An attempt to observe vertical land motion along the norwegian coast by CryoSat-2 and tide gauges

Present-day climate-change-related ice-melting induces elastic glacial isostatic adjustment (GIA) effects, while paleo-GIA effects describe the ongoing viscous response to the melting of late-Pleistocene ice sheets. The unloading initiated an uplift of the crust close to the centers of former ice sheets. Today, vertical land motion (VLM) rates in Fennoscandia reach values up to around 10 mm/year and are dominated by GIA. Uplift signals from GIA can be computed by solving the sea-level equation (SLE), S ˙ = N ˙ − U ˙ . All three quantities can also be determined from geodetic observations: relative sea-level variations ( S ˙ ) are observed by means of tide gauges, while rates of absolute sea-level change ( N ˙ ) can be observed by satellite altimetry; rates of VLM ( U ˙ ) can be determined by GPS (Global Positioning System). Based on the SLE, U ˙ can be derived by combining sea-surface measurements from satellite altimetry and relative sea-level records from tide gauges. In the present study, we have combined 7.5 years of CryoSat-2 satellite altimetry and tide-gauge data to estimate linear VLM rates at 20 tide gauges along the Norwegian coast. Thereby, we made use of monthly averaged tide-gauge data from PSMSL (Permanent Service for Mean Sea Level) and a high-frequency tide-gauge data set with 10-min sampling rate from NMA (Norwegian Mapping Authority). To validate our VLM estimates, we have compared them with the independent semi-empirical land-uplift model NKG2016LU_abs for the Nordic-Baltic region, which is based on GPS, levelling, and geodynamical modeling. Estimated VLM rates from 1 Hz CryoSat-2 and high-frequency tide-gauge data reflect well the amplitude of coastal VLM as provided by NKG2016LU_abs. We find a coastal average of 2.4 mm/year (average over all tide gauges), while NKG2016LU_abs suggests 2.8 mm/year; the spatial correlation is 0.58

Postglacial gravity change in Fennoscandia - three decades of repeated absolute gravity observations

For the first time, we present a complete, processed compilation of all repeated absolute gravity (AG) observations in the Fennoscandian postglacial land uplift area and assess their ability to accurately describe the secular gravity change, induced by Glacial Isostatic Adjustment (GIA). The dataset spans over more than three decades and consists of 688 separate observations at 59 stations. Ten different organisations have contributed with measurements using 14 different instruments. The work was coordinated by the Nordic Geodetic Commisson (NKG). Representatives from each country collected and processed data from their country, respectively, and all data were then merged to one dataset. Instrumental biases are considered and presented in terms of results from international comparisons of absolute gravimeters. From this dataset, gravity rates of change (g_dot) are estimated for all stations with more than two observations and a timespan larger than two years. The observed rates are compared to predicted rates from a global GIA model as well as the state of the art semi-empirical land uplift model for Fennoscandia, NKG2016LU. Linear relations between observed g_dot and the land uplift, h_dot (NKG2016LU), are estimated from the absolute gravity observations by means of weighted least squares adjustment (WLSA) as well as weighted orthogonal distance regression (WODR). The empirical relations are not significantly different from the modelled, geophysical relation g_dot = 0:03 - 0:163(+-0.016)h_dot. We also present a g_dot -model for the whole Fennoscandian land uplift region. At many stations, the observational estimates of g_dot still suffer from few observations and/or unmodelled environmental effects (e.g. local hydrology). We therefore argue that, at present, the best predictions of GIA-induced gravity rate of change in Fennoscandia are achieved by means of the NKG2016LU land uplift model, together with the geophysical relation between g_dot and h_dot

An Accuracy Assessment of Absolute Gravimetric Observations in Fennoscandia

We compare a suite of absolute gravimeters used to monitor the temporal changes of gravity at a number of sites in Fennoscandia. Direct comparisons are made from simultaneous observations at selected sites within and outside of the postglacial uplift region. We also compare results at sites visited by two instruments with some separation in time. We conclude from four years of data that gravity differences are obtained within an rms error of ± 3 Gal. The data reveal no systematic biases between the instruments, but occasional shifts from one year to another are noted. We consider that annual instrument comparisons are required to ensure data integrity in a regional observing program that extends over more than a decade

Undersøkelser av belastningsfenomener på jordens overflate - geometriske deformasjoner og endringer i tyngdefeltet

