Session: Methodology III

Study of Arctic sea ice is of particular interest in the context of climate change, since it is one of the main parameters in the ocean-atmosphere interaction. In this work, a new method is proposed to monitor and characterize the Arctic sea ice. The method uses as inputs passive microwave observations at window frequencies ranging from 23.8 to 89 GHz from the Advanced Microwave Sounding Unit (AMSU) -A and –B. Several studies deal with the sea ice surface characterization and use simplified assumptions to retrieve and/or infer sea ice surface emissivities. However, assumptions on the relative roughness of the surface have rarely been taken into account, while they can provide accurate information on sea ice extent and sea ice surface properties.
In this work, the effect of a diffuse 'Lambertian' and a flat 'Specular' surface assumption is analysed over the Arctic sea ice at different window frequencies, using a Specular/Lambertian emissivity ratio (noted Emis_ratio hereafter) over a one-year database of near-nadir observations. A monthly ice/no ice delimitation over the Arctic region is obtained using emissivity ratio at low frequencies and is found to be in very good agreement with other available sea ice products (from the EUMETSAT Open Sea and Ice Satellite Application Facilities and CERSAT-IFREMER). The relatively high sensitivity of Emis_ratio at 50.3 GHz to surface roughness as well as the potential
combined use of emissivities at 23.8 and 89 GHz, turn out to be a good indicator for sea ice type classification and characterization.

Satellite ocean color remote sensing evolved rapidly since the 1978 launch of the Color Zone Coastal Scanner (CZCS). Since that launch, the Naval Research Laboratory (NRL) has developed and transitioned tactical ocean optical products (diver visibility, laser penetration depth, chlorophyll concentration, and inherent optical products) from polar-orbiting ocean color sensors to the Naval Oceanographic Office (NAVOCEANO).  Beginning with CZCS, NRL exploited the succession of ocean color sensors, including Sea-viewing Wide Field-of-view Sensor (SeaWiFS), Moderate Resolution Imaging Spectrometer (Aqua MODIS), Medium Resolution Imaging Spectrometer (MERIS), Suomi National Polar-orbiting Partnership Visible Infrared Imager Radiometer Suite (SNPP VIIRS) and Communication, Ocean, and Meteorological Satellite Geostationary Ocean Color Imager (COMS GOCI).  Future sensors of interest include the Sentinel 3 series Ocean and Land Color Imager (OLCI) and the Joint Polar Satellite System (JPSS) VIIRS.

NRL’s Automated Optical Processing System (AOPS) processes ocean color satellite data to provide an operational near-real time depiction of the bio-optical ocean environment.  These products are also used for validation of/or assimilation into ocean forecast models and to predict the impact of the environment on Navy coastal operations.  NRL contributes to advancements in satellite processing techniques, atmospheric correction for coastal waters, enhanced resolution optical properties using imaging bands, cloud masking, and sensor merging for optimal operational products.  Multiple satellites are necessary to provide changing conditions throughout the day allowing for detection of rapid optical temporal and spatial changes due to tides, winds, and river outflow.  The Sentinel 3A and 3B OLCIs are critical to Navy coastal operations due to the quality of the data and its AM orbit that complements the MODIS Aqua and SNPP VIIRS PM orbits. 

In order to track satellite sensor performance NRL developed a semi-automated on-orbit vicarious calibration technique fed by real-time data streams from the Coastal Ocean Color Aerosol Robotic Network (AERONET-OC) and the Marine Optical Buoy (MOBY).  All current ocean color sensors are monitored using this technique by comparing satellite radiances to in situ radiances.  The monitoring capability allows long- and short-term assessment of sensor stability and defines the uncertainty of products in coastal and open ocean waters resulting in consistent product generation from each of the sensors.  Additional NRL coastal in situ and cruise of opportunity measurements that complement the AERONET-OC and MOBY data are also analyzed.

Sensor characterization is critical in order to use the end products for Navy support because of the low total radiance signal acquired by satellite sensors at the top of the atmosphere.  Errors in sensor calibration, out-of-spectral band response, polarization and atmospheric correction are considered and compensated so that operational products among sensors will be accurate and consistent.  New sensors become operational more quickly using this capability.    

