Scientific studies and operational applications of SARAL/AltiKa
SARAL/AltiKa's secondary objectives include the monitoring of the main continental water levels (lakes, rivers, enclosed seas), the monitoring of large-scale sea level variations, the observation of polar oceans (thanks to the high inclination of its orbit), the analysis and forecast of wave and wind ﬁelds, the study of continental ice (thanks to the lower penetration in Ka-band) and sea ice, the access to low rains climatology (enabled in counterpart to the sensitivity of Ka-band to clouds and low rains) and the marine biogeochemistry (notably through the role of the meso- and submeso-scale physics).
Calibration and validation investigations have shown that the quality of the data meets the expectations and initial mission requirements. Data have been delivered to users in a very rapid manner. The quality of all products is in line with mission requirements. Many scientiﬁc investigations have been undertaken and a large part of the scientiﬁc community has quickly seized the opportunity offered by these data. This is exemplified by the special Issue dedicated to SARAL/AltiKa (The SARAL/AltiKa Satellite Altimetry Mission, Volume 38, Supp. 1, Marine Geodesy, 2015). These activities were widely realized within the framework of the previous OSTST program selected in 2012 under the title "Scientific Studies for the SARAL/AltiKa Ka-band satellite altimetric mission".
Nowadays, the SARAL/AltiKa Ka-band altimetric mission has taken a full position in the altimetric satellite constellation that has been built over years providing a major push to oceanographic sciences. It is likely that innovations brought by the Ka-band may bring some opportunities to understand Ku better. Extended capabilities that are offered by the Ka-band allow to open even more widely some new frontiers of altimetry such as coastal oceanography, cryosphere, hydrology, beyond the traditional scope of the open ocean investigations. Ka-band altimetry, with SARAL/AltiKa as the today's most emblematic declination, can be seen also as a step towards improved resolution altimetry and a preparation for the SWOT NASA-CNES mission project.
SARAL was launched on February 25, 2013 at 12:31 UTC. Cycle #1 began on March 14, 2013. The 3 year lifetime requirement has been met in February 25, of this year 2016. At this stage, the present proposal will aim at a double objective:
The proposal gather all the main actors in the French scientific and operational community that is concerned by the use of SARAL/AltiKa data in cooperation with Indian and international colleagues. Most of them have had an important involvement in the use of SARAL/AltiKa data and published their results in the Marine Geodesy special issue.
The present project addresses 18 different scientific projects relevant of the SARAL/AltiKa scientific and operational objectives:
1- Data processing and improvements
Summary: The SARAL/AltiKa mission has already brought high-quality observations to the ocean, the cryosphere and the coastal regions; however, waveform analysis and inversion techniques of 40 Hz high resolution SARAL/AltiKa data can be used to further improve the estimates of sea surface height, sea state bias and sigma0 and more generally estimates of the height of continental water bodies, ice-sheets or icebergs.
2- Reaching kilometric scales with AltiKa radiometer
Summary: To explore, compare and assess different methods, instrumental or algorithmic, allowing to retrieve a wet tropospheric correction (WTC) and surface characteristics at scales below 10 km, with a goal at 1 km. An operational objective is to monitor and validate the radiometer measurements (classical objective, not detailed here)
3- Data improvements and observability in the prospect of SWOT
Summary: The goal of this work is to analyze the small scale content measured by AltiKa and to investigate the nature of the new signals (e.g. internal tides, coastal currents) that become -in theory- accessible thanks to the recent development of new altimetry processing, and the unprecedented precision of SARAL/AltiKa.
4- SARAL/AltiKa data into AVISO/DUACS
Summary: We propose to carry out activities to optimize the use of SARAL data in the DUACS (Data Unification and Altimeter Combination System) system to improve the along track and gridded multimission products.
5- Ka-band sea state bias (SSB) analysis
Summary: To provide the scientific underpinning for production of the best possible satellite-derived ocean surface topography data sets by improving the sea state bias estimate and physical background
6- AltiKest: AltiKa for estuaries and near-shore ocean
Summary: We propose to add value to the AltiKa archive across the continent-to-ocean hydraulic continuum of Bengal delta, to retrieve the signature of tides and river discharge on water level variability.
