Surface Height Changes Research Articles

Study on Defect Detection Method using Surface Height Change due to Low Loading

Study on Defect Detection Method using Surface Height Change due to Low Loading

Dokriani glacier(Dingarh glacier) and its geological analysis

The study of geomorphological system not only gives information about environmental processes operating there but also relate them whith global environmental system. The geomorphological analysis of Dokriani glacier, Garhwal Himalaya shows five phases of glacial advancement and retreat in the form of well preserved lateral and terminal moraines. The observed retreat rate of glacier in last two decades is about 17.2 m/yr which represents its negative mass balance followed by change in snout position, area and surface height. The farthest glacier expansion of the valley represented as terminal moraine is situated at 8.3 km from present day snout. Whereas, the other sucsessive glacial stages has followed the similar fashion of glacial advancement due to climatic sensitiveness.

ROCK GLACIER MONITORING USING AERIAL PHOTOGRAPHS: CONVENTIONAL VS. UAV-BASED MAPPING – A COMPARATIVE STUDY

Abstract. Rock glaciers are creep phenomena of mountain permafrost. Typically, these landforms look like lava flows from a bird’s eye view. Active rock glaciers move downslope with flow velocities in the range of few centimeters to several meters per year. Thus, large masses of rock and ice can be gradually transported down-valley. In this paper we present a comparative study analyzing surface change for Tschadinhorn rock glacier, a relatively fast moving rock glacier located in the Hohe Tauern Range of the Austrian Alps. Aerial photographs (1954–2017) of both metric (conventional) and non-metric (UAV-based) aerial surveys were compared to derive multi-annual to annual flow vector fields and surface height change. For each time interval given we computed a single representative value for flow velocity and, if applicable, also for area-wide surface height change, i.e. volume change. The velocity graph obtained represents the temporal evolution of the kinematics of the rock glacier with good discrimination. Volume change was difficult to quantify since temporal changes were rather small and close to insignificance. The precision and accuracy of the results obtained were numerically quantified. Our study showed that for the Tschadinhorn rock glacier UAV-based aerial surveys can substitute conventional aerial surveys as carried out by national mapping agencies, such as the Austrian Federal Office of Metrology and Surveying (BEV). Thus, UAV-based aerial surveys can help to bridge the data gap between regular aerial surveys. The high accuracy of the UAV-derived results would even allow intra-annual change detection of flow velocity.

Anthropometric factors and Breslow thickness

Melanoma is the most rapidly increasing cancer form in Norway, and in 2016, 2114 men and women were diagnosed with the disease. This form of cancer is often diagnosed at an early stage, and if you follow changes in your skin you will be able to detect the cancer at an early stage. Melanoma with metastasis (spreading elsewhere in the body), however, is a serious disease that is difficult to treat, and more than 300 people die in Norway from melanoma each year. The thickness of the tumour (called Breslow thickness) is an important factor for survival after melanoma; those with a thin tumour at diagnosis live longer than those with a thick tumour. In a new Norwegian study, researchers explored associations between certain factors (called anthropometric factors ‐ such as body mass index (BMI), body surface area (BSA), height, weight and weight change) and Breslow thickness, overall and by anatomical site and histological subtype (type of melanoma). Nearly 300,000 Norwegian men and women who were enrolled into a study group called the Janus population‐based cohort 1972–2003 were followed for melanoma. By 2014, 2570 cases of melanoma with information on tumour thickness were identified. This large case‐series of incident melanomas, demonstrated positive associations between BMI, BSA, weight and Breslow thickness, and showed that Breslow thickness increased with increasing BMI, BSA and weight, before levelling off or declining at high values, suggesting that behavioural or other mechanisms apply at high values.

