Frank Siegismund - Academia.edu (original) (raw)
Papers by Frank Siegismund
A major task of the GOCINA project is the determination of a Mean Dynamic Topography (MDT) for th... more A major task of the GOCINA project is the determination of a Mean Dynamic Topography (MDT) for the region between Greenland and the UK, north and south of the Greenland-Scotland Ridge. The MDT is the temporal average of the ocean's surface as measured by satellite altimetry relative to the geopotential height (or Geoid). The MDT gradient determines the geostrophic part of the surface circulation.
Journal of Geophysical Research: Oceans, 2013
ABSTRACT
Senckenbergiana maritima, 2001
ABSTRACT
As part of the GOCE User Toolbox Specification (GUTS) project, the GUTS team has carried out a sc... more As part of the GOCE User Toolbox Specification (GUTS) project, the GUTS team has carried out a scientific trade-off study, to select the best algorithms to fulfil the user requirements for the toolbox. In some cases the selection is straightforward. However, in other cases, the choice depends on scientific applications as well as the algorithm efficiency and more practical considerations. Studies need to be performed to demonstrate the best possible algorithm. One example is the selection of filtering functions and the choice of interpolation schemes used in calculation of a mean dynamic topography from combined GOCE and satellite altimeter data. The trade-off study has also selected the functionality of the toolbox, given the user requirements and the recommended algorithms. In this paper we present the proposed functionality of the toolbox, and the most important algorithm selections. As part of the scientific trade-off, a pilot application is investigating validation of mean dynamic topography, generated from pseudo observations using proposed toolbox algorithms, against ocean model data. The study includes results from research into methods of calculation of mean dynamic topography and filtering methods presented at this workshop.
While variations in the sea surface height and thus in the ocean currents can be derived directly... more While variations in the sea surface height and thus in the ocean currents can be derived directly from satellite altimeter data, an assessment of the absolute value of the ocean dynamic topography (and hence the absolute surface circulation) requires that the elevation of a hypothetical ocean at rest, i.e., the geoid, be subtracted from the altimetric mean sea surface height. The typical elevation scale of the dynamic topography is of the order of 0.1 to 1 m. The dilemma is that the precision of present geoid models is of the same size on the scale of many ocean-circulation features. Hence, the calculation of the mean dynamic topography using mean sea surface height and geoid information can not be satisfactory performed for wavelengths less than about 1000 km. In that spatial domain, the geoid model error becomes equal to or larger than the dynamic topography signals. The application of imprecise geoid models to the determination of dynamic topography at shorter spatial-scales can consequently result in the computation of false topographic signals (< 1m) which, in turn, correspond to erroneous transport calculations of several Sv (where 1 Sv = 10 6 m 3 /s). Transport uncertainties of this magnitude are of significance in climate studies. In this context the Gravity and Ocean Circulation Explorer (GOCE) mission planned for launch in early 2007 is therefore considered to be of significant importance.
Advanced Technologies in Earth Sciences, 2013
Advanced Technologies in Earth Sciences, 2010
ABSTRACT Results from several numerical ocean models were used in combination with available ocea... more ABSTRACT Results from several numerical ocean models were used in combination with available ocean bottom pressure data to quantitatively examine the skill of ocean models in simulating fast and slow bottom pressure variations and to test the quality of GRACE monthly fields of bottom pressure variations. The comparison between model simulations and bottom pressure data does reveal a substantial agreement between models and pressure measurements on high frequencies, but also some clear differences on longer time scales (> 1 year) that need to be corrected in order to improve estimates of the barotropic circulation. We also find a good agreement between monthly GRACE solutions and ECCO/GECCO syntheses that encourage us now to use the GRACE fields as constraints in ocean syntheses efforts. Differences of the order of a few centimeters appear consistent with previously estimated uncertainties provided by Quinn and Ponte (2008). There appears to be a large potential for assimilating GRACE data into ocean circulation models and thereby correct the seasonally varying barotropic circulation in the models, but results also highlight remaining uncertainties in the GRACE data.