This thesis addresses deformations and gravity changes due to surface loads like the ocean tides, hydrology and glaciers. These phenomena are discussed in light of height and gravity observations collected by GPS and gravimeters of the FG5 and LaCoste & Romberg types. A surface load is here defined as a mass resting at the surface of the Earth. Body loading due to the Earth tides is consequently outside the scope of this thesis. The analysis is further restricted to address elastic processes only, i.e. loading effects in phase with the time history of the load. Viscoelastic processes like glacial isostatic adjustment are not discussed in depth. A significant part of this thesis addresses ocean tide loading (OTL). The phenomenon is theoretically discussed and observational results are provided. A suite of global OTL models was compared to gravity and GPS time series at coastal stations in Norway. It was found that global models are in phase with the observations and only millimeter discrepancies exist between the magnitude of GPS observations and OTL models. When it comes to the magnitude of the gravity signals, best agreement was obtained by OTL corrections calculated from FES2004 and NAO99b. However, at several stations we observe periodic residuals of nearly 10 μgal amplitude. To reduce the weighted standard deviation of the gravity time series, an alternative method was developed for calculating OTL corrections. The method was based on locally observed ocean tides and a global OTL model for vertical displacement. Compared to global models, the alternative method reduced the RMS by up to 40 %. The gravitational effect of hydrology was investigated in Trysil. Trysil is located inland Norway and our observations have revealed seasonal gravity changes of nearly 20 μgal. A hydrological model was developed from snow depth readings, well readings, and precipitation data. Compared to a three year long gravity time series, the model explained 64 % of the variation and reduced the amplitude of the seasonal signal strongly. More than 90 % of the gravity signal from the hydrology was formed by the snow cover within 200 m of the gravity laboratory. The thesis also presents a high accuracy gravity network for Norway. The network includes 16 stations with gravity estimates accurate to 3-4 μgal. Compared to previously published values, this is an improvement of one order of magnitude. The gravity values will change by up to 1 μgal annually due to glacial isostatic adjustment. Finally, attempts were made to use ground based relative gravity observations to measure the mass balance of a glacier. Preliminary results show that the method can resolve the mass balance within 10 % of the loss determined by conventional mass balance measurements. It still remains to fully validate the methodology in field.Denne avhandlingen tar for seg jordskorpedeformasjoner og endringer i jordens tyngdefelt som skyldes belastningsfenomener på jordens overflate. Belastninger kan oppstå som et resultat av for eksempel tidevann, snø, grunnvann og overflatevann etter et kraftig regnfall. Belastningsfenomenene er diskutert i lys av observasjoner samlet ved hjelp av GPS og gravimetre av FG5 og LaCoste & Romberg typen. Avhandlingen begrenser seg til belastninger som finner sted på jordens overflate og kun elastiske prosesser. Tidejordsfenomenet og viskoelastisk landhevning faller derfor utenfor avhandlingens tematiske avgrensning. En vesentlig del av avhandlingen fokuserer på fenomenet ocean tide loading (OTL). For en samling stasjoner langs norskekysten er tidsserier av tyngde og GPS observasjoner sammenliknet med OTL signaler beregnet ut fra fritt tilgjengelige globale tidevannsmodeller. Tidsforløpet til modellene (fasen) er i godt samsvar med observasjonene. Det samme gjelder størrelsen til modellerte vertikale deformasjoner. Derimot underestimerer modellene OTL signalene i tyngdeobservasjonene på flere stasjoner. Resultatet er periodiske residualer med oppmot 10 μgal amplitude. Alt i alt fungerer modellene FES2004 og NAO99b best langs norskekysten. Likevel etterlyses bedre globale OTL modeller for tyngde i dette området. En alternativ metode for å beregne endringer i tyngdekraften som skyldes OTL har blitt utviklet. Metoden kombinerer lokalt observert tidevann med en global OTL modell for vertikale deformasjoner. Sammenliknet med de beste globale OTL modellene, gir denne tilnærmingen opptil 40 % lavere RMS. Hydrologisk innvirkning på tyngdemålinger ble undersøkt i Trysil. I Trysil observerer vi at tyngdekraften varierer med nesten 20 μgal gjennom et år. Dette skyldes i hovedsak varierende hydrologi. En hydrologisk modell basert på observerte snødybder, grunnvannstand og nedbørsmålinger ble utviklet. Modellen forklarer 64 % av tyngdemålingenes variasjon. Det største bidraget kommer fra snødekket innenfor 200 m fra tyngdeobservatoriet. Alene utgjør denne komponenten 90 %. Avhandlingen presenterer også et førsteordens nettverk av tyngdestasjoner i Norge. Nettverket består av 16 stasjoner med tyngdeverdier av nøyaktighet 3 til 4 μgal. Dette er en størrelsesorden bedre enn tidligere publiserte verdier for området. Det forventes at tyngdeverdiene vil endre seg med oppimot 1 μgal årlig på grunn av landhevning. Til sist diskuteres forsøk på å måle isbreers massebalanse ved hjelp av et bakkebasert relativgravimeter (LaCoste & Romberg). Det gjenstår fremdeles å teste metoden fullt ut i felt. Foreløpige resultater tyder likevel på at den utviklede metoden stemmer innenfor 10 % med tradisjonelle massebalansemålinger

