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Greenland Ice Sheet mass loss is impacting connected terrestrial and marine hydrologic systems with global consequences. Groundwater is a key component of water cycling in the Arctic, underlying the 1.7e6km2ice sheet and forming offshore freshwater reserves. However, despite its vast extent, the response of Greenland’s groundwater to ongoing ice sheet change is unknown. Here we present in-situ observations of deep groundwater conditions under the Greenland Ice Sheet, obtained in a 651-metre-long proglacial bedrock borehole angled under the ice sheet margin. We find that Greenland’s groundwater system responds rapidly and sensitively to relatively minor ice sheet forcing. Hydraulic head clearly varies over multi-annual, seasonal and diurnal timescales, which we interpret as a response to fluid pressure forcing at the ice/bed interface associated with changes in overlying ice loading and ice sheet hydrology. We find a systematic decline in hydraulic head over the eight-year observational period is linked primarily to ice sheet mass loss. Ongoing and future ice thinning will probably reduce groundwater discharge rates, with potential impacts to submarine freshwater discharge, freshwater delivery to fjords and biogeochemical fluxes in the Arctic.

The link between subglacial hydrology and basal sliding has prompted work on basal hydrology models with water pressure and drainage capacity as prognostic variables. We find that the Glacier Drainage System model, which belongs to a commonly used family of subglacial hydrology models that include both channelized and distributed drainage components, underpredicts winter water pressure when compared to borehole observations from western Greenland given a wide range of plausible parameter values and inputs. This problem, though previously noted by other modelers, has not been addressed. Possible causes for the discrepancy including idealized model inputs or unconstrained parameters are investigated through a series of modeling experiments on both synthetic and realistic ice sheet geometries. Numerical experiments reveal that englacial storage and hydraulic conductivity in the distributed system are the primary controls on winter water pressure in Glacier Drainage System model. Observations of temperate layer thickness and englacial water content from western Greenland imply an upper bound on englacial storage, suggesting that a reduction in hydraulic conductivity is the most plausible cause of high winter water pressure. We conclude that hydraulic conductivity acts as a proxy for the subgrid‐scale connectivity of the linked cavity system and should therefore change seasonally in correspondence with melt water availability. A commonly used drainage model formulation underpredicts observations of water pressure in winterEnglacial storage elevates modeled winter pressure, but observations indicate that storage is limitedDecreasing hydraulic conductivity is physically plausible and reproduces winter pressure observations

Ice in the ablation zone of the Greenland ice sheet is known to contain vertical temperature gradients that arise from conduction at the boundaries, the addition of strain and latent heat, and advective heat transport. A three‐dimensional array of temperature measurements in a grid of boreholes reveals horizontal ice temperature gradients that challenge the present conceptualization of heat transfer. We measure two distinct types of temperature variability in the horizontal direction, one impacting a confined region where ice temperatures span a range of 5°C, and another with temperatures consistently varying by approximately 2°C across the entire 3‐D block. We suggest the first demonstrates the localized and limited nature of latent heat input, and the second demonstrates that vertical heat advection outpaces diffusion. These findings imply that ice flow is highly variable over sub‐ice‐thickness length scales, which in turn generates contrasts in ice temperature that may impact ice deformation and fracturing. The 3‐D thermal structure of an ~8 × 107m3block of ice in the Greenland ice sheet ablation zone was measured with over 300 sensorsUnexpected horizontal temperature gradients are present across length scales below one ice thickness and through the full ice columnSlow rates of diffusion relative to advective heat transfer can generate a heterogeneous temperature structure similar to what we observe

The hydrological processes and sliding dynamics of the Greenland Ice Sheet are closely linked to the structural framework of its bed. Establishing whether the ice mass rests on bedrock relatively clear of rock debris, a thick deformable till layer, or some intermediate combination, is therefore key to understanding the ice sheet response to changes in meltwater inputs. Here we report on direct observations of the ice‐bed interface along a flow line transect in the ablation zone of the western Greenland Ice Sheet. Our measurements are derived from a network of 32 boreholes that vary from ~100 m deep near the ice margin to 830 m deep, 46 km upflow from the terminus. We performed a suite of experiments and sampling techniques in the holes to investigate the bed conditions. In contrast to previous geophysical and drilling studies elsewhere in Greenland which have suggested that the ice rests on a thick layer of sediment, we find no evidence for a thick sediment cover on the bed of the Kangerlussuaq sector of the ice sheet. Our observations imply that this area has a relatively clean and hard bed and that this facilitates a basal hydraulic system governed by ice/bedrock interactions. The lack of sediment cover on the bed could be due to recent deglaciation and advance of this sector of the ice sheet and/or variable rates of erosion and preferential routing of sediment through bedrock troughs. Mechanical, thermal, and hydrological measurements in 32 boreholes indicate that the ice/bed interface is a hard and impenetrable surfaceThe ice/bed interface has high‐transmissivity meltwater flow pathways that are spatially discrete and maintain pressure gradientsDirect observations reveal subglacial hydrology and sliding conditions consistent with a hard bed classification

Context: A sports injury has both physical and psychological consequences for the athlete. A common postinjury psychological response is elevated fear of reinjury.Objective: To provide an overview of the implications of fear of reinjury on the rehabilitation of athletes, including clinical methods to measure fear of reinjury; the impact of fear of reinjury on rehabilitation outcomes, including physical impairments, function, and return to sports rate; and potential interventions to address fear of reinjury during rehabilitation.Evidence Acquisition: PubMed was searched for articles published in the past 16 years (1990-2016) relating to fear of reinjury in athletes. The reference lists of the retrieved articles were searched for additionally relevant articles.Study Design: Clinical review.Level of Evidence: Level 3.Results: Fear of reinjury after a sports injury can negatively affect the recovery of physical impairments, reduce self-report function, and prevent a successful return to sport. Athletes with high fear of reinjury might benefit from a psychologically informed practice approach to improve rehabilitation outcomes. The application of psychologically informed practice would be to measure fear of reinjury in the injured athletes and provide interventions to reduce fear of reinjury to optimize rehabilitation outcomes.Conclusion: Fear of reinjury after a sports injury can lead to poor rehabilitation outcomes. Incorporating principles of psychologically informed practice into sports injury rehabilitation could improve rehabilitation outcomes for athletes with high fear of reinjury.