Quantifying variable rates of postglacial relative sea level fall from a cluster of 24 isolation basins in southern Norway

Sediment sequences obtained from 24 isolation basins located on the Skagerrak coastline are used to develop new sea level index points for southern Norway. The basins are located near a common shoreline isobase and extend from marine limit at c. 83 m asl. to present sea level. The abundance of bedrock isolation basins within the small study area enables a high-resolution reconstruction of postglacial relative sea-level change. For most of the basin records, 3–5 samples of terrestrial plant macrofossils were radiocarbon dated across the biostratigraphically-defined isolation boundary. Eight of the isolation events were further constrained by Bayesian modelling of the radiocarbon age results. The overall reconstructed sea-level history is as expected for a formerly glaciated coastline; consisting of a rapid regression from marine limit followed by a slower pace of relative sea level fall from early-mid Holocene to present. The new dataset of sea level index points, however, further delineates century-scale oscillations in rates of sea level regression, which record past changes in sea surface height and vertical land motion (glacial isostatic adjustments, GIA). Results provide new constraints for future numerical models aimed at reducing uncertainty in century-scale relative sea-level projections for Fennoscandia.

Anthropometric factors and Breslow thickness: prospective data on 2570 cases of cutaneous melanoma in the population-based Janus Cohort.

Breslow thickness is the most important prognostic factor of localized cutaneous melanoma (CM), but associations with anthropometric factors have been sparsely and incompletely investigated. To examine prediagnostic body mass index (BMI), body surface area (BSA), and height, weight and weight change in relation to Breslow thickness, overall and by anatomical site and histological subtype; and to assess possible nonlinear associations between these anthropometric factors and Breslow thickness. CMs in the Janus Cohort were identified between 1972 and 2014. Linear regression was used to estimate geometric mean ratios (GMRs) of Breslow thickness with 95% confidence intervals (CIs) according to anthropometric factors. Restricted cubic splines in generalized linear models predicted adjusted mean Breslow thickness, and were used to assess possible nonlinear relationships. Of 2570 cases of CM, obese patients had a GMR of 1·16 (95% CI 1·04-1·30) of Breslow thickness vs. normal-weight patients. For BSA and weight, quintile 5 showed GMRs of 1·13 (95% CI 1·00-1·27) and 1·17 (95% CI 1·03-1·33) of Breslow thickness vs. quintile 1, respectively. Associations seemed restricted to superficial spreading melanomas and CMs on the trunk and lower limbs. The associations plateaued at an adjusted mean Breslow thickness of about 2·5 mm (BMI 29 kg m-2 , BSA 2·05 m2 and weight 90 kg), before declining for the highest values. No associations were found for height and weight change. This large case-series of incident CM demonstrated positive associations between BMI, BSA, weight and Breslow thickness, and suggested that behavioural or other mechanisms apply at high values.

Bias in Estimates of Global Mean Sea Level Change Inferred from Satellite Altimetry

Estimates of regional and global average sea level change remain a focus of climate change research. One complication in obtaining coherent estimates is that geodetic datasets measure different aspects of the sea level field. Satellite altimetry constrains changes in the sea surface height (SSH; or absolute sea level), whereas tide gauge data provide a measure of changes in SSH relative to the crust (i.e., relative sea level). The latter is a direct measure of changes in ocean volume (and the combined impacts of ice sheet melt and steric effects), but the former is not since it does not account for crustal deformation. Nevertheless, the literature commonly conflates the two estimates by directly comparing them. We demonstrate that using satellite altimetry records to estimate global ocean volume changes can lead to biases that can exceed 15%. The level of bias will depend on the relative contributions to sea level changes from the Antarctic and Greenland Ice Sheets. The bias is also more sensitive to the detailed geometry of mass flux from the Antarctic Ice Sheet than the Greenland Ice Sheet due to rotational effects on sea level. Finally, in a regional sense, altimetry estimates should not be compared to relative sea level changes because radial crustal motions driven by polar ice mass flux are nonnegligible globally.

Variability of the Ross Gyre, Southern Ocean: Drivers and Responses Revealed by Satellite Altimetry

AbstractYear‐round variability in the Ross Gyre (RG), Antarctica, during 2011–2015, is derived using radar altimetry. The RG is characterized by a bounded recirculating component and a westward throughflow to the south. Two modes of variability of the sea surface height and ocean surface stress curl are revealed. The first represents a large‐scale sea surface height change forced by the Antarctic Oscillation. The second represents semiannual variability in gyre area and strength, driven by fluctuations in sea level pressure associated with the Amundsen Sea Low. Variability in the throughflow is also linked to the Amundsen Sea Low. An adequate description of the oceanic circulation is achieved only when sea ice drag is accounted for in the ocean surface stress. The drivers of RG variability elucidated here have significant implications for our understanding of the oceanic forcing of Antarctic Ice Sheet melting and for the downstream propagation of its ocean freshening footprint.

