Tonie Van Dam

University of Utah | UOU · Department of Geology and Geophysics

Ongoing climate change leads to an increase in prolonged drought and severe weather events in the United States, particularly pronounced in semi-arid regions such as the western United States. It could have lasting social and environmental impacts. Continuous monitoring of near-surface hydrological processes and groundwater resources will provide h...

The 2020 M5.7 Magna earthquake within the Salt Lake Valley is a reminder that devastating earthquakes are a real threat in the Intermountain West. Approximately 80% of Utah’s population and much of its economic infrastructure are located within the seismically active Wasatch Front urban corridor. Although much has been learned about the Salt Lake b...

The effective isotropic radiated power (EIRP) is the measured radiated power of an antenna pointed in a specific direction. For the Global Positioning System (GPS), the EIRP is a function of the transmitted power and the gain of the transmitting antenna. It is a fundamental observation used for estimating surface reflectivity that can be used to es...

Time-dependent loading deformations of the Earth’s surface, due to nontidal changes in the atmosphere, ocean, land water/ice, etc., contribute significantly to the seasonal and secular Global Positioning System (GPS) site displacements, especially for the up component. While loading deformations derived from general circulation model (GCM) outputs...

Observations of changes in terrestrial water storage (TWS) obtained from the satellite mission GRACE (Gravity Recovery and Climate Experiment) have frequently been used for water cycle studies and for the improvement of hydrological models by means of calibration and data assimilation. However, due to a low spatial resolution of the gravity field m...

To be published in: Reports on Astronomy 2018-2021 (IAU Transactions A; Vol. xx), as part of the report of IAU Commission A2 (Rotation of the Earth).

Autonomous formation flight enables new satellite missions for novel applications. The cost and limits of propulsion systems can be overcome if environmental resources are being benefitted of. Currently, atmospheric drag is used in low Earth orbit to this end. Solar radiation pressure, which is of similar order of magnitude as aerodynamic ram press...

The irregular interannual variations observed in the Greenland ice sheet (GrIS) mass balance can be interpreted as stochastic. These variations often have large amplitudes, and, if not accounted for correctly in the mass change model parameterization, could have profound impacts on the estimate of the secular trend and acceleration. Here we propose...

Observations of changes in terrestrial water storage obtained from the satellite mission GRACE (Gravity Recovery and Climate Experiment) have frequently been used for water cycle studies and for the improvement of hydrological models by means of calibration and data assimilation. However, due to a low spatial resolution of the gravity field models...

We retrieve sea levels in polar regions via GNSS reflectometry (GNSS-R), using signal-to-noise ratio (SNR) observations from eight POLENET GNSS stations. Although geodetic-quality antennas are designed to boost the direct reception from GNSS satellites and to suppress indirect reflections from natural surfaces, the latter can still be used to estim...

We retrieve sea levels in polar regions via GNSS reflectometry (GNSS-R), using signal-to-noise ratio (SNR) observations from eight POLENET GNSS stations. Although geodetic-quality antennas are designed to boost the direct reception from GNSS satellites and to suppress indirect reflections from natural surfaces, the latter can still be used to estim...

Vertical crustal displacements induced by atmospheric, hydrological, cryospheric, and oceanic load changes are detectable with sub-cm accuracy by precise continuous GPS measurements. Areas subjected to rapid load changes due to ice sheet melt, drought, massive groundwater extraction, or lake level drop, are characterized by a dominant non-linear ve...

To remove atmospheric pressure loading (ATML) effect from GNSS coordinate time series, surface pressure (SP) models are required to predict the displacements. In this paper, we modeled the 3D ATML surface displacements using the latest MERRA-2 SP grids, together with four other products (NCEP-R-1, NCEP-R-2, ERA-Interim and MERRA) for 596 globally d...

To advance geodynamics and geophysical research, high precision Global Navigation Satellite System (GNSS) observations of velocity and position are needed. In this study, we construct a 3‐D model of a ~5.9‐year periodic signal for the Earth surface displacements. This model is mainly based on the determined time sequences and the Y22 spatial patter...

Mass redistribution within the Earth system deforms the surface elastically. Loading theory allows us to predict loading induced displacement anywhere on the Earth’s surface using environmental loading models, e.g., Global Land Data Assimilation System. In addition, different publicly available loading products are available. However, there are dif...

