W. Boot

Large and rapid melt-induced velocity changes in the ablation zone of the Greenland Ice Sheet.

Continuous Global Positioning System observations reveal rapid and large ice velocity fluctuations in the western ablation zone of the Greenland Ice Sheet. Within days, ice velocity reacts to increased meltwater production and increases by a factor of 4. Such a response is much stronger and much faster than previously reported. Over a longer period of 17 years, annual ice velocities have decreased slightly, which suggests that the englacial hydraulic system adjusts constantly to the variable meltwater input, which results in a more or less constant ice flux over the years. The positive-feedback mechanism between melt rate and ice velocity appears to be a seasonal process that may have only a limited effect on the response of the ice sheet to climate warming over the next decades.

Drifting snow measurements on the Greenland Ice Sheet and their application for model evaluation

This paper presents autonomous drifting snow observations performed on the Greenland Ice Sheet in the fall of 2012. High-frequency snow particle counter (SPC) observations at∼ 1 m above the surface provided drifting snow number fluxes and size distributions; these were combined with meteorological observations at six levels. We identify two types of drifting snow events: katabatic events are relatively cold and dry, with prevalent winds from the southeast, whereas synoptic events are short lived, warm and wet. Precipitating snow during synoptic events disturbs the drifting snow measurements. Output of the regional atmospheric climate model RACMO2, which includes the drifting snow routine PIEKTUK-B, agrees well with the observed near-surface climate at the site, as well as with the frequency and timing of drifting snow events. Direct comparisons with the SPC observations at 1 m reveal that the model overestimates the horizontal snow transport at this level, which can be related to an overestimation of saltation and the typical size of drifting snow particles.

A Photoelectric Detector Suspended under a Balloon for Actinic Flux Measurements

Abstract An instrument to measure ultraviolet actinic flux in and outside clouds was constructed and calibrated. Characteristics of the instrument axe: 1) it is equipped with gallium phosphide photodiodes, 2) its isotropic directional response is almost perfect due to the special design of the optical diffusers, and 3) it is a lightweight construction (150 g) allowing the use of a balloon or a kit as an observation platform. Ground-level observations and vertical profiles, measured with this UV photometer in clear air and in stratocumulus during the Atlantic Stratocumulus Transition Experiment in the Azores, confirm that the Madronich radiation transfer model is valid for clouds.

Surface and tethered‐balloon observations of actinic flux: Effects of arctic stratus, surface albedo, and solar zenith angle

As part of the First ISCCP Regional Experiment (FIRE III) Arctic Cloud Experiment actinic flux measurements were made above the Arctic Sea ice during May 1998. The actinic flux, which is also referred to as the 4π radiative flux, is the relevant radiative parameter needed to determine photodissociation rates. It is shown that for a plane-parallel cloud the change in the net irradiance as a function of the optical depth is proportional to the magnitude of the actinic flux. Continuous actinic flux measurements were made just above the snow-covered ice surface by a UV-A and a visible 4π radiometer (wavelengths ∼365 and ∼550 nm, respectively). In addition, vertical profiles of the actinic flux through low arctic stratus clouds were obtained by means of a visible 4π radiometer suspended under a tethered balloon. The cloud thermodynamic and microphysical structure was determined from observations made by the National Center for Atmospheric Research C-130 aircraft. In addition, the phase and liquid water path of the cloud was assessed from microwave radiometer, lidar, and radar data. During clear-sky conditions the diurnal variation of the magnitude of actinic flux was controlled mainly by Rayleigh scattering and surface reflection. Above a stratus cloud layer the actinic flux was found to be almost the same as during clear-sky conditions. This could be attributed to the fact that the effective albedo of the arctic sea ice and the cloud is only slightly higher than the ground albedo alone. In the arctic stratus clouds the actinic flux was found to be nearly constant with height, except in a shallow layer near the cloud top where the actinic flux increased significantly with height. The vertical profiles that were observed in arctic stratus differed from those measured in Atlantic stratocumulus; in the latter the actinic flux was found to increase gradually from cloud base to cloud top. A delta-Eddington model is utilized to illustrate that the exact shape of the vertical profile is very sensitive to the solar zenith angle. During the arctic experiments the solar zenith angle was generally much larger than during the observations in Atlantic stratocumulus.

Hyaluronic Acid-Based Hydrogel Coating Does Not Affect Bone Apposition at the Implant Surface in a Rabbit Model