Thirteen years of subglacial lake activity in Antarctica from multi-mission satellite altimetry

ABSTRACTThe ability to detect the surface expression of moving water beneath the Antarctic ice sheet by satellite has revealed a dynamic basal environment, with implications for regional ice dynamics, grounding-line stability, and fluxes of freshwater and nutrients to the Southern Ocean. Knowledge of subglacial activity on timescales important for near-term prediction of ice-sheet fluctuations (decadal to century) is limited by the short observational record of NASA's Ice, Cloud, and land Elevation Satellite (ICESat) laser altimetry mission used to generate the last continent-wide survey (2003–08). Here, we use synthetic aperture radar-interferometric-mode data from ESA's CryoSat-2 radar altimetry mission (2010–present), which samples 45 of the ICESat-derived subglacial lakes, to extend their time series to the end of 2016. The extended time series show that there have been surface-height changes at 20 of the 45 lakes since 2008, indicating that some of these features are persistent and potentially cyclic, while other features show negligible changes, suggesting these may be transient or nonhydrological features. Continued monitoring of active lakes for both height and velocity changes, as well as developing methods for identifying additional lakes, is critical to quantifying the full distribution of active subglacial lakes in Antarctica.

The influence of ice sheets on temperature during the past 38 million years inferred from a one-dimensional ice sheet–climate model

Abstract. Since the inception of the Antarctic ice sheet at the Eocene–Oligocene transition (∼ 34 Myr ago), land ice has played a crucial role in Earth's climate. Through feedbacks in the climate system, land ice variability modifies atmospheric temperature changes induced by orbital, topographical, and greenhouse gas variations. Quantification of these feedbacks on long timescales has hitherto scarcely been undertaken. In this study, we use a zonally averaged energy balance climate model bidirectionally coupled to a one-dimensional ice sheet model, capturing the ice–albedo and surface–height–temperature feedbacks. Potentially important transient changes in topographic boundary conditions by tectonics and erosion are not taken into account but are briefly discussed. The relative simplicity of the coupled model allows us to perform integrations over the past 38 Myr in a fully transient fashion using a benthic oxygen isotope record as forcing to inversely simulate CO2. Firstly, we find that the results of the simulations over the past 5 Myr are dependent on whether the model run is started at 5 or 38 Myr ago. This is because the relation between CO2 and temperature is subject to hysteresis. When the climate cools from very high CO2 levels, as in the longer transient 38 Myr run, temperatures in the lower CO2 range of the past 5 Myr are higher than when the climate is initialised at low temperatures. Consequently, the modelled CO2 concentrations depend on the initial state. Taking the realistic warm initialisation into account, we come to a best estimate of CO2, temperature, ice-volume-equivalent sea level, and benthic δ18O over the past 38 Myr. Secondly, we study the influence of ice sheets on the evolution of global temperature and polar amplification by comparing runs with ice sheet–climate interaction switched on and off. By passing only albedo or surface height changes to the climate model, we can distinguish the separate effects of the ice–albedo and surface–height–temperature feedbacks. We find that ice volume variability has a strong enhancing effect on atmospheric temperature changes, particularly in the regions where the ice sheets are located. As a result, polar amplification in the Northern Hemisphere decreases towards warmer climates as there is little land ice left to melt. Conversely, decay of the Antarctic ice sheet increases polar amplification in the Southern Hemisphere in the high-CO2 regime. Our results also show that in cooler climates than the pre-industrial, the ice–albedo feedback predominates the surface–height–temperature feedback, while in warmer climates they are more equal in strength.

Understanding the spatial variation of sea level rise in the North Sea using satellite altimetry

AbstractThis paper examines the spatial variation of sea surface height trends in the North Sea Basin as seen by satellite altimetry and assesses its underlying causes. Changes in the potential temperature and salinity of the North Sea are transposed into corresponding changes in sea surface height and regional anomalies of linear sea level trend calculated. The same is carried out for the meteorological processes which act on the sea surface. The steric and meteorological regional sea level rise anomalies are summed with those from contributions from land ice and compared against the values seen by satellite altimetry over the period 1993–2014. Results show that there is good agreement between the observations and the reconstruction. The local meteorological contribution appears to be most important in describing regional variation in linear sea level rise and is reinforced with a local halosteric contribution which shows a similar spatial pattern.