International Association of Geodesy Symposia, 2007
Initially, existing mean dynamic topography (MDT) models were collected and reviewed. The models ... more Initially, existing mean dynamic topography (MDT) models were collected and reviewed. The models were corrected for the differences in averaging period using the annual anomalies computed from satellite altimetry. Then a composite MDT was derived as the mean value in each grid node together with a standard deviation to represent its error. A new synthetic MDT was obtained from the new mean sea surface (MSS) KMS04 combined with a regional geoid updated using GRACE gravity and gravimetric data from a recent airborne survey. Compared with the composite MDT the synthetic MDT showed very similar results.
sletter
In the framework of the ESA HPF (High Processing Facility), a number of gravity models have been ... more In the framework of the ESA HPF (High Processing Facility), a number of gravity models have been computed from the GOCE data since the beginning of the mission in 2009. In addition to the classical method (the so-called direct approach) that combines orbit and gravity modelling using the orbit perturbation theory, two alternative methods have been newly developed dedicated to the GOCE mission, i.e. the time-wise and the space-wise approaches. Also, after preliminary models based on 71 days of GOCE data were delivered in June 2010, new models have been made available recently, based on more than six months of data.
geodesy.unr.edu, 2007
The OCTAS project, Ocean Circulation and Transport Between North Atlantic and the Arctic Sea, fun... more The OCTAS project, Ocean Circulation and Transport Between North Atlantic and the Arctic Sea, funded by the Norwegian Research Council, is a multidisciplinary project combining geodesy, satellite altimetry and oceanography. The main objective is to ...
Journal of Geophysical Research, 2009
Journal of Geophysical Research, 2012
1] The impact of assimilating global ocean bottom pressure (OBP) information from the Gravity Rec... more 1] The impact of assimilating global ocean bottom pressure (OBP) information from the Gravity Recovery and Climate Experiment (GRACE) gravity anomalies on the circulation estimate was investigated. For this an estimate of the ocean circulation is being inferred by extending the 50-year-long German part of the Estimating the Circulation and Climate of the Ocean (GECCO) ocean synthesis into recent years. The assimilation system is an improved version of the previous GECCO optimization, which now includes a sea ice model, has enhanced resolution on a truly global domain including the Arctic Ocean. By analyzing differences to a synthesis that additionally assimilated OBP, the GRACE data was found to provide complementary information to standard ocean data sets including satellite altimetry when assimilated. Although in principle standard ocean data sets include the OBP information, the reason why this cannot be extracted is the much larger prior errors for hydrographic and altimeter data in comparison to OBP data owing to the fact that only the former two need to include the unresolved eddy signal. The largest impact of gravity data is found to be on the barotropic circulation, particularly in the subtropical gyres and the polar latitudes. Remaining differences between the simulated and observed OBP information are associated with meridional stripes in the GRACE gravity maps and with the leakage of terrestrial hydrological information into the ocean. Additional differences close to the continental boundaries are related to the self-attraction and loading, processes that are not included in the models.
Journal of Geophysical Research, 2011
Journal of Geophysical Research, 2007
Journal of Coastal Research, 2005
SIMIONATO, C.G.; VERA, C.S., and SIEGISMUND, F., 2005. Surface wind variability on seasonal and i... more SIMIONATO, C.G.; VERA, C.S., and SIEGISMUND, F., 2005. Surface wind variability on seasonal and interannual scales over Rio de la Plata area. Journal of Coastal Research, 21(4), 770-783. West Palm Beach (Florida), ISSN 0749-0208.
International Journal of Climatology, 2005
Existing studies on recent global warming are almost exclusively based on environmental data from... more Existing studies on recent global warming are almost exclusively based on environmental data from the Earth's surface. Seasonal information on the effects of climate change on subsurface settings of mid to high latitudes is extremely scarce. Here, we present the first temperature proxy record from bottom (c. 50 m) water settings of the North Sea employing the oxygen isotope composition of ocean quahog shells. Results indicate that δ 18 O aragonite measured across shells of Arctica islandica can provide reliable estimates (±0.25 to ±0.4°C) of the ambient bottom water temperatures. Over the period AD 1880-2001, warming trends in bottom waters are of the order of 0.042 to 0.138°C/decade. Apparently, the annual maximum-temperature trend shows a twofold increase over the past four decades (0.236°C/decade) while the minimum-temperature trend has remained relatively stable (0.042°C/decade). During the same time interval, however, annual maximum temperatures at the sea surface quadrupled. Shell oxygen-isotope-derived winter temperatures also provide a proxy for the winter North Atlantic oscillation index (WNAO). Some 28 to 50% of the variability in minimum temperatures below the thermocline can be explained by changes of the WNAO. Our new tool enables testing and verification of climate models prior to the 20th century greenhouse forcing.
