Geodetic Institute

Hønefoss, Norway

Geodetic Institute

Hønefoss, Norway
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Simpson M.J.R.,Geodetic Institute | Breili K.,Geodetic Institute | Kierulf H.P.,Geodetic Institute | Kierulf H.P.,University of Oslo
Climate Dynamics | Year: 2014

In this work we establish a framework for estimating future regional sea-level changes for Norway. Following recently published works, we consider how different physical processes drive non-uniform sea-level changes by accounting for spatial variations in (1) ocean density and circulation (2) ice and ocean mass changes and associated gravitational effects on sea level and (3) vertical land motion arising from past surface loading change and associated gravitational effects on sea level. An important component of past and present sea-level change in Norway is glacial isostatic adjustment. Central to our study, therefore, is a reassessment of vertical land motion using a far larger set of new observations from a permanent GNSS network. Our twenty-first century sea-level estimates are split into two parts. Firstly, we show regional projections largely based on findings from the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR4) and dependent on the emission scenarios A2, A1B and B1. These indicate that twenty-first century relative sea-level changes in Norway will vary between -0.2 to 0.3 m (1-sigma ± 0.13 m). Secondly, we explore a high-end scenario, in which a global atmospheric temperature rise of up to 6 °C and emerging collapse for some areas of the Antarctic ice sheets are assumed. Using this approach twenty-first century relative sea-level changes in Norway are found to vary between 0.25 and 0.85 m (min/max ± 0.45 m). We attach no likelihood to any of our projections owing to the lack of understanding of some of the processes that cause sea-level change. © 2014 The Author(s).

Omang O.C.D.,Geodetic Institute | Kierulf H.P.,Geodetic Institute | Kierulf H.P.,University of Oslo
Geophysical Research Letters | Year: 2011

We use Superconducting Gravimeter (SG) and Global Positioning System (GPS) measurements from Ny-lesund, Svalbard, to infer changes in ice mass loss between September 1999 and September 2010. We find that during this period, the gravity rate and vertical crustal velocities are changing with time, adding to evidence about varying rates of ice mass loss. The gravity rate varies through 10 years of observation;-0.23 μGal/yr in 2000-2002,-3.22 μGal/yr in 2002-2005 and-1.10 μGal/yr in 2005-2010. The gravity changes agree well with the observed uplift rates measured by GPS, which are 4.4, 11.3 and 7.4 mm/yr, over the same periods. In addition, we generate model predictions which account for past and present-day ice mass variation. We find that the models under predict both the observed uplift rates and gravity changes. Copyright 2011 by the American Geophysical Union.

Kierulf H.P.,Geodetic Institute | Kierulf H.P.,University of Oslo | Ouassou M.,Geodetic Institute | Simpson M.J.R.,Geodetic Institute | Vestol O.,Geodetic Institute
Journal of Geodesy | Year: 2013

In Norway, as in the rest of Fennoscandia, the process of Glacial Isostatic Adjustment causes ongoing crustal deformation. The vertical and horizontal movements of the Earth can be measured to a high degree of precision using GNSS. The Norwegian GNSS network has gradually been established since the early 1990s and today contains approximately 140 stations. The stations are established both for navigation purposes and for studies of geophysical processes. Only a few of these stations have been analyzed previously. We present new velocity estimates for the Norwegian GNSS network using the processing package GAMIT. We examine the relation between time-series length and precision. With approximately 3. 5 years of data, we are able to reproduce the secular vertical rate with a precision of 0. 5 mm/year. To establish a continuous crustal velocity field in areas where we have no GNSS receivers or the observation period is too short to obtain reliable results, either interpolation or modeling is required. We experiment with both approaches in this analysis by using (i) a statistical interpolation method called Kriging and (ii) a GIA forward model. In addition, we examine how our vertical velocity field solution is affected by the inclusion of data from repeated leveling. Results from our geophysical model give better estimates on the edge of the network, but inside the network the statistical interpolation method performs better. In general, we find that if we have less than 3. 5 years of data for a GNSS station, the interpolated value is better than the velocity estimate based on a single time-series. © 2012 The Author(s).

