Time Scale for Rapid Draining of a Surficial Lake Into the Greenland Ice Sheet

2015 ◽  
Vol 82 (7) ◽  
Author(s):  
James R. Rice ◽  
Victor C. Tsai ◽  
Matheus C. Fernandes ◽  
John D. Platt
Keyword(s):  
Hydraulic Fracture ◽  
Liquid Water ◽  
Time Scale ◽  
Strong Dependence ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Time Dependent ◽  
Hydraulic Fractures ◽  
Average Temperature ◽  
Parameter Ranges

A 2008 report by Das et al. documented the rapid drainage during summer 2006 of a supraglacial lake, of approximately 44×106 m3, into the Greenland ice sheet over a time scale moderately longer than 1 hr. The lake had been instrumented to record the time-dependent fall of water level and the uplift of the ice nearby. Liquid water, denser than ice, was presumed to have descended through the sheet along a crevasse system and spread along the bed as a hydraulic facture. The event led two of the present authors to initiate modeling studies on such natural hydraulic fractures. Building on results of those studies, we attempt to better explain the time evolution of such a drainage event. We find that the estimated time has a strong dependence on how much a pre-existing crack/crevasse system, acting as a feeder channel to the bed, has opened by slow creep prior to the time at which a basal hydraulic fracture nucleates. We quantify the process and identify appropriate parameter ranges, particularly of the average temperature of the ice beneath the lake (important for the slow creep opening of the crevasse). We show that average ice temperatures 5–7  °C below melting allow such rapid drainage on a time scale which agrees well with the 2006 observations.

scholarly journals Hydraulic Fracture Propagation in a Poro-Elastic Medium with Time-Dependent Injection Schedule Using the Time-Stepped Linear Superposition Method (TLSM)

Energies ◽  
2020 ◽  
Vol 13 (24) ◽  
pp. 6474
Author(s):  
Tri Pham ◽  
Ruud Weijermars
Keyword(s):  
Hydraulic Fracture ◽  
Fracture Propagation ◽  
Fracture Network ◽  
Time Dependent ◽  
Reservoir Rock ◽  
Hydraulic Fractures ◽  
Superposition Method ◽  
Stress Concentrations ◽  
Linear Superposition ◽  
Dynamic Fractures

The Time-Stepped Linear Superposition Method (TLSM) has been used previously to model and analyze the propagation of multiple competitive hydraulic fractures with constant internal pressure loads. This paper extends the TLSM methodology, by including a time-dependent injection schedule using pressure data from a typical diagnostic fracture injection test (DFIT). In addition, the effect of poro-elasticity in reservoir rocks is accounted for in the TLSM models presented here. The propagation of multiple hydraulic fractures using TLSM-based codes preserves infinite resolution by side-stepping grid refinement. First, the TLSM methodology is briefly outlined, together with the modifications required to account for variable time-dependent pressure and poro-elasticity in reservoir rock. Next, real world DFIT data are used in TLSM to model the propagation of multiple dynamic fractures and study the effect of time-dependent pressure and poro-elasticity on the development of hydraulic fracture networks. TLSM-based codes can quantify and visualize the effects of time-dependent pressure, and poro-elasticity can be effectively analyzed, using DFIT data, supported by dynamic visualizations of the changes in spatial stress concentrations during the fracture propagation process. The results from this study may help develop fracture treatment solutions with improved control of the fracture network created while avoiding the occurrence of fracture hits.

Time Scale and Ice Accumulation During the Last 125,000 Years as Indicated by the Greenland 018 curve

1972 ◽  
Vol 109 (1) ◽  
pp. 17-24 ◽  
Author(s):  
N. A. Mörner
Keyword(s):  
North America ◽  
Time Scales ◽  
Time Scale ◽  
Ice Core ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Climatic Changes ◽  
Dynamic Changes ◽  
Ice Accumulation

SummaryThe 18 curve from the 1390 m long ice core from Camp Century, Greenland, shows climatic changes that are easily correlated with known glacial and non-glacial events of North America and north Europe and are thus indirectly dated. With a known chronology, the glacial dynamic changes of the Greenland Ice Sheet can be calculated for the last 125,000 years. It is concluded that the dynamics of the Greenland Ice Sheet have changed drastically during this period and that these changes are directly related to major changes of climate and extension of the Wisconsin and Weichselian glaciations. Logarithmic time scales earlier applied to this curve must therefore be incorrect.

