Hansheng Wang

Load Love numbers and Green's functions for elastic Earth models PREM, iasp91, ak135, and modified models with refined crustal structure from Crust 2.0

Load Love numbers and Greens functions are computed for elastic Earth models PREM, iasp91 and ak135, and their modified models with refined crustal structure from Crust 2.0. It is found that the differences of results between iasp91 or ak135 and PREM, and the effects of refinement of crustal structure are significant for the Love numbers of degrees from around 200 to very high numbers, and for the Greens functions in the near-field. The results of the models given in this paper are applicable to the studies related to loading processes (present surface mass transport as measured by GRACE and GPS, ocean tide loading, etc.), making it possible to use different models or assess the uncertainties of solutions of the loading problems under investigation. In order to ensure the stability of the solutions for degrees larger than 360 (or when the resolution is less than 55km), a variable transformation on the solution vector is used in this paper and proved to work effectively.

Increased water storage in North America and Scandinavia from GRACE gravity data

Changes in continental water storage have been difficult to constrain from space-borne gravity data in regions experiencing both ice melting and glacial isostatic adjustment. Separation of the hydrologic and isostatic signals reveals increases in water storage in both North America and Scandinavia over the past decade. Space-borne gravity data from the Gravity Recovery and Climate Experiment (GRACE) have revealed trends in present-day continental water storage in many parts of the world. In North America and northern Europe, it has been difficult to provide reliable estimates because of the strong background signals of glacial isostatic adjustment1. Attempts to separate the hydrologic signal from the background with numerical models2 are affected by uncertainties in our understanding of the precise glacial history and mantle viscosity3,4. Here we use a combination of GRACE data and measurements from the global positioning system to separate the hydrological signals without any model assumptions. According to our estimates, water storage in central North America increased by 43.0±5.0 Gt yr−1 over the past decade. We attribute this increase to a recovery in terrestrial water storage after the extreme Canadian Prairies drought between 1999 and 2005. We find a smaller rise in water storage in southern Scandinavia, by 2.3±0.8 Gt yr−1. In both North America and Scandinavia, our computed increases in water storage are consistent with long-term observations of terrestrial water level. We suggest that the detected mass gains in terrestrial water storage need to be taken into account in studies on global sea-level rise.

A GPS velocity field for Fennoscandia and a consistent comparison to glacial isostatic adjustment models

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.

An approach for spherical harmonic analysis of non-smooth data

A method is proposed to evaluate the spherical harmonic coefficients of a global or regional, non-smooth, observable dataset sampled on an equiangular grid. The method is based on an integration strategy using new recursion relations. Because a bilinear function is used to interpolate points within the grid cells, this method is suitable for non-smooth data; the slope of the data may be piecewise continuous, with extreme changes at the boundaries. In order to validate the method, the coefficients of an axisymmetric model are computed, and compared with the derived analytical expressions. Numerical results show that this method is indeed reasonable for non-smooth models, and that the maximum degree for spherical harmonic analysis should be empirically determined by several factors including the model resolution and the degree of non-smoothness in the dataset, and it can be several times larger than the total number of latitudinal grid points. It is also shown that this method is appropriate for the approximate analysis of a smooth dataset. Moreover, this paper provides the program flowchart and an internet address where the FORTRAN code with program specifications are made available.

Spatiotemporal Characterization of Land Subsidence and Uplift (2009–2010) over Wuhan in Central China Revealed by TerraSAR-X InSAR Analysis

The effects of ground deformation pose a significant geo-hazard to the environment and infrastructure in Wuhan, the most populous city in Central China, in the eastern Jianghan Plain at the intersection of the Yangtze and Han rivers. Prior to this study, however, rates and patterns of region-wide ground deformation in Wuhan were little known. Here we employ multi-temporal SAR interferometry to detect and characterize spatiotemporal variations of ground deformation in major metropolitan areas in Wuhan. A total of twelve TerraSAR-X images acquired during 2009–2010 are used in the InSAR time series analysis. InSAR-derived results are validated by levelling survey measurements and reveal a distinct subsidence pattern within six zones in major commercial and industrial areas, with a maximum subsidence rate up to −67.3 mm/year. A comparison analysis between subsiding patterns and urban developments as well as geological conditions suggests that land subsidence in Wuhan is mainly attributed to anthropogenic activities, natural compaction of soft soil, and karst dissolution of subsurface carbonate rocks. However, anthropogenic activities related to intensive municipal construction and industrial production have more significant impacts on the measured subsidence than natural factors. Moreover, remarkable signals of secular land uplift are found along both banks of the Yangtze River, especially along the southern bank, with deformation rates ranging mostly from +5 mm/year to +17.5 mm/year. A strong temporal correlation is highlighted between the detected displacement evolutions and the water level records of the Yangtze River, inferring that this previously unknown deformation phenomenon is likely related to seasonal fluctuations in water levels of the Yangtze River.