BackgroundUncemented orthopaedic implants rely on the bone-implant interface to provide stability, therefore it is essential that a coating does not interfere with the bone-forming processes occurring at the implant interface. In addition, local application of high concentrations of antibiotics for prophylaxis or treatment of infection may be toxic for osteoblasts and could impair bone growth.Questions/Purposes In this animal study, we investigated the effect of a commercially available hydrogel, either unloaded or loaded with 2% vancomycin. We asked, does unloaded hydrogel or hydrogel with vancomycin (1) interfere with bone apposition and timing of bone deposition near the implant surface; and (2) induce a local or systemic inflammatory reaction as determined by inflammation around the implant and hematologic parameters.MethodsIn 18 New Zealand White rabbits, an uncoated titanium rod (n = 6), a rod coated with unloaded hydrogel (n = 6), or a rod coated with 2% vancomycin-loaded hydrogel (n = 6) was implanted in the intramedullary canal of the left tibia. After 28 days, the bone volume fraction near the implant was measured with microCT analysis, inflammation was semiquantitatively scored on histologic sections, and timing of bone apposition was followed by semiquantitative scoring of fluorochrome incorporation on histologic sections. Two observers, blinded to the treatment, scored the sections and reconciled their scores if there was a disagreement. The hematologic inflammatory reaction was analyzed by measuring total and differential leukocyte counts and erythrocyte sedimentation rates in blood. With group sizes of six animals per group, we had 79% power to detect a difference of 25% in histologic scoring for infection and inflammation.ResultsNo differences were found in the amount of bone apposition near the implant in the No Gel group (48.65% ± 14.95%) compared with the Gel group (59.97% ± 5.02%; mean difference [MD], 11.32%; 95% CI, −3.89% to 26.53%; p = 0.16) or for the Van2 group (56.12% ± 10.06%; MD, 7.46; 95% CI, −7.75 to 22.67; p = 0.40), with the numbers available. In addition, the scores for timing of bone apposition did not differ between the No Gel group (0.50 ± 0.55) compared with the Gel group (0.33 ± 0.52; MD, −0.17; 95% CI, −0.86 to 0.53; p = 0.78) or the Van2 group (0.83 ± 0.41; MD, 0.33; 95% CI, −0.36 to 1.03; p = 0.42). Furthermore, we detected no differences in the histopathology scores for inflammation in the No Gel group (2.33 ± 1.67) compared with the Gel group (3.17 ± 1.59; MD, 0.83; 95% CI, −0.59 to 2.26; p = 0.31) or to the Van2 group (2.5 ± 1.24; MD, 0.17; 95% CI, −1.26 to 1.59; p = 0.95). Moreover, no differences in total leukocyte count, erythrocyte sedimentation rate, and neutrophil, monocyte, eosinophil, basophil, and lymphocyte counts were present between the No Gel or Van2 groups compared with the Gel control group, with the numbers available.ConclusionThe hydrogel coated on titanium implants, unloaded or loaded with 2% vancomycin, had no effect on the volume or timing of bone apposition near the implant, and did not induce an inflammatory reaction in vivo, with the numbers available.Clinical relevanceAntibiotic-loaded hydrogel may prove to be a valuable option to protect orthopaedic implants from bacterial colonization. Future clinical safety studies will need to provide more evidence that this product does not impair bone formation near the implant and prove the safety of this product.

The K-transect in west Greenland: Automatic weather station data (1993–2016)

ABSTRACT We present twenty-three years (1993–2016) of automatic weather station (AWS) data, collected along the K-transect near Kangerlussuaq in west Greenland. The transect runs from east to west, roughly perpendicular to the ice sheet edge at about 67° N. The K-transect originated from the Greenland Ice Margin Experiments (GIMEX), held in the summers of 1990 and 1991. Until recently, surface mass balance and ice velocity measurements were performed at nine locations along the K-transect, of which four are equipped with AWS: two in the ablation zone at approximately 500 m and 1,000 m asl, one at the approximate equilibrium-line altitude (~1,500 m asl), and one in the lower accumulation zone (~1,850 m asl) at distances of 5, 38, 88, and 140 km from the ice edge, respectively. Here, we present an overview of the various AWS types and their data corrections, quality, and availability, including a preliminary trend analysis. Recent increases in temperature and radiation components are associated with the frequent occurrence of anti-cyclonic conditions in west Greenland, resulting in clear skies and relatively warm summers. Strong melt concurs with a decrease in winter accumulation, lowering the surface albedo of the ice sheet. The AWS situated at 1,500 m asl, the former equilibrium-line altitude (ELA), observed almost a doubling of the summertime net shortwave radiation since 2004; as a result, the ELA along the K-transect has been steadily increasing and is currently situated well above 1,700 m asl.

A Multiple Direction Radiation Sensor, DIRAM

The Directional Radiance Distribution Measurement (DIRAM) device was designed and built to determine the angular distribution of shortwave radiance as a function of height in cloudy and clear-sky conditions at various surface albedos. The construction contains 42 sensors, consisting of a collimation system and a detector, which are mounted in two domes (21 in each). The collimators are made of solid PERSPEX cylinders, 8 mm in diameter and ;24 mm long, which are fully transparent in the visible range. Three diaphragms are carved in each cylinder to reduce the field of view to ;78 and to reduce the stray light. The detector is a commerially available silicon photodiode for the visible spectral range. The domes can be placed at ground level, or on top and below a research aircraft. The collimators collect broadband visible radiation in 42 regularly distributed viewing angles. The aperture of each sensor is about 78. The 42 signals are continuously stored by a datalogger with variable frequency up to 10 Hz. The angular dependence of solar radiation scattered from clouds (or the earth’s surface) can be determined. The purpose is to collect data on the scattering of solar radiation by clouds (and surface) in order to gain more insight in the shortwave atmospheric radiation budget. Optical properties such as angular discrimination of the collimators and sensitivity of the device were investigated and summarized. During a test flight the operational aspects were investigated. It appeared that the device was able to collect data without interruption in severe conditions (vibrations, strong temperature, and humidity changes).

Dynamic Response of a High Arctic Glacier to Melt and Runoff Variations

The dynamic response of High Arctic glaciers to increased runoff in a warming climate remains poorly understood. We analyze a 10-year record of continuous velocity data collected at multiple sites on Nordenskioldbreen, Svalbard, and study the connection between ice flow and runoff within and between seasons. During the melt season, the sensitivity of ice motion to runoff at sites in the ablation and lower accumulation zone drops by a factor of 3 when cumulative runoff exceeds a local threshold, which is likely associated with a transition from inefficient (distributed) to efficient (channelized) drainage. Average summer (June-August) velocities are found to increase with summer ablation, while subsequent fall (September-November) velocities decrease. Spring (March-May) velocities are largely insensitive to summer ablation, which suggests a short-lived impact of summer melt on ice flow during the cold season. The net impact of summer ablation on annual velocities is found to be insignificant.