Observations of changes in terrestrial water storage obtained from the satellite mission GRACE (Gravity Recovery and Climate Experiment) have frequently been used for water cycle studies and for the improvement of hydrological models by means of calibration and data assimilation. However, due to a low spatial resolution of the gravity field models...

As changes in gravity are directly related to mass variability, satellite missions observing the Earth’s time varying gravity field are a unique tool for observing mass transport processes in the Earth system, such as the water cycle, rapid changes in the cryosphere, oceans, and solid Earth processes, on a global scale. The observation of Earth’s g...

Measurements of the spatio-temporal variations of Earth’s gravity field from the Gravity Recovery and Climate Experiment (GRACE) mission have led to new insights into large spatial mass redistribution at secular, seasonal and subseasonal timescales. GRACE solutions from various processing centres, while adopting different processing strategies, res...

Earth observation satellites yield a wealth of data for scientific, operational and commercial exploitation. However, the redistribution of mass in the system Earth is not yet part of the standard inventory of Earth Observation (EO) data products to date. It is derived from the Gravity Recovery and Climate Experiment (GRACE) mission and its Follow-...

Earth observation satellites yield a wealth of data for scientific, operational and commercial exploitation. However, the redistribution of mass in the system Earth is not yet part of the standard inventory of Earth Observation (EO) data products to date. It is derived from the Gravity Recovery and Climate Experiment (GRACE) mission and its Follow-...

Much of the research to understand the ice mass changes of Greenland ice sheet (GrIS) has focused on detecting linear rates and accelerations at decadal or longer periods. The transient (short-term, non-secular) mass changes show large variability, and if not properly accounted for, can introduce significant biases into estimates of long-term ice m...

Significance The recent deglaciation of Greenland is a response to both oceanic and atmospheric forcings. From 2000 to 2010, ice loss was concentrated in the southeast and northwest margins of the ice sheet, in large part due to the increasing discharge of marine-terminating outlet glaciers, emphasizing the importance of oceanic forcing. However, t...

Over the 15 years of the Gravity Recovery and Climate Experiment (GRACE) mission, various data processing approaches were developed to derive time-series of global gravity fields based on sensor observations acquired from the two spacecrafts. In this paper, we compare GRACE-based mass anomalies provided by various processing groups against Global N...

The Global Positioning System (GPS) and Gravity Recovery and Climate Experiment (GRACE) provide important geodetic datasets to study glacial mass change. Applying the multichannel singular spectral analysis to the GPS-measured vertical and horizonal crustal displacement and GRACE-derived vertical displacement near Jakobshavn Isbræ (JI) in western G...

The continuously operating Global Positioning System (GPS) sites mounted on bedrock around the coast of Greenland provide important geodetic datasets to quantify the solid Earth's response to historical and present-day ice mass variations. The presence of colored noise and irregular seasonal signals makes it difficult to detect transient changes in...

Changes in J2, resulting from past and present changes in Earth’s climate, are traditionally observed by Satellite Laser ranging (SLR). Assuming an elastic Earth, it is possible to infer changes in J2 from changes in Earth’s shape observed by GPS. We compare estimates of non-secular J2 changes from GPS, SLR, GRACE and a load model. The GPS and SLR...

Inversion of geodetic site displacement data to infer surface mass loads has previously been demonstrated using a spherical harmonic representation of the load. This method suffers from the continent-rich, ocean-poor distribution of the geodetic data, coupled with the predominance of the continental load (water storage and atmospheric pressure) com...

The three-part paper deals with energy-minimal multiple crack propagation in a linear elastic solid under quasi-static conditions. The principle of minimum total energy, i.e. the sum of the potential and fracture energies, which stems directly from the Griffith’s theory of cracks, is applied to the problem of arbitrary crack growth in 2D. The propo...

We present and discuss JTRF2014, the Terrestrial Reference Frame (TRF) the Jet Propulsion Laboratory constructed by combining space-geodetic inputs from Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), Global Navigation Satellite Systems (GNSS) and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) sub...

The three-part paper deals with energy-minimal multiple crack propagation in a linear elastic solid under quasi-static conditions. The principle of minimum total energy, i.e. the sum of the potential and fracture energies, which stems directly from the Griffith’s theory of cracks, is applied to the problem of arbitrary crack growth in 2D. The propo...

