Full Stokes ice-flow modeling of the high-Alpine glacier saddle Colle Gnifetti, Monte Rosa: Flow field characterization for an improved interpretation of the ice-core records

The high-Alpine glacier saddle Colle Gnifetti (CG), Monte Rosa massif, is the only cold glacier archive in the European Alps offering detailed ice-core records on the millennial-scale. However, the highly irregular snow deposition pattern and the complex flow regime produce depositional noise and upstream effects, which hinder the full interpretation of the ice-core records in terms of past atmospheric changes. In this context, this work focuses on establishing a three-dimensional full Stokes ice-flow model of the CG saddle, with the main objective to calculate precise backward trajectories of existing ice-core sites, which is necessary to evaluate potential upstream effects. The developed full Stokes model is fully thermomechanically coupled and includes firn rheology, firn densification and enthalpy transport, with consideration of atmospheric temperature changes of the last century, strain heating and surface meltwater refreezing. The simulations are performed using the state-of-the-art Finite Element software Elmer/Ice. The CG full Stokes model is validated by comparison with measurements of surface velocities, accumulation, annual layer thickness, borehole inclination angles, density and temperature. Estimated using different bedrock topographies, the error of the calculated source point positions on the glacier surface amounts to ~10% of the distance to the corresponding drill site. Moreover, the three-dimensional age field of the glacier is calculated with an uncertainty of ~20%. The calculated chronologies of four out of five ice cores are consistent with experimental dating results, based among others on layer counting and 14C measurements

A full Stokes ice-flow model to assist the interpretation of millennial-scale ice cores at the high-Alpine drilling site Colle Gnifetti, Swiss/Italian Alps

The high-Alpine ice-core drilling site Colle Gnifetti (CG), Monte Rosa, Swiss/Italian Alps, provides climate records over the last millennium and beyond. However, the full exploitation of the oldest part of the existing ice cores requires complementary knowledge of the intricate glacio-meteorological settings, including glacier dynamics. Here, we present new ice-flow modeling studies of CG, focused on characterizing the flow at two neighboring drill sites in the eastern part of the glacier. The3-D full Stokes ice-flow model is thermo-mechanically coupled and includes firn rheology, firn densification and enthalpy transport, and is implemented using the finite element software Elmer/Ice. Measurements of surface velocities, accumulation, borehole inclination, density and englacial temperatures are used to validate the model output. We calculate backward trajectories and map the catchment areas. This constrains, for the first time at this site, the so-called upstream effects for the stable water isotope time series of the two ice cores drilled in 2005 and 2013. The model also provides a 3-D age field of the glacier and independent ice-core chronologies for five ice-core sites. Model results are a valuable addition to the existing glaciological and ice-core datasets. This especially concerns the quantitative estimate of upstream conditions affecting the interpretation of the deep ice-core layers

Full Stokes ice-flow modeling of the high-Alpine glacier saddle Colle Gnifetti, Monte Rosa

The high-Alpine glacier saddle Colle Gnifetti (CG), Monte Rosa massif, is a unique drilling site in the European Alps offering continuous ice-core records on the millennial time-scale. However, the full interpretation ofthe ice-core time series is challenging due to the highly irregular (spatial and temporal) snow deposition pattern and, together with a complex flow regime, upstream effects. In this context, we present results of a new three-dimensional full Stokes ice-flow model of the CG saddle. The main objectives of the modeling tool concern (a) the calculation of backward trajectories of existing ice-core drill sites, which is required in order to evaluate potential upstream effects, and (b) provide a reliable age–depth relation, in order to support experimental methods in ice-core dating.The established full Stokes model is fully thermo-mechanically coupled. The model includes firn rheology and firn densification. The temperature field is calculated using the enthalpy method, with consideration of atmospheric temperature changes of the last century, strain heating and surface meltwater refreezing. The simulations are performed using the state-of-the-art Finite Element software Elmer/Ice. The CG full Stokes model is validated by comparison with glaciological measurements of surface velocities, snow accumulation, borehole inclination angles, density and englacial temperatures. Using the calculated backwards trajectories, the locations on the glacier surface of the ice-core source points are identified with an uncertainty of∼10% of the distance to the corresponding drill site. Moreover, the three-dimensional age field of the glacier is calculated with an uncertainty of∼20%. The calculated ice-core chronologiesare consistent with experimental dating results, based among others on annual layer counting and 14C measurements

Ice-flow modeling results to assist the interpretation of millennial-scale ice cores recovered at the high-Alpine ice drilling site Colle Gnifetti, Swiss/Italian Alps

Ice-flow modeling studies were conducted to assist the interpretation of millennial-scale ice cores recovered at the high-Alpine drilling site Colle Gnifetti, Swiss/Italian Alps. Backwards trajectories starting from the borehole sites at different depths are calculated in order to estimate ice-core catchment areas. The position of source points identified on the glacier surface is provided in the Swiss coordinate system (LV03) