Coastal Sea Level from CryoSat-2 SARIn Altimetry in Norway

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Forecast uncertainty and ensemble spread in surface currents from a regional ocean model

An operational ocean Ensemble Prediction System (EPS) for the coastal seas off Northern Norway is evaluated by comparing with high-frequency radar current speed estimates. The EPS is composed of 24 members for which the ocean current is not perturbed nor constrained but forced with an atmosphere ensemble. The ocean ensemble spread stems from (i) accumulated differences in wind-forcing history and (ii) constraints of sea surface temperature by data assimilation. The intention of the ensemble is to reflect the actual uncertainty in initial conditions, which are largely unknown in terms of mesoscale circulation. We find a low but pronounced predictive skill in surface currents along with a good statistic skill. Additionally, current speeds show deterioration of the validation metrics over the forecast range. Further, high-resolution wind forcing seems to provide better forecast skill in currents compared to lower resolution forcing. In general, the ensemble exhibits the ability to predict forecast uncertainty

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

Global geopotential models from 2006 to 2014

Globalni geopotencijalni modeli visoke rezolucije imaju ključnu ulogu u geodeziji i geoznanostima, od praktične upotrebe, poput preciznog određivanja orbita, do znanstvene primjene, poput istraživanja gustoće struktura Zemljine unutrašnjosti. Današnji globalni modeli gravitacijskog polja, dobiveni uglavnom satelitskim mjerenjima, postaju sve detaljniji i precizniji. Važan zadatak geodezije je osigurati dostupnost funkcionala gravitacijskog polja ostalim geoznanostima. Nužno je izračunati odgovarajuće funkcionale što je moguće točnije iz danih globalnih modela te, ukoliko je potrebno, uzimanjem u obzir topografskih modela određenih modernim satelitskim metodama neovisno o gravitacijskom polju. Ovaj je rad nastao na temelju seminarskih radova iz kolegija “Određivanje oblika Zemlje” u akademskoj godini 2013./2014. (diplomski studij Geodetskog fakulteta, Zavod za geomatiku, Katedra za državnu izmjeru). Prikazane su usporedbe undulacija geoida i anomalija slobodnog zraka geopotencijalnih modela iz 2006., 2008., 2011., 2012., 2013. i 2014. godine.High-resolution global gravity field models play a fundamental role in geodesy and Earth sciences, ranging from practical purposes, like precise orbit determination, to scientific applications, like investigation of the density structure of the Earth’s interior. Nowadays, global gravity field models, mainly derived from satellite measurements, become more and more detailed and accurate. An important task of geodesy is to make the gravity field functionals available to other geosciences. It is necessary to calculate the corresponding functionals as accurately as possible from a given global gravity field models and, if required, with simultaneous consideration of the topography models determined by modern satellite methods independently from the gravity field. This paper is based on seminar papers which were made within the subject “Determination of the Earth’s shape” in academic year 2013/2014 (Master Studies, Department of Geomatics at the Faculty of Geodesy, Chair for State Survey). Comparisons of geoid undulations and free air gravity anomalies between geopotential models of 2006, 2008, 2011, 2012, 2013 and 2014 are presented

The coastal mean dynamic topography in Norway observed by CryoSat-2 and GOCE

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Altimetry for the future: Building on 25 years of progress

In 2018 we celebrated 25 years of development of radar altimetry, and the progress achieved by this methodology in the fields of global and coastal oceanography, hydrology, geodesy and cryospheric sciences. Many symbolic major events have celebrated these developments, e.g., in Venice, Italy, the 15th (2006) and 20th (2012) years of progress and more recently, in 2018, in Ponta Delgada, Portugal, 25 Years of Progress in Radar Altimetry. On this latter occasion it was decided to collect contributions of scientists, engineers and managers involved in the worldwide altimetry community to depict the state of altimetry and propose recommendations for the altimetry of the future. This paper summarizes contributions and recommendations that were collected and provides guidance for future mission design, research activities, and sustainable operational radar altimetry data exploitation. Recommendations provided are fundamental for optimizing further scientific and operational advances of oceanographic observations by altimetry, including requirements for spatial and temporal resolution of altimetric measurements, their accuracy and continuity. There are also new challenges and new openings mentioned in the paper that are particularly crucial for observations at higher latitudes, for coastal oceanography, for cryospheric studies and for hydrology. The paper starts with a general introduction followed by a section on Earth System Science including Ocean Dynamics, Sea Level, the Coastal Ocean, Hydrology, the Cryosphere and Polar Oceans and the ‘‘Green” Ocean, extending the frontier from biogeochemistry to marine ecology. Applications are described in a subsequent section, which covers Operational Oceanography, Weather, Hurricane Wave and Wind Forecasting, Climate projection. Instruments’ development and satellite missions’ evolutions are described in a fourth section. A fifth section covers the key observations that altimeters provide and their potential complements, from other Earth observation measurements to in situ data. Section 6 identifies the data and methods and provides some accuracy and resolution requirements for the wet tropospheric correction, the orbit and other geodetic requirements, the Mean Sea Surface, Geoid and Mean Dynamic Topography, Calibration and Validation, data accuracy, data access and handling (including the DUACS system). Section 7 brings a transversal view on scales, integration, artificial intelligence, and capacity building (education and training). Section 8 reviews the programmatic issues followed by a conclusion