Background: In patients who have undergone anterior cruciate ligament reconstruction (ACLR), the effect of neuromuscular re-education (NMR) programs on standard hop tests outcomes, including limb symmetry indices (LSIs), is unknown.Hypothesis: Both legs will show improvement in hop test–measured units after neuromuscular training, but the involved leg will show relatively greater improvement leading to improved limb symmetry. Patients younger than 18 years will show more improvement than patients who are older.Study Design: Retrospective cohort study.Level of Evidence: Level 3.Methods: Patients self-selected their participation in this NMR program, which was completed after traditional outpatient physical therapy. Pre– and post–hop test scores were recorded as the primary outcome measure.Results: Seventy-one patients met the inclusion criteria and completed hop testing. Overall, the involved leg showed significant improvements (pretest/posttest) for single-leg hop (138.30 cm/156.89 cm), triple crossover hop (370.05 cm/423.11 cm), and timed hop (2.21 s/1.99 s). Similarly, on the uninvolved leg, improvements were seen for the single-leg hop (159.30 cm/171.87 cm) and triple crossover hop (427.50 cm/471.27 cm). Overall mean limb symmetry improved across all 4 hop tests, but there was significant improvement only on the single-leg hop (87% pretest to 92% posttest). Patients younger than 18 years showed mean significant LSI improvement on the triple crossover hop.Conclusion: Utilizing an intensive 6-week NMR program after ACLR prior to return to sport can improve quantitative hop test measurements. Patients younger than 18 years had greater improvement than those 18 years and older.Clinical Relevance: Advanced NMR programs can be successfully utilized in the postoperative ACLR setting to improve quantitative limb symmetry.

Following on our earlier single‐station, 2‐dimensional (2D) broadband interferometric mapping and polarization (BIMAP) observations of lightning discharges, we recently deployed two BIMAP stations at Los Alamos National Laboratory to map the lightning sources and their polarization in full 3‐dimensional (3D) space (BIMAP‐3D). The two stations are separated by 11.5‐km and each station consists of four antenna sets (instead of three for the original BIMAP) that form a Y‐shaped array for improved interferometric performance. In this paper, we report the BIMAP‐3D system design, a generalized and analytical 2D interferometry technique for noncoplanar antenna array, a two‐stage 3D mapping technique based on geometric triangulation and baseline‐based differential time of arrival, and a technique to reconstruct the polarization orientation in 3D space by combining the 2D polarization results from the two‐station observations. Along with description of the techniques, we demonstrate and discuss the initial lightning results, including 3D maps for a hybrid intracloud and cloud‐to‐ground flash and for a normal intracloud flash, development of abnormal K‐change leaders, and polarization signatures for a K‐change leader. We find that with the two‐stage 3D mapping techniques, the sources can be located to meters accuracy for a favorable event that occurs between the two stations. We also find the polarization vectors for the example K leader are mostly orthogonal to the leader channel after the full 3D polarization analysis. The main purpose of this paper is to report the BIMAP‐3D techniques and capabilities. Detailed analysis of more specific discharge processes will be reported in later studies. A new 3‐dimensional broadband radio frequency interferometric mapping and polarization system (BIMAP‐3D) is developed and deployed at Los Alamos National Laboratory for lightning research. BIMAP‐3D provides an unprecedented capability in high‐resolution, time‐evolving 3D lightning source mapping and 3D source polarization detection for detailed study of lightning discharge physics. In this research, we described the BIMAP‐3D system, introduced a suite of advanced data processing techniques, and demonstrated BIMAP‐3D's capabilities with actual lightning observations. This new capability is expected to lead to a range of new understandings and discoveries for a variety of lightning discharge processes. A new 3D broadband interferometric mapping and polarization system for lightning study is introducedA suite of new data process algorithms is reported, and 3D lightning results for overall flashes and K‐change leaders are demonstratedPolarization orientations for a K‐change leader are found mostly orthogonal to the leader channel A new 3D broadband interferometric mapping and polarization system for lightning study is introduced A suite of new data process algorithms is reported, and 3D lightning results for overall flashes and K‐change leaders are demonstrated Polarization orientations for a K‐change leader are found mostly orthogonal to the leader channel

Intense rainfall events significantly affect Alpine and Alaskan glaciers through enhanced melting, ice-flow acceleration and subglacial sediment erosion, yet their impact on the Greenland ice sheet has not been assessed. Here we present measurements of ice velocity, subglacial water pressure and meteorological variables from the western margin of the Greenland ice sheet during a week of warm, wet cyclonic weather in late August and early September 2011. We find that extreme surface runoff from melt and rainfall led to a widespread acceleration in ice flow that extended 140 km into the ice-sheet interior. We suggest that the late-season timing was critical in promoting rapid runoff across an extensive bare ice surface that overwhelmed a subglacial hydrological system in transition to a less-efficient winter mode. Reanalysis data reveal that similar cyclonic weather conditions prevailed across southern and western Greenland during this time, and we observe a corresponding ice-flow response at all land- and marine-terminating glaciers in these regions for which data are available. Given that the advection of warm, moist air masses and rainfall over Greenland is expected to become more frequent in the coming decades, our findings portend a previously unforeseen vulnerability of the Greenland ice sheet to climate change.