Ward van Pelt

Fieldwork 2017

Pictures of the 2017 field campaign to Nordenskiöldbreen, Svalbard, are now online!


An electronic copy of my PhD-thesis, defended in 2014, can be found here!

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 Nordenskiöldbreen, 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. Please read more about this work here: link

The changing impact of snow conditions and refreezing on the mass balance of an idealized Svalbard glacier

Glacier surface melt and runoff depend strongly on seasonal and perennial snow (firn) conditions. Not only does the presence of snow and firn directly affect melt rates by reflecting solar radiation, it may also act as a buffer against mass loss by storing melt water in refrozen or liquid form. In Svalbard, ongoing and projected amplified climate change with respect to the global mean change has severe implications for the state of snow and firn and its impact on glacier mass loss. Model experiments with a coupled surface energy balance-firn model were done to investigate the climatic mass balance and the changing role of snow and firn conditions for an idealized Svalbard glacier. A climate forcing for the past, present and future (1984-2104) is constructed, based on observational data from Svalbard Airport and a seasonally dependent projection scenario. With this forcing we mimic conditions for a typical inland Svalbard glacier. Results illustrate ongoing and future firn degradation in response to an elevational retreat of the equilibrium line altitude (ELA) of 31 m per decade. The temperate firn zone is found to retreat and expand, while cold ice in the ablation zone warms considerably. In response to pronounced winter warming and an associated increase in winter rainfall, the current prevalence of refreezing during the melt season gradually shifts to the winter season in a future climate. Sensitivity tests reveal that in a present and future climate the density and thermodynamic structure of Svalbard glaciers are heavily influenced by refreezing. Refreezing acts as a net buffer against mass loss. However, the net mass balance change after refreezing is substantially smaller than the amount of refreezing itself, which can be ascribed to melt-enhancing effects after refreezing, which partly offset the primary mass-retaining effect of refreezing. Please read more about this work here: link

Multi-decadal climate and seasonal snow conditions in Svalbard

Svalbard climate is undergoing amplified change with respect to the global mean. Changing climate conditions directly affect the evolution of the seasonal snowpack, through its impact on accumulation, melt and moisture exchange. We analyze long-term trends and spatial patterns of seasonal snow conditions in Svalbard between 1961 and 2012. Downscaled regional climate model output is used to drive a snow modeling system (SnowModel), with coupled modules simulating the surface energy balance and snowpack evolution. The precipitation forcing is calibrated and validated against snow depth data on a set of glaciers around Svalbard. Climate trends reveal seasonally inhomogeneous warming and a weakly positive precipitation trend, with strongest changes in the north. In response to autumn warming the date of snow onset increased (2 days per decade), whereas in spring/summer opposing effects cause a non-significant trend in the snow disappearance date. Maximum snow water equivalent (SWE) in winter/spring shows a modest increase (+0.01 m w.e. per decade), while the end-ofsummer minimum snow area fraction declined strongly (from 48% to 36%). The equilibrium line altitude is highest in relatively dry inland regions and time-series show a clear positive trend (25 m per decade) as a result of summer warming. Finally, rain-on-snow in the core winter season, affecting ground-ice formation and limiting access of grazing animals to food supplies, peaks during specific years (1994, 1996, 2000 and 2012) and is found to be concentrated in the lower-lying coastal regions in south-western Svalbard. Please read more about this work here: link

The mass balance and firn evolution of glaciers around Kongsfjorden, Svalbard

In this work, a coupled modeling approach is applied to simulate the long-term (1961-2012) surface mass balance and subsurface evolution of the Kongsvegen and Holtedahlfonna glacier systems in western Svalbard. Principle aims are: 1) to quantify and analyze the distributed surface mass balance evolution, 2) to estimate the contribution of melt water refreezing and internal accumulation to the mass balance, and 3) to detect changes in firn conditions over the simulation period. In order to achieve this, HIRLAM regional climate model output for 1961-2012 is projected onto the 100-m model grid and serves as input for a coupled surface energy balance - firn model. Available stake measurements since 1987, together with weather station data and snow profiling observations, are used for parameter estimation, as well as validation of the model results. Extensive spin-up is performed to provide initialized subsurface conditions at the start of the experiments. Results indicate a slightly positive area-averaged surface mass balance, which only fractionally compensates for mass loss by calving. Refreezing provides a strong buffer for mass loss and substantial internal accumulation, i.e. refreezing below the previous years summer surface, adds uncertainty to mass balance observations in the accumulation zone. Increasing melt rates over the last two decades has led to enhanced mass loss and a retreat of the firn line. Together with a negative trend in the albedo and elevated runoff this could mark the onset of accelerated near-future mass loss. Please read more about this work here: link

Snow accumulation variability along a snow radar transect on Nordenskiöldbreen, Svalbard

Substantial uncertainty in mass balance modelling as well as in the interpretation of local mass balance observations stems from the lack of detailed knowledge of how snow accumulation varies in space and time. We present a novel inverse modelling approach to reconstruct annual accumulation patterns from Ground Penetrating Radar (GPR) data. A coupled surface energy balance - snow model is used to simulate surface melt and the evolution of subsurface density, temperature and water content. The inverse problem consists of iteratively calibrating accumulation, serving as input for the model, by finding a match between modelled and observed radar travel times. Accounting for melt water percolation, refreezing and runoff facilitates accumulation reconstruction in temperate firn. The inverse method is applied to a 16-km long GPR transect on Nordenskiöldbreen, Svalbard, yielding annual accumulation patterns for 2007-2012. Accumulation patterns contain substantial spatial variability, with annual standard deviations ranging from 13 to 27% of the mean, and show considerable year-to-year variations. Compared to traditional methods, accounting for horizontal density variability along the transect is shown to dampen spatial variability in reconstructed accumulation, whereas incorporating irreducible water storage reduces absolute values. Correlating normalised accumulation to terrain characteristics in the dominant wind direction indicates a strong preference of snow deposition on leeward slopes. The negative impact of small-scale accumulation variability on the mean net mass balance is quantified, yielding a negligible impact in the accumulation zone and a negative impact of -0.09 m w.e. a-1 in the ablation area. Please read more about this work here: link

