Abstract's details
Synergistic use of the Sentinel-3A SRAL/MWR and SLSTR Sensors for the Wet Tropospheric Correction Retrieval
CoAuthors
Event: 2022 Ocean Surface Topography Science Team Meeting
Session: Instrument Processing: Propagation, Wind Speed and Sea State Bias
Presentation type: Type Oral
Contribution: PDF file
Abstract:
The computation of sea surface height measurements from satellite altimetry is nowadays a straightforward procedure, enabling sea level variation studies globally. Amongst the various range corrections that need to be applied to the range measured by the altimeter, the wet tropospheric path delay is still one of the most significant error sources.
The most accurate way to retrieve the corresponding wet tropospheric correction (WTC) is through the measurements of microwave radiometers (MWR) on board altimetry missions, collocated with those from the altimeter. For the altimeter reference missions, three-band MWR have been used, with frequency bands around 18 GHz, 23 GHz and 36 GHz. In the case of the dual-band MWR on board European Space Agency’s (ESA) missions, the lack of the lowest frequency channel, which provides mainly information on the surface emissivity and its contribution in the measured MWR brightness temperatures (TB), is currently taken into account by considering additional parameters, namely the altimeter backscatter coefficient, σ0, the sea surface temperature (SST), and the atmospheric temperature vertical decrease rate (γ800).
Although the altimeter σ0 parameter is collocated with the MWR measurements, the SST is currently extracted from external static seasonal tables. Recent studies show that the use of a dynamic SST extracted from Numerical Weather Models (ERA5) improves the WTC retrieval, whereas the γ800 parameter provides redundant information.
The Copernicus Sentinel-3 mission payload, besides the Synthetic Aperture Radar Altimeter (SRAL) and MWR sensors, includes the Sea and Land Surface Temperature Radiometer (SLSTR), among other collocated sensors, from which gridded SST observations are derived over ocean, simultaneously with observations from the SRAL and MWR sensors.
In this context, the objective of the present work is the development of a synergistic approach for the ESA Sentinel-3 mission between the SRAL and MWR sensors with the SLSTR instrument. The aim is to derive the SST measurement from the SLSTR sensor for each SRAL observation and assess its impact in the WTC retrieval over open ocean, thus removing the need for the extraction of the SST parameter from external sources.
In a first stage, the SLSTR-derived SST are evaluated against the ERA5 model; their impact in the WTC retrieval is assessed in a second stage. The results show that the use of the SLSTR-derived SST, compared to those from ERA5, has no significant impact on the WTC retrieval over open ocean, both globally and regionally. Thus, for the WTC retrieval, there seems to be no advantage in having collocated SST and altimeter and radiometer observations. Additionally, this study reinforces that the use of dynamic SST leads to a significant improvement over the current Sentinel-3 WTC operational algorithms.
The most accurate way to retrieve the corresponding wet tropospheric correction (WTC) is through the measurements of microwave radiometers (MWR) on board altimetry missions, collocated with those from the altimeter. For the altimeter reference missions, three-band MWR have been used, with frequency bands around 18 GHz, 23 GHz and 36 GHz. In the case of the dual-band MWR on board European Space Agency’s (ESA) missions, the lack of the lowest frequency channel, which provides mainly information on the surface emissivity and its contribution in the measured MWR brightness temperatures (TB), is currently taken into account by considering additional parameters, namely the altimeter backscatter coefficient, σ0, the sea surface temperature (SST), and the atmospheric temperature vertical decrease rate (γ800).
Although the altimeter σ0 parameter is collocated with the MWR measurements, the SST is currently extracted from external static seasonal tables. Recent studies show that the use of a dynamic SST extracted from Numerical Weather Models (ERA5) improves the WTC retrieval, whereas the γ800 parameter provides redundant information.
The Copernicus Sentinel-3 mission payload, besides the Synthetic Aperture Radar Altimeter (SRAL) and MWR sensors, includes the Sea and Land Surface Temperature Radiometer (SLSTR), among other collocated sensors, from which gridded SST observations are derived over ocean, simultaneously with observations from the SRAL and MWR sensors.
In this context, the objective of the present work is the development of a synergistic approach for the ESA Sentinel-3 mission between the SRAL and MWR sensors with the SLSTR instrument. The aim is to derive the SST measurement from the SLSTR sensor for each SRAL observation and assess its impact in the WTC retrieval over open ocean, thus removing the need for the extraction of the SST parameter from external sources.
In a first stage, the SLSTR-derived SST are evaluated against the ERA5 model; their impact in the WTC retrieval is assessed in a second stage. The results show that the use of the SLSTR-derived SST, compared to those from ERA5, has no significant impact on the WTC retrieval over open ocean, both globally and regionally. Thus, for the WTC retrieval, there seems to be no advantage in having collocated SST and altimeter and radiometer observations. Additionally, this study reinforces that the use of dynamic SST leads to a significant improvement over the current Sentinel-3 WTC operational algorithms.