Seasonal to multi-annual speedup and slowdown of Greenland outlet glaciers inferred from time-dependent remote sensing observations

2020 ◽  
Author(s):  
Lizz Ultee ◽  
Bryan Riel ◽  
Brent Minchew
Keyword(s):  
Time Series ◽  
Flow Velocity ◽  
Ice Sheet ◽  
Surface Air Temperature ◽  
Greenland Ice Sheet ◽  
Time Dependent ◽  
Surface Velocity ◽  
Short Term ◽  
Ice Flow

<p>The rate of ice flux from the Greenland Ice Sheet to the ocean depends on the ice flow velocity through outlet glaciers. Ice flow velocity, in turn, evolves in response to multiple geographic and environmental forcings at different timescales. For example, velocity may vary daily in response to ocean tides, seasonally in response to surface air temperature, and multi-annually in response to long-term trends in climate. The satellite observations processed as part of the NASA MEaSUREs Greenland Ice Sheet Velocity Map allow us to analyse variations in ice surface velocity at multiple timescales. Here, we decompose short-term and long-term signals in time-dependent velocity fields for Greenland outlet glaciers based on the methods of Riel et al. (2018). Patterns found in short-term signals can constrain basal sliding relations and ice rheology, while the longer-term signals hint at decadal in/stability of outlet glaciers. We present example velocity time series for outlets including Sermeq Kujalleq (Jakobshavn Isbrae) and Helheim Glacier, and we highlight features indicative of dynamic drawdown or advective restabilization. Finally, we comment on the capabilities of a time series analysis software under development for glaciological applications.</p>

scholarly journals Seasonal monitoring of melt and accumulation within the deep percolation zone of the Greenland Ice Sheet and comparison with simulations of regional climate modeling

2018 ◽  
Author(s):  
Achim Heilig ◽  
Olaf Eisen ◽  
Michael MacFerrin ◽  
Marco Tedesco ◽  
Xavier Fettweis
Keyword(s):  
Water Content ◽  
Liquid Water ◽  
Pore Space ◽  
Regional Climate ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Liquid Water Content ◽  
Water Infiltration ◽  
Near Surface ◽  
Percolation Zone

Abstract. Increasing melt over the Greenland ice sheet (GrIS) recorded over the past years has resulted in significant changes of the percolation regime of the ice sheet. It remains unclear whether Greenland's percolation zone will act as meltwater buffer in the near future through gradually filling all pore space or if near-surface refreezing causes the formation of impermeable layers, which provoke lateral runoff. Homogeneous ice layers within perennial firn, as well as near-surface ice layers of several meter thickness are observable in firn cores. Because firn coring is a destructive method, deriving stratigraphic changes in firn and allocation of summer melt events is challenging. To overcome this deficit and provide continuous data for model evaluations on snow and firn density, temporal changes in liquid water content and depths of water infiltration, we installed an upward-looking radar system (upGPR) 3.4 m below the snow surface in May 2016 close to Camp Raven (66.4779° N/46.2856° W) at 2120 m a.s.l. The radar is capable to monitor quasi-continuously changes in snow and firn stratigraphy, which occur above the antennas. For summer 2016, we observed four major melt events, which routed liquid water into various depths beneath the surface. The last event in mid-August resulted in the deepest percolation down to about 2.3 m beneath the surface. Comparisons with simulations from the regional climate model MAR are in very good agreement in terms of seasonal changes in accumulation and timing of onset of melt. However, neither bulk density of near-surface layers nor the amounts of liquid water and percolation depths predicted by MAR correspond with upGPR data. Radar data and records of a nearby thermistor string, in contrast, matched very well, for both, timing and depth of temperature changes and observed water percolations. All four melt events transferred a cumulative mass of 56 kg/m2 into firn beneath the summer surface of 2015. We find that continuous observations of liquid water content, percolation depths and rates for the seasonal mass fluxes are sufficiently accurate to provide valuable information for validation of model approaches and help to develop a better understanding of liquid water retention and percolation in perennial firn.