Glacier elevation changes (2012–2016) of the Puruogangri Ice Field on the Tibetan Plateau derived from bi-temporal TanDEM-X InSAR data

ABSTRACT This study aims to evaluate the potential of TerraSAR-X (TSX) add-on for Digital Elevation Measurement (TanDEM-X) bi-static synthetic aperture radar (SAR) data sets for the retrieval of glacier digital elevation models (DEMs) and elevation changes over mountain regions. We exploited two pairs of TanDEM-X SAR data sets acquired in 2012 and 2016 over the Puruogangri Ice Field (PIF), which is the largest modern glacier on the Tibetan Plateau (TP). Two fine-detail and high-precision DEMs for 2012 and 2016 over the PIF were generated by differential interferometric processing, and were validated against height measurements from global positioning system (GPS) and Ice, Cloud, and land Elevation Satellite (ICESat) altimetry, yielding a vertical accuracy of 1.91 ± 0.76 m and 1.69 ± 0.83 m, respectively. The elevation changes were derived by differencing the bi-temporal TanDEM-X DEMs and revealed predominant glacier surface thinning on the PIF. An annual surface thinning rate of −0.317 ± 0.027 m year−1 was estimated in the period 2012–2016, which is much larger than the estimate of −0.049 ± 0.200 m year−1 for the period 2000–2012 reported in previous studies. This accelerating trend of glacier surface thinning might be attributable to the continued increase in summer temperature since the 1980s and decrease in annual precipitation between two periods of investigation. This study demonstrates that comparison of the bi-temporal TanDEM-X DEMs is an efficient method for accurate and detailed retrieval of the latest surface elevation changes of mountain glaciers.

Recent high-resolution Antarctic ice velocity maps reveal increased mass loss in Wilkes Land, East Antarctica

We constructed Antarctic ice velocity maps from Landsat 8 images for the years 2014 and 2015 at a high spatial resolution (100 m). These maps were assembled from 10,690 scenes of displacement vectors inferred from more than 10,000 optical images acquired from December 2013 through March 2016. We estimated the mass discharge of the Antarctic ice sheet in 2008, 2014, and 2015 using the Landsat ice velocity maps, interferometric synthetic aperture radar (InSAR)-derived ice velocity maps (~2008) available from prior studies, and ice thickness data. An increased mass discharge (53 ± 14 Gt yr−1) was found in the East Indian Ocean sector since 2008 due to unexpected widespread glacial acceleration in Wilkes Land, East Antarctica, while the other five oceanic sectors did not exhibit significant changes. However, present-day increased mass loss was found by previous studies predominantly in west Antarctica and the Antarctic Peninsula. The newly discovered increased mass loss in Wilkes Land suggests that the ocean heat flux may already be influencing ice dynamics in the marine-based sector of the East Antarctic ice sheet (EAIS). The marine-based sector could be adversely impacted by ongoing warming in the Southern Ocean, and this process may be conducive to destabilization.

Spatial-Temporal Characteristics of Glacier Velocity in the Central Karakoram Revealed with 1999–2003 Landsat-7 ETM+ Pan Images

The situation of stable and slightly advancing glaciers in the Karakoram is called the “Karakoram anomaly”. Glacier surface velocity is one of the key parameters of glacier dynamics and mass balance, however, the response of glacier motion to this regional anomaly is not fully understood. Here, we characterize the spatial-temporal variations in glacier velocity over the Central Karakoram from 1999–2003. The inter-annual glacier velocity fields were retrieved using a cross-correlation-based algorithm applied to four Landsat-7 Enhanced Thematic Mapper Plus (ETM+) panchromatic image pairs. We find that most of the glaciers on the southern slope flowed faster than those on the northern slope, which might be attributed to the differences in glacier sizes. Furthermore, ice motion observations over four years reveal that most of the glaciers were quasi-stable or experienced small fluctuations of flow velocity during our study period. We identify a new surging event for the South Skamri Glacier in the study period by investigating the glacier frontal changes and the longer-term time series of surface velocities between 1996 and 2006. From the transverse velocity profiles of seven typical glaciers, we infer that basal sliding is the predominant motion mechanism of the middle and upper glaciers, whereas internal deformation dominates closest to the glacier terminus.

Detection and analysis of surface velocity over Baltoro glacier with ENVISAT ASAR data

As global warming intensifies, most of mountain glaciers are in retreat in the worldwide. However, glaciers in Karakoram have been observed to be slightly thickening and advancing. Because the Karakoram Mountains are one of the most remote and less accessible mountain ranges in the world, there are very few in situ observations of glacier dynamics in the Karakoram. In this study, we use 3 ENVISAT ASAR images acquired during the period 2004-2006 to detect surface velocity of the Baltoro glacier. Glacier surface velocity is derived from the SAR offset tracking method which is widely used to estimate surface displacements. Our results indicate that the glacier surface velocity is relatively stable during 2004 to 2006. Moreover, we discuss ice dynamics mechanisms by using transverse velocity profiles.

Annual Glacier-Wide Mass Balance (2000–2016) of the Interior Tibetan Plateau Reconstructed from MODIS Albedo Products

Glaciers in the Tibetan Plateau (TP) play a crucial role in regulating agriculture irrigation, river discharge and the regional/global climate system. However, mass balance records of TP glaciers have remained scarce due to challenging mountainous terrain and harsh weather conditions, which limits our understanding of the influence of melting glaciers on local water resources and responses to climate change. Here, we present and assess an albedo-based method to derive annual mass balance for three glaciers in the interior TP from Moderate Resolution Imaging Spectroradiometer (MODIS) albedo data during 2000–2016. A strong linear correlation (R2 = 0.941, P < 0.001) is found between annual minimum-averaged glacier-wide albedo (AMGA) values and annual mass balance measurements on the Xiao Dongkemadi glacier. Furthermore, the 17-year-long annual mass balance series of the Xiao Dongkemadi glacier and the Geladandong mountain region glaciers, and the Purogangri ice cap are reconstructed for the first time, with a mass loss rate of 535 ± 63 mm w.e.a−1, 243 ± 66 mm w.e.a−1 and 113 ± 68 mm w.e.a−1, respectively. The results are verified by geodetic estimates, with relative error ranging from 4.55% to 11.80%, confirming that the albedo-based method can be used to estimate specific mass budgets for interior TP glaciers. A strong correlation between the mass balance series and air temperature infers that increasing summer air temperature may be one of main reasons for glacier shrinkage of the three studied glaciers.