Climate Research, 2001
ABSTRACT Using data from the NCEP/NCAR reanalysis for the 40 yr period from 1958 to 1997, a wind ... more ABSTRACT Using data from the NCEP/NCAR reanalysis for the 40 yr period from 1958 to 1997, a wind statistic for the North Sea is derived. This analysis includes consideration of the monthly mean wind speed and decadal variability in angular distribution. The wind density function is introduced, which combines frequency of occurence and mean wind speed for a given direction, The North Sea is subdivided into 3 sectors. Changes in the wind density function for the 3 regions are analysed for 2 seasons, October to January and February to March. The annual mean wind speed for the North Sea shows a rising trend of similar to 10 % during the last 40 yr, mainly restricted to the period from October to March. For October to January, west-southwesterly wind directions are enhanced in the last 3 decades compared to the period from 1958 to 1967 for the whole North Sea area. The last decade from 1988 to 1997 is outstanding: The duration of typical winter wind conditions, with high wind speeds and the prevalence of west-southwesterly winds, is extended from October to January in the first 3 decades towards February and March in the last one. The wind density functions for both seasons investigated and all of the 3 sectors resemble each other in the last decade. In the first 3 decades an enhancement of southerly wind directions is found in the northern part of the North Sea, northward of 59 degreesN. This trend is broken in the last decade when west-southwesterly directions for the whole North Sea area predominate.
As part of the GOCE User Toolbox Specification (GUTS) project, the GUTS team has carried out a sc... more As part of the GOCE User Toolbox Specification (GUTS) project, the GUTS team has carried out a scientific trade-off study, to select the best algorithms to fulfil the user requirements for the toolbox. In some cases the selection is straightforward. However, in other cases, the choice depends on scientific applications as well as the algorithm efficiency and more practical considerations. Studies need to be performed to demonstrate the best possible algorithm. One example is the selection of filtering functions and the choice of interpolation schemes used in calculation of a mean dynamic topography from combined GOCE and satellite altimeter data. The trade-off study has also selected the functionality of the toolbox, given the user requirements and the recommended algorithms. In this paper we present the proposed functionality of the toolbox, and the most important algorithm selections. As part of the scientific trade-off, a pilot application is investigating validation of mean dynamic topography, generated from pseudo observations using proposed toolbox algorithms, against ocean model data. The study includes results from research into methods of calculation of mean dynamic topography and filtering methods presented at this workshop.
A major task of the GOCINA project is the determination of a Mean Dynamic Topography (MDT) for th... more A major task of the GOCINA project is the determination of a Mean Dynamic Topography (MDT) for the region between Greenland and the UK, north and south of the Greenland-Scotland Ridge. The MDT is the temporal average of the ocean's surface as measured by satellite altimetry relative to the geopotential height (or Geoid). The MDT gradient determines the geostrophic part of the surface circulation.
Journal of Geophysical Research: Oceans, 2013
ABSTRACT
Senckenbergiana maritima, 2001
ABSTRACT
As part of the GOCE User Toolbox Specification (GUTS) project, the GUTS team has carried out a sc... more As part of the GOCE User Toolbox Specification (GUTS) project, the GUTS team has carried out a scientific trade-off study, to select the best algorithms to fulfil the user requirements for the toolbox. In some cases the selection is straightforward. However, in other cases, the choice depends on scientific applications as well as the algorithm efficiency and more practical considerations. Studies need to be performed to demonstrate the best possible algorithm. One example is the selection of filtering functions and the choice of interpolation schemes used in calculation of a mean dynamic topography from combined GOCE and satellite altimeter data. The trade-off study has also selected the functionality of the toolbox, given the user requirements and the recommended algorithms. In this paper we present the proposed functionality of the toolbox, and the most important algorithm selections. As part of the scientific trade-off, a pilot application is investigating validation of mean dynamic topography, generated from pseudo observations using proposed toolbox algorithms, against ocean model data. The study includes results from research into methods of calculation of mean dynamic topography and filtering methods presented at this workshop.