The Sentinel missions have been designed to support the operational services of Copernicus program, ensuring long-term availability in a wide range of spectral, spatial and temporal resolutions. In particular, Sentinel-2 (S-2) data with the improved features of high spatial resolution and high revisit frequency (5 days with the pair of satellites in operation) will play a fundamental role in recording the land cover types and monitoring the land cover changes in regular time intervals. Nevertheless, there can be limitations to the generation of high-level operational products, such as land-change detection maps and geophysical variables, when cloud free images are not available for a certain period. Cloud coverage is usually the main problem of the optical remotely sensed data, hindering the time series availability and consequently the constant land surface monitoring. In an attempt to face this inefficiency, the synergistic use of Sentinel-2 MSI and Sentinel-3 OLCI instruments is investigated in this research and mainly its prospective contribution to consistent retrieval of biophysical parameters and improvement of land cover and land change products.

To that end, an unmixing-based model is proposed with the aim to integrate in one composite image the benefits of the two Sentinel missions. In essence, the main goal is to fill the gaps in S-2 time series, based on the more frequent information of S-3 time series. The unmixing process is applied on a sliding-window, where the central low spatial resolution (S-3 like data) pixel is unmixed based on the information of all the pixels in the window. The fusion output is resulted by implementing three basic steps: the fuzzy clustering of the high spatial resolution image (S-2 like data), the calculation of the pure spectra (endmember) for every class and the restoration of the unmixed image pixel. Temporal weights are introduced for the efficient handling of the temporal variability between the images and fuzzy clustering is implemented for the preservation of the pixels’ spectral variability in the analysed window. The resulting time-series fused products, with the spatial resolution of S-2 and the temporal as well as the spectral resolution of S-3 bands, are intended to be the input data for land monitoring applications in a recurrent temporal framework. The proposed fusion model has been applied on MODIS (MOD09GA L2G) and SPOT4 (Take 5) data and the experimental results have demonstrated the high potential of the approach, though some improvements are required for the effective analysis of the changed areas through the study period.   

As part of the Sentinel-3 mission and in order to ensure the highest quality of products, ESA in cooperation with EUMETSAT has set up the Sentinel-3 Mission Performance Centre (S-3 MPC). This facility is part of the Payload Data Ground Segment (PDGS) and aims at controlling the quality of all generated products, from L0 to L2. The S-3 MPC is composed of a Coordinating Centre (CC), where the core infrastructure is hosted, which is in charge of the main routine activities (especially the quality control of data) and the overall service management. Expert Support Laboratories (ESLs) are involved in calibration and validation activities and provide specific assessment of the products (e.g., analysis of trends, ad hoc analysis of anomalies, etc.). The S-3 MPC interacts with the Processing Archiving Centres (PACs) and the Marine centre at EUMETSAT.

The S-3 MPC service contract is currently carried out by a consortium composed of 23 partners and led by ACRI-ST, France. The S-3 MPC contract was kick-offed in September 2014 with a first set-up phase of 12 months. After the launch of S3-A (planned before end of 2015), the S-3 MPC will start its second phase to support commissioning activities. Then a routine operation phase of up to 5 years will begin, including the commissioning activities related to S3-B when this unit will be launched.

Though all instruments and products of the S-3 mission are in the scope of the S-3 MPC, a certain number of activities are specific to the Optical mission, which includes OLCI, SLSTR and SYN products, some other are specific to the Surface Topography Mission (STM), which addresses validation of topography products over all surfaces.

In the frame of the S-3 MPC, 2 types of activities will be done:

Routine Quality Control, done by dedicated operators based at the MPC/CC; for the optical mission, there will be 3 operators who will monitor the quality of OLCI, SLSTR and SYN products; for STM, one operator will be based at CLS which host the MPC/CC secondary centre

Detailed analysis and specific expertise done by the ESLs (Expert Support Laboratories) based in their own institutes but closely linked to the MPC/CC; 14 ESLs dedicated to the optical mission and 7 ESLs dedicated to STM are part of the S-3 MPC consortium

The role of ESLs is essential as they are responsible for the Instrument and Level 1 processing performance and configuration and for the Level 2 processing and configuration, as well as products validation. More specifically, they are in charge of:

Maintaining the Cal/Val and Cal/Val Implementation plans

Supporting the Satellite Commissioning Team during the S3-A commissioning, get ready and perform S3-B commissioning

Ensuring the Cal/Val Implementation Plan execution, while synthetizing results, and  reporting on achieved performances

