RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 10, ES1002, doi:10.2205/2007ES000262, 2008
Exploiting satellite altimetry in coastal ocean through the ALTICORE projectS. Lebedev, A. Sirota, D. Medvedev, and S. KhlebnikovaGeophysical Center, Russian Academy of Sciences, Moscow, Russia S. Vignudelli Consiglio Nazionale delle Ricerche, Istituto di Biofisica, Area Ricerca CNR San Cataldo, Pisa, Italy H. M. Snaith, P. Cipollini, and F. Venuti National Oceanography Centre, Southampton, UK F. Lyard, J. Bouffard, J. F. Cretaux, and F. Birol Laboratoire d'Etudes Géophysique et Ocèanographie Spatiale, Toulouse Cedex 9, France L. Roblou Noveltis, Parc Technologique du Canal 2, Ramonville-Saint-Agne, France A. Kostianoy, A. Ginzburg, N. Sheremet, and E. Kuzmina P. P. Shirshov Institute of Oceanology, Moscow, Russia R. Mamedov, K. Ismatova, A. Alyev, and B. Mustafayev Institute of Geography, Center for Caspian Sea Problems, Baku, Azerbaijan Contents
Abstract[1] Altimeter-derived information on sea level and sea state could be extremely important for resolving the complex dynamics of the coastal ocean. Satellite altimetry was not originally conceived with coastal ocean in mind, but future missions (AltiKa and CryoSat-2) promise much improved nearshore capabilities. A current priority is to analyze the existing, under-exploited, 15-year global archive of coastal altimeter data to draw recommendations for these missions. There are intrinsic difficulties in processing and interpretation of the data, e.g.: the proximity of land, control by the seabed, and rapid variations due to tides and atmospheric effects. But there are also unexploited possibilities, including higher along track data rates and multi-altimetry scenarios that need to be explored. There are also difficulties of accessing and manipulating data from multiple sources, many of which undergo regular revision and enhancement. In response to these needs, the ALTICORE (ALTImetry for COastal REgions - www.alticore.eu) project started in December 2006, funded for two-years by the European INTAS scheme (www.intas.be). The overall aim of ALTICORE is to build up capacity for provision of altimeter-based information in support of coastal ocean studies in some European Seas (Mediterranean, Black, Caspian, White and Barents). ALTICORE will also contribute to improved cooperation between Europe and Eastern countries and enhance networking of capacity in the area of satellite altimetry. This paper discusses the approach, summarizes the planned work and shows how the coastal community should eventually benefit from better access to improved altimeter-derived information. Introduction[2] A number of studies [e.g., Crout, 1997] have provided a general understanding of the difficulties and challenges in interpreting altimeter-derived measurements in marginal seas such as those surrounding Europe; these studies have confirmed the great value of the altimetric products for large- and meso-scale applications. For instance, in the Mediterranean Sea, altimetry has been successfully applied at basin scale [e.g., Ayoub et al., 1998; Larnicol et al., 1995; Vignudelli, 1997] and in specific open sea regions [e.g., Buongiorno Nardelli et al., 1999; Vignudelli et al., 2000, 2003]. Comparison of satellite altimetry data with tide gauge data and hydrodynamic simulation results for the Barents and White Seas [e.g., Lebedev and Tikhonova, 2002; Lebedev et al., 2003] has shown that the remotely sensed data satisfactorily describe the hydrodynamic regime of these seas, including tides. Satellite altimetry data have allowed investigation of both mesoscale dynamics and water balances of the Black Sea [e.g., Eremeev et al., 2004; Ginzburg et al., 2003]. They are indispensable for investigation of significant seasonal and interannual variability of Caspian Sea level, including Kara Bogaz Gol Bay [e.g., Lebedev and Kostianoy, 2005]. The usefulness of standard altimetric products, however, is greatly reduced in coastal areas due to sampling issues, inaccurate corrections and other data quality problems [Anzenhofer et al., 1999]. This has motivated recent investigations into ad-hoc data screening and processing to recover valuable information near the coast, which have yielded partial but encouraging results at a specific Mediterranean site [Vignudelli et al., 2005]. [3] The coastal environment varies from the open sea in many ways. It is a region where sea conditions can vary quickly, both in time and space. Altimeter processing over these areas requires accurate knowledge of tides and of the hi-frequency atmospheric (wind and pressure) effects at the sea surface. Working next to land also poses the challenge of retrieving data