While variations in the sea surface height and thus in the ocean currents can be derived directly... more While variations in the sea surface height and thus in the ocean currents can be derived directly from satellite altimeter data, an assessment of the absolute value of the ocean dynamic topography (and hence the absolute surface circulation) requires that the elevation of a hypothetical ocean at rest, i.e., the geoid, be subtracted from the altimetric mean sea surface height. The typical elevation scale of the dynamic topography is of the order of 0.1 to 1 m. The dilemma is that the precision of present geoid models is of the same size on the scale of many ocean-circulation features. Hence, the calculation of the mean dynamic topography using mean sea surface height and geoid information can not be satisfactory performed for wavelengths less than about 1000 km. In that spatial domain, the geoid model error becomes equal to or larger than the dynamic topography signals. The application of imprecise geoid models to the determination of dynamic topography at shorter spatial-scales can consequently result in the computation of false topographic signals (< 1m) which, in turn, correspond to erroneous transport calculations of several Sv (where 1 Sv = 10 6 m 3 /s). Transport uncertainties of this magnitude are of significance in climate studies. In this context the Gravity and Ocean Circulation Explorer (GOCE) mission planned for launch in early 2007 is therefore considered to be of significant importance.
Advanced Technologies in Earth Sciences, 2013
Advanced Technologies in Earth Sciences, 2010
ABSTRACT Results from several numerical ocean models were used in combination with available ocea... more ABSTRACT Results from several numerical ocean models were used in combination with available ocean bottom pressure data to quantitatively examine the skill of ocean models in simulating fast and slow bottom pressure variations and to test the quality of GRACE monthly fields of bottom pressure variations. The comparison between model simulations and bottom pressure data does reveal a substantial agreement between models and pressure measurements on high frequencies, but also some clear differences on longer time scales (> 1 year) that need to be corrected in order to improve estimates of the barotropic circulation. We also find a good agreement between monthly GRACE solutions and ECCO/GECCO syntheses that encourage us now to use the GRACE fields as constraints in ocean syntheses efforts. Differences of the order of a few centimeters appear consistent with previously estimated uncertainties provided by Quinn and Ponte (2008). There appears to be a large potential for assimilating GRACE data into ocean circulation models and thereby correct the seasonally varying barotropic circulation in the models, but results also highlight remaining uncertainties in the GRACE data.
International Association of Geodesy Symposia, 2007
Initially, existing mean dynamic topography (MDT) models were collected and reviewed. The models ... more Initially, existing mean dynamic topography (MDT) models were collected and reviewed. The models were corrected for the differences in averaging period using the annual anomalies computed from satellite altimetry. Then a composite MDT was derived as the mean value in each grid node together with a standard deviation to represent its error. A new synthetic MDT was obtained from the new mean sea surface (MSS) KMS04 combined with a regional geoid updated using GRACE gravity and gravimetric data from a recent airborne survey. Compared with the composite MDT the synthetic MDT showed very similar results.
sletter
In the framework of the ESA HPF (High Processing Facility), a number of gravity models have been ... more In the framework of the ESA HPF (High Processing Facility), a number of gravity models have been computed from the GOCE data since the beginning of the mission in 2009. In addition to the classical method (the so-called direct approach) that combines orbit and gravity modelling using the orbit perturbation theory, two alternative methods have been newly developed dedicated to the GOCE mission, i.e. the time-wise and the space-wise approaches. Also, after preliminary models based on 71 days of GOCE data were delivered in June 2010, new models have been made available recently, based on more than six months of data.
geodesy.unr.edu, 2007
The OCTAS project, Ocean Circulation and Transport Between North Atlantic and the Arctic Sea, fun... more The OCTAS project, Ocean Circulation and Transport Between North Atlantic and the Arctic Sea, funded by the Norwegian Research Council, is a multidisciplinary project combining geodesy, satellite altimetry and oceanography. The main objective is to ...
Journal of Geophysical Research, 2009
Journal of Geophysical Research, 2012
1] The impact of assimilating global ocean bottom pressure (OBP) information from the Gravity Rec... more 1] The impact of assimilating global ocean bottom pressure (OBP) information from the Gravity Recovery and Climate Experiment (GRACE) gravity anomalies on the circulation estimate was investigated. For this an estimate of the ocean circulation is being inferred by extending the 50-year-long German part of the Estimating the Circulation and Climate of the Ocean (GECCO) ocean synthesis into recent years. The assimilation system is an improved version of the previous GECCO optimization, which now includes a sea ice model, has enhanced resolution on a truly global domain including the Arctic Ocean. By analyzing differences to a synthesis that additionally assimilated OBP, the GRACE data was found to provide complementary information to standard ocean data sets including satellite altimetry when assimilated. Although in principle standard ocean data sets include the OBP information, the reason why this cannot be extracted is the much larger prior errors for hydrographic and altimeter data in comparison to OBP data owing to the fact that only the former two need to include the unresolved eddy signal. The largest impact of gravity data is found to be on the barotropic circulation, particularly in the subtropical gyres and the polar latitudes. Remaining differences between the simulated and observed OBP information are associated with meridional stripes in the GRACE gravity maps and with the leakage of terrestrial hydrological information into the ocean. Additional differences close to the continental boundaries are related to the self-attraction and loading, processes that are not included in the models.