Altimetry for the future: building on 25 years of progress

In 2018 we celebrated 25 years of development of radar altimetry, and the progress achieved by this methodology in the fields of global and coastal oceanography, hydrology, geodesy and cryospheric sciences. Many symbolic major events have celebrated these developments, e.g., in Venice, Italy, the 15th (2006) and 20th (2012) years of progress and more recently, in 2018, in Ponta Delgada, Portugal, 25 Years of Progress in Radar Altimetry. On this latter occasion it was decided to collect contributions of scientists, engineers and managers involved in the worldwide altimetry community to depict the state of altimetry and propose recommendations for the altimetry of the future. This paper summarizes contributions and recommendations that were collected and provides guidance for future mission design, research activities, and sustainable operational radar altimetry data exploitation. Recommendations provided are fundamental for optimizing further scientific and operational advances of oceanographic observations by altimetry, including requirements for spatial and temporal resolution of altimetric measurements, their accuracy and continuity. There are also new challenges and new openings mentioned in the paper that are particularly crucial for observations at higher latitudes, for coastal oceanography, for cryospheric studies and for hydrology. The paper starts with a general introduction followed by a section on Earth System Science including Ocean Dynamics, Sea Level, the Coastal Ocean, Hydrology, the Cryosphere and Polar Oceans and the “Green” Ocean, extending the frontier from biogeochemistry to marine ecology. Applications are described in a subsequent section, which covers Operational Oceanography, Weather, Hurricane Wave and Wind Forecasting, Climate projection. Instruments’ development and satellite missions’ evolutions are described in a fourth section. A fifth section covers the key observations that altimeters provide and their potential complements, from other Earth observation measurements to in situ data. Section 6 identifies the data and methods and provides some accuracy and resolution requirements for the wet tropospheric correction, the orbit and other geodetic requirements, the Mean Sea Surface, Geoid and Mean Dynamic Topography, Calibration and Validation, data accuracy, data access and handling (including the DUACS system). Section 7 brings a transversal view on scales, integration, artificial intelligence, and capacity building (education and training). Section 8 reviews the programmatic issues followed by a conclusion