The impact of mobile phone use on where we look and how we walk when negotiating floor based obstacles.

Pedestrians regularly engage with their mobile phone whilst walking. The current study investigated how mobile phone use affects where people look (visual search behaviour) and how they negotiate a floor based hazard placed along the walking path. Whilst wearing a mobile eye tracker and motion analysis sensors, participants walked up to and negotiated a surface height change whilst writing a text, reading a text, talking on the phone, or without a phone. Differences in gait and visual search behaviour were found when using a mobile phone compared to when not using a phone. Using a phone resulted in looking less frequently and for less time at the surface height change, which led to adaptations in gait by negotiating it in a manner consistent with adopting an increasingly cautious stepping strategy. When using a mobile phone, writing a text whilst walking resulted in the greatest adaptions in gait and visual search behaviour compared to reading a text and talking on a mobile phone. Findings indicate that mobile phone users were able to adapt their visual search behaviour and gait to incorporate mobile phone use in a safe manner when negotiating floor based obstacles.

The surface roughness of Mercury from the Mercury Laser Altimeter: Investigating the effects of volcanism, tectonism, and impact cratering

AbstractSurface roughness is a statistical measure of change in surface height over a given spatial horizontal scale after the effect of broad‐scale slope has been removed and can be used to understand how geologic processes produce and modify a planet's topographic character at different scales. The statistical measure of surface roughness employed in this study of Mercury was the root‐mean‐square deviation and was calculated from 45 to 90°N at horizontal baselines of 0.5–250 km with detrended topographic data from individual Mercury Laser Altimeter tracks. As seen in previous studies, the surface roughness of Mercury has a bimodal spatial distribution, with the cratered terrain (dominated by the intercrater plains) possessing higher‐surface roughness than the smooth plains. The measured surface roughness for both geologic units is controlled by a trade‐off between impact craters generating higher‐surface roughness values and flood‐mode volcanism decreasing surface roughness. The topography of the two terrain types has self‐affine‐like behavior at baselines from 0.5 to 1.5 km; the smooth plains collectively have a Hurst exponent of 0.88 ± 0.01, whereas the cratered terrains have a Hurst exponent of 0.95 ± 0.01. Subtle variations in the surface roughness of the smooth plains can be attributed to differences in regional differences in the spatial density of tectonic landforms. The northern rise, a 1000 km wide region of elevated topography centered at 65°N, 40°E, is not distinguishable in surface roughness measurements over baselines of 0.5–250 km.

Parameterizing Deep Water Percolation Improves Subsurface Temperature Simulations by a Multilayer Firn Model

Deep preferential percolation of melt water in snow and firn brings water lower along the vertical profile than a laterally homogeneous wetting front. This widely recognized process is an important source of uncertainty in simulations of subsurface temperature, density and water content in seasonal snow and in firn packs on glaciers and ice sheets. However, observation and quantification of preferential flow is challenging and therefore it is not accounted for by most of the contemporary snow/firn models. Here we use temperature measurements in the accumulation zone of Lomonosovfonna, Svalbard, done in April 2012 – 2015 using multiple thermistor strings to describe the process of water percolation in snow and firn. Effects of water flow through the snow and firn profile are further explored using a coupled surface energy balance - firn model forced by the output of the regional climate model WRF. In situ air temperature, radiation and surface height change measurements are used to constrain the surface energy and mass fluxes. To account for the effects of preferential water flow in snow and firn we test a set of depth-dependent functions allocating a certain fraction of the melt water available at the surface to each snow/firn layer. Experiments are performed for a range of characteristic percolation depths and results indicate a reduction in root mean square difference between the modeled and measured temperature by up to a factor of two compared to the results from the default water infiltration scheme. This illustrates the significance of accounting for preferential water percolation to simulate subsurface conditions. The suggested approach to parameterization of the preferential water flow requires low additional computational cost and can be implemented in layered snow/firn models applied both at local and regional scales, for distributed domains with multiple mesh points.