Journal of Geophysical Research, 2011
Journal of Geophysical Research, 2007
Journal of Coastal Research, 2005
SIMIONATO, C.G.; VERA, C.S., and SIEGISMUND, F., 2005. Surface wind variability on seasonal and i... more SIMIONATO, C.G.; VERA, C.S., and SIEGISMUND, F., 2005. Surface wind variability on seasonal and interannual scales over Rio de la Plata area. Journal of Coastal Research, 21(4), 770-783. West Palm Beach (Florida), ISSN 0749-0208.
International Journal of Climatology, 2005
Existing studies on recent global warming are almost exclusively based on environmental data from... more Existing studies on recent global warming are almost exclusively based on environmental data from the Earth's surface. Seasonal information on the effects of climate change on subsurface settings of mid to high latitudes is extremely scarce. Here, we present the first temperature proxy record from bottom (c. 50 m) water settings of the North Sea employing the oxygen isotope composition of ocean quahog shells. Results indicate that δ 18 O aragonite measured across shells of Arctica islandica can provide reliable estimates (±0.25 to ±0.4°C) of the ambient bottom water temperatures. Over the period AD 1880-2001, warming trends in bottom waters are of the order of 0.042 to 0.138°C/decade. Apparently, the annual maximum-temperature trend shows a twofold increase over the past four decades (0.236°C/decade) while the minimum-temperature trend has remained relatively stable (0.042°C/decade). During the same time interval, however, annual maximum temperatures at the sea surface quadrupled. Shell oxygen-isotope-derived winter temperatures also provide a proxy for the winter North Atlantic oscillation index (WNAO). Some 28 to 50% of the variability in minimum temperatures below the thermocline can be explained by changes of the WNAO. Our new tool enables testing and verification of climate models prior to the 20th century greenhouse forcing.
Climate Research, 2001
ABSTRACT Using data from the NCEP/NCAR reanalysis for the 40 yr period from 1958 to 1997, a wind ... more ABSTRACT Using data from the NCEP/NCAR reanalysis for the 40 yr period from 1958 to 1997, a wind statistic for the North Sea is derived. This analysis includes consideration of the monthly mean wind speed and decadal variability in angular distribution. The wind density function is introduced, which combines frequency of occurence and mean wind speed for a given direction, The North Sea is subdivided into 3 sectors. Changes in the wind density function for the 3 regions are analysed for 2 seasons, October to January and February to March. The annual mean wind speed for the North Sea shows a rising trend of similar to 10 % during the last 40 yr, mainly restricted to the period from October to March. For October to January, west-southwesterly wind directions are enhanced in the last 3 decades compared to the period from 1958 to 1967 for the whole North Sea area. The last decade from 1988 to 1997 is outstanding: The duration of typical winter wind conditions, with high wind speeds and the prevalence of west-southwesterly winds, is extended from October to January in the first 3 decades towards February and March in the last one. The wind density functions for both seasons investigated and all of the 3 sectors resemble each other in the last decade. In the first 3 decades an enhancement of southerly wind directions is found in the northern part of the North Sea, northward of 59 degreesN. This trend is broken in the last decade when west-southwesterly directions for the whole North Sea area predominate.
As part of the GOCE User Toolbox Specification (GUTS) project, the GUTS team has carried out a sc... more As part of the GOCE User Toolbox Specification (GUTS) project, the GUTS team has carried out a scientific trade-off study, to select the best algorithms to fulfil the user requirements for the toolbox. In some cases the selection is straightforward. However, in other cases, the choice depends on scientific applications as well as the algorithm efficiency and more practical considerations. Studies need to be performed to demonstrate the best possible algorithm. One example is the selection of filtering functions and the choice of interpolation schemes used in calculation of a mean dynamic topography from combined GOCE and satellite altimeter data. The trade-off study has also selected the functionality of the toolbox, given the user requirements and the recommended algorithms. In this paper we present the proposed functionality of the toolbox, and the most important algorithm selections. As part of the scientific trade-off, a pilot application is investigating validation of mean dynamic topography, generated from pseudo observations using proposed toolbox algorithms, against ocean model data. The study includes results from research into methods of calculation of mean dynamic topography and filtering methods presented at this workshop.