The three-part paper deals with energy-minimal multiple crack propagation in a linear elastic solid under quasi-static conditions. The principle of minimum total energy, i.e. the sum of the potential and fracture energies, which stems directly from the Griffith’s theory of cracks, is applied to the problem of arbitrary crack growth in 2D. The propo...

As the global climate changes, understanding short-term variations in water storage is increasingly important. Continuously operating Global Positioning System (cGPS) stations in Iceland record annual periodic motion—the elastic response to winter accumulation and spring melt seasons—with peak-to-peak vertical amplitudes over 20 mm for those sites...

Large-scale mass redistribution in the terrestrial water storage (TWS) leads to changes in the low-degree spherical harmonic coefficients of the Earth’s surface mass density field. Studying these low-degree fluctuations is an important task that contributes to our understanding of continental hydrology. In this study, we use global GNSS measurement...

We study fluctuations in the degree-2 zonal spherical harmonic coefficient of the Earth's gravity potential, C20, over the period 2003-2015. This coefficient is related to the Earth's oblateness and studying its temporal variations,ΔC20, can be used to monitor large-scale mass movements between high and low latitude regions. We examine ΔC20 inferre...

In response to present-day ice mass loss on and near the Greenland Ice Sheet, steady crustal uplifts have been observed from the network of Global Positioning System (GPS) stations mounted on bedrock. In addition to the secular uplift trends, the GPS time series also show prominent annual variability. Here we examine the annual changes of the verti...

Measurements of vertical crustal uplift from bedrock sites around the edge of the Greenland ice sheet (GrIS) can be used to constrain present day mass loss. Interpreting any observed crustal displacement around the GrIS in terms of present day changes in ice is complicated, however, by the glacial isostatic adjustment (GIA) signal. With GPS observa...

Accurate quantification of the millennial-scale mass balance of the Greenland ice sheet (GrIS) and its contribution to global sea-level rise remain challenging because of sparse in situ observations in key regions. Glacial isostatic adjustment (GIA) is the ongoing response of the solid Earth to ice and ocean load changes occurring since the Last Gl...

Since 2009, we have installed five GNSS stations around the Princess Elisabeth Base in Antarctica. Four stations have been installed on stable bedrock, while the fifth station is installed at the Derwael Ice rise. The stations are used in the frame of the BELSPO projects, GIANT-LISSA and IceCon, to provide information for monitoring the horizontal...

Most GPS coordinate time series, surface displacements derived from the Gravity Recovery and Climate Experiment (GRACE), and loading models display significant annual signals at many regions. This paper compares the annual signals of the GPS position time series from the Crustal Dynamics Data Information System (CDDIS), estimates of loading from GR...

Observations over the past 2 decades show substantial ice loss associated with the speed-up of marine-terminating glaciers in Greenland. Here we use a regional three-dimensional outlet glacier model to simulate the behaviour of Jakobshavn Isbræ (JI) located in western Greenland. Our approach is to model and understand the recent behaviour of JI wit...

Continental hydrology has a large influence on the excitation of polar motion (PM). However, these effects are far from being completely understood. Current global water storage models differ significantly from one another and are unable to completely represent the complex hydrological cycle, particularly at interannual scales. A promising alternat...

European Gravity Service for Improved Emergency Management - Status and project highlights

Observations over the past two decades show substantial ice loss associated with the speedup of marine terminating glaciers in Greenland. Here we use a regional 3-D outlet glacier model to simulate the behaviour of Jakobshavn Isbræ (JI) located in west Greenland. Using atmospheric and oceanic forcing we tune our model to reproduce the observed fron...

To remove continental water storage (CWS) signals from the GPS data, CWS mass models are needed to obtain predicted surface displacements. We compared weekly GPS height time series with five CWS models: (1) the monthly and (2) three-hourly Global Land Data Assimilation System (GLDAS); (3) the monthly and (4) one-hourly Modern-Era Retrospective Anal...

Temporal aliasing is expected to add up to the error budget of future gravity satellite missions of low-low satellite-to-satellite tracking (LL-SST) type in such a way, that could act as a constraining factor on their way to achieve the expected accuracy that new generation sensors could provide. Within the scope of the ESA-SC4MGV project, we inves...

Global Positioning System (GPS) signal-to-noise ratio (SNR) measurements can be employed to retrieve environmental variables in multipath reception conditions, whereby direct or line-of-sight transmission is received simultaneously with coherent reflections thereof. Previous GPS SNR multipath studies of soil moisture and snow depth have focused on...