Havnivå, havstrømmer og vertikal landhevning langs Norskekysten

Ocean circulation plays a fundamental role in climate and sea-level related studies due to the ocean’s large heat-storage and transport capacity. Ocean circulation can be derived from numerical ocean models, which might be driven by various sets of observations, such as wind fields or water salinity and temperature. One of the most important ocean-observing systems is satellite altimetry, which allows to construct maps of the mean sea surface (MSS). The ocean’s mean dynamic topography (MDT) is the height of MSS above the geoid and its inclination reveals magnitude and direction of ocean currents. A detailed picture of the marine geoid in combination with an altimetry derived MSS leads to an increased understanding of ocean circulation. The application of satellite altimetry is mostly limited to the open or deep ocean because of its peculiarities close to the coast. The presence of land in altimetric footprints makes the retrieval of radar echos difficult. Also, tidal models used to correct altimetric observations are degraded along the continental shelf border and in the coastal zone. However, coastal zones have gained increased interest in recent years by cause of their high relevance to society considering sea-level rise, shipping, and other off-shore activities. Thus, there are increased efforts in coastal altimetry, and its applicability to monitor the coastal environment was identified. The application of satellite altimetry in coastal zones has become possible, among others, due to the European Space Agency’s CryoSat-2 (CS2) satellite. CS2 carries a radar altimeter, which enables the determination of coastal MDT due to its smaller footprint and delay-Doppler processing. Precise monitoring of sea-level changes is essentially important for understanding not only climate but also social and economic aspects of sea-level rise, especially in coastal zones. Coastal cities are built upon the Earth’s crust, which can be subject to uplift or subsidence. Today, vertical land motion (VLM) rates in Fennoscandia reach values of up to ~10 mm/year and are dominated by glacial isostatic adjustment (GIA), while additional signals caused, e.g., by the elastic rebound from contemporary melting of glaciers, tectonic processes or hydrological loading contribute less. GIA is the ongoing response of the Earth and oceans to the melting of late-Pleistocene ice sheets. The unloading initiated an uplift of the crust close to the centers of former ice sheets. This phenomenon affects the national height systems directly as well as observations of regional sea level and its temporal changes as measured by tide gauges along the coast. The thesis consist of two major blocks, namely, satellite altimetry and GIA. The first part of the thesis investigates the possibilities of CS2 SAR(In) altimetry to provide observations in the Norwegian coastal zone and addresses the determination and quality assessment of the coastal MDT. The second part comprises the quantification of the Earth’s response to melting of late-Pleistocene ice sheets by either modelling (i.e., solving the sea-level equation) or combining sea-surface measurements from CS2 and sea-level records from tide gauges. It is shown that CS2 is able to provide valid observations in Norwegian coastal areas that were previously not monitored by conventional altimetry. CS2 sea-level anomalies within 45 km×45 km boxes were compared with in situ sea level at 22 tide gauges. Over all tide gauges, CS2 shows a standard deviation of differences of 16 cm with a correlation of 0.61. Ocean-tide and inverse barometer geophysical corrections were identified as most crucial, and it was noted that a large amount of observations at land-confined tide gauges were not assigned an ocean-tide value. Due to the availability of local air-pressure observations and ocean-tide predictions, the standard inverse barometer and ocean-tide corrections were replaced with local ones. The refined corrections give an improvement of 24% (to 12.2 cm) and 12% (to 0.68) in terms of standard deviations of difference and correlations, respectively. Using new regional geoid models as well as CS2, three geodetic coastal MDT models in Norway were determined and validated against independent tide-gauge measurements as well as the operational coastal ocean model NorKyst800. The CS2 MDT models agree on a ~3-5 cm level with both tide-gauge and ocean MDT models. In addition, geostrophic surface currents were computed in order to identify errors in the used geoid models. Even though the regional geoids are all based on the latest satellite gravity data provided by GOCE (Gravity field and steady-state Ocean Circulation Explorer), the resulting circulation patterns are dependent upon geoids they were based on. It is demonstrated