Interannual variability in net accumulation on the Greenland Ice Sheet: Observations and implications for mass balance measurements

This is the published version, also available here: http://dx.doi.org/10.1029/1998JD200082.Nine 24-year accumulation records from the Summit region in central Greenland are analyzed to separate the effects of spatial noise and interannual fluctuations on the variability in each core. The study shows that both processes are equally important, with standard deviations of 25 mm water equivalent per year and 24 mm water equivalent per year, respectively. A comparison with estimates of surface roughness based on high-resolution laser altimetry of the surface indicates that in the studied region the spatial noise can be reliably estimated from surface roughness. The response of the ice-sheet surface to the interannual fluctuations can be estimated using a simple zero-dimensional ice-sheet response model. For the Summit region of central Greenland, a change in surface elevation of ∼20 mm water equivalent per year measured over a 5-year period, can be attributed with 95% confidence to a trend in climate. This probability decreases rapidly as the observation period is shortened. For intervals greater than ∼5 year, the probability depends only weakly on the measurement interval. This suggests an optimum spacing of ∼5 years between repeat elevation measurements

Are we overusing IVF?

Peer reviewedPublisher PD

An assessment of a days off decomposition approach to personnel scheduling

This paper studies a two-phase decomposition approach to solve the personnel scheduling problem. The first phase creates a days off schedule, indicating working days and days off for each employee. The second phase assigns shifts to the working days in the days off schedule. This decomposition is motivated by the fact that personnel scheduling constraints are often divided in two categories: one specifies constraints on working days and days off, while the other specifies constraints on shift assignments. To assess the consequences of the decomposition approach, we apply it to public benchmark instances, and compare this to solving the personnel scheduling problem directly. In all steps we use mathematical programming. We also study the extension that includes night shifts in thefirst phase of the decomposition. We present a detailed results analysis, and analyze the effect of various instance parameters on the decompositions' results. In general, we observe that the decompositions significantly reduce the computation time, and that they produce good solutions for most instances

Controls on advance of tidewater glaciers: results from numerical modeling applied to Columbia Glacier

This is the published version, also available here: http://dx.doi.org/10.1029/2006JF000551.A one-dimensional numerical ice flow model is used to study the advance of a tidewater glacier into deep water. Starting with ice-free conditions, the model simulates glacier growth at higher elevations followed by advance on land to the head of the fjord. Once the terminus reaches a bed below sea level, calving is initiated. A series of simulations was carried out with various boundary conditions and parameterizations of the annual mass balance. The results suggest that irrespective of the calving criterion and accumulation rate in the catchment area, it is impossible for the glacier terminus to advance into deeper water (>300 m water depth) unless sedimentation at the glacier front is included. The advance of Columbia Glacier, Alaska, is reproduced by the model by including “conveyor belt” recycling of subglacial sediment and the formation of a sediment bank at the glacier terminus. Results indicate slow advance through the deep fjord and faster advance in shallow waters approaching the terminal moraine shoal and the mouth of the fjord