An iterative inverse approach to obtain basal topography

Detailed knowledge of basal topography is relevant both for estimating ice volume contained in ice masses as well as for accurate time-dependent modelling of glacier dynamics. As direct observations of basal topography are scarce, inverse methods in which more widely available surface data are used to learn about basal conditions have become of increasing interest. We evaluate an inverse modelling approach to reconstruct distributed subglacial topography. The inverse method involves an iterative procedure in which an ice dynamical model (PISM) is run multiple times over a prescribed period, while being forced with space and time-dependent climate input. After every iteration bed heights are adjusted using information of the remaining misfit between observed and modelled surface topography. The inverse method is first applied in synthetic experiments with a constant climate forcing to verify convergence and robustness of the approach in three dimensions. In a next step, the inverse approach is applied to Nordenskiöldbreen, Svalbard, forced with height- and time-dependent climate input since 1300 AD. An L-curve stopping criterion is used to prevent over-fitting. Validation against radar data reveals a high correlation (up to R = 0.89) between modelled and observed thicknesses. Remaining uncertainties can mainly be ascribed to inaccurate model physics, in particular uncertainty in the description of sliding. Results demonstrate the applicability of this inverse method to reconstruct the ice thickness distribution of glaciers and ice caps. In addition to reconstructing bedrock topography, the method provides a direct tool to initialise ice flow models for forecasting experiments. Please read more about this work here: link

A new subglacial hydrology model for PISM

This research provides first steps towards implementation of a more realistic subglacial hydrology model in the Parallel Ice Sheet Model (PISM). Ice flow by sliding of the ice over the bed is known to depend strongly on the presence and evolution of subglacial water. In particular, sliding laws link the basal shear stress to subglacial water pressure. To model water pressures ice sheet models require a hydrology component that: 1) incorporates reasonable physics of water transport and evolving drainage morphology, 2) is applicable at a wide range of spatial and temporal scales, 3) contains a limited number of poorly-constrained constants. With this in mind, we developed and tested a subglacial hydrology model connecting distributed drainage to englacial water storage and discuss its potential for implementation in ice sheet models. The model is applied to the geometry of Nordenskiöldbreen, Svalbard. We analyse steady-state behaviour with respect to model parameter choices and discuss the crucial role of englacial transport on the pressure evolution in transient summer melt conditions. Steady-state experiments indicate a high sensitivity of model output to poorly-constrained parameters affecting cavity opening and closure as well as parameters affecting melt input and transmissivity of the drainage system. Transient experiments show the strong dependence of pressure variability on englacial porosity and motivate the use of the porosity as a regularisation parameter. This work is done in cooperation with Ed Bueler (University of Alaska, Fairbanks). More details can be found here: link

Mass balance, runoff and refreezing on Nordenskiöldbreen

A distributed energy balance model is coupled to a multi-layer snow model in order to study the mass balance evolution and the impact of refreezing on the mass budget of Nordenskiöldbreen, Svalbard. The model is forced with output of the regional climate model RACMO and meteorological data from Svalbard Airport. Extensive calibration and initialisation are performed to increase the model accuracy. For the period 1989-2010, we find a mean net mass balance of -0.39 m w.e. a-11. Refreezing contributes on average 0.27 m w.e. a-1 o the mass budget and is most pronounced in the accumulation zone. The simulated mass balance, radiative fluxes and subsurface profiles are validated against observations and are generally in good agreement. Climate sensitivity experiments reveal a non-linear, seasonally dependent response of the mass balance, refreezing and runoff to changes in temperature and precipitation. It is shown that including seasonality in climate change, with less pronounced summer warming, reduces the sensitivity of the mass balance, refreezing and the equilibrium line altitude (ELA) in a future climate. Please read more about this work here: link.

Cyclicity in PISM

At the ice-bed interface, the rate of basal motion depends crucially and in a complex manner on the interplay of stresses, thermodynamics and hydrology. Regardless of variability in the external climate forcing, these internal interactions may lead to feedbacks inducing periodic changes in ice geometry and dynamics. To study cyclic behaviour, numerical experiments were conducted on a synthetic topography with a three-dimensional thermo-mechanically coupled ice-sheet model, the Parallel Ice Sheet Model (PISM). Within the model, combined stress balances are connected to evolving thermodynamics and hydrology. The sensitivity of cyclic behaviour to changes in sliding-law parameters and the climate input is studied. Multiple types of oscillations were found with strong variations in both amplitude and frequency of ice volume and sliding velocities. A physical description is given in which these variations and transitions from one oscillation type to another are linked to the interplay of stresses, heat transport and hydrology at the ice-bed interface. High-frequency oscillations are linked to interaction of sliding and a redistribution of water at the base. Low-frequency cycles additionally rely on changes in the thermal regime, thereby mimicking surging of Svalbard-type. Oscillation characteristics are shown to be strongly sensitive to changes in sliding-law parameters and inclusion of a surface height dependent mass balance. Please read more about this work here: link