Kierulf H.P.,Geodetic Institute | Kierulf H.P.,University of Oslo | Steffen H.,Lantmateriet | Simpson M.J.R.,Geodetic Institute | And 3 more authors.
Journal of Geophysical Research B: Solid Earth | Year: 2014

In Fennoscandia, the process of Glacial Isostatic Adjustment (GIA) drives ongoing crustal deformation. Crustal velocities from GPS observations have proved to be a useful tool in constraining GIA models. However, reference frame uncertainties, plate tectonics, intraplate deformations as well as other geophysical processes contaminate the results. Former studies have shown that different International Terrestrial Reference Frames have large discrepancies, especially in the vertical component, which hamper geophysical interpretation. We present new velocity estimates for the Fennoscandian and North European GPS network. Our GPS velocity field is directly realized in a GIA reference frame. Using this method (named the GIA frame approach) we are able to constrain GIA models with minimal influence of errors in the reference frame or biasing signals from plate tectonics. The drawbacks are more degrees of freedom that might mask real but unmodeled signals. Monte Carlo tests suggest that our approach is robust at the 97% level in terms of correctly separating different models of ice history but, depending on deformation patterns, the identified Earth model may be slightly biased in up to 39% of cases. We compare our results to different one- and three-dimensional GIA models employing different global ice-load histories. The GIA models generally provide good fit to the data but there are still significant discrepancies in some areas. We suggest that these differences are mainly related to inaccuracies in the ice models and/or lateral inhomogeneities in the Earth structure under Fennoscandia. Thus, GIA models still need to be improved, but the GIA frame approach provides a base for further improvements. © 2014. American Geophysical Union.

Lecavalier B.S.,University of Ottawa | Milne G.A.,University of Ottawa | Simpson M.J.R.,Geodetic Institute | Wake L.,Northumbria University | And 9 more authors.
Quaternary Science Reviews | Year: 2014

An ice sheet model was constrained to reconstruct the evolution of the Greenland Ice Sheet (GrIS) from the Last Glacial Maximum (LGM) to present to improve our understanding of its response to climate change. The study involved applying a glaciological model in series with a glacial isostatic adjustment and relative sea-level (RSL) model. The model reconstruction builds upon the work of Simpson etal. (2009) through four main extensions: (1) a larger constraint database consisting of RSL and ice extent data; model improvements to the (2) climate and (3) sea-level forcing components; (4) accounting for uncertainties in non-Greenland ice. The research was conducted primarily to address data-model misfits and to quantify inherent model uncertainties with the Earth structure and non-Greenland ice. Our new model (termed Huy3) fits the majority of observations and is characterised by a number of defining features. During the LGM, the ice sheet had an excess of 4.7m ice-equivalent sea-level (IESL), which reached a maximum volume of 5.1m IESL at 16.5calka BP. Modelled retreat of ice from the continental shelf progressed at different rates and timings in different sectors. Southwest and Southeast Greenland began to retreat from the continental shelf by ~16 to 14calka BP, thus responding in part to the Bølling-Allerød warm event (c. 14.5calka BP); subsequently ice at the southern tip of Greenland readvanced during the Younger Dryas cold event. In northern Greenland the ice retreated rapidly from the continental shelf upon the climatic recovery out of the Younger Dryas to present-day conditions. Upon entering the Holocene (11.7calka BP), the ice sheet soon became land-based. During the Holocene Thermal Maximum (HTM; 9-5calka BP), air temperatures across Greenland were marginally higher than those at present and the GrIS margin retreated inland of its present-day southwest position by 40-60km at 4calka BP which produced a deficit volume of 0.16m IESL relative to present. In response to the HTM warmth, our optimal model reconstruction lost mass at a maximum centennial rate of c. 103.4 Gt/yr. Our results suggest that remaining data-model discrepancies are affiliated with missing physics and sub-grid processes of the glaciological model, uncertainties in the climate forcing, lateral Earth structure, and non-Greenland ice (particularly the North American component). Finally, applying the Huy3 Greenland reconstruction with our optimal Earth model we generate present-day uplift rates across Greenland due to past changes in the ocean and ice loads with explicit error bars due to uncertainties in the Earth structure. Present-day uplift rates due to past changes are spatially variable and range from 3.5 to -7 mm/a (including Earth model uncertainty). © 2014 Elsevier Ltd.