scholarly journals Surface mass budget and meltwater discharge from the Kangerlussuaq sector of the Greenland ice sheet during record-warm year 2010

2011 ◽  
Vol 5 (5) ◽  
pp. 2319-2347 ◽  
Author(s):  
D. van As ◽  
A. Hubbard ◽  
B. Hasholt ◽  
A. B. Mikkelsen ◽  
M. van den Broeke ◽  
...  
Keyword(s):  
High Temperatures ◽  
Time Lag ◽  
Ice Sheet ◽  
Drainage System ◽  
Greenland Ice Sheet ◽  
Western Coast ◽  
Positive Anomaly ◽  
Ablation Zone ◽  
Surface Mass ◽  
Average Temperature

Abstract. The year 2010 has been anomalously warm in most of Greenland, most notably in the south and along the western coast. Our study targets the Kangerlussuaq region around 67° N in Southwest Greenland, where the temperature anomalies were record setting. In 2010, the average temperature was 5 °C (2.7 standard deviations) above the 1974–2010 average in the town of Kangerlussuaq. High temperatures were also observed over the ice sheet, with the positive anomaly increasing with altitude. Also surface albedo, from calibrated MODIS measurements, was anomalously low in 2010, chiefly in the upper ablation zone. The low albedo was caused by the high ablation in 2010, which profited in turn from high temperatures, low albedo, and of low wintertime accumulation. The largest melt excess (166%) was found in the upper ablation zone, where higher temperatures and lower albedo contributed equally to the melt anomaly. In total, we estimate that 6.6 km3 of surface meltwater ran off the ice sheet in the Kangerlussuaq catchment area in 2010, exceeding "normal" year 2009 by 145%. When compared to discharge estimated from discharge measurements in the proglacial river we find good agreement. The time lag between the records is caused by storage within and underneath the ice sheet, and suggests adaption of the subglacial drainage system to meltwater availability, with more efficient drainage occurring after the peak of the melt season.

scholarly journals The modelled liquid water balance of the Greenland Ice Sheet

2017 ◽  
Author(s):  
Christian R. Steger ◽  
Carleen H. Reijmer ◽  
Michiel R. van den Broeke
Keyword(s):  
Water Balance ◽  
Liquid Water ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Special Focus ◽  
Southeastern Part ◽  
Upward Migration ◽  
Near Surface ◽  
Surface Mass ◽  
Surface Melt

Abstract. Recent studies indicate that the surface mass balance will dominate the Greenland Ice Sheet's (GrIS) contribution to 21st century sea level rise. Consequently, it is crucial to understand the liquid water balance (LWB) of the ice sheet and its response to increasing surface melt. We therefore analyse a firn simulation conducted with SNOWPACK for the GrIS and over the period 1960–2014 with a special focus on the LWB and refreezing. An indirect evaluation of the simulated refreezing climate with GRACE and firn temperature observations indicate a good model performance. Results of the LWB analysis reveal a spatially uniform increase in surface melt during 1990–2014. As a response, refreezing and runoff also indicate positive trends for this period, where refreezing increases with only half the rate of runoff, which implies that the majority of the additional liquid input runs off the ice sheet. However, this pattern is spatially variable as e.g. in the southeastern part of the GrIS, most of the additional liquid input is buffered in the firn layer due to relatively high snowfall rates. The increase in modelled refreezing leads to a general decrease in firn air content and to a substantial increase in near-surface firn temperature in some regions. On the western side of the ice sheet, modelled firn temperature increases are highest in the lower accumulation zone and are primarily caused by the exceptional melt season of 2012. On the eastern side, simulated firn temperature increases more gradually and with an associated upward migration of firn aquifers.