In this paper, we compare the efficiency of two models to estimate the surface displacements due to continental water storage (CWS) variations over continental North America. The first model, the monthly North America Land Data Assimilation System Phase 2 Noah (NLDAS-2 Noah) is a model of CWS restricted to North America. The second data set, the Gl...

Seasonal signals (annual and semi-annual) in GPS time series are of great importance for understanding the evolution of regional mass, e.g. ice and hydrology. Conventionally, these signals are derived by least-squares fitting of harmonic terms with a constant amplitude and phase. In reality, however, such seasonal signals are modulated, i.e., they...

Observations of the redistribution of environmental mass in the Earth system are to date not part of the inventory of Earth observation (EO) data although they constitute a unique remote sensing opportunity. The recently selected Horizon2020 project “European Gravity Service for Improved Emergency Management” strives to change this and will demonst...

The SLR observations to spherical geodetic satellites, e.g., LAGEOS-1/2, Starlette, Stella, AJISAI and LARES, provide remarkable information about the temporal variations of the very long wavelength part of the Earth's gravity field. As opposed to the low Earth orbiting satellites tracked by GPS high-low satellite-to-satellite tracking (GPS hl-SST)...

GRACE is undoubtedly one of the most important sources to observe mass transport on global scales. However, GRACE has outlived its predicted life time and the satellite system is showing signs of fatigue. As the value of any geophysical or environmental record is proportional to the length of the time series, the geo-scientific communities are seri...

Observations over the past decade show significant ice loss associated with the speed-up of glaciers in southeast Greenland from 2003, followed by a deceleration from 2006. These short-term, episodic, dynamic perturbations have a major impact on the mass balance on the decadal scale. To improve the projection of future sea level rise, a long-term d...

Surface displacements due to temporal changes in environmental mass redistributions are observable in the coordinate time series of many Global Navigation Satellite System (GNSS) sites. In this study, we investigated the effect of loading on estimates of tectonic velocity computed from campaign-style GNSS observations. The study region is in the Py...

The interaction and coupling of the Earth system components that include the atmosphere, hydrosphere, cryosphere, lithosphere, and other fluids in Earth's interior, influence the Earth's shape, gravity field and its rotation (the three pillars of geodesy). The effects of global climate change, such as sea level rise, glacier melting, and geoharzard...

Variations in the degree-2 Stokes coefficients C-20, C-21 and S-21 can be used to understand long- and short-term climate forcing. Here we derive changes in these coefficients for the period 2003 January-2012 April using Earth rotation data. Earth rotation data contain contributions from motion terms (the effects of winds and currents) and contribu...

Three different environmental loading methods are used to estimate surface displacements and correct non-linear variations in a set of GPS weekly height time series. Loading data are provided by (1) Global Geophysical Fluid Center (GGFC), (2) Loading Model of Quasi-Observation Combination Analysis software (QLM) and (3) our own daily loading time s...

[1] In the event of a termination of the Gravity Recovery and Climate Experiment (GRACE) mission before the launch of GRACE Follow-On (due for launch in 2017), high-low satellite-to-satellite tracking (hl-SST) will be the only dedicated observing system with global coverage available to measure the time-variable gravity field (TVG) on a monthly or...

Satellite altimeter measurements of sea surface height include a small contribution from vertical motion of the seafloor caused by crustal loading. Loading by ocean tides is routinely allowed for in altimeter data processing. Here, loading by nontidal fluids of the atmosphere, ocean, and terrestrial hydrosphere is examined. The crustal deformation...

Advances in gravity instrumentation have allowed for the determination of the absolute acceleration of gravity to a precision of 3-5 μGal and observations of tidally driven changes in gravity on the order of nanogals. With observations of gravity and changes in gravity at these levels of precision we are able to investigate problems such as the res...

In 2012, the International Earth Rotation and Reference Systems Service (IERS) released a call for space geodetic solutions corrected for non-tidal atmospheric loading at the observation level. The main objective of this analysis campaign is to evaluate the impact of non-tidal atmospheric loading corrections on Terrestrial Reference Frame (TRF), ge...

In the last decade, temporal variations of the global gravity field have become an ubiquitous and invaluable source of information for geophysical and environmental studies. It is important that the time series of observations is not interrupted as some geophysical phenomena, e.g. postglacial rebound or long term ice mass trends, are only beginning...