Omang O.C.,Geodetic Institute | Tscherning C.C.,Copenhagen University | Forsberg R.,Danish National Space Center
International Association of Geodesy Symposia | Year: 2012

In gravity field modeling measurements are usually located on or above the terrain. However, when using the residual topographic modeling (RTM) method, measurements may end up inside the masses after adding the mean topography. These values do not correspond to values evaluated using a harmonic function. A so-called harmonic correction has been applied to gravity anomalies to solve this problem. However, for height anomalies no correction has been applied. To generalize the correction to e.g. height anomalies we interprete that the vertical gravity gradient inside the masses multiplied by height equals the correction. In principle the procedure is applicable to all gravity field functionals. We have tested this generalization of the procedure which consist in determining equivalent quantities in points Q on the mean surface if this surface is in free air. The procedure has as data the reduced values in P inside the masses but considered as being located at the mean surface. Numerical tests with height anomaly data from New Mexico and Norway as control data show that for gravity anomalies the general procedure is better than using the original harmonic correction procedure. © Springer-Verlag Berlin Heidelberg 2012.

Simpson M.J.R.,Geodetic Institute | Wake L.,University of Calgary | Milne G.A.,University of Ottawa | Huybrechts P.,Vrije Universiteit Brussel
Journal of Geophysical Research: Solid Earth | Year: 2011

We show predictions of present-day vertical land motion in Greenland using a recently developed glacial isostatic adjustment (GIA) model, calibrated using both relative sea level (RSL) observations and geomorphological constraints on ice extent. Predictions from our GIA model are in agreement with the relatively small number of GPS measurements of absolute vertical motion from south and southwest Greenland. This suggests that our model of ice sheet evolution over the Holocene period is reasonably accurate. The uplift predictions are highly sensitive to variations of upper mantle viscosity. Thus, depending on the Earth model adopted, different periods of ice loading change dominate the present-day response in particular regions of Greenland. We also consider the possible influence of more recent changes in the ice sheet by applying a second ice model; specifically, a surface mass balance (SMB) model, which covers the period 1866 to 2005. Predictions from this model suggest that decadal-scale SMB changes over the past ∼140 years play only a small role in determining the present-day viscous response (at the sub-mm/yr level in most locations for a range of Earth model parameters). High rates of peripheral thinning from 1995 to 2005 predicted using the SMB model produce large elastic uplift rates (∼6 mm/yr) in west and southwest Greenland. This suggests that in some areas close to the ice margin, modern surface mass balance changes have a dominant control on present-day vertical land motion. Copyright © 2011 by the American Geophysical Union.

Mysen E.,Geodetic Institute
Journal of Geodesy | Year: 2016

The Kalman filter is derived directly from the least-squares estimator, and generalized to accommodate stochastic processes with time variable memory. To complete the link between least-squares estimation and Kalman filtering of first-order Markov processes, a recursive algorithm is presented for the computation of the off-diagonal elements of the a posteriori least-squares error covariance. As a result of the algebraic equivalence of the two estimators, both approaches can fully benefit from the advantages implied by their individual perspectives. In particular, it is shown how Kalman filter solutions can be integrated into the normal equation formalism that is used for intra- and inter-technique combination of space geodetic data. © 2016 Springer-Verlag Berlin Heidelberg

Mysen E.,Geodetic Institute
Journal of Applied Geodesy | Year: 2015

We have subtracted known contributions from the GOCE gradiometer data that forms the basis of the fourth generation time-wise gravity field solution (TIM4), and mapped the residuals onto a local mascon model covering the south of Norway. Quasigeoids derived from combinations of the mascon adjusted TIM4 solution and EGM08 were then compared to height anomalies of a precise and dense GPS/levelling network. It was found that a mascon adjustment of TIM4 based on the first satellite cycle could improve the consistency between the corresponding quasigeoid and the network by about 0.5 cm. If the data foundation of the mascon solution was expanded to include also the next two-three satellite cycles then an improved consistency between the corresponding quasigeoid and the network height anomalies appeared on all spatial scales. © 2015 by Walter de Gruyter.

Mysen E.,Geodetic Institute
International Journal of Applied Earth Observation and Geoinformation | Year: 2015

We have compared the EGM08/GOCE quasigeoid with the height anomalies of a precise GPS/levelling network in the south of Norway where the GOCE ground tracks are dense and the gravitational signal has been described as rough. It was found that the inclusion of the GOCE gravity potential can improve the quasigeoid fit with the GPS/levelling network by 1.3 cm, and that this improvement takes place onspatial scales larger than 80 km. We therefore expect that GOCE will improve our knowledge of the marinegeoid. It is argued that the obtained results cannot be used in a direct way to increase the precision of the Norwegian height system in general due to the short distances between the network points. © 2013 Elsevier B.V.

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