Dados de altimetria por satélite contêm informações importantes para o mapeamento de estruturas tectônicas em regiões oceânicas, como falhas, zonas de fratura e montes submarinos. A grande disponibilidade e densidade desses dados permitem a identificação de feições do assoalho oceânico com boa precisão. Este trabalho tem como objetivo investigar as características estruturais da Elevação do Rio Grande, no Atlântico Sul, através de elementos do campo de gravidade terrestre. Para isso, são utilizados dados de altura da superfície do mar (SSH) provenientes dos satélites das missões ERS1-GM, Geosat-GM e Seasat. Com o cálculo da derivada direcional da SSH ao longo das trilhas ascendentes e descendentes dos satélites, foram obtidos os gradientes de superfície do mar (SSG), que ressaltam os curtos comprimentos de ondas associados às importantes feições oceânicas estudadas. Também foram calculados os gradientes da altura geoidal (desvio da vertical) para auxiliar na interpretação do sinal da SSG. Através da SSH foi possível identificar estruturas de maior porte, como o rift da elevação, e algumas fraturas e montes submarinos maiores. Por outro lado, a SSG forneceu maiores detalhes sobre as feições já caracterizadas pela SSH e de toda a região, revelando também diversas outras estruturas de menor dimensão. O posicionamento das feições identificadas por ambas as grandezas, SSH e SSG, é bastante preciso. Entretanto, fatores como a direção e a orientação das trilhas dos satélites e a presença de estruturas adjacentes podem influenciar a resposta da SSG para uma determinada feição tectônica, por isso, as trilhas ascendentes e descendentes dos três satélites podem apresentar respostas diferenciadas. Sendo assim, recomenda-se analisar os dois conjuntos de trilhas de várias missões altimétricas para obter maiores informações das características estruturais das feições investigadas. O estudo também permitiu identificar possíveis estruturas com uma resposta característica de montes submarinos, nas trilhas descendentes de SSH, que não foram caracterizados anteriormente na literatura e não possuem correspondente nos modelos topográficos/batimétricos. / Satellite altimetry data contain important information for mapping tectonic structures in oceanic regions, such as faults, fracture zones and seamounts. The great availability and spatial density of these data allow one to identify ocean floor features with good accuracy. This work aims to investigate structural characteristics of the Rio Grande Rise, in South Atlantic, through elements of the terrestrial gravity field. We used sea surface height (SSH) data from satellite missions ERS1-GM, Geosat-GM and Seasat to calculate sea surface gradients (SSG), which are the SSH directional derivative along the ascending and descending satellite tracks. SSG emphasize the short wavelengths associated with the important oceanic features studied. Geoid gradients (deflection of the vertical) were also calculated to assist in the interpretation of the SSG signal. By analyzing sea surface heights, it is possible to identify larger structures, such as the rift of the rise, some fractures and large seamounts. In contrast, sea surface gradients provide greater details of the features characterized by the SSH and the entire area, also revealing several smaller seamounts. The positioning of the structures identified by both SSH and SSG is fairly accurate. However, factors such as direction and orientation of the satellite tracks and the presence of adjacent structures may influence the SSG response to a given tectonic feature. For this reason, the ascending and descending tracks of the three satellites may have different responses. Therefore, it is recommended that one analyzes the two sets of tracks from the several altimetric missions to obtain more information on the structural characteristics of the features. The study also allowed us to identify possible structures with a characteristic response of seamounts on SSH descending tracks, which were not previously characterized in the literature and do not have a similar correspondent in topographic/bathymetric models.

A variação do nível do mar em relação ao geóide é conhecida como Topografia Dinâmica do oceano, cuja determinação é importante no estudo da circulação geostrófica, vórtices e outros fenômenos. O sinal do geóide predomina na definição da topografia dinâmica em todos os comprimentos de onda, ou seja, o nível do mar está intimamente ligado ao campo geopotencial. Os modelos geoidais globais anteriores ao GRACE eram precisos para comprimentos de onda com centenas de quilômetros, já em curtos comprimentos de onda (menores que 100 km) esses modelos não possuíam resolução suficiente, prejudicando assim a determinação da TD e, conseqüentemente, o estudo de fenômenos com comprimentos de ondas na faixa de 100 a 200 km. No desenvolvimento do modelo EGM96, foram inseridos novos dados gravimétricos de superfície, além de dados de órbitas de satélites e altimetria, obtendo-se assim melhora considerável em relação ao modelo OSO91A (incorporado aos dados do TOPEX/Poseidon); ainda sim, o modelo EGM96 possui erro de aproximadamente 18 cm, que é considerado alto para várias aplicações oceanográficas. Recentemente, foram divulgados os modelos globais do campo de gravidade EIGEN-5C (obtido através dos dados do GRACE) e EGM2008, completos para grau e ordem 360 e 2159 em termos de coeficientes harmônicos esféricos, respectivamente; estes modelos possibilitaram a estimativa de correntes com resolução e precisão superior ao modelo EGM96. No presente trabalho, os modelos geoidais citados acima, juntamente com o modelo de nível médio do mar DNSC08, foram usados na determinação da topografia dinâmica média e das correntes geostróficas absolutas médias (no período de 2003 a 2008) utilizando