Variability of the Pacific North Equatorial Current from 1993 to 2012 based on a 1/8° Pacific model simulation

AbstractBased on a multidecadal Pacific basin model simulation, interannual variations of the North Equatorial Current (NEC) are investigated. The model reproduces well the characteristics of the NEC and its interannual variability. The NEC transport is calculated as an integral of the westward velocity from 6°N to 21°N and from the surface to the 1200 m depth. The magnitude and standard deviation of the NEC transport increase from 46.5 and 3.9 Sv at 175°E to 66.7 and 6.5 Sv at 130°E, respectively, and both peak around 132°E prior to entering the bifurcation region. The NEC transport tends to be higher during positive Oceanic Niño Index (ONI) years but lower during negative ONI years with the maximum difference of more than 20 Sv. The interannual variability of the NEC transport is closely related to changes of the sea surface height in the tropical Pacific Ocean, and the increase of the NEC is mostly balanced by the increase in the North Equatorial Counter Current (NECC) on the tropical gyre side. The present study further suggests a long‐term decline of the NEC transport from 1993 to 2012, which is consistent with the patterns in the trend of wind stress curl. Transport anomalies reconstructed from the normal modes of zonal velocity suggest that the first baroclinic mode captures about 95% of the variance in the NEC transport, while the second mode adds only additional 3–4%. A 1.5‐layer reduced gravity model further reveals that the first (second) baroclinic mode is driven primarily by the wind (thermal) forcing. The wind forcing plays a predominant role in determining the interannual variability in the NEC transport while the effect of the thermal forcing is rather limited.

Subaperture stitching algorithms: A comparison

With the research focus of subaperture stitching interferometry shifting from flat wavefronts to aspheric ones, a variety of algorithms for stitching optimization have been proposed. We try to category and compare the algorithms in this paper by their modeling of misalignment-induced subaperture aberrations which are of low orders. A simple way is to relate the aberrations to misalignment by linear approximation with small angle assumption. But it can not exactly model the induced aberrations of aspheres. In general, the induced aberrations can be fitted to free polynomials and then removed from subaperture measurements. However, it is at the risk of mixing up the surface error and the induced aberrations. The misalignment actually introduces different terms of aberrations with certain proportions. The interrelation is then determined through analytical modeling or ray tracing. The analytical model-based algorithm and the ray tracing-based algorithm both are tedious, aperture shape-related and surface type-specific. While the configuration space-based algorithm we proposed numerically calculates the surface height change under rigid body transformation, it is generally applicable to various surface types and different aperture shapes. Simulations and experiments are presented to compare the stitching results when different algorithms are applied to null cylindrical subapertures measured with a computer generated hologram. The configuration space-based algorithm shows superior flexibility and accuracy.

Active subglacial lakes and channelized water flow beneath the Kamb Ice Stream

Abstract. We identify two previously unknown subglacial lakes beneath the stagnated trunk of the Kamb Ice Stream (KIS). Rapid fill-drain hydrologic events over several months are inferred from surface height changes measured by CryoSat-2 altimetry and indicate that the lakes are probably connected by a subglacial drainage network, whose structure is inferred from the regional hydraulic potential and probably links the lakes. The sequential fill-drain behavior of the subglacial lakes and concurrent rapid thinning in a channel-like topographic feature near the grounding line implies that the subglacial water repeatedly flows from the region above the trunk to the KIS grounding line and out beneath the Ross Ice Shelf. Ice shelf elevation near the hypothesized outlet is observed to decrease slowly during the study period. Our finding supports a previously published conceptual model of the KIS shutdown stemming from a transition from distributed flow to well-drained channelized flow of subglacial water. However, a water-piracy hypothesis in which the KIS subglacial water system is being starved by drainage in adjacent ice streams is also supported by the fact that the degree of KIS trunk subglacial lake activity is relatively weaker than those of the upstream lakes.

