Funcionamiento de mareógrafos modernos: hacia la precisión milimétrica

Considerable efforts are being made worldwide to upgrade tide gauge networks using new technologies. Because of the unique location of the Kerguelen Islands, the measurement of sea level there has received particular attention, with up to four systems equipped with modern sensors functioning simultaneously (two pressure tide gauges, a radar tide gauge, and a GPS-equipped buoy). We analysed and compared the sea level data obtained with these systems from 2003 to 2010, together with a time series of tide pole observations. This is the first time that a multi-comparison study with tide gauges has been undertaken over such a long time span and that the stability of modern radar tide gauges has been examined. The multi-comparison enabled us to evaluate the performance of the tide gauges in several frequency ranges, identify errors and estimate their magnitude. The drift of the pressure sensors (up to 8.0 mm/yr) was found to be one of the most relevant sources of systematic error. Other sources of difference such as clock drift, scale error and different locations of the instruments were also detected. After correcting the time series of sea level for these errors we estimated an upper bound for the radar instrumental error in field condition at ~0.3 cm.Actualmente se están realizando muchos esfuerzos para renovar las redes maregráficas utilizando nuevas tecnologías. En este contexto, la monitorización del nivel del mar en las islas Kerguelen ha recibido una atención particular debido a su localización única. Hasta cuatro equipos han realizado medidas simultáneamente: dos mareógrafos de presión, un mareógrafo radar y una boya GPS. En este trabajo se analizan y comparan los datos obtenidos con dichos equipos desde 2003 hasta 2010, complementándolos con observaciones realizadas con una escala de marea. Es la primera vez que se plantea una comparación de estas características durante un periodo de tiempo tan largo, y que se aborda el estudio de la estabilidad de los mareógrafos radar a largo plazo. La comparación permitió evaluar el comportamiento de los mareógrafos en distintos rangos de frecuencia, identificar errores y estimar su magnitud. La deriva del sensor de presión apareció como la fuente de error más relevante (hasta 8 mm/año). También se detectaron otras fuentes de diferencias como derivas en el reloj, el error de escala o la diferente localización de los instrumentos. Tras corregir esos errores fue posible estimar un límite superior de ~0.3 cm para el error instrumental del radar en condiciones de campo

Measuring Sea Level with GPS-Equipped Buoys: A Multi-Instruments Experiment at Aix Island

Measuring sea-level in a global reference frame with sub-centimeter accuracy is a relevant challenge in the context of current global warming and associated sea-level rise. Global Navigation Satellite Systems (GNSS) can provide sea-level measurements directly referenced in an absolute geocentric frame. We present here the results of a multi-instruments experiment with three buoys equipped with Global Positioning System (GPS), a radar tide gauge and a tide pole. This experiment was carried out at Aix Island (West coast of France) on the 27-28 March 2012. The GPS buoys were evaluated against conventional tide gauge measurements through a Van de Casteele test. The Root Mean Square Error (RMSE) computed from the difference between the GPS-buoys and radar tide gauge data ranges from 1 cm to 2.2 cm, which is suitable for tidal applications and offers interesting perspectives for future sea-level variations studies.La medición del nivel del mar en un marco de referencias globales con una precisión subcentimétrica es un desafío importante en el contexto del calentamiento mundial actual y del aumento del nivel del mar asociado al mismo. Los Sistemas Mundiales de Navegación por Satélite (GNSS) pueden proporcionar medidas del nivel del mar directamente referenciadas en una estructura geocéntrica absoluta. Presentamos aquí los resultados de un experimento multi-instrumentos con tres boyas equipadas de un Sistema de Posiciona-miento Global (GPS), un mareógrafo con sistema de radar y una escala de mareas. Este experimento fue llevado a cabo en la Isla de Aix (Costa Occidental de Francia), los días 27 y 28 de Marzo del 2012. Las boyas GPS fueron evaluadas comparándolas con las medidas de los mareógrafos convencionales mediante un test Van de Casteele. El Error Cuadrático Medio (RMSE) calculado a partir de la diferencia entre los datos de las boyas GPS y los datos de mareógrafo, oscila de 1 a 2,2 cm, lo que es apropiado para las aplicaciones de mareas y ofrece perspectivas interesantes para futuros estudios de variaciones del nivel del mar.La mesure du niveau de la mer dans un référentiel mondial avec une précision sub-centimétrique est un défi pertinent dans le contexte actuel du réchauffement climatique et de l’élévation du niveau des mers qui en résulte. Les systèmes mondiaux de navigation par satellite (GNSS) peuvent fournir des mesures du niveau de la mer directement rapportées à un référentiel géocentrique absolu. Nous présentons ici les résultats d’une expérience multi-instruments avec trois bouées équipées d’un système de positionnement par satellite (GPS), un marégraphe à radar et une échelle de marée. Cette expérience a été effectuée à l’île d’Aix (côte ouest de la France) les 27 et 28 mars 2012. Les bouées GPS ont été éva-luées par rapport aux mesures du marégraphe conventionnel au moyen d’un test de Van de Casteele. L’erreur quadratique moyenne (RMSE) calculée à partir de la différence entre les données des bouées GPS et celles du marégraphe radar est comprise entre 1 cm et 2,2 cm, ce qui convient pour les applications marégraphique et offre d’intéressantes pers-pectives pour les futures études des variations du niveau de la mer

Doped TiO2 aerogels as alternative catalyst supports for proton exchange membrane fuel cells: A comparative study of Nb, V and Ta dopants

International audienceNb, Ta and V-doped TiO2 aerogels and xerogels have been synthesized as possible new alternatives to carbon blacks for Proton Exchange Membrane Fuel Cells catalyst supports. A comparative study of different dopants was realized in a single study. Nb, Ta and V showed different behaviors with respect to the final material structure and morphology, composition and electronic conductivity. They are all prone to surface segregation, to different extents. V-doped TiO2 apart, the rutile structure could only be obtained after calcination in a reducing atmosphere at 800 °C for Nb or Ta-doped TiO2. The electronic conductivity exhibited a maximum at 10 at.% for Nb and Ta, 5 at.% for V. Nb revealed to be the most appropriate dopant to increase the electronic conductivity of TiO2, followed by Ta and V. 4 to 5 orders of magnitude were gained after Nb doping for xerogels conductivity to reach almost 0.1 S cm−1. The role of point defects was discussed to account for phase transition and evolution of conductivity

New modeling of the Vostok ice flow line and implication for the glaciological chronology of the Vostok ice core

International audienceWe have used new spaceborne (elevation) and airborne (ice thickness) data to constrain a 2D1/2 model of snow accumulation and ice flow along the Ridge B‐Vostok station ice flow line (East Antarctica). We show that new evaluations of the ice flow line geometry (from the surface elevation), ice thickness (from low‐frequency radar data), and basal melting and sliding change significantly the chronology of the Vostok ice core. This new Vostok dating model reconciles orbital and glaciological timescales and is in good agreement with the Dome Fuji glaciological timescale. At the same time, the new model shows significantly older ages than the previous GT4 timescale for the last glacial part, being thus in better agreement with the GRIP and GISP2 chronologies

Extension of high temporal resolution sea level time series at Socoa (Saint Jean-de-Luz, France) back to 1875

In this data paper sea level time series at Socoa (Saint Jean-de-Luz, Southwestern France) is extended in a data archaeology exercise. We have catalogued water level records stored in ledgers and charts, as well as other associated documents (metadata) in thorough research of national and local archives. An extensive effort was made to rescue these documents by archiving them in digital formats. Based on this large set of rescued documents, the Socoa time series is further extended back in time by about 40 years, at hourly (for ledgers) to 5-minutes (for charts) sampling. Analysis of the precise levelling information reveals that the datum of the tide gauge site has been stable. We assessed the consistency of this new century-long time series based on nearby tide gauge data. Although the overall timeseries is generally consistent, siltation is found to be a recurrent problem of the stilling well which impacted some part of the extended data. However, being a high temporal resolution sea level time series spanning more than 100 years, this new dataset will be useful for advancing climate research, particularly the decadal scale variations in the North Atlantic, as well as the storminess and extreme events along the French Basque coastal region.</p

Comparative assimilation of Topex/poseidon and ERS altimetric data and of TAO temperature data in the tropical Pacific Ocean during 1994-1998, and the mean sea-surface height issue

International audienceFive years of Topex/Poseidon (T/P) and ERS sea level anomaly (SLA) data (1994–1998) are assimilated every 10 days into a primitive equation model of the tropical Pacific ocean. The data assimilation technique used here is a reduced-order Kalman filter derived from the Singular Evolutive Extended Kalman (SEEK) filter [J. Mar. Syst. 16(3–4) (1998) 323] with an error covariance matrix parameterised by a subset of multivariate 3D global empirical orthogonal functions (EOFs). The assimilation run is compared to the free run and to independent data from the TAO network. The impact of sea-surface height (SSH) assimilation on surface and subsurface temperature and currents is estimated in the equatorial band. In a second stage, temperature data from the TAO array are assimilated in the same conditions as in the first stage. The comparison between the results of the two assimilation experiments is made mainly with a view to gaining insights into the mean sea-surface height (MSSH) for the assimilation of altimeter data, and more generally, into the question of biases. XBT observations and TAO array data are then used to build a physically more consistent mean sea-surface height for assimilation of SLA data. Results from the assimilation of altimeter data referenced to this new MSSH show significant improvements

Tide gauges

Tide gauge measurements provide data for routine tidal predictions in ports as well as for extreme events such as storm surges and tsunamis. Along with satellite altimeter measurements, tide gauges also provide measurements used for sea-level rise estimates. This is particularly important for impact assessment in low-lying coastlines of south Asia as well as islands such as the Maldives in the Indian Ocea