o método de filtragem SSA no Atlântico Sul (20o N a 55o S, 80o W a 20o E) . Os resultados foram comparados com produtos do modelo numérico HYCOM, demonstrando que os modelos geoidais recentes (EIGEN-5C e EGM2008) apresentaram resultados satisfatórios na determinação da TDM e correntes, com a plena identificação das principais feições de grande e meso escala, o que não ocorre com a TDM do EGM96. A Corrente do Brasil, entre 20°S e 30°S, na isóbata de 200 m, apresentou velocidades resultantes médias de aproximadamente 0.20 m/s (desvio padrão de 0.09 m/s) quando determinada com a TDM-EGM2008, 0.22 m/s (desvio padrão de 0.12 m/s) vii utilizando a TDM-EIGEN-5C, embora os ruídos interfiram na estimativa, e 0.30 m/s (desvio padrão de 0.17 m/s) considerando a TDM-EGM96; o modelo numérico HYCOM forneceu velocidade de 0.25 m/s (desvio padrão de 0.13 m/s) na mesma região. Estatisticamente, as correntes do modelo TDM-EGM08 possuem maior correlação espacial com o modelo numérico HYCOM, 0.7 para u (componente EW) e 0.69 para v (componente NS); por outro lado, as correntes oriundas da TDM-EIGEN-5C apresentaram uma correlação de 0.62 para u e 0.64 para v, enquanto que as correntes do modelo TDM-EGM96 apresentaram uma correlação de 0.10 para u e 0.11 para v. Uma alternativa no uso de dados de altimetria de satélites (Jason, por exemplo) se encontra em dados de altimetria multi-satélites combinados com resultados de medições in-situ, submetidos a análise objetiva (são portanto dados combinados, MERGED, fornecidos pela AVISO); com esses dados se tem significativo aumento de precisão e resolução dos dados de altimetria, da topografia dinâmica e das correntes geostróficas. Na região que engloba a Confluência Brasil Malvinas (50o S a 20o S , 70 o W a 30o W) foram analisadas as variações temporais da topografia dinâmica e correntes oriundas de altimetria e do modelo geoidal EGM2008. Na análise específica desta região, verificouse que, apesar das principais feições terem sido identificadas, os dados de altimetria ainda carecem de resolução necessária no estudo das mesmas. O modelo EGM2008 apresentou os melhores resultados que os outros modelos (comparando com o modelo HYCOM e dados combinados de AVISO), devido à sua resolução espacial. / The variations of sea level relative to the geoid are known as Dynamic Ocean Topography (DOT), whose determination is important in studies of the geostrophic circulation, eddies and other phenomena. The sign of the geoid predominates in the definition of dynamic topography at all wavelengths, ie, sea level is closely linked to the geopotential field. Global geoid models prior to GRACE were precise for wavelengths of hundreds of kilometers, but in short wavelengths (less than 100 km) these models did not have enough resolution, thus impairing the determination of DOT and therefore the study of phenomena with wavelengths in the range of 100-200 km. In developing the EGM96, new surface gravity data were incorporated into the modeling, from satellite orbits and altimetry, resulting in a considerable improvement over the previous OSO91A (incorporated to TOPEX / Poseidon data); still EGM96 had a standard deviation of about 18 cm, considered too high for many oceanographic applications. Recently, global models of the gravity field were published, EIGEN-5C (obtained from GRACE data) and EGM2008, complete to degree and order 360 and 2159 in terms of spherical harmonic coefficients, respectively; these models allowed the estimation of currents with resolution and accuracy better than model EGM96. In this work, the geoid models mentioned above, along with models of mean sea level such as DNSC08, were used in the determination of DOT and absolute geostrophic currents (in 2003-2008) using the filtering method SSA (Singular Spectrum Analysis) in South Atlantic (20o N - 55o S , 80 o W - 20o E) . The results were compared with products from HYCOM hydrodynamic numerical model and show that recent geoid models (EIGEN-5C and EGM2008) lead to satisfactory results in determining the Average Dynamic Topography (ADT) and currents, with full identification of the main features of large and meso scales, which does not occur with ADT_EGM96. The Brazil Current (20°S-30°S), in the 200 m isobath, had average speed results of approximately 0.20 m/s (standard deviation 0.09 m/s) when computed with ADT-EGM2008, 0.22 m/s (standard deviation 0.12 m/s) by using ADT_EIGEN-5C, despite noise interference with the estimate, and 0.30 m/s (standard deviation 0.17 m/s) considering the ADT_EGM96; the numerical model HYCOM provided speed of 0.25 m/s (standard deviation 0.13 m/s) in the same region. Statistically, the currents based on model ADT_EGM08 have higher correlation with the numerical model HYCOM, 0.70 for u (EW component) and 0.69 for v (NS component); on the other hand, the currents computed with ADT_EIGEN-5C had correlations of 0.62 for u and 0.64 for v, while currents estimated from ADT_EGM96 showed correlations of 0.10 for u and 0.11 for v. An alternative in the use altimetry data (Jason, for example) is found in altimetry multi-satellites data combined with results of measurements in-situ, submitted to objective analysis; with these data, a significant increase of precision and resolution is reached for the altimetry data, dynamic topography and geostrophyc currents. In the region encompassing the Brazil Malvinas Confluence (50o S - 20o S , 70 o W - 30o W) were analyzed temporal variations of DOT and currents derived from altimetry and geoid model EGM2008. In the specific analysis of this region, it was found that although the main features have been identified, the altimetry data still lacks resolution to study them.The model EGM2008 showed the best results (comparing with the HYCOM model and combined data of AVISO), due to their spatial resolutions.

A variação do nível do mar em relação ao geóide é conhecida como Topografia Dinâmica do oceano, cuja determinação é importante no estudo da circulação geostrófica, vórtices e outros fenômenos. O sinal do geóide predomina na definição da topografia dinâmica em todos os comprimentos de onda, ou seja, o nível do mar está intimamente ligado ao campo geopotencial. Os modelos geoidais globais anteriores ao GRACE eram precisos para comprimentos de onda com centenas de quilômetros, já em curtos comprimentos de onda (menores que 100 km) esses modelos não possuíam resolução suficiente, prejudicando assim a determinação da TD e, conseqüentemente, o estudo de fenômenos com comprimentos de ondas na faixa de 100 a 200 km. No desenvolvimento do modelo EGM96, foram inseridos novos dados gravimétricos de superfície, além de dados de órbitas de satélites e altimetria, obtendo-se assim melhora considerável em relação ao modelo OSO91A (incorporado aos dados do TOPEX/Poseidon); ainda sim, o modelo EGM96 possui erro de aproximadamente 18 cm, que é considerado alto para várias aplicações oceanográficas. Recentemente, foram divulgados os modelos globais do campo de gravidade EIGEN-5C (obtido através dos dados do GRACE) e EGM2008, completos para grau e ordem 360 e 2159 em termos de coeficientes harmônicos esféricos, respectivamente; estes modelos possibilitaram a estimativa de correntes com resolução e precisão superior ao modelo EGM96. No presente trabalho, os modelos geoidais citados acima, juntamente com o modelo de nível médio do mar DNSC08, foram usados na determinação da topografia dinâmica média e das correntes geostróficas absolutas médias (no período de 2003 a 2008) utilizando o método de filtragem SSA no Atlântico Sul (20o N a 55o S, 80o W a 20o E) . Os resultados foram comparados com produtos do modelo numérico HYCOM, demonstrando que os modelos geoidais recentes (EIGEN-5C e EGM2008) apresentaram resultados satisfatórios na determinação da TDM e correntes, com a plena identificação das principais feições de grande e meso escala, o que não ocorre com a TDM do EGM96. A Corrente do Brasil, entre 20°S e 30°S, na isóbata de 200 m, apresentou velocidades resultantes médias de aproximadamente 0.20 m/s (desvio padrão de 0.09 m/s) quando determinada com a TDM-EGM2008, 0.22 m/s (desvio padrão de 0.12 m/s) vii utilizando a TDM-EIGEN-5C, embora os ruídos interfiram na estimativa, e 0.30 m/s (desvio padrão de 0.17 m/s) considerando a TDM-EGM96; o modelo numérico HYCOM forneceu velocidade de 0.25 m/s (desvio padrão de 0.13 m/s) na mesma região. Estatisticamente, as correntes do modelo TDM-EGM08 possuem maior correlação espacial com o modelo numérico HYCOM, 0.7 para u (componente EW) e 0.69 para v (componente NS); por outro lado, as correntes oriundas da TDM-EIGEN-5C apresentaram uma correlação de 0.62 para u e 0.64 para v, enquanto que as correntes do modelo TDM-EGM96 apresentaram uma correlação de 0.10 para u e 0.11 para v. Uma alternativa no uso de dados de altimetria de satélites (Jason, por exemplo) se encontra em dados de altimetria multi-satélites combinados com resultados de medições in-situ, submetidos a análise objetiva (são portanto dados combinados, MERGED, fornecidos pela AVISO); com esses dados se tem significativo aumento de precisão e resolução dos dados de altimetria, da topografia dinâmica e das correntes geostróficas. Na região que engloba a Confluência Brasil Malvinas (50o S a 20o S , 70 o W a 30o W) foram analisadas as variações temporais da topografia dinâmica e correntes oriundas de altimetria e do modelo geoidal EGM2008. Na análise específica desta região, verificouse que, apesar das principais feições terem sido identificadas, os dados de altimetria ainda carecem de resolução necessária no estudo das mesmas. O modelo EGM2008 apresentou os melhores resultados que os outros modelos (comparando com o modelo HYCOM e dados combinados de AVISO), devido à sua resolução espacial. / The variations of sea level relative to the geoid are known as Dynamic Ocean Topography (DOT), whose determination is important in studies of the geostrophic circulation, eddies and other phenomena. The sign of the geoid predominates in the definition of dynamic topography at all wavelengths, ie, sea level is closely linked to the geopotential field. Global geoid models prior to GRACE were precise for wavelengths of hundreds of kilometers, but in short wavelengths (less than 100 km) these models did not have enough resolution, thus impairing the determination of DOT and therefore the study of phenomena with wavelengths in the range of 100-200 km. In developing the EGM96, new surface gravity data were incorporated into the modeling, from satellite orbits and altimetry, resulting in a considerable improvement over the previous OSO91A (incorporated to TOPEX / Poseidon data); still EGM96 had a standard deviation of about 18 cm, considered too high for many oceanographic applications. Recently, global models of the gravity field were published, EIGEN-5C (obtained from GRACE data) and EGM2008, complete to degree and order 360 and 2159 in terms of spherical harmonic coefficients, respectively; these models allowed the estimation of currents with resolution and accuracy better than model EGM96. In this work, the geoid models mentioned above, along with models of mean sea level such as DNSC08, were used in the determination of DOT and absolute geostrophic currents (in 2003-2008) using the filtering method SSA (Singular Spectrum Analysis) in South Atlantic (20o N - 55o S , 80 o W - 20o E) . The results were compared with products from HYCOM hydrodynamic numerical model and show that recent geoid models (EIGEN-5C and EGM2008) lead to satisfactory results in determining the Average Dynamic Topography (ADT) and currents, with full identification of the main features of large and meso scales, which does not occur with ADT_EGM96. The Brazil Current (20°S-30°S), in the 200 m isobath, had average speed results of approximately 0.20 m/s (standard deviation 0.09 m/s) when computed with ADT-EGM2008, 0.22 m/s (standard deviation 0.12 m/s) by using ADT_EIGEN-5C, despite noise interference with the estimate, and 0.30 m/s (standard deviation 0.17 m/s) considering the ADT_EGM96; the numerical model HYCOM provided speed of 0.25 m/s (standard deviation 0.13 m/s) in the same region. Statistically, the currents based on model ADT_EGM08 have higher correlation with the numerical model HYCOM, 0.70 for u (EW component) and 0.69 for v (NS component); on the other hand, the currents computed with ADT_EIGEN-5C had correlations of 0.62 for u and 0.64 for v, while currents estimated from ADT_EGM96 showed correlations of 0.10 for u and 0.11 for v. An alternative in the use altimetry data (Jason, for example) is found in altimetry multi-satellites data combined with results of measurements in-situ, submitted to objective analysis; with these data, a significant increase of precision and resolution is reached for the altimetry data, dynamic topography and geostrophyc currents. In the region encompassing the Brazil Malvinas Confluence (50o S - 20o S , 70 o W - 30o W) were analyzed temporal variations of DOT and currents derived from altimetry and geoid model EGM2008. In the specific analysis of this region, it was found that although the main features have been identified, the altimetry data still lacks resolution to study them.The model EGM2008 showed the best results (comparing with the HYCOM model and combined data of AVISO), due to their spatial resolutions.

Die Bestimmung von Variationen des Meeresspiegels ist heute wichtiger Gegenstand geodätischer Forschungen. So stellt der mittlere Meeresspiegel eine geometrische Grenzfläche der Erdfigur dar. Seine säkulare Änderung ist darüber hinaus eine wichtige Größe u.a. bei der Planung künftiger Küstenschutzmaßnahmen. Die zuverlässige Schätzung dieser Größen aus den relativ kurzen Zeitreihen aktueller Satellitenaltimeterbeobachtungen wird jedoch durch lang- und kurzzeitig wirkende Meeresspiegelvariationen erschwert. Am Beispiel der Ostsee wurde ein Verfahren entwickelt, diese Variationen in den Beobachtungsdaten mit Hilfe modellierter Oberflächenauslenkungen, die einem hochauflösenden ozeanographischen Modell der Ostsee entnommen wurden, zu reduzieren. Die Streuung der beobachteten Höhen wurde so von ursprünglich etwa 15 bis 25cm auf unter 10cm reduziert. Vor der Nutzung des Modells ergab der Vergleich der modellierten mit beobachteten Variationen des Meeresspiegels eine Genauigkeit der modellierten Meeresspiegelauslenkungen von 5 bis 15cm. Darüber hinaus konnte unter Verwendung von Pegelbeobachtungen eine Modellverbesserung erzielt werden. Die Kombination der Beobachtungen unterschiedlicher Altimetermissionen erforderte des Weiteren eine Homogenisierung der Reihen und die Überprüfung ihrer Langzeitstabilität. Hierzu erfolgte eine absolute Altimetervalidierung durch Vergleich mit Pegelmessungen in der südlichen Ostsee. Relative Biaswerte wurden durch Vergleich der Beobachtungen unterschiedlicher Altimeter im Bereich der gesamten Ostsee ermittelt. Die korrigierten und um die modellierten Meeresspiegelvariationen reduzierten Altimetermeeresspiegelhöhen bildeten im Weiteren die Grundlage zur Bestimmung des mittleren Meeresspiegels der Ostsee und seiner säkularen Änderung. Der lineare Trend des Meeresspiegelanstieges in der Ostsee, bezogen auf das Geoid, ergab sich zu (+1,2+-0,8)mm/Jahr. Die Kombination dieses Wertes mit der an Küstenpegeln beobachteten relativen Meeresspiegeländerung erlaubte weiterhin die Schätzung postglazialer Landhebungsraten im Küstenbereich an den Pegelstationen. Die mittlere Meerestopographie innerhalb der offenen Ostsee wurde, bezogen auf ein regionales Geoidmodell, mit einer Genauigkeit von etwa 3 bis 10cm bestimmt. Das erarbeitete Verfahren zeigt das Potential der Kombination geodätischer Beobachtungsverfahren mit den Ergebnissen einer präzisen Modellierung der beobachteten Größe. Die Arbeit stellt eine regionale Studie dar, die - bei Vorhandensein entsprechender Daten - auch auf andere Regionen übertragbar ist. / The determination of sea level variations is a major topic of present geodetic research. The mean sea surface is a geometric boundary surface to describe the figure of the Earth. Its secular change has, for instance, to be considered in future coastal protection projects. However, the estimation of reliable values for these parameters from the relatively short altimeter observation time series is hampered by long- and short-term sea level variations. For the Baltic Sea, a processing scheme was developed that uses sea surface elevations taken from a high-resolution oceanographic model of the Baltic Sea to reduce these sea level variations in the observations. Thus, the standard deviation of the observed sea level heights was reduced from original values of about 15 to 25cm to final values of less than 10cm. A comparison of the modelled and the observed sea level variations was performed before the model was employed. From this, the accuracy of the modelled sea surface elevations was found to be in the order of 5 to 15cm. Moreover, the oceanographic model could be improved by the assimilation of tide gauge observations. The combination of the observations of different satellite altimeters requires also the homogenisation of the time series and the examination of their temporal stability. The comparison with tide gauge observations at the southern coast of the Baltic Sea allowed the determination of the absolute bias of each altimeter mission. Relative bias values were estimated by comparison of altimetric heights of different missions within the entire Baltic Sea. Finally, the time series of the corrected and reduced altimetric sea level heights formed the basis on which the mean sea surface of the Baltic Sea and its secular change were determined. The linear rate of the sea level rise in the Baltic Sea with respect to the geoid was estimated to (+1,2+-0,8)mm/year. This absolute rate was then combined with the relative sea level change observed at coastal tide gauges, allowing the estimation of the rates of the postglacial rebound in the vicinity of the gauges. The mean sea surface topography in the open Baltic was determined with an accuracy of about 3 to 10cm. A regional geoid model served as the reference surface. The presented procedure shows the potential of combining geodetic observations with the results of a precise model of the observed parameter. It can be regarded as a regional study that can be applied also to other regions where observational and modelled data are available.

Die Bestimmung von Variationen des Meeresspiegels ist heute wichtiger Gegenstand geodätischer Forschungen. So stellt der mittlere Meeresspiegel eine geometrische Grenzfläche der Erdfigur dar. Seine säkulare Änderung ist darüber hinaus eine wichtige Größe u.a. bei der Planung künftiger Küstenschutzmaßnahmen. Die zuverlässige Schätzung dieser Größen aus den relativ kurzen Zeitreihen aktueller Satellitenaltimeterbeobachtungen wird jedoch durch lang- und kurzzeitig wirkende Meeresspiegelvariationen erschwert. Am Beispiel der Ostsee wurde ein Verfahren entwickelt, diese Variationen in den Beobachtungsdaten mit Hilfe modellierter Oberflächenauslenkungen, die einem hochauflösenden ozeanographischen Modell der Ostsee entnommen wurden, zu reduzieren. Die Streuung der beobachteten Höhen wurde so von ursprünglich etwa 15 bis 25cm auf unter 10cm reduziert. Vor der Nutzung des Modells ergab der Vergleich der modellierten mit beobachteten Variationen des Meeresspiegels eine Genauigkeit der modellierten Meeresspiegelauslenkungen von 5 bis 15cm. Darüber hinaus konnte unter Verwendung von Pegelbeobachtungen eine Modellverbesserung erzielt werden. Die Kombination der Beobachtungen unterschiedlicher Altimetermissionen erforderte des Weiteren eine Homogenisierung der Reihen und die Überprüfung ihrer Langzeitstabilität. Hierzu erfolgte eine absolute Altimetervalidierung durch Vergleich mit Pegelmessungen in der südlichen Ostsee. Relative Biaswerte wurden durch Vergleich der Beobachtungen unterschiedlicher Altimeter im Bereich der gesamten Ostsee ermittelt. Die korrigierten und um die modellierten Meeresspiegelvariationen reduzierten Altimetermeeresspiegelhöhen bildeten im Weiteren die Grundlage zur Bestimmung des mittleren Meeresspiegels der Ostsee und seiner säkularen Änderung. Der lineare Trend des Meeresspiegelanstieges in der Ostsee, bezogen auf das Geoid, ergab sich zu (+1,2+-0,8)mm/Jahr. Die Kombination dieses Wertes mit der an Küstenpegeln beobachteten relativen Meeresspiegeländerung erlaubte weiterhin die Schätzung postglazialer Landhebungsraten im Küstenbereich an den Pegelstationen. Die mittlere Meerestopographie innerhalb der offenen Ostsee wurde, bezogen auf ein regionales Geoidmodell, mit einer Genauigkeit von etwa 3 bis 10cm bestimmt. Das erarbeitete Verfahren zeigt das Potential der Kombination geodätischer Beobachtungsverfahren mit den Ergebnissen einer präzisen Modellierung der beobachteten Größe. Die Arbeit stellt eine regionale Studie dar, die - bei Vorhandensein entsprechender Daten - auch auf andere Regionen übertragbar ist. / The determination of sea level variations is a major topic of present geodetic research. The mean sea surface is a geometric boundary surface to describe the figure of the Earth. Its secular change has, for instance, to be considered in future coastal protection projects. However, the estimation of reliable values for these parameters from the relatively short altimeter observation time series is hampered by long- and short-term sea level variations. For the Baltic Sea, a processing scheme was developed that uses sea surface elevations taken from a high-resolution oceanographic model of the Baltic Sea to reduce these sea level variations in the observations. Thus, the standard deviation of the observed sea level heights was reduced from original values of about 15 to 25cm to final values of less than 10cm. A comparison of the modelled and the observed sea level variations was performed before the model was employed. From this, the accuracy of the modelled sea surface elevations was found to be in the order of 5 to 15cm. Moreover, the oceanographic model could be improved by the assimilation of tide gauge observations. The combination of the observations of different satellite altimeters requires also the homogenisation of the time series and the examination of their temporal stability. The comparison with tide gauge observations at the southern coast of the Baltic Sea allowed the determination of the absolute bias of each altimeter mission. Relative bias values were estimated by comparison of altimetric heights of different missions within the entire Baltic Sea. Finally, the time series of the corrected and reduced altimetric sea level heights formed the basis on which the mean sea surface of the Baltic Sea and its secular change were determined. The linear rate of the sea level rise in the Baltic Sea with respect to the geoid was estimated to (+1,2+-0,8)mm/year. This absolute rate was then combined with the relative sea level change observed at coastal tide gauges, allowing the estimation of the rates of the postglacial rebound in the vicinity of the gauges. The mean sea surface topography in the open Baltic was determined with an accuracy of about 3 to 10cm. A regional geoid model served as the reference surface. The presented procedure shows the potential of combining geodetic observations with the results of a precise model of the observed parameter. It can be regarded as a regional study that can be applied also to other regions where observational and modelled data are available.

Mit der Mission des Ice, Cloud and Land Elevation Satellite (ICESat) gelangte erstmals ein Laseraltimetersystem in einen erdgebundenen Orbit. Die vorliegende Arbeit verdeutlicht anhand von drei verschiedenen Anwendungen das Potenzial dieser Altimeterdaten zur Überwachung des Antarktischen und des Grönländischen Eisschilds. Beide Schilde bilden ein Schlüsselglied im globalen Klimasystem der Erde. In einem ersten Hauptabschnitt werden die ICESat-Altimeterdaten für das Gebiet des Lake Vostok, des größten Vertreters subglazialer Seen in der Antarktis, untersucht. Dieses Gebiet eignet sich durch die Höhenstabilität des über dem See liegenden Eisschilds insbesondere als Validierungsgebiet für Altimeterdaten. Diese werden hinsichtlich der zwischen den Lasern auftretenden Offsets umfassend analysiert. Die ermittelten Offsets variieren in einem Bereich zwischen -7.5 und +13.9 cm und erreichen damit die angestrebte Messgenauigkeit der Mission. Im Hinblick auf eine Bestimmung von zeitlich linearen Höhenänderungen der Eisschilde stellen sie den größten genauigkeits-limitierenden Faktor dar. Aus den um die Offsets korrigierten Altimeterdaten wird ein rasterförmiges Topographiemodell der Eisoberfläche erstellt. Dieses wird umfassend untersucht. Im Anschluss werden glaziologische Anwendungen vorgestellt, für welche das Topographiemodell eine zentrale Grundlage bildet. Unter anderem erfolgt in der Kombination mit Eisdicken- und Geoidinformationen der Nachweis, dass sich das Eis über dem See im hydrostatischen Gleichgewicht befindet. Im Zuge dieser Untersuchung wird aber auch deutlich, dass an einigen Stellen des Sees das Gleichgewicht verletzt wird. Mögliche Ursachen hierfür werden näher untersucht und eingehend diskutiert. Für den Grönländischen Eisschild erfolgt die Analyse der um die Laseroffsets korrigierten Altimeterdaten zur Ableitung zeitlich linearer Höhenänderungen. Die methodische Basis hierfür bildet eine Wiederholspuranalyse der Altimeterdaten. Zur Minimierung des Einflusses der lokalen Topographie und zur besseren Separation der saisonalen Höhenvariation werden die korrespondierenden Altimetermessungen entlang der Referenzspuren an ein drei-komponentiges mathematisches Modell durch Ausgleichung bestmöglich angepasst. Die für den ICESat-Missionszeitraum bestimmte mittlere Höhenrate des Eisschilds beträgt -13.0±0.5 cm/a. Die stärkste Höhenabnahme verzeichnet der Eisschild in den westlichen und südöstlichen küstennahen Randbereichen. Unter Verwendung der Eisdichte für die Volumen-Massen-Umrechnung entspricht dies einer Massenänderung von -209.5±35.6 Gt/a. Dies entspricht einem eustatischen Meeresspiegelanstieg von +0.6±0.1 mm/a. In einer dritten Anwendung werden die ICESat-Altimeterdaten über dem Amery-Schelfeises untersucht. Es wird eine Methodik vorgestellt, welche auf der Kreuzkorrelation von Höhenprofilen verschiedener Epochen beruht und zur Ableitung von Fließgeschwindigkeiten des Schelfeises dient. Der entwickelte Ansatz wird auf die ICESat-Referenzspur 49 angewendet. Sie verläuft entlang der zentralen Achse des Schelfeises. Im Bereich zwischen -71.6° und -70.1° Breite wächst die Fließgeschwindigkeit von +0.83±0.09 m/d auf +1.02±0.06 m/d an. Das Ergebnis steht im Einklang mit einem unabhängigen Geschwindigkeitsmodell, welches zur Validierung herangezogen wurde. / The Ice, Cloud and Land Elevation Satellite (ICESat) was the first Earth-orbiting laser altimeter mission in space. The following work is dedicated to the ICESat-altimetry data in order to demonstrate their full potential for the investigation of glaciological implications in polar regions. The primary science objective of the mission was to focus on the mass balances of the Greenland Ice Sheet and the Antarctic Ice Sheet. Both of them play a key role in the Earth's climate system. Firstly, the ICESat elevation profiles covering the Lake Vostok region are analysed in more detail. The Lake Vostok is the largest known subglacial lake in Antarctica to date. Due to a fast and strong degradation of the laser energy, the ICESat elevation measurements are affected by offsets. The estimated offsets between the laser operational periods vary between -7.5 und +13.9 cm. Therefore, they can't be neglected in the view of precise mass change determinations for ice sheets. In addition, a Digital Elevation Model (DEM) of the ice surface topography is generated on the basis of the adjusted elevation profiles. The DEM is analysed in more detail. Furthermore, the DEM forms the basis for the investigation of glaciological implications. In combination with an ice-thickness model and a regional geoid model the hydrostatic equilibrium condition is evaluated. It turns out, that the ice sheet covering the lake fulfils the hydrostatic equilibrium condition within ±1 m for large parts of the lake. Beside this, positive and negative deviations are found in the northern and southern part of the lake. Secondly, ice surface height changes and their temporal variations are inferred for the Greenland ice sheet. This investigation is based on a refined repeat-track analysis in order to exploit the full potential of ICESat's altimetry data. To reduce the influence of the local topography corresponding measurements along the track are fitted to a mathematical model, consisting of three components. For the entire ice sheet a mean surface height trend of -13.0±0.5 cm/yr is determined. The largest changes are identified at the coastal margins of the ice sheet. Using the ice surface height changes long-term volume- and mass-change rates are inferred. For this purpose the density of pure ice is used for the volume-mass-conversion. The overall long-term mass change rate amounts to -209.5±35.6 Gt/yr. This is equivalent to an eustatic sea level rise of +0.6±0.1 mm/yr. A third approach analyses ICESat elevation profiles over the Amery ice shelf. The method is based on a cross-correlation analysis of different ICESat repeat cycle in order to determine the ice flow velocity along the track. This method is applied to reference track 49. The investigation reveals that between 71.7° S and 70.1° S along the reference track, the ice-flow velocity increases from about +0.83±0.09 m/d to +1.02±0.06 m/d. These results are in general good agreement with velocities derived from an independent velocity field.

Dams and reservoirs with multiple purposes require effective management to fully realize their purposes and maximize efficiency. For instance, a reservoir intended mainly for the purposes of flood control and hydropower generation may result in a system with primary objectives that conflict with each other. This is because higher hydraulic heads are required to achieve the hydropower generation objective while relatively lower reservoir levels are required to fulfill flood control objectives. Protracted imbalances between these two could increase the susceptibility of the system to risks of water shortage or flood, depending on inflow volumes and operational policy effectiveness. The magnitudes of these risks can become even more pronounced when upstream use of the river is unregulated and uncoordinated so that upstream consumptions and releases are arbitrary. As a result, safe operational practices and risk management alternatives must be structured after an improved understanding of historical and anticipated inflows, actual and speculative upstream uses, and the overall hydrology of catchments upstream of the reservoir. One of such systems with an almost yearly occurrence of floods and shortages due to both natural and anthropogenic factors is the dual reservoir system of Kainji and Jebba in Nigeria. To analyze and manage these risks, a methodology that combines a stochastic and deterministic approach was employed. Using methods outlined by Box and Jenkins (1976), autoregressive integrated moving average (ARIMA) models were developed for forecasting Niger river inflows at Kainji reservoir based on twenty-seven-year-long historical inflow data (1970-1996). These were then validated using seven-year inflow records (1997-2003). The model with the best correlation was a seasonal multiplicative ARIMA (2,1,1)x(2,1,2)12 model. Supplementary iv validation of this model was done with discharge rating curves developed for the inlet of the reservoir using in situ inflows and satellite altimetry data. By comparing net inflow volumes with storage deficit, flood and shortage risk factors at the reservoir were determined based on (a) actual inflows, (b) forecasted inflows (up to 2015), and (c) simulated scenarios depicting undefined competitive upstream consumption. Calculated highrisk years matched actual flood years again suggesting the reliability of the model. Monte Carlo simulations were then used to prescribe safe outflows and storage allocations in order to reduce futuristic risk factors. The theoretical safety levels achieved indicated risk factors below threshold values and showed that this methodology is a powerful tool for estimating and managing flood and shortage risks in reservoirs with undefined competitive upstream consumption

No description available.

Der regionalen Schwerefeldmodellierung in polaren Gebieten kommt in vielerlei Hinsicht eine besondere und wachsende Bedeutung zu. Einerseits sind hochauflösende und präzise Geoidmodelle eine wichtige Eingangsgröße bei der Untersuchung und Quantifizierung geophysikalischer, ozeanographischer bzw. glaziologischer Phänomene, z. B. bei der Bestimmung der mittleren dynamischen Ozeantopographie oder der Anwendung des Schwimmgleichgewichts im Bereich von Schelfeisen, Meereis oder subglazialen Seen. Zudem werden sie allgemein zur Referenzierung von Höhenmodellen benötigt. Andererseits sind, aufgrund der unvermeidbaren polaren Datenlücken von Satellitenbeobachtungen jenseits der Grenzbreite (sog. „Polloch“), terrestrische Schweredaten auch für die globale Schwerefeldmodellierung unerlässlich. Jedoch sind die verfügbaren terrestrischen (bodennahen) Schwerebeobachtungen insbesondere im Gebiet der Antarktis äußerst lückenhaft und heterogen. So entspricht das tatsächliche Auflösungsvermögen selbst aktueller kombinierter Schwerefeldmodelle wie EGM2008 oder EIGEN-6C über dem antarktischen Kontinent lediglich dem der reinen Satellitenmodelle aus GRACE bzw. GOCE. Des Weiteren sind Standardverfahren der regionalen Geoidmodellierung hier nicht ohne Weiteres anwendbar. Neben der Heterogenität der Daten als praktischer Herausforderung muss aus theoretischer Sicht dem zusätzlichen Dichtekontrast durch das Eis Rechnung getragen werden. Die vorliegende kumulative Dissertation greift diese Problematik auf. Während die einzelnen Publikationen die Ergebnisse ausgewählter regionaler Fallstudien präsentieren, soll die folgende zusammenfassende Abhandlung einen doppelten Bogen spannen, indem die geophysikalischen Phänomene gleichzeitig als zu untersuchende Anwendungsgebiete und als Einflussfaktoren im Kontext der regionalen Geoidmodellierung beschrieben werden. So wird am Beispiel der Weddellsee gezeigt, wie die Meereisbedeckung die Qualität und Zuverlässigkeit der mithilfe der Satellitenaltimetrie abgeleiteten Schwerefeldmodelle beeinträchtigt. Diese Modelle bilden derzeit die alleinige Datengrundlage für die hochauflösenden globalen Modelle im Gebiet des Antarktischen Ozeans. Zugleich wird anhand des verfeinerten regionalen Modells und daraus abgeleiteter geostrophischer Geschwindigkeiten demonstriert, dass selbst lückenhafte und heterogene terrestrische Daten hier einen wesentlichen Beitrag zur simultanen Kalibrierung und Vereinheitlichung des Datenbestandes leisten können. Im Ergebnis konnten in den küstennahen Gewässern Differenzen von mehreren Dezimetern gegenüber Geoidhöhen aus EGM2008 festgestellt werden, welche teils auf systematische Abweichungen und teils auf Rauschen im globalen Modell zurückzuführen sind. Über dem Festland erreicht dessen Vernachlässigungsfehler im quadratischen Mittel sogar 0,75 m und Maxima von über 3 m. Ein weiteres verfeinertes und, dank geeigneter Eingangsdaten, sehr genaues und hochauflösendes Geoidmodell wird für die Region um den Vostoksee in der Ostantarktis abgeleitet. In Kombination mit Eisoberflächenhöhen und Eisdicken gelingt es, das Schwimmgleichgewicht des subglazialen Sees nachzuweisen. Das gegenüber GOCE zusätzlich gewonnene Geoidsignal ist hier mit 0,56 m Standardabweichung zwar etwas kleiner, jedoch wird im Vergleich mit der residualen Auslenkung des Seespiegels (0,26 m Standardabweichung) auch für diese Anwendung der signifikante und gegenüber dem Auflösungsvermögen von GOCE auch notwendige Beitrag eines regionalen Geoidmodells deutlich. Für das hydrostatische Gleichgewicht eines subglazialen Sees ist streng genommen das tatsächliche Schwerepotential in Höhe des Seespiegels maßgeblich. Dessen Berechnung erfordert eine Fortsetzung des Störpotentials nach unten innerhalb der Topographie, welche konzeptionell in engem Zusammenhang mit dem bekannten Geoid-Quasigeoid-Separationsterm steht. Dessen oft angenommene Approximation mithilfe der Bougueranomalie kann, angesichts der heutigen Anforderungen an ein modernes zentimetergenaues Geoid, gerade in rauem Gelände zu ungenau sein. In Anlehnung an aktuelle Arbeiten auf diesem Gebiet wird ein verallgemeinerter und zugleich verfeinerter Ansatz zur praktischen Berechnung des Terms erarbeitet. Am Beispiel des Himalaya werden die einzelnen Anteile im Rahmen einer Simulationsstudie quantifiziert und insbesondere ihre Sensitivität gegenüber dem Integrationsradius der Topographie untersucht. Besonderes Augenmerk liegt ebenso auf dem indirekten Effekt der Topographie in Bezug auf das Potential, welcher, im Gegensatz zur Anwendung eines planaren Modells, in sphärischer Betrachtungsweise nicht verschwindet. / In many respects, regional gravity field modeling in polar areas is of special, and growing, interest. On the one hand, high-resolution and precise geoid models are an important input parameter to investigate and quantify manifold geophysical, oceanographical and glaciological phenomena, e.g., the determination of the mean dynamic ocean topography, or the application of the hydrostatic equilibrium condition in the areas of ice shelves, sea ice, or subglacial lakes. Moreover, geoid models are in general needed as a reference for height models. On the other hand, because of the unavoidable polar data gaps in satellite measurements due to the inclination (the so-called “polar gap”), terrestrial gravity data are indispensable also for global gravity field modeling. However, the available terrestrial (ground-based) gravity datasets, in particular of Antarctica, are very sparse and heterogeneous. For example, over the Antarctic continent the true resolution of even the most recent combined global geopotential models such as EGM2008 or EIGEN-6C only corresponds to that of the satellite-only models derived from GRACE and GOCE, respectively. Furthermore, standard techniques of regional geoid modeling cannot be readily used in this area. Apart from the heterogeneity of the data as a practical challenge the additional density contrast implied by the covering sheet needs to be accounted for from the theoretical point of view. This complex situation is the starting point for the present cumulative dissertation. Whereas the individual publications present the results of selected regional case studies, the intention of the following summary is to draw an integrated picture aiming at explaining the geophysical phenomena as both applications and influencing factors in the context of regional geoid modeling. Using the example of the Weddell Sea it is shown how sea-ice coverage affects the quality and reliability of marine gravity field models derived from radar satellite altimetry. At present, these models are the only input data to the high-resolution global geopotential models. At the same time, the refined regional model and geostrophic velocities derived thereof are employed to demonstrate how even sparse and heterogeneous terrestrial gravity data may provide a contribution to simultaneously calibrate and unify the available datasets. As a result, near the coast differences at the order of some decimeters could be observed in comparison with EGM2008, originating partly from systematic effects and noise in the global model. In the continental areas, its omission error even yields a standard deviation of 0.75 m and attains a maximum of more than 3 m. Another refined and, owing to appropriate input data, very precise and highly resolving geoid model is derived for the region around subglacial Lake Vostok. In combination with ice-surface heights and ice thickness data it is used to provide observational evidence that the lake is in a state of hydrostatic equilibrium. There, the additional geoid signal w.r.t. GOCE is a bit smaller (0.56 m standard deviation). However, considering the residual deviations of the apparent lake level (0.26 m standard deviation) the significant and necessary, as compared to the resolution of GOCE, contribution of a regional geoid model to this application is shown. In a strict sense, the relevant quantity to evaluate the hydrostatic equilibrium condition of a subglacial lake is the actual geopotential at the anticipated lake level. Its computation requires a downward continuation of the disturbing potential inside the topography, which is closely related to the concept of the well-known geoid-quasigeoid separation term. In the past, this term was frequently described as an approximation by means of the Bouguer anomaly. However, considering the modern requirements of the “one-centimeter geoid” this approximation may be too coarse over rough terrain. Following recent works in this field, a generalized yet refined approach for practical implementation of the term is developed. The individual constituents of the term are quantified. In particular, their sensitivity against the radius up to which topography is taken into account is investigated. For this simulation study, the Himalaya mountain region served as test area. Furthermore, special focus is given to the indirect of topography on the potential which, contrary to applying a planar model, does not vanish in the spherical approach.

The dynamics captured in the ocean surface current data provided by satellite altimetry has been a subject of debate since the past decade. In particular, the contribution of surface and interior dynamics to altimetry remains unclear. One avenue to settling this issue is to compare the turbulence (for example, the nature of spectra and interscale fluxes) captured by altimetry to theories of two-dimensional, surface and interior quasigeostrophic turbulence. In this thesis, we focus on mesoscales (i.e., scales of the order of few hundred kms) that are well resolved by altimetry data. Aspects of two dimensional, three dimensional, geotropic and surface quasigeostrophic turbulence are revisited and compared with the observations. Specifically, we compute kinetic energy (KE) spectra and fluxes in five geographical regions (all over the globe) using 21 years of 0.25◦resolution daily data as provided by the AVISO project. We report a strong forward cascade of KE at small scales (accompanied by a spectral scaling of the form k−3) and a robust inverse cascade at larger scales. Further, we show that the small diver-gent part in horizontal velocity data drives the strong forward flux of KE. Indeed, on considering only the non-divergent part of the flow, in accord with incompressible two-dimensional turbulence, the inverse cascade is unaffected, but the forward transfer becomes very weak and the spectral slopes over this range of scales tend to a relatively steeper k−3.5scaling. We note that our results do not agree with interior first bar clinic mode quasigeostrophic (incorrect strength of forward flux) or surface-quasigeostrophic (incorrect spectral slopes) turbulence. Rather, the results are compatible with rotating shallow water and rotating stratified Boussinesq models in which condition of geostrophic balance is dominant but the divergence of horizontal velocity field is not exactly zero. Having seen the “mean” picture of fluxes and spectra from altimetry, in the second part of the thesis we investigate the variability of these entities. In particular, we employ Empirical Or-thogonal Function (EOF) analysis and focus on the variability in the spectral flux. Remarkably, over the entire globe, irrespective of the region under consideration, we see that the first two EOFs explain a large part of the variability in flux anomalies. The geometry of these modes is distinct, the first represents a single signed transfer across scales (i.e. large to small or small to large depending on the sign of the associated principal component), while the second is a mixed mode in that it exhibits a forward/inverse transfer at large/small scales.