flagged as unreliable but potentially recoverable after more careful, specific screening. There is clear scope for investigating the limitations of the current 1 Hz data stream and assessing the advantages and feasibility of the adoption of higher data stream rates (typically 10 Hz or 20 Hz). The standard data treatment also needs to be remodelled, minimizing the inclusion of spurious values and gaps and monitoring the processing chain from beginning to end. This need for better screening or interpolation/extrapolation of missing values does not only apply to the raw altimetric measurements, but also to the necessary corrections, that may suffer from similar (or worse) problems of land contamination and inadequate models. From a calibration and validation point of view, there is also the question of evaluating how improved averages over satellite footprints may be compared to point-wise values (in situ data) normally collected at different temporal scales; this involves non-trivial sampling and averaging issues, as well as assumptions on the local scales of variability of the altimeter-derived products. Research Objectives and Strategy[4] The central focus of the ALTICORE project is the improvement of the monitoring capabilities of satellite altimeters in the coastal region. The specific goals are: [5] (1) to define the quality standards required for altimetry products in coastal regions based on the potential applications and user requirements; [6] (2) to research new screening and processing strategies to recover data meeting the quality standards defined in (1); [7] (3) to generate an improved altimetric data stream for the Mediterranean coastlines with these methods; [8] (4) to carry out validation tests of the new product at a pilot and opportunity sites; [9] (5) to apply the validated methodology to coastal areas in the Black, Caspian, White and Barents seas; [10] (6) to setup a Grid-compliant system for data provision to modellers, forecasters and data-integrators in NIS countries; [11] (7) to promote the added-value product amongst the user community. [12] Altimetric missions in the last 15 years (TOPEX/ Poseidon, ERS 1 and 2, GFO, Envisat and Jason 1) have resulted in great advance in marine research and operational oceanography, providing accurate sea level data (at cm error level) and high-value information products (including waves and wind) for fisheries planning, ship routing and offshore operations [Cotton et al., 2004]. Satellite altimetry is now a mature technology and a routine component of operational earth observation systems. However, the utility of altimetric data near the coasts, where the impact on a number of economic activities could be significant, has been neither completely explored nor addressed from a user perspective. The main problem is that existing altimetric products are not optimised for coastal retrievals [Vignudelli et al., 2000] owing to some processing and quality control issues, for instance the fidelity of corrective terms in coastal areas and possible contamination due to the presence of land in the footprint [Lebedev and Kostianoy, 2005]. These issues are currently impeding the effective use of altimeter-derived products in coastal areas [Vignudelli et al., 2005]. This proposal aims at mitigating or removing, where possible, the obstacles to operational use of altimetry over coastal areas, with particular reference to the European seas (Mediterranean, Black, Caspian, White and Barents), through a concerted action of Western European and NIS Researchers in line with the framework of cooperation set by the INTAS initiative. We expect this project to advance coastal altimetry, from the present underexploited status, into pre-operational use with a fully implemented system for the production of quality controlled data and their dissemination, through a Grid-compliant interface, to the end-users. This will impact significantly on the use of altimetry for coastal research and coastal management. [13] The project will initially seek to improve 1 Hz data by [14] 1) analyzing the corrective terms and providing the best solutions, including those derived from proper local modelling; [15] 2) developing a set of algorithms to automate quality control and gap-filling functions; [16] 3) determining more thorough testing and validation strategies. [17] These improved products will be delivered to users using Grid technology, allowing a deep assessment of the 1 Hz data performance and limitation over a wider range of coastal conditions (e.g., land topography, waves, winds, tides, etc.). The work outlined above will provide the input to a second phase of the project, where the feasibility and advantages of extending