16:45 - 17:05
Validation of Sentinel-3 Wet Tropospheric Correction
1Universidade do Porto, Faculdade de Ciências, Portugal; 2Centro Interdisciplinar de Investigação Marinha e Ambiental (CIIMAR/CIMAR), Universidade do Porto, Porto, Portugal
Launched on 16 February 2016, Sentinel-3 (S3) carries a two-band microwave radiometer (MWR) similar to that of Envisat, aimed at the precise retrieval of the wet tropospheric correction (WTC) through collocated measurements withe the SRAL instrument.
Due to their instrumental characteristics and retrieval algorithms, the two-band MWR deployed on the European Space Agency (ESA) altimeter missions are known for their good performance in the open-ocean. However, when they approach the coast, the retrieval algorithm, which was designed for surfaces with ocean emissivity, generates very noisy values as the footprint encounters surfaces with different emissivity. The same happens at high latitudes in regions covered with ice.
This study is a contribution to the Sentinel-3 Validation Team project ID 13769, titled “Validation Of Coastal ALtimetry from Sentinel-3, by assessing the MWR-derived WTC present on S3 products, released for validation purposes.
The validation is performed by means of comparisons with independent data sets namely: scanning imaging microwave radiometers (SI-MWR) such as the Special Sensor Microwave Image Sounder (SSM/IS) or the GPM Microwave Imager (GMI); Global Navigation Satellite Systems (GNSS) derived path delays determined at coastal stations; wet path delays from the MWR on board Jason-2 and Jason-3.
Moreover, the overall along-track performance is compared against estimates obtained from the GNSS-derived Path Delay Plus (GPD+) algorithm and from atmospheric models. For this purpose, GPD+ wet path delays are derived by combining, through space-time objective analysis, all available measurements but not including S3 MWR, i.e. using only SI-MWR and GNSS observations.
Once the S3 MWR-derived WTC has been examined, the GPD+ algorithm is tuned to S3 by an appropriate detection of the invalid measurements due to land or ice contamination or any additional error source. Subsequently, new GPD+ estimates are obtained, now only for the points with invalid MWR values, by combining all available observations, this time also including valid measurements from the S3 MWR.
In addition to the statistical comparisons between the S3 MWR-derived WTC and the various WTC sources, the correction is also evaluated by means of sea level anomaly variance, both along-track, at crossovers and function of distance from coast.
17:05 - 17:25
Performance of Sentinel-3 Surface Topography Mission Microwave Radiometer in Coastal Areas
1CLS, France; 2ESA (ESRIN); 3CNES; 4ESA (ESTEC)
The Sentinel-3A Surface Topography Mission has been launched in February 2016. The topography payload aboard Sentinel-3A has measurements requirements over ocean as well as coastal areas, sea ice, and inland waters, these latter with enhanced performances benefitting from the dual-frequency SAR altimeter (SRAL). We present here the measurements acquired by the microwave radiometer (MWR) supporting the SRAL for the retrieval of the wet tropospheric correction.
The MWR is a two-channel microwave radiometer (23.8 and 36.5 GHz) similar to the Envisat MWR. The radiometer will perform measurements of brightness temperatures in both bands interpolated at the location of the altimeter footprint.
For the retrieval of the wet tropospheric correction, two algorithms based on simulated parameters (brightness temperatures and altimeter backscattering ratio) and neural networks are proposed in the Sentinel-3A products. First a classical approach, similar to Envisat’s algorithm is using both brightness temperatures and altimeter backscattering coefficient to take into account the surface roughness. Secondly, an innovative algorithm is using additional inputs such as the sea surface temperature and temperature lapse rate to improve the retrieval over specific areas such as upwelling regions.
We will present the performance of the Sentinel-3A MWR performances over coastal areas and compare its measurements to other radiometers in these regions.
17:25 - 17:45
Exploitation of AIRWAVE for Retrieving the Wet Tropospheric Correction for Coastal Altimetry
1Universidade do Porto, Faculdade de Ciências, Portugal; 2Centro Interdisciplinar de Investigação Marinha e Ambiental (CIIMAR/CIMAR), Universidade do Porto, Porto, Portugal; 3SERCO, ESA/ESRIN, Frascati, Italy; 4CNR, Istituto di Scienze dell'Atmosfera e del Clima, Bologna, Italy; 5EUMETSAT, Darmstadt, Germany; 6RHEA System SA, ESA
AIRWAVE (Advanced Infra-Red WAter Vapour Estimator) Version 1 is an algorithm that has been expressly developed for the retrieval of Total Column Water Vapour (TCWV) from the measurements of the Along Track Scanning Radiometer (ATSR) on board ERS and Envisat missions. It is fast and independent from external constrains. The current version of the algorithm works on ocean/cloud-free scenes by combining advanced radiative transfer models and a sea surface spectral emissivity database. The simultaneous use of forward and nadir measurements minimise the impact of the knowledge of the sea surface temperature and of the atmospheric radiation on the quality of the retrieved TCWV. Exploiting only the TIR channels of the instrument, the algorithm enables the estimation of TCWV both in the day and night part of the orbits and the full exploitation of the ATSR instrument series, spanning from 1991 to 2012.
