Exploiting the high spatial resolution of AIRWAVE TCWV data to retrieve the WTC for coastal altimetry in view to its application to Sentinel-3

Clara Lázaro (Universidade do Porto, Faculdade de Ciências; Centro Interdisciplinar de Investigação Marinha e Ambiental (CIIMAR), Portugal)


M. Joana Fernandes (Universidade do Porto, Faculdade de Ciências; Centro Interdisciplinar de Investigação Marinha e Ambiental (CIIMAR), Portugal); Stefano Casadio (SERCO, ESA/ESRIN, Italy); Elisa Castelli (CNR, Istituto di Scienze dell'Atmosfera e del Clima, Italy); Enzo Papandrea (SERCO, ESA/ESRIN; CNR, Istituto di Scienze dell'Atmosfera e del Clima, Italy); Bianca Maria Dinelli (CNR, Istituto di Scienze dell'Atmosfera e del Clima, Italy); Alessandro Burini (EUMETSAT, Germany); Bojan Bojkov (EUMETSAT, Germany); Jérôme Bouffard (RHEA System SA, ESA, Italy)

Event: 2017 Ocean Surface Topography Science Team Meeting

Session: Instrument Processing: Propagation, Wind Speed and Sea State Bias

Presentation type: Type Oral

The AIRWAVE dataset contains Total Column Water Vapour (TCWV) values derived from the measurements of the (Advanced) Along-Track Scanning Radiometers ((A)ATSR) on board ERS and ENVISAT missions. The algorithm for the AIRWAVE data retrieval uses day and night dual-view thermal infrared observations from the (A)ATSR above cloud-free ocean. The (A)ATSR instrument series has been fully exploited and the dataset spans the period 1991-2012. One of the main advantages of using the AIRWAVE dataset to compute the wet tropospheric correction (WTC) over coastal regions is its high spatial resolution, allowing the retrieval of cloud-free TCWV very close to the coastline, which is of utmost importance for Coastal Altimetry.
The focus of this study is twofold. On the one hand, it exploits the AIRWAVE dataset to retrieve an improved WTC in coastal regions by using the GNSS-derived Path Delay Plus (GPD+) algorithm. On the other, it attempts to evaluate whether and to what extent AIRWAVE observations actually improve the WTC in the NW Mediterranean Sea.
To conduct the research, a new GPD+ WTC using AIRWAVE Version 2 (V2) data has been calculated for the entire ENVISAT period and for the NW Mediterranean Sea. Taking advantage of the AATSR and microwave radiometer (MWR) data simultaneity, the inclusion of AIRWAVE data is expected to contribute to a more accurate sea level anomaly (SLA) signal and thus to a better oceanographic characterization of the region.
The GPD+ WTC with AIRWAVE for ENVISAT has been evaluated through statistical analysis of SLA variance (along-track, at crossovers and function of distance from coast) and by direct comparison with other available WTC, namely those from ERA Interim and the GPD+ WTC computed with all data (GNSS, third party scanning imaging MWR and on-board MWR) but excluding AIRWAVE, which has been fully validated in the scope of several ESA projects (e.g., Sea Level Climate Change Initiative). The results from this assessment and the main findings will be provided in detail.
It has been concluded that AIRWAVE data still could be improved by refining the capability of the cloud screening algorithm used in the AIRWAVE generation to detect thin clouds, which in the current version leads to dry biased TCWV values. While a new version of AIRWAVE is not released, strategies to filter out cloud-contamination from data prior to their input in the GPD+ algorithm are being developed. Since the AIRWAVE algorithm can be adapted to Copernicus Sentinel-3 (S3) SLSTR (Sea and Land Surface Temperature Radiometer) data, this study is expected to contribute to the generation of a coastal-improved WTC for S3.
Aiming to evaluate if the high spatial resolution signals present in AIRWAVE actually improve the WTC, the GPD+ WTC with AIRWAVE for ENVISAT has also been computed for high-rate 20 Hz altimetry data. This high rate WTC has been evaluated using sea level anomalies derived from improved ranges from e.g. the Adaptive Leading Edge Subwaveform (ALES) rectracker, available up to 50 km distance from the coast. Results from the validation of this high-rate WTC are also shown for comparison.

Oral presentation show times:

RoomStart DateEnd Date
Symphony Ballroom IV Tue, Oct 24 2017,14:54 Tue, Oct 24 2017,15:12
Clara Lázaro
Universidade do Porto, Faculdade de Ciências; Centro Interdisciplinar de Investigação Marinha e Ambiental (CIIMAR)