that some of these differences are due to erroneous or lack of marine gravity data. In addition, the coincidence of the CS2 geographical mode mask with the Norwegian Coastal Current makes it challenging to distinguish between artifacts in CS2 observations that arise during mode switches and real ocean signal. Using ice histories from the ICE-x series (ICE-5G and ICE6G_C) along with related Earth models (VMx), vertical velocity fields as well as time series of relative sea level (RSL) change were predicted. Computations were performed with the open-source sea-level equation solver (SELEN) and validated against external data, i.e., the semi-empirical land-uplift model NKG2016LU_abs and geological RSL reconstructions. In addition, SELEN solutions were compared with published grids of vertical velocities derived by other authors in order to quantify the significance of software’s assumptions and approximations. In general, all software solutions agree on a ~1mm/year level with NKG2016LU_abs in terms of standard deviations of differences. In view of ice models, all uplift rates as well as RSL predictions calculated with ICE6G_C show a considerably better fit to NKG2016LU_abs and RSL data than model results of ICE-5G, which confirms an improvement within the ICE-x series. For both ice models, predictions of present-day vertical velocity fields based on VMx rheologies agree better with observations than predictions based on NKG rheologies. On the other hand, predictions with NKG rheologies fit better RSL data than predictions with VMx rheologies. Applying a well known method for the determination of VLM by combining satellite altimetry and tide-gauge observations, for the first time, CS2 data (within 45 km×45 km boxes) were used for this purpose, bridging thereby the two major thesis’ blocks. Hence, 7.5 years of CS2 and tide-gauge data were combined to estimate linear VLM trends at 20 tide gauges along the Norwegian coast. Monthly-averaged tide-gauge data from PSMSL (Permanent Service for Mean Sea Level) and a high-frequency tide-gauge data set with 10-minute sampling rate from NMA (Norwegian Mapping Authority) were used. Estimated VLM rates from 1 Hz CS2 and high-frequency tide-gauge data reflect well the amplitude of coastal VLM as provided by NKG2016LU_abs. A coastal average of 2.4 mm/year (average over all tide gauges) was found, while NKG2016LU_abs suggests 2.8 mm/year; the spatial correlation is 0.58.Havstrømmer spiller en grunnleggende rolle i klima- og havnivårelaterte studier på grunn av havets transportevne og store varmekapasitet. Havstrømmer kan avledes fra numeriske havmodeller basert på ulike observasjoner, så som vindfelt eller vannets saltinnhold og temperatur. Satellittaltimetri er en av de viktigste observasjonssystemene for å konstruere den geografiske fordelingen av midlere havnivå (MSS - mean sea surface). Havets midlere dynamiske topografi (MDT - mean dynamic topography) er høyden til MSS over geoiden. MSS-flatens helning i forhold til geoiden avgjør havstrømmenes styrke og retning. Et detaljert bilde av den marine geoiden kombinert med MSS avledet fra altimetri-observasjoner fører til en bedret forståelse av havstrømmene. Satellittaltimetri kan anvendes direkte over åpent dyphav, men må underkastes spesiell oppmerksomhet for data nær kysten. Radarekko fra hav og land samtidig gjør tolkningen av observasjonene vanskelig. Altimetrihøyder må korrigeres for tidevannseffekter og nær kontinentalsokler og kystsoner er modellene for tidevannsberegning mer usikre. Kystsoner fått økt oppmerksom i de senere år på grunn av den samfunnsmessige betydning for befolkning, skipsfart og off-shore virksomheter, som vil bli påvirket av endringer i havnivået. Følgelig har det vært økende aktivitet innen kystsonealtimetri med påvisning avmetodens anvendelse for overvåking av kystmiljøet. Dette har særlig utviklet seg med den europeiske romfartsorganisasjonen ESAs CryoSat-2 (CS2) satellitt. CS2 har et radar-altimeter som gjør det mulig å bestemme MDT nær kysten fordi instrumentet har mindre fotavtrykk enn tidligere versjoner og ved å benytte forsinket-Doppler-prosessering av dataene. Presis overvåking av havnivåets endringer er avgjørende viktig for å forstå ikke bare klimavariasjoner, men også samfunnsmessige og økonomiske konsekvenser av havnivåøkning, spesielt i kystsoner. Byer i kystsonen er bygget på jordklodens faste overflate, og den kan være underkastet både landhevning og innsynkning. I dag er den vertikale bevegelsen i Fennoskandia opptil ~10 mm/år og domineres av postglasial isostatisk landhevning (GIA - glacial isostatic adjustment). I tillegg er detmindre bidrag fra jordoverflatens elastiske respons forårsaket av dagens nedsmelting av isbreer, tektoniske prosesser og belastninger fra hydrologiske prosesser. GIA er jordoverflatens og havets langsomme respons