the processing chain to higher rate streams will be investigated. This is an extension based on the fact that currently operating altimeters such as Jason 1 and Envisat missions have been designed to provide a 20 Hz data stream which lends itself to be employed for coastal-oriented processing. High rate data streams (10 Hz) were also available from Geosat, GFO and ERS satellites. However these were much noisier than Jason and Envisat so the real utility of the high rate data from these instruments will be assessed. Research Programme[18] This two-year program is specifically oriented to: a) improving satellite altimetry along the coasts of the Mediterranean, Caspian, Black, White and Barents seas and b) to allowing access to the improved data by a Grid-compliant architecture. As such, it has two complementary and interconnected components: one is the definition and development of an advanced altimeter data processing system, and the other is the design and implementation of a structure for efficient access to distributed archives of data. [19] The altimeter processing stream will be built upon the Radar Altimeter Data System (RADS) [Schrama et al., 2000] and will extend that system to address coastal user requirements and specific processing issues, by employing corrections optimized for the coastal environment and all available complementary local metocean information from various data sources (e.g., tide gauges and wave sensors; weather stations; high-resolution models; etc.). For example, regional corrective models for the atmospheric and tidal effects will be used, e.g., MOG2D, [Carrère and Lyard, 2003] for the Mediterranean Sea. Any improvement in the quality of the data will be benchmarked against independent in situ measurements. The ALBICOCCA site at Capraia Island (NW Mediterranean), for which there is a long continuity of altimetry monitoring, will be the pilot benchmark site; in addition to it, some coastal sites of opportunity will be selected on the basis of ground-based data availability from cooperative efforts like ESEAS, SELF, SONEL or national networks (e.g., Italian, Russian, Azerbaijan, etc.) in proximity of altimeter ground tracks. For instance, historical and current data on the Caspian Sea level from several coastal meteo stations in Azerbaijan [Mamedov, 1997, 2000; Mamedov et al., 1999] will be used for comparison over that basin. Finally, the processing system will be applied to the whole length of the coast in the major basins. [20] The system for efficient access to distributed archives of data will be based upon the Grid concept. Its structure will consist of regional data centres, each one with primary responsibility for maintaining its regional-archives by selecting the best corrections and ensuring quality control. Centres will operate a Grid-compliant set of web-services allowing access to the full functionality of data extraction, and a central web server will provide a simple interface to the web services to give interactive access to users. Particular attention will be devoted to the visualization and dissemination of the product to users such as modellers and forecasters, and to this effect a grid compliant application will be built and demonstrated over a number of case studies. The project will also made available a dissemination package in the form of a DVD, in a fashion similar to what the European Space Agency has done with the "River&Lake Product from Altimetry'' (ENVISAT-ERS, 2004, http://earth.esa.int/riverandlake/docs/Product-Handbook-1-2.pdf). Approach and Methodology[21] The project will adopt an approach starting from the indications of the users, which entails the following points: [22] 1) identifying and understanding the user needs in terms of problems (e.g., coastal protection from increasing sea level and/or changing wave climate), procedures (e.g., calibrate models), products (e.g., sea level; wave height) and specifications (e.g., accuracy level), on the basis of what is currently available (e.g., tide gauges; wave sensors), and highlighting how altimetry can help (e.g., better coverage); [23] 2) evaluating what can be done with the "official'' products and what are the major issues, e.g., deficiencies in existing data streams and their handling, quality controls, error components; [24] 3) determining what data processing steps need to be improved for adding value, e.g., use of available retracked products, use of 10/20 Hz data, correction updates, new processing functions; [25] 4) addressing the required validation exercises; [26] 5) going from the sensor measurement to a product, e.g., quality-controlled coastal sea level anomalies and significant wave height; [27] 6) raising the