Brian C. Gunter

Data assimilation of GRACE terrestrial water storage estimates into a regional hydrological model of the Rhine River basin

(1) Department of Geoscience and Remote Sensing, Delft University of Technology, Delft, The Netherlands (n.tangdamrongsub@tudelft.nl), (2) Water Resources Management, Delft University of Technology, Delft, The Netherlands, (3) School of Aerospace Engineering, Georgia Institute of Technology, Atlanta, The United States, (4) Operational Water Management, Deltares, Delft, The Netherlands, (5) Hydrology and Quantitative Water Management Group, Department of Environmental Sciences, Wageningen University, The Netherlands

Mass balance of the Antarctic Ice Sheet from 1992 to 2017

The Antarctic Ice Sheet is an important indicator of climate change and driver of sea-level rise. Here we combine satellite observations of its changing volume, flow and gravitational attraction with modelling of its surface mass balance to show that it lost 2,720u2009±u20091,390 billion tonnes of ice between 1992 and 2017, which corresponds to an increase in mean sea level of 7.6u2009±u20093.9 millimetres (errors are one standard deviation). Over this period, ocean-driven melting has caused rates of ice loss from West Antarctica to increase from 53u2009±u200929 billion to 159u2009±u200926 billion tonnes per year; ice-shelf collapse has increased the rate of ice loss from the Antarctic Peninsula from 7u2009±u200913 billion to 33u2009±u200916 billion tonnes per year. We find large variations in and among model estimates of surface mass balance and glacial isostatic adjustment for East Antarctica, with its average rate of mass gain over the period 1992–2017 (5xa0±xa046 billion tonnes per year) being the least certain.The Antarctic Ice Sheet is an important indicator of climate change and driver of sea-level rise. Here we combine satellite observations of its changing volume, flow and gravitational attraction with modelling of its surface mass balance to show that it lost 2,720u2009±u20091,390 billion tonnes of ice between 1992 and 2017, which corresponds to an increase in mean sea level of 7.6u2009±u20093.9 millimetres (errors are one standard deviation). Over this period, ocean-driven melting has caused rates of ice loss from West Antarctica to increase from 53u2009±u200929 billion to 159u2009±u200926 billion tonnes per year; ice-shelf collapse has increased the rate of ice loss from the Antarctic Peninsula from 7u2009±u200913 billion to 33u2009±u200916 billion tonnes per year. We find large variations in and among model estimates of surface mass balance and glacial isostatic adjustment for East Antarctica, with its average rate of mass gain over the period 1992–2017 (5xa0±xa046 billion tonnes per year) being the least certain.

Using Satellite Constellations for Improved Determination of Earth's Time-Variable Gravity

The spatiotemporal resolution of the time-variable gravity field models derived from current dedicated gravity field missions is inherently limited by their ground-track coverage. Furthermore, the results are subject to aliasing effects caused by submonthly mass transport signals, such as those caused by atmospheric and ocean processes. To address these issues, this study explores the feasibility of using nondedicated satellite constellations, such as those from commercial communication networks or a low-cost array of custom-built microsatellites, as a complementary data source. The positioning receivers onboard the constellation’s satellites would ideally provide a high density of observations in the form of derived accelerations that, while much less accurate than those obtained from dedicated gravitymissions, are still sufficient to observe the longest wavelength gravity signals at even subdaily intervals. Using a series of simulated mission scenarios, as well as a limited amount of real-data analysis, it is shown that such constellations, acting either independently or when combined with dedicated gravity field missions, may offer a noticeable improvement in the recovery of the large-scale (greater than 1000 km) high-frequency (less than 1month) components of the global gravity field.

ESA ice sheet CCI: derivation of the optimal method for surface elevation change detection of the Greenland ice sheet – round robin results

For more than two decades, radar altimetry missions have provided continuous elevation estimates of the Greenland ice sheet (GrIS). Here, we propose a method for using such data to estimate ice-sheet-wide surface elevation changes (SECs). The final data set will be based on observations acquired from the European Space Agency’s Environmental Satellite (ENVISAT), European Remote Sensing (ERS)-1 and -2, CryoSat-2, and, in the longer term, Sentinel-3 satellites. In order to find the best-performing method, an intercomparison exercise has been carried out in which the scientific community was asked to provide their best SEC estimates as well as feedback sheets describing the applied method. Due to the hitherto few radar-based SEC analyses as well as the higher accuracy of laser data, the participants were asked to use either ENVISAT radar or ICESat (Ice, Cloud, and land Elevation Satellite) laser altimetry over the Jakobshavn Isbræ drainage basin. The submissions were validated against airborne laser-scanner data, and intercomparisons were carried out to analyse the potential of the applied methods and to find whether the two altimeters were capable of resolving the same signal. The analyses found great potential of the applied repeat-track and cross-over techniques, and, for the first time over Greenland, that repeat-track analyses from radar altimetry agreed well with laser data. Since topography-related errors can be neglected in cross-over analyses, it is expected that the most accurate, ice-sheet-wide SEC estimates are obtained by combining the cross-over and repeat-track techniques. It is thus possible to exploit the high accuracy of the former and the large spatial data coverage of the latter. Based on CryoSat’s different operation modes, and the increased spatial and temporal data coverage, this shows good potential for a future inclusion of CryoSat-2 and Sentinel-3 data to continuously obtain accurate SEC estimates both in the interior and margin ice sheet.