One of the features of AIRWAVE V1 is that it enables the retrieval of cloud-free TCWV very close to the coastline, which is of paramount importance in Coastal Altimetry science.
This study focus on the exploitation of AIRWAVE for retrieving the wet tropospheric correction (WTC) for coastal altimetry. Wet path delays (WPD) derived from the AIRWAVE TCWV dataset have been incorporated in the Global Navigation Satellite System (GNSS) derived Path Delay Plus (GPD+) algorithm aiming at the generation of a new WTC for each altimetry mission. They are combined by optimal interpolation with WPD observations derived from the microwave radiometer (MWR) on board each altimeter missions, except for CryoSat-2, from a set of scanning imaging MWR on board Remote Sensing missions (e.g. the Special Sensor Microwave Imager (SSM/I) and the SSM/I Sounder (SSM/IS)) and from a network of coastal and island GNSS stations, with the representation and quality of each dataset taken into account by the interpolation. In this way, the GPD+ WTC is expected to benefit from the high spatial resolution of AIRWAVE data, with an impact on the effectiveness of the retrieval of coastal altimetry parameters such as Sea Level Anomaly (SLA).
The AIRWAVE-derived GPD+ WTC has been firstly released for 1-Hz ENVISAT data, taking advantage of AATSR and MWR data simultaneity. Results from the validation of this new GPD+ WTC by statistical analyses of SLA variance (along-track, at crossovers and function of distance from coast) and by direct comparison with independent sources such as GNSS data not used in the computations are shown. To fully exploit the high spatial resolution of AIRWAVE data and their availability closer to the coast, the GPD+ WTC for ENVISAT has also been computed for high-rate altimetry data from e.g. the Adaptive Leading Edge Subwaveform (ALES) rectracker. Results from the validation of this high-rate WTC are also shown for comparison.
17:45 - 18:05
ACCRA : A Study on Future Microwave Radiometers for Atmospheric Correction of Radar Altimeters on Coastal Regions
1CLS, France; 2JCR, UK; 3IPSL, France; 4CNRM, France; 5ESA/ESTEC, Netherlands
The wet tropospheric correction (WTC) is a major source of uncertainty in altimetry budget error, due to its large spatial and temporal variability: this is why the main altimetry missions include a microwave radiometer (MR) The commonly agreed requirement on WTC for current missions is to retrieve WTC with an error better than 1cm rms.
With the introduction of the along-track synthetic aperture processing, first implemented in CryoSat-2, and now in the upcoming operational altimetry missions such as Sentinel-3 and Jason-CS, more accurate altimetry data are anticipated for coastal and inland waters.
Nevertheless, the quality of data in those areas are expected to be degraded with respect to those of the open oceans due to the rather wide field of view of the MR (-3 dB beam-width of ~20 km). As a matter of fact, the MR observations over those waters are subject to contamination by land brightness temperatures which fall within the MR footprint.
The present team has been selected by ESA/ESTEC to work on a MR instrument design for future operational radar altimetry missions. Such a design shall include the classical MR channels for ensuring observation continuity, augmented by a set of high frequency channels for enabling accurate altimetry over coastal and inland waters.
In this study team, the extensive systems and radiometric engineering experience of JCR Systems is complemented by a significant expertise of LOCEAN , CLS and CNRM in water vapour retrievals, a considerable experience in design and development of microwave and millimeter wave radiometer front ends from RAL and a substantial knowledge of SMT Consultancies in microwave and millimeter-wave antenna design.
We will present the final results of this study. First, the selection of an optimal set of observation frequencies based on an analysis of both potential horizontal resolution and the value of the physical information provided. Then, the instrument design selected in consistency with the selected channels. And finally, the results of a dedicated 1D-VAR retrieval approach with a specific strategy over coastal areas, using the best of each observation frequency. These results using the ACCRA configuration are discussed against the configuration of Jason-CS.