awareness of altimetry data amongst the user community via a dissemination package; [28] 7) distributing the products efficiently via a Grid-compliant portal which allows fully functional and custom extraction of optimized data to the users. [29] In methodological terms, the process will include: [30] 1) acquiring all available satellite data (1 Hz streams to start with, possibly higher rate streams later) over the regions of interest; [31] 2) compiling local data sets, including tide gauges, metocean observations and model output; [32] 3) characterizing the coastal region by taking into account the non-uniform conditions, e.g., bathymetry, land morphology, tides, wind; [33] 4) analyzing initial data capabilities, e.g., anomalies, critical factors; [34] 5) building processing chain, e.g., adjust corrections, add new or improved local corrective terms; [35] 6) defining data match-up exercises, e.g., discrepancies, confidence levels; [36] 7) building a server for the improved products. [37] This methodology has two main benefits: a) it will make coastal altimetry data of higher quality than the currently available products, and b) it will make the data immediately available to modellers and data integrators. Preliminary Results[38] With respect to planned work the following results were achieved during the first year of the project: Mediterranean Sea
Black Sea
Caspian Sea
White and Barents Seas
[44] The physical characteristics of the Barents Sea and exchange with the nearest seas have been studied. The average depth of the basin is > 200 m and the deepest point is > 600 m. The White Sea is located at the southernmost part of the Barents Sea. Many factors influence the hydrodynamic regime of these seas, including the tidal regime (up to 8 m amplitude in some places). In RADS these tides can be corrected with FES99 or GOT00 (the latter is the choice recommended by RADS authors). But both models have 0.5o grid - too coarse to properly resolve the White Sea. GC suggests that in this area we should use the HRCRF (Hydrometeorological Research Centre of the Russian Federation) tidal model. This differs from GOT by up to 4 m in some places, for instance at the entrance of the White Sea (Figure 7). Another issue in this region is the Earth's crust uplift, which can be up to 4 mm yr -1. Storm surges are also important and may reach 2 m in the strongest events. The coverage of this area by different satellites was discussed - T/P and Jason only marginally touch it. GFO is good for the White Sea, while ERS and Envisat cover all the White and Barents seas. The ice-free period is Apr/May to Oct/Nov for the White Sea. There was consensus that this is an appropriate area to show the possible improvements due to the adoption of regional tidal models as opposed to global models. Correlations of sea level from altimetry and tide gauge data can be fairly high ( > 0.9) for the Barents Sea using ERS data. For Geosat in the Barents Sea the correlations are noticeably lower, probably due to orbit errors and more inaccurate corrections. Correlations are extremely high when ERS-2 and Envisat are used in combination with two Norwegian tide gauges for which there are long, high-quality time series
Conclusion[45] The expected outcomes of the ALTICORE project will be: [46] (1) a set of quality protocols for coastal altimetry; [47] (2) a set of improved altimetric corrections, algorithms and quality control procedures, optimized for coastal targets; [48] (3) a system for the production of the improved, protocol-compliant altimetric products along the European coasts; the system will be configured as a Grid application and will include a portal for access to the improved data; [49] (4) an outreach and dissemination package, in the form of a brochure plus a DVD with demonstrative data, targeted to users such as modellers, data integrators and forecasters. Acknowledgments[50] This study was supported by the INTAS No 05-1000008-7927 Project "ALTImetry for COastal REgions'' (ALTICORE) and a series of grants of the Russian Foundation for Basic Research (06-05-64871-a, 07-05-00141-a). ReferencesAnzenhofer, M., C. K. Shum, and M. Rentsh (1999), Costal altimetry and applications, Tech. Rep., no. 464, Geodetic Science and Surveying, 40 pp., The Ohio State University, Columbus, USA. Ayoub, N., P. Y. Le Traon, and P. De Mey (1998), A description of the Mediterranean surface variable circulation from combined ERS-1 and TOPEX/Poseidon altimeter data, J. Mar. Syst., 18, 3, doi:10.1016/S0924-7963(98)80004-3. [CrossRef] Buongiorno Nardelli, B., R. Santoleri, S. Marullo, D. Iudicone, and S. Zoffoli (1999), Altimetric sea level anomalies and three-dimensional structure of the sea in the