An Assessment of the Precision and Accuracy of Altimetry Retrievals for a Monterey Bay GNSS-R Experiment

Global navigation satellite systems (GNSS) provide signals of opportunity for bistatic radar remote sensing, called GNSS reflectometry (GNSS-R), that have potential to be used for ocean altimetry. These signals have advantages over traditional mono-static radar that include reduced cost, high density of measurements in time and space, and an inherent reference to a highly accurate time-space frame. Here, we examine GNSS-R data collected from an aircraft flying over Monterey Bay, California. A downward-looking dual-frequency left-hand circularly polarized patch antenna recorded reflected signals. An upward-looking commercial antenna recorded the direct signals. Dual-frequency carrier phase data from this antenna were also used to produce precise coordinates for the aircraft. The L1 P-code GPS data were collected over four days with two flights per day and consist of nine transects covering a 1 deg× 1 deg grid. The performance of three timing retrieval algorithms has been evaluated based on measurement precision. From the observed cross-correlation waveform, the specular reflection timing was derived from the delay of the 70% peak correlation power (HALF method), the waveform leading edge peak first derivative (DER method), or the delay associated with a best fit function approximating the nominal waveform shape (PARA3 method). It was found that the HALF method produced the most precise measurements for a 5 s integration time with a standard deviation of σ = 0.6 m. The measurement accuracy is characterized by comparison with well-established models including neutral atmospheric delay, mean sea surface height, and ocean and solid Earth tides. Biases on the order of 1-4 m are observed with respect to a modeled mean sea surface and between each flight. However, the measurements are shown to track changes in sea surface height along the ground track to within 0.6 m.

Object-based analysis of unmanned aerial vehicle imagery to map and characterise surface features on a debris-covered glacier

Debris-covered glaciers in the Himalaya may have spatially-averaged rates of surface height change that are similar to those observed on bare-ice glaciers, despite the insulating effects of thick debris. Spatially heterogeneous melt patterns caused by the development and evolution of ice cliffs and supraglacial pond systems result in substantial mass losses over time. However, mechanisms controlling the formation and survival of cliffs and ponds remain largely unknown. To study the distribution and characteristics of these surface features we deploy an unmanned aerial vehicle (UAV) over a stretch of the debris-covered Langtang Glacier, Nepal. Acquired images are processed into high-resolution orthomosaics and elevation models with the Structure from Motion (SfM) photogrammetry algorithm. Ice cliffs and ponds are classified using object-based image analysis (OBIA) and their morphology and spatial distribution are analysed and evaluated using object, pixel and point cloud approaches. Results show that ice cliffs are predominantly north-facing, and larger ice cliffs are generally coupled with supraglacial ponds, which may affect their evolution considerably. The spatial distribution of ice cliffs indicates that they are more likely to form in areas where high strain rates are expected. The spatial configuration of ponds over the entire tongue reveals high pond density near confluences, possibly due to closure of conduits via transverse compression. We conclude that the combination of OBIA and UAV imagery is a valuable tool in the semi-automatic and objective analysis of surface features on debris-covered glaciers. The technique may also have potential for upscaling to the use of spaceborne imagery, and the use of UAV-derived point clouds to analyse ice cliff undercuts is promising.

The threat of a support surface translation affects anticipatory postural control

This study examined how postural threat in the form of a potential perturbation affects an individual's ability to perform a heel raise. Seventeen adults completed three conditions: i) low threat, where participants performed a heel raise in response to a “go” tone, ii) high threat, where participants either heard the same “go” tone, for which they performed a heel raise, or experienced a support surface translation in the medio-lateral direction that disturbed their balance, and iii) choice reaction time task, where participants either completed a heel raise in response to the same “go” tone or a toe raise in response to a lower pitched tone. For all heel raise trials, anticipatory postural adjustments (APAs) were quantified from center of pressure (COP) recordings and electromyographic (EMG) activity from the tibialis anterior (TA) and soleus (SOL). Results indicated that participants exhibited larger APAs, as reflected by the greater backward COP displacement (p=0.038) and velocity (p=0.022) as well as a larger TA EMG amplitude (p=0.045), during the high threat condition. During the execution phase of the heel raise, an earlier (p=0.014) and larger (p=0.041) SOL EMG activation were observed during the high threat condition. These results contrast with previous findings of reduced APAs when the postural threat was evoked through changes in surface height. Therefore, the characteristics of the postural threat must be considered to isolate the effects of threat on anticipatory movement control.