Estimation des biais et précisions des marégraphes par combinaison de plusieurs séries temporelles co-localisées.

International audienc

宋代三級行政体制の形成 : 元豊帳法の分析から(東洋史部会)(平成一五年度早稲田大学史学会大会)

International audienceSea level extremes and their temporal variability have been explored based on the hourly measurements at Marseille tide gauge for the period 1885–2008. A careful quality check has first been applied to the observations to ensure consistency of the record by eliminating outliers and datum shifts. Yearly percentiles have been used to investigate long-term trends of extremes revealing that secular variations in extremes are linked to mean sea level changes. The associated decadal changes show discrepancies between mean sea level trend and extreme fluctuations, due to the influence of the atmospheric forcing. A local regression model based on the generalized Pareto distribution has been applied to derive trends in return levels. The 50-years return levels reach values between 80 and 120 cm. The most significant changes in return levels are characterized by an increase since the 1970s

Sea level changes at Tenerife Island (NE Tropical Atlantic) since 1927

Hourly sea level observations measured by five tide gauges at Santa Cruz harbor (Tenerife Island), in the Northeastern Tropical Atlantic, have been merged to build a consistent and almost continuous sea level record starting in 1927. Datum continuity was ensured using high precision leveling information. The time series underwent a detailed quality control in order to remove outliers, time drifts, and datum shifts. The resulting sea level record was then used to describe the low frequency (interannual to decadal) sea level variability at Tenerife. It was found that at interannual and longer time scales, the observed sea level changes are primarily driven by steric sea level variations. Such steric changes are originated by coastal trapped waves induced by longshore winds along the continental coast and propagate poleward. Observed sea level rise at Tenerife was 2.09?±?0.04 mm/yr since 1927. According to the hydrographic observations in the area, only half of this trend was attributed to steric sea level changes for the top 500 m, at least since 1950

Rattachement géodésique des marégraphes dans un système de référence mondial par techniques de géodésie spatiale

The study of the Earth's climate system shows that its modeling is complex. The complexity of the models is not so much related to the diversity of the physical processes considered as to their complicated mathematical formulation. In this context, the sea level is of scientific interest as a climatic indicator. Sea level data are varied: geomorphological, archaeological, isotopic, and so on. The review shows that each is of interest in studying the causes of long-term sea-level variation. Many authors have analyzed tide gauge data to estimate secular changes in sea levels and study their relationship to climate change. We conducted as critical a review as possible of these studies, while again processing the PSMSL data. We have identified the problems inherent in these measures. Conscious of these problems, we have estimated a value of the recent secular trend of the sea level: (1.6 0.3) mm / year. We examined the data of the century-old tide gauge of Marseilles. The analysis of calibrations shows that the regular settings of the instrumental reference are essential to guard against a significant drift in the problematic of interest. In the absence of an absolutely stable altimetric reference, any attempt to precisely determine eustatic variations from tide gauges seems uncertain. After a panorama of vertical references, we devoted ourselves to studying the quality of the terrestrial reference system established by IERS. The results are encouraging, but still insufficient here. The basic elements of the geodesic attachment of tide gauges are studied, distinguishing the different independent quantities. The success of the synergistic process lies in recent advances in space-based radio positioning techniques. Within each link in the chain of the process, the sources of systematic error are numerous and varied. Neglect of one of the magnitudes of the synergistic process affects the results at the end of the chain. Experts in each area of ​​the process must intervene. It is on this observation that our SONEL project was built.L'étude du système climatique de la Terre montre que sa modélisation est complexe. La complexité des modèles n'est pas tant liée à la diversité aux processus physiques considérés, qu'à leur formulation mathématique compliquée. Dans ce cadre, le niveau de la mer présente un intérêt scientifique certain comme un indicateur climatique. Les données sur le niveau de la mer sont variées : géomorphologiques, archéologiques, isotopiques, etc. La revue montre que chacune présente un intérêt dans l'étude des causes de variation du niveau des mers à long terme. De nombreux auteurs ont analysé les données des marégraphes afin d'estimer les variations séculaires du niveau des mers et étudier leur relation avec le changement climatique. Nous avons effectué une analyse aussi critique que possible de ces études, tout en traitant à nouveau les données du PSMSL. Nous avons dégagé les problèmes inhérents à ces mesures. Conscients de ces problèmes, nous avons estimé une valeur de la tendance séculaire récente du niveau des mers : (1.6 0.3) mm/an. Nous avons examiné les données du marégraphe centenaire de Marseille. L’analyse des étalonnages montre que les calages réguliers de la référence instrumentale sont indispensables pour se prémunir d'une dérive non négligeable dans la problématique qui nous intéresse. Faute d'une référence altimétrique stable de façon absolue, toute tentative de détermination précise des variations eustatiques à partir des marégraphes semble incertaine. Après un panorama des références verticales, nous nous sommes consacrés à étudier la qualité du système de référence terrestre établi par l'IERS. Les résultats sont encourageants, mais encore insuffisants ici. Les éléments de base du rattachement géodésique des marégraphes sont étudiés, en distinguant les différentes grandeurs indépendantes. Le succès du processus synergique réside dans les progrès récents réalisés dans les techniques spatiales radioélectriques de positionnement. Au sein de chaque maillon de la chaine du processus les sources d'erreur systématiques sont nombreuses et variées. Une négligence sur l'une des grandeurs du processus synergique affecte les résultats au bout de la chaine. Des experts en chaque domaine du processus doivent intervenir. C’est sur ce constat que s'est bâti notre projet SONEL

