GOCE for coastal ocean dynamics and geodesy

ESAs GOCE satellite orbited the Earth between 2009 and 2013. It was designed to observe the gravitational field of the planet at an elevation of 250 km. Six accelerometers made repeated measurements of the derivative of gravitation in three directions in space. The resolution at the surface of the Earth corresponds to a circle with a diameter of 80 km. In polar orbit GOCE homogeneously mapped a slice of the gravitational field during each 90 minutes revolution. The data allow derivation of a reference surface called the geoid by space geodesists. It is the potential surface of the Earth's gravity field which best coincides with mean sea level during the observed time interval. This reference surface is widely used in geodesy and oceanography. It gives rise to measurements of height above sea level, used officially in Norway and around the world. When compared to observations of ocean topography from altimetry satellites the reference surface helps reveal current patterns at open sea. These large scale currents are the heat transport mechanisms of nature and are essential for regional climate. From a research standpoint, such studies are difficult to do near the coast because the altimetry signal is then reflected off both water and land, thus contaminating the pure ocean signal. Development of regional geoid models based on GOCE data assimilated with terrestrial data meet with special mathematical challenges. Methodological problems have been raised and discussed at several conferences. The last release of data from the GOCE satellite have been employed to compute satellite-only gravity field models of the Earth. This has been combined with regional geoids along the Norwegian coast. Validated coastal altimetry data products have been employed to determine the mean dynamic topography (MDT) at tide gauge stations along the coast of Norway. Comparisons have also been made to independent oceanographic models. Numerical stepwise and sequential collocation techniques were employed for regional geoid modeling and to determine the formal error propagation in a complex system. There are challenges due to topographic effects along the coast and also connected to the choice of filters (based on error propagation and/or comparison with independent data) when combining satellite data and terrestrial data of the gravity field. Altimetry data with high spatial resolution obtained by the CryoSat-2 satellite were validated in the Norwegian coastal zone using time series data from tide gauges. Special mathematical approaches were developed to analyze the altimetry signal. MDT-values correspond within 3-5 cm for various observing techniques. Analyzes of SAR altimetry with CryoSat-2 have detected ocean currents along the Norwegian coast with improved spatial resolution. This has created expectations for even better future results with Sentinel-3. A challenge is that marine gravity data are inhomogeneously distributed and show variable quality. The results suggest that the MDT-signal is significant for length scales smaller than observed by GOCE. Norway has risen vertically since the end of the last ice age, which continually changes the gravity field because of mass transport. Data from several coastal stations observed by the absolute gravimeter of NMBU have been analyzed. The changes of the gravity field is different from site to site. A pattern emerges which compares well with the observed postglacial vertical uplift. This phenomenon affects the national height system directly as well as the observations of regional sea level and its temporal behavior as measured by coastal tide gauges. Three models of the time history of postglacial isostatic adjustment have been computed and compared to present time observations in Fennoscandia. They reveal that the viscosity of the lower mantle determines the uplift rate, while the thickness of the lithosphere determines relative sea level rates. The uplift rates determined by GPS time series at tide gauge stations have been compared to time series observations of the sea level, also derived from satellite altimetry with CryoSat-2. The average uplift rate for 20 tide gauges along the Norwegian coast is 2.8 mm/year, while CryoSat-2 yields 2.4 mm/year and 3.3 mm/year, for long and short sampling rates, respectively. The project has resulted in two doctoral dissertations and two master theses. The stipendiates have attended international summer schools, presented results at scientific conferences, and have had extended research visits at foreign institutions (The Academy of Science in Munich and Space Research Institute at the Technical University of Denmark).

Prosjektet har etablert et sterkt internasjonalt samarbeid og har inkludert flere nasjonale aktører i forvaltningen. De internasjonale forskningsoppholdene hadde avgjørende betydning. Akademisk utfall: 2 doktorgrader, 2 mastergrader, internasjonal erfaring med publisering, konferansedeltakelse og foredrag. Nansen Environmental and Remote Sensing Center (NERSC) og Meteorologisk Institutt ønsker å benytte prosjektets MDT-resultater til prediksjoner av havstrømmer og i studier av den norske kyststrømmen. Det vil gi økt samarbeid mellom flere disipliner av geovitenskapene.

The main applications of ESA's satellite mission GOCE are in oceanography and geodesy. Ocean circulation is of relevance for climate and sea-level related studies due to the large heat-storage and transport capacity of the world's oceans. Ocean currents c an be inferred from the height of the mean sea surface (observed by means of satellite altimetry) above a horizontal surface at mean sea level (in geodetic terminology called the geoid). GOCE-derived geoid surfaces have already improved our knowledge of o cean circulation on fine spatial scales and the results have been validated by comparison with numerical dynamic ocean models. Such comparisons are usually limited to the deep ocean, because both satellite altimetry and ocean models have peculiarities clo se to the coast. Coastal currents have gained increased interest in recent years due to their societal impacts related to coastal sea-level rise, shipping, fishery and various kinds of off-shore activities. There exist dedicated coastal dynamic models as well as dedicated altimetric products. However, not all models and data are available along the Norwegian coast, and for those available reliable quality assessment is hardly found. Therefore GOCODYN aims at comparison of all available data and models to derive reliable quality estimates. This is of fundamental importance for their future application and it has not been investigated along the Norwegian coast which is challenging due to its many fjords and islands. The required GOCE-based geoid model (refi ned by additional terrestrial data) does not only serve as reference surface for the ocean's mean dynamic topography, but also for topographic heights on land. The second aim of GOCODYN is to assess the quality of the current Norwegian vertical reference frame which is based on spirit leveling and to relate it to a globally consistent and homogenous frame. Applications are in geodesy, engineering and all fields where precise height information is required.