Time-Domain Reflectometry Observations of Meltwater Percolation and Retention in the Firn Layer of the Greenland Ice Sheet

2020 ◽  
Author(s):  
Samira Samimi ◽  
Shawn Marshall ◽  
Michael McFerrin
Keyword(s):  
Energy Balance ◽  
Surface Energy ◽  
Time Domain ◽  
Liquid Water ◽  
Surface Energy Balance ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Time Domain Reflectometry ◽  
Latent Heat Release ◽  
Near Surface

<p>Mass loss from the Greenland Ice Sheet has increased in recent decades due to significant increases in surface melt and runoff. The fraction of summer melt retains as a liquid water or refreezes as it percolates into the underlying cold firn, acting as a buffer to the summer runoff. There are challenges to quantifying both infiltration and refreezing of meltwater in this complex heterogeneous cold firn and to understand the spatial variability of these processes. In this study we present continuous in situ measurements of near-surface temperature and dielectric permittivity, a proxy for volumetric water content, using TDR (Time Domain Reflectometry) methods in the percolation zone of the southern Greenland Ice Sheet. We established two observation sites near Dye 2 in April, 2016, excavating firn pits to depths of 2.2 and 5.3 m. The two sites are 650 m apart to quantify the percolation and refreezing of meltwater and to observe the spatial variability of these processes through summer 2016. Thermistor arrays were used to track the thermal signature of meltwater penetration in firn, through the effects of latent heat release when meltwater refreezes. Through the addition of TDR probes, we attempt to directly quantify meltwater volume as well as hydraulic conductivity of the near-surface snow and firn. An automatic weather station (AWS) configured for surface energy balance monitoring was also installed. AWS data were used to calculate the surface energy balance and model meltwater production. The melting front, characterized by 0°C conditions and direct evidence of liquid water, penetrated to a depth of between 1.8 and 2.1 m in summer 2016; at depths of 2.1 m and greater, temperatures remained below 0°C, there was no evidence of abrupt warming (i.e. latent heat release), and dielectric permittivities remained at their background levels. Meltwater penetrated several thick ice layers, but not until temperatures reached the melting point at these depths, implying that ice layers may transition to a permeable ‘slush’ layer, given enough conductive and latent heating, permitting progressive penetration of meltwater to depth. Firn temperatures (sub-zero conditions below ~2 m) appear to have been the main barrier to deep penetration of meltwater during summer 2016.</p>

APPLICATION OF THE EMPIRICAL MODE DECOMPOSITION TO FIELD TILTMETER DATA FOR HYDRAULIC FRACTURE MAPPING

2009 ◽  
Vol 01 (03) ◽  
pp. 407-424 ◽  
Author(s):  
ZUORONG CHEN ◽  
ROBERT G. JEFFREY
Keyword(s):  
Empirical Mode Decomposition ◽  
Hydraulic Fracture ◽  
Time Scale ◽  
Perfect Reconstruction ◽  
Hydraulic Fractures ◽  
Characteristic Time Scale ◽  
Intrinsic Mode Functions ◽  
Mode Decomposition ◽  
Starting Point ◽  
Fracture Mapping

Empirical Mode Decomposition (EMD) is a fully data-driven, adaptive technique for analyzing time series from nonlinear and nonstationary processes. The starting point of EMD is to treat the signal as a superposition of different intrinsic modes of oscillations (fast oscillations superimposed on slow oscillations). The essence of this method is to empirically identify the intrinsic oscillatory modes by their characteristic time scale imbedded in a signal, and then decompose the signal into a collection of a finite and often small number of intrinsic mode functions (IMF) through a so-called sifting process. Each IMF component then represents only one mode of both amplitude and frequency modulated oscillation of the signal at a certain time scale or frequency band, and the sum of all the IMF components as well as a residual produces a perfect reconstruction of the original signal. Partial reconstruction can be achieved by selectively removing fast or slowly varying IMFs, which provides a method to remove unwanted (noise) parts of the signal. In this paper, the EMD is applied to quantitative analysis of field tiltmeter data collected to monitor and map hydraulic fractures. The fracture-related tilt components are extracted by identifying the relevant IMFs that contribute to them, which allows removing the noise and background trend components from the raw data. The extracted tilt data are then inverted to obtain the volume and orientation of the hydraulic fractures. Physically reasonable prediction of the hydraulic fracture volume is used to demonstrate that the application of EMD to field tiltmeter data analyzing can be successfully carried out.

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