Investigations of surface loads of the Earth - geometrical deformations and gravity changes

This thesis addresses deformations and gravity changes due to surface loads like the ocean tides, hydrology and glaciers. These phenomena are discussed in light of height and gravity observations collected by GPS and gravimeters of the FG5 and LaCoste & Romberg types. A surface load is here defined as a mass resting at the surface of the Earth. Body loading due to the Earth tides is consequently outside the scope of this thesis. The analysis is further restricted to address elastic processes only, i.e. loading effects in phase with the time history of the load. Viscoelastic processes like glacial isostatic adjustment are not discussed in depth. A significant part of this thesis addresses ocean tide loading (OTL). The phenomenon is theoretically discussed and observational results are provided. A suite of global OTL models was compared to gravity and GPS time series at coastal stations in Norway. It was found that global models are in phase with the observations and only millimeter discrepancies exist between the magnitude of GPS observations and OTL models. When it comes to the magnitude of the gravity signals, best agreement was obtained by OTL corrections calculated from FES2004 and NAO99b. However, at several stations we observe periodic residuals of nearly 10 μgal amplitude. To reduce the weighted standard deviation of the gravity time series, an alternative method was developed for calculating OTL corrections. The method was based on locally observed ocean tides and a global OTL model for vertical displacement. Compared to global models, the alternative method reduced the RMS by up to 40 %. The gravitational effect of hydrology was investigated in Trysil. Trysil is located inland Norway and our observations have revealed seasonal gravity changes of nearly 20 μgal. A hydrological model was developed from snow depth readings, well readings, and precipitation data. Compared to a three year long gravity time series, the model explained 64 % of the variation and reduced the amplitude of the seasonal signal strongly. More than 90 % of the gravity signal from the hydrology was formed by the snow cover within 200 m of the gravity laboratory. The thesis also presents a high accuracy gravity network for Norway. The network includes 16 stations with gravity estimates accurate to 3-4 μgal. Compared to previously published values, this is an improvement of one order of magnitude. The gravity values will change by up to 1 μgal annually due to glacial isostatic adjustment. Finally, attempts were made to use ground based relative gravity observations to measure the mass balance of a glacier. Preliminary results show that the method can resolve the mass balance within 10 % of the loss determined by conventional mass balance measurements. It still remains to fully validate the methodology in field

Observed sea-level changes along the Norwegian coast

publishedVersio

An Attempt to Observe Vertical Land Motion along the Norwegian Coast by CryoSat-2 and Tide Gauges

Present-day climate-change-related ice-melting induces elastic glacial isostatic adjustment (GIA) effects, while paleo-GIA effects describe the ongoing viscous response to the melting of late-Pleistocene ice sheets. The unloading initiated an uplift of the crust close to the centers of former ice sheets. Today, vertical land motion (VLM) rates in Fennoscandia reach values up to around 10 mm/year and are dominated by GIA. Uplift signals from GIA can be computed by solving the sea-level equation (SLE), ˙S = ˙N ˙U . All three quantities can also be determined from geodetic observations: relative sea-level variations (˙S ) are observed by means of tide gauges, while rates of absolute sea-level change ( ˙N ) can be observed by satellite altimetry; rates of VLM ( ˙U) can be determined by GPS (Global Positioning System). Based on the SLE, ˙Ucan be derived by combining sea-surface measurements from satellite altimetry and relative sea-level records from tide gauges. In the present study, we have combined 7.5 years of CryoSat-2 satellite altimetry and tide-gauge data to estimate linear VLM rates at 20 tide gauges along the Norwegian coast. Thereby, we made use of monthly averaged tide-gauge data from PSMSL (Permanent Service for Mean Sea Level) and a high-frequency tide-gauge data set with 10-min sampling rate from NMA (Norwegian Mapping Authority). To validate our VLM estimates, we have compared them With the independent semi-empirical land-uplift model NKG2016LU_abs for the Nordic-Baltic region, which is based on GPS, levelling, and geodynamical modeling. Estimated VLM rates from 1 Hz CryoSat-2 and high-frequency tide-gauge data reflect well the amplitude of coastal VLM as provided by NKG2016LU_abs. We find a coastal average of 2.4 mm/year (average over all tide gauges), while NKG2016LU_abs suggests 2.8 mm/year; the spatial correlation is 0.58