Channel of Sicily, J. Geophys. Res., 104, 20,585, doi:10.1029/1999JC900103. [CrossRef] Carrère, L., and F. Lyard (2003), Modeling the barotropic response of the global ocean to atmospheric wind and pressure forcing - comparisons with observations, Geophys. Res. Lett., 30, (6), 1275, doi:10.1029/2002GL016473. [CrossRef] Cotton, D., T. Allan, Y. Menard, P. Y. le Traon, L. Cavaleri, E. Doombos, and P. Challenor (2004), Global Altimeter Measurements By Leading Europeans, Requirements for future satellite altimetry, Tech. Rep. European Project EVR1-CT2001-20009, 47 pp., Brussels, Belgium. Crout, R. L. (1997), Coastal currents from satellite altimetry, Sea Technology, 8, 33. Eremeev, V. N., G. K. Korotaev, and L. N. Radaikina (2004), Monitoring of the Black Sea dynamics based on satellite technologies, Physical Oceanography, 14, (2), 114, doi:10.1023/B:POCE.0000037874.11966.dc. [CrossRef] Ginzburg, A. I., A. G. Kostianoy, and N. A. Sheremet (2003), Mesoscale variability of the Black Sea as revealed from TOPEX/POSEIDON and ERS-2 altimeter data, Issledovaniya Zemli iz Kosmosa (in Russian), 3, 34. Larnicol, G., P. Y. Le Traon, N. Ayoub, and P. De Mey (1995), Mean sea level and surface circulation variability of the Mediterranean Sea from 2 years of TOPEX/POSEIDON altimetry, J. Geophys. Res., 100, 25,163, doi:10.1029/95JC01961. [CrossRef] Lebedev, S. A., and O. V. Tikhonova (2002), Application of satellite altimetry for investigation of sea level variation of the southeastern Barents Sea, in: Proceedings of 4th International Scientific and Technical Conference "Modern Methods and Technology of Oceanologic Researches'' November, 2002, vol. 2 (in Russian), p. 58, IO RAS, Moscow. Lebedev, S. A., O. Zilberstein, S. Popov, and O. Tikhonova (2003), Analysis of temporal sea level variation in the Barents and the White Seas from altimetry, tide gauges and hydrodynamic simulation, in: International Workshop on Satellite Altimetry, IAG Symposia, vol. 126, edited by C. Hwang, C. K. Shum, J. C. Li, p. 243-250, Springer Verlag, Berlin, Heidelberg. Lebedev, S. A., and A. G. Kostianoy (2005), Satellite Altimetry of the Caspian Sea (in Russian), 366 pp., Sea Publ., Moscow. Mamedov, R. M. (1997), Long-Term prognosis of the Caspian Sea level, Proceedings of Regional Workshop on Integrated Coastal Zone Management (ICZM), Chabahar-I.R. Iran (February 24-29 1996), 79 pp., INCO Publication, Iran. Mamedov, R. M. (2000), Changing Caspian Sea level and assessment of the Azerbaijan coastal zone vulnerability, Int. Symp. on "Integrated Water Resources Management'', USA 9-12 April 2000, 84 pp., University of California, Davis. Mamedov, R. M., L. I. Kulizade, and Y. V. Hadiyev (1999), Impact of climate anomalies on the level of Caspian Sea, Proc. of the Second International Conference on Climate and Water, vol. 2, Espoo, Finland, 17-20 August 1998, edited by J. C. I. Dooge, E. Kuusisto and R. A. Feddes, 972 pp., UNESCO, Paris. Schrama, E., R. Scharroo, and M. Naeije (2000), Radar altimeter database system (RADS): Toward a generic multi-satellite altimeter database system., Final Report, USP-2, 88 pp., SRON/BCRS, Delft, Netherlands. Vignudelli, S. (1997), Analysis of ERS-1 altimeter collinear passes in the Mediterranean Sea during 1992-1993, Int. J. Remote Sens., 18, 573, doi:10.1080/014311697218953. [CrossRef] Vignudelli, S., P. Cipollini, M. Astraldi, G. P. Gasparini, and G. M. R. Manzella (2000), Integrated use of altimeter and in situ data for understanding the water exchanges between the Tyrrhenian and Ligurian Seas, J. Geophys. Res., 105, 19,649, doi:10.1029/2000JC900083. [CrossRef] Vignudelli, S., P. Cipollini, F. Reseghetti, G. Fusco, G. Gasparini, and G. M. R. Manzella (2003), Comparison between XBT data and TOPEX/Poseidon satellite altimetry in the Ligurian-Tyrrhenian area, Ann. Geophys., 21, 123. Vignudelli, S., P. Cipollini, L. Roblou, F. Lyard, G. Gasparini, G. M. R. Manzella, and M. Astraldi (2005), Improved satellite altimetry in coastal systems: case study of the Corsica Channel (Mediterranean Sea), Geophys. Res. Lett., 32, L07608, doi:10.1029/2005GL022602. [CrossRef] Received 21 November 2007; accepted 27 December 2007; published 24 January 2008. Keywords: satellite altimetry, coastal ocean, ALTICORE project. Index Terms: 1240 Geodesy and Gravity: Satellite geodesy: results; 1241 Geodesy and Gravity: Satellite geodesy: technical issues; 1641 Global Change: Sea level change; 4217 Oceanography: General: Coastal processes. ![]() Citation: 2008), Exploiting satellite altimetry in coastal ocean through the ALTICORE project, Russ. J. Earth Sci., 10, ES1002, doi:10.2205/2007ES000262. (Copyright 2008 by the Russian Journal of Earth SciencesPowered by TeXWeb (Win32, v.2.0). |