Vertical land motion as a key to understanding sea level change and variability

International audienceVertical land motions are a key element in understanding how sea levels have changed over the past century and how future sea levels may impact coastal areas. Ideally, to be useful in long-term sea level studies, vertical land motion should be determined with standard errors that are 1 order of magnitude lower than the contemporary climate signals of 1 to 3 mm/yr observed on average in sea level records, either using tide gauges or satellites. This metrological requirement constitutes a challenge in geodesy. Here we review the most successful instrumental methods that have been used to determine vertical displacements at the Earth’s surface, so that the objectives of understanding and anticipating sea levels can be addressed adequately in terms of accuracy. In this respect, the required level of uncertainty is examined in two case studies (global and local). A special focus is given to the use of the Global Positioning System (GPS) and to the combination of satellite radar altimetry with tide gauge data. We update previous data analyses and assess the quality of global satellite altimetry products available to the users for coastal applications. Despite recent advances, a near-plateau level of accuracy has been reached. The major limitation is the realization of the terrestrial reference frame, whose physical parameters, the origin and the scale factor, are beyond the scope of a unique technique such as the GPS. Additional practical but nonetheless important issues are associated with the installation of GPS antennas, such as ensuring that there is no unknown differential vertical motion with the tide gauge

Land motion estimates from GPS at tide gauges: a geophysical evaluation

International audienceSpace geodesy applications have mainly been limited to horizontal deformations due to a number of restrictions on the vertical component accuracy. Monitoring vertical land motion is nonetheless of crucial interest in observations of long-term sea level change or postglacial rebound measurements. Here, we present a global vertical velocity field obtained with more than 200 permanent GPS stations, most of them colocated with tide gauges (TGs). We used a state of the art, homogeneous processing strategy to ensure that the reference frame was stable throughout the observation period of almost 10 yr. We associate realistic uncertainties to our vertical rates, taking into account the time-correlation noise in the time-series. The results are compared with two independent geophysical vertical velocity fields: (1) vertical velocity estimates using long-term TG records and (2) postglacial model predictions from the ICE-5G (VM2) adjustment. The quantitative agreement of the GPS vertical velocities with the ‘internal estimates’ of vertical displacements using the TG record is very good, with a mean difference of −0.13 ± 1.64 mm yr−1 on more than 100 sites. For 84 per cent of the GPS stations considered, the vertical velocity is confirmed by the TG estimate to within 2 mm yr−1. The overall agreement with the glacial isostatic adjustment (GIA) model is good, with discrepancy patterns related either to a local misfit of the model or to active tectonics. For 72 per cent of the sites considered, the predictions of the GIA model agree with the GPS results to within two standard deviations. Most of the GPS velocities showing discrepancies with respect to the predictions of the GIA model are, however, consistent with previously published space geodesy results. We, in turn, confirm the value of 1.8 ± 0.5 mm yr−1 for the 20th century average global sea level rise, and conclude that GPS is now a robust tool for vertical land motion monitoring which is accurate at least at 1 mm yr−1