An approach for estimating time-variable rates from geodetic time series

There has been considerable research in the literature focused on computing and forecasting sea-level changes in terms of constant trends or rates. The Antarctic ice sheet is one of the main contributors to sea-level change with highly uncertain rates of glacial thinning and accumulation. Geodetic observing systems such as the Gravity Recovery and Climate Experiment (GRACE) and the Global Positioning System (GPS) are routinely used to estimate these trends. In an effort to improve the accuracy and reliability of these trends, this study investigates a technique that allows the estimated rates, along with co-estimated seasonal components, to vary in time. For this, state space models are defined and then solved by a Kalman filter (KF). The reliable estimation of noise parameters is one of the main problems encountered when using a KF approach, which is solved by numerically optimizing likelihood. Since the optimization problem is non-convex, it is challenging to find an optimal solution. To address this issue, we limited the parameter search space using classical least-squares adjustment (LSA). In this context, we also tested the usage of inequality constraints by directly verifying whether they are supported by the data. The suggested technique for time-series analysis is expanded to classify and handle time-correlated observational noise within the state space framework. The performance of the method is demonstrated using GRACE and GPS data at the CAS1 station located in East Antarctica and compared to commonly used LSA. The results suggest that the outlined technique allows for more reliable trend estimates, as well as for more physically valuable interpretations, while validating independent observing systems.

Realization of a consistent set of vertical reference surfaces in coastal areas

We present a combined approach for the realization of the (quasi-)geoid as a height reference surface and the vertical reference surface at sea (chart datum). This approach, specifically designed for shallow seas and coastal waters, provides the relation between the two vertical reference surfaces without gaps down to the coast. It uses a regional hydrodynamic model, which, after vertical referencing, provides water levels relative to a given (quasi-)geoid. Conversely, the hydrodynamic model is also used to realize a (quasi-)geoid by providing corrections to the dynamic sea surface topography, which are used to reduce radar altimeter-derived sea surface heights to the (quasi-)geoid. The coupled problem of vertically referencing the hydrodynamic model and computing the (quasi-)geoid is solved iteratively. After convergence of the iteration process, the vertically referenced hydrodynamic model is used to realize the chart datum. In this way, consistency between the chart datum and (quasi-)geoid is ensured. We demonstrate the feasibility and performance of this approach for the Dutch mainland and North Sea. We show that in the Dutch part of the North Sea, the differences between modeled and observed instantaneous and mean dynamic sea surface topography is 8–10 and 5.8xa0cm, respectively. On land, we show that the methodology provides a quasi-geoid which has a lower standard deviation (SD) than the European Gravimetric Geoid 2008 (EGG08) and the official Netherlands quasi-geoid NLGEO2004-grav when compared to GPS-levelling data. The root mean square at 81 GPS-levelling points is below 1.4xa0cm; no correction surface is needed. Finally, we show that the chart datum (lowest astronomical tide, LAT) agrees with the observed chart datum at 92 onshore tide gauges to within 21.5xa0cm (SD).

Comparison of Elevation Change Detection Methods From ICESat Altimetry Over the Greenland Ice Sheet

Estimation of the surface elevation change of the Greenland Ice Sheet (GrIS) is essential for understanding its response to recent and future climate change. Laser measurements from the NASA’s Ice, Cloud, and land Elevation Satellite (ICESat) created altimetric surveys of GrIS surface elevations over the 2003–2009 operational period of the mission. This paper compares four change detection methods using Release 634 ICESat laser altimetry data: repeat tracks (RTs), crossovers (XOs), overlapping footprints (OFPs), and triangulated irregular networks (TINs). All four methods begin with a consistently edited data set and yield estimates of volumetric loss of ice from the GrIS ranging from −193 to −269 km³/yr. Using a uniform approach for quantifying uncertainties, we find that volume change rates at the drainage system scale from the four methods can be reconciled within 1-<inline-formula> <tex-math notation=LaTeX>

Empirical estimation of present-day Antarctic glacial isostatic adjustment and ice mass change

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Assessing total water storage and identifying flood events over Tonlé Sap basin in Cambodia using GRACE and MODIS satellite observations combined with hydrological models

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Uplift rates from a new high-density GPS network in Palmer Land indicate significant late Holocene ice loss in the southwestern Weddell Sea

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