.
This paper presents a review of these advances and how they contributed to the development of altimetry products tuned to coastal applications.
Various methods have been proposed and tested to limit the effect of land in the retrievals of geophysical parameters from altimetry. For conventional altimetry (for which the waveforms are a given, sub-waveform retrackers have been shown to work well to retrieve results closer to the coast, even within the dimension of the footprint. To make these measurements unbiased with respect to the usual Brown waveform retrackers is an additional challenge.
In the case of SAR altimetry, one has two approaches: (1) devise a way to exclude echoes that show contamination and build a waveform only from the remaining, and (2) focus the waveform to such a narrow beam, that contamination can be excluded. Both techniques are still in their infancy but show some promising results.
Regarding the range geophysical and range corrections, great progress has been achieved in the wet tropospheric correction (WTC), by means of various approaches to improve the retrieval of the WTC near the coast (e.g. mixed pixel, land decontamination and GNSS-derived path delay algorithms) and in the development of better ocean tide models such as FES2014. Other corrections such as sea state bias still require further research to better understand sea state effects in the coastal zone, while various studies are taking place in modelling these effects in open-ocean and for the various altimeter modes (LRM, SAR). Advances in the modelling of other corrections such as the dynamic atmospheric correction and other atmospheric corrections driven by improved models will be presented. In addition, due to the height dependence on the tropospheric (dry and wet) corrections, the importance of providing them at high rate (20 Hz) in coastal products will be stressed.
In the last ten years, largely fostered by the set of coastal altimetry and OSTST workshops, great progress has been achieved in these topics, namely due to the advent of new instruments—Ka band and Synthetic Aperture Radar (SAR) altimetry—, new retracking algorithms, handling of the coastal contamination of radiometers, and improved corrections.
This paper presents a review of these advances and how they contributed to the development of altimetry products tuned to coastal applications.
Various methods have been proposed and tested to limit the effect of land in the retrievals of geophysical parameters from altimetry. For conventional altimetry (for which the waveforms are a given, sub-waveform retrackers have been shown to work well to retrieve results closer to the coast, even within the dimension of the footprint. To make these measurements unbiased with respect to the usual Brown waveform retrackers is an additional challenge.
In the case of SAR altimetry, one has two approaches: (1) devise a way to exclude echoes that show contamination and build a waveform only from the remaining, and (2) focus the waveform to such a narrow beam, that contamination can be excluded. Both techniques are still in their infancy but show some promising results.
Regarding the range geophysical and range corrections, great progress has been achieved in the wet tropospheric correction (WTC), by means of various approaches to improve the retrieval of the WTC near the coast (e.g. mixed pixel, land decontamination and GNSS-derived path delay algorithms) and in the development of better ocean tide models such as FES2014. Other corrections such as sea state bias still require further research to better understand sea state effects in the coastal zone, while various studies are taking place in modelling these effects in open-ocean and for the various altimeter modes (LRM, SAR). Advances in the modelling of other corrections such as the dynamic atmospheric correction and other atmospheric corrections driven by improved models will be presented. In addition, due to the height dependence on the tropospheric (dry and wet) corrections, the importance of providing them at high rate (20 Hz) in coastal products will be stressed.