på nedsmeltingen av store iskapper i sen- Pleistocene, etter siste istid. Avtagende belastning fra disse massene forårsaket en landhevning av jordoverflaten der iskappen var. Dette fenomenet påvirker nasjonale høydesystemer direkte. Observasjoner med tidevannsmålere langs kysten av regionalt havnivå og dets forandringer påvirkes også når landet hever seg med tiden. Denne doktoravhandlingen har to hovedtemaer, nemlig satellittaltimetri og GIA. I den første delen undersøkes mulighetene for å utnytte CS2 SAR(In) interferometriske altimetri-observasjoner til bestemmelse av MDT i den norske kystsonen med kvalitetsvurderinger av resultatet. I den andre delen kvantifiseres jordoverflatens respons på avsmelting av iskapper etter siste istid, både ved modellering (dvs. løsning av havnivåligningen) og ved kombinasjon av havnivåmålinger fra CS2 satellitten og fra tidevannsmålere langs kysten. Vi viser at CS2 bidrar med observasjoner av den norske kystsonen i områder som tidligere ikke kunne observeres med konvensjonell altimetri fra andre satellitter. Havnivå-anomalier innenfor kvadrater på 45 km×45 km avledet fra CS2 data ble sammenlignet med in situ havnivå bestemt ved 22 tidevannsmålere. Forskjellene har et standard avvik på 16 cm med en korrelasjon på 0.61. Korreksjoner for tidevannsvariasjoner og geofysiske invers barometer effekter ble identifisert som helt nødvendige for resultatet. Mange observasjonsserier på tidevannsstasjoner inne i fjorder hadde ikke tilordnede tidevannsverdier. Siden både lokalt lufttrykk og tidevannsprediksjoner var tilgjengelig, ble disse benyttet i stedet for standardmodeller for invers barometer og tidevannskorreksjoner. Det førte til en forbedring på 24% (til 12.2 cm) i standardavviket og 12% (til 0.68) i korrelasjon. Vi benyttet tre nye regionale geoidemodeller sammen med data fra CS2 til å bestemme tre geodetiske MDT-modeller for den norske kystsonen. De ble validert mot både uavhengige tidevannsmålinger og den operasjonelle havmodellen NorKyst800. MDT-modellene overensstemmer innenfor 3-5 cm med både tidevannsmålinger og havmodell. Vi beregnet også geostrofiske overflatestrømmer i et forsøk på å identifisere feil i de anvendte geoidemodellene. Selv om alle de regionale geoidemodellene er basert på de siste gravitasjonsdataene fra GOCE-satellitten, så avhenger de beregnede strømningsmønstrene av de enkelte geoidemodellene. Vi viser at noen av forskjellene skyldes feilaktige eller mangelfulle marine tyngdedata. Dessuten har CS2 en geografisk modemaskering som faller sammen med den norske kyststrømmen. Det gjør det vanskelig å skille mellom havsignalet og særegenheter i CS2 dataene når satellitten foretar mode-endringer. Vi benyttet tidsforløpene i ICE-x modellene (ICE-5G og ICE6G_C) sammen med geofysiske jordmodeller (VMx) til å beregne vertikale hastighetsfelt og tidsserier for relativt havnivå. Beregningene ble gjort ved hjelp av tilgjengelig (open-source) programvare til løsning av havnivåligningen (SELEN) og ble validert mot eksterne data, nemlig den semi-empiriske landhevningsmodellen NKG2016LU_abs og geologiske rekonstruksjoner av relativt havnivå. SELEN-løsningene ble også sammenlignet med vertikale hastigheter publisert av andre forfattere (som benyttet annen programvare) i et forsøk på å kvantifisere betydningen av de antagelser og tilnærminger som programvaren var bygget på. Forskjellen fra våre løsninger overensstemmer innenfor et standardavvik på ~1 mm/år med vertikalhastighetene i NKG2016LU_abs. Ismodellen ICE6G_C gir vertikale hastigheter og havnivåforløp som overensstemmer mye bedre med NKG2016LU_abs og dataserier for relative havnivåendringer enn den tidligere modellen ICE-5G. Det antyder en mer treffende beskrivelse av ishistorien. Prediksjoner av dagens vertikale hastighetsfelt basert på VMx-rheologier og ismodellene gir bedre overensstemmelse med observasjonene enn med rheologiene benyttet i NKGmodellen. Derimot gir prediksjoner av relativt havnivå bedre overensstemmelse med NKG-rheologier enn med VMx-rheologier. Med utgangspunkt i havnivåligningen har vi for første gang bestemt den vertikale landhevningen ved å kombinere data fra satellittaltimetri og tidevannsmålere. CS2 data (i 45 km×45 kmkvadrater) knytter dermed avhandlingens to temaer sammen. Til sammen 7.5 år med CS2 data ble kombinert med data fra 20 tidevannsmålere langs norskekysten for å estimere lineære trender for vertikale hastigheter. Tidevannsmålinger ble analysert som tidsserier av månedsmidler fra PSMSL (Permanent Service for Mean Sea Level) og som tidsserier med 10 minutter oppløsning fra Kartverket. De beregnede vertikale hastigheter fra 1 Hz CS2 og den høyfrekvente tidevannsserien gjenspeiler verdiene langs kysten i NKG2016LU_abs. Et gjennomsnitt for alle tidevannsmålerne er 2.4 mm/år, mens NKG2016LU_abs gir 2.8 mm/år; den romlige korrelasjonen er 0.58