7- Connections between open-ocean and coastal sea level in the Northern Indian Ocean
Summary: This subproject will take advantage of the SARAL-AltiKa good performance in coastal regions to explore how large-scale sea level variability in the Indian Ocean transmit signals to the coastal regions (shelf and shelf break) over the entire Northern Indian Ocean.
8- Coastal sea level - Mediterranean Sea
Summary: SARAL/AltiKa’s high quality observations in the coastal ocean will be leveraged with two objectives: 1) extend previous scientific studies by coupling multi-mission altimetry and in-situ data with high-resolution numerical modeling over the NW Mediterranean Sea and 2) analyse the coastal sea level change at regional scale.
9- Meso-scale Activity around New Caledonia: from AltiKa to SWOT (MANCAS)
Summary: We propose to analyze the in situ observations collected in the framework of the 2012-2015 CNES-TOSCA AltiGlidEx project, along with more recent altimetric data (AltiKa, Sentinel-3), to document the properties of the mesoscale and sub-mesoscale variability around New Caledonia.
10- Assimilation in coastal and regional wave models
Summary: SARAL/AltiKa plays an important role in the operational wave system at Météo-france for global and regional scales. In previous work (Aouf et al. 2014) it has been shown that the assimilation of SARAL/AltiKa in regional models MFWAM induces a significant impact on the estimated wave heights, in particular under storm conditions. In shallow water the impact for small range of wave heights was also well identified (great lakes, Java sea and Japan sea). In this research proposal we will focus on the assimilation of SARAL/AltiKa from global to coastal areas.
11- Data assimilation of AltiKa SLA in a global operational ocean analysis and forecast system
Summary: We will assess the role of SARAL/AltiKa in constraining the open ocean variability of a 1/12° real time global ocean analysis and forecasting system within the Mercator operational system.
12- Wave period retrieval/estimation from SARAL Ka band altimeter
Summary: Retrieval of wave period from SARAL/AltiKa wave data, comparison with different models of wave period and intercomparison of data of Ku- and Ka-band altimeters.
13- Assessment of fine scales SSH Statistics in the North Atlantic and in the Solomon Sea, observability in the prospect of SWOT.
Summary: The purpose of this activity is to assess fine scale (1-100km) SSH fields resolved by the ocean models used in preparation for SWOT mission with innovative statistical metrics based on SARAL/AltiKa products and future delay Doppler/SAR altimetry products (Sentinel-3).
Summary: With the new drifting orbit, SARAL measurements are not collected at fixed locations, which is a major drawback for the dissemination of the water level series, or for the prediction of other hydrological variables from the stage values, such as discharge (predicted from stage from a polynomial relationship). The purpose of this project is to relocate the water level of river free surface gained by SARAL at fixed positions in order that the time series can be appended to the previous series on the 35 day orbit flown by SARAL in its first phase mission. Relocating of the measurements will be based on seasonal profiles of the free surface slope. These slopes will be estimated by combination of satellite altimetry (for an absolute long wavelength reference) and DEM such as SRTM (for the short wavelength changes in elevation).
15- Lake studies
Summary: Establishment of a global lake database merging contours inferred from existing database (GLWD, GLOBAWO) and altimetry data from the drifting orbit of SARAL/AltiKa. It will serve as a priori onboard DEM for current and future altimetry mission and a priori lake database for the SWOT mission.
16- Continental snow and ice
Summary: SARAL/AltiKa allows the temporal survey of ice sheets and provides also an opportunity to better constraint both this error and the restitution of the surface characteristics. In the other hand, several studies have shown the interest of radar altimetry for other kind of snow and ice cover.
17- Sea ice icebergs
Summary: AltiKa combines a low penetration (because of the use of Ka band) and a high along track resolution (because of the 40 Hz rate) which already demonstrates a high potential for both icebergs studies (detection and elevation profile) ans sea ice freeboard estimates wil be further analyzed in conjonction with Icesat-2 data (for sea ice) and GPM for icebergs.
18- High-resolution geoid of the Mediterranean Sea
Summary: SARAL drifting phase data in the form of gravity anomalies or Mean Sea Surface (MSS) are used to evaluate and subsequently enhance a high-resolution geoid of the Mediterranean.