Proposing sensor and processing baseline evolutions (algorithms & database)

Performing anomaly investigations 

The team has been sized to tackle the specificities and the challenges of the optical mission which can be briefly summarised:

The scope of activities is wide due to the size of the mission, with 2 instruments and tens of products to calibrate and validate;

Sentinel-3 is an innovative mission, designed to better fit with the wide spectrum of users’ expectation in terms of accuracy, timeliness and reliability of measurements: if for OLCI, we rely on a strong MERIS background, the Synergy products are new and the SLSTR instrument is radically different from AATSR, leading to specific and new calibration activities;

The high number of ESLs, necessary to cover all aspects of the optical mission, has led to an organisation facilitating cross-cutting discussions and activities between OLCI and SLSTR, as well as identifying synergies between the 2 instruments;

On the STM side, a high qualified team is ready to address the different issues of Sentinal-3 STM payload:

Quality Control activities

The calibration, characterisation and performance of the altimeter (SRAL) and the microwave radiometer (MWR) sensors

Validation of the products and ground processing for topography observations over all the different surfaces (ocean, sea ice, coastal, river and lakes, land ice sheet), but also for wind and wave data.

Assessment of the mission performance, both at global and regional scales, based on comparison with other altimeters  and with in-situ data

Support for the continuous improvement of the S-3 STM performance taking into account both operational needs from the Copernicus Service and needs expressed by the scientific community.

During the commissioning phase, the S-3 MPC will progressively start its activities of control and validation of PDGS products. Meanwhile, some partners are involved in phase E1 activities done by the ESTEC teams, which should ensure a seamless transition towards the routine operations phase.

The MPC/CC and the ESLs exchange products, calibration data and reports. The main tool operated at the MPC/CC is the S-3 Mission Performance Monitoring Facility (MPMF), complemented by plug-ins, and by any other additional tools and facilities, software and hardware required to perform the mission performance activities.

Due to the high volume of data which must be exchanged and in order to facilitate the analysis to be performed by the expert scientists, an innovative facility is being implemented as part of the MPC/CC. We propose to all ESLs to use a collaborative platform which is a secured IT environment mixing hardware and software elements enabling users to work remotely. The main benefit is that users do not need to download huge amount of data by performing their processing and analysis where the products are located. Moreover, some specific optical tools, such as the S3 toolbox, ODESA or MERMAID (adapted to OLCI) will be made available. First tests of the platform have been successfully done in last December.

The Atlantic Meridional Transect (AMT) is a yearly research expedition that sails over 100 degrees of latitude in the Atlantic ocean from the UK to the tip of South America. Since its inception in 1995, AMT has had a strong focus on collecting bio-optical measurements in support of satellite calibration/validation activities. Core bio-optical measurements include underway and in-situ inherent optical properties, underway above-water remote-sensing reflectance, as well discrete samples for coloured dissolved organic matter, phytoplankton pigment analysis by high performance liquid chromatography, particulate organic carbon, total suspended solids, and net primary production. In this presentation we will describe the methods employed for collecting and quality-controlling optical data, as well as those adopted for characterizing and reducing uncertainties. We will also present and discuss recent comparisons between AMT measurements and ESA Ocean Colour Climate Change Initiative products.

In the framework of the eSurge project, funded by ESA, the capabilities of altimeter and scatterometer data to improve storm surge forecasts made by a hydrodynamic model were tested. The model is based on the finite element technique and was set-up to reproduce storm surge events in the Mediterranean Sea, with a focus on Adriatic. Wind and pressure analysis fields from ECMWF atmospheric model are used as forcing and the modelled sea level (i.e., the surge) is compared with the surge computed from observations near Venice, in the northern Adriatic Sea. Satellite data are tested in 18 cases of extreme storm surge events, recorded in Venice from 2004 to 2012. Scatterometer wind data are used to correct the wind speed, which is often underestimated in Adriatic, with a different correction depending on the last available scatterometer data. Altimeter TWLE without tide is assimilated by means of a dual 4d-var technique. Results show the two methods are perfectly compatible. The wind correction brings improvements in all the cases analyzed, reducing the RMS error on the maximum surge estimation from 16 cm to 10 cm. Altimeter assimilation needs improvements but still reduces the error to 14 cm. The combined application of both methodologies is able to reduce the RMS error to 9 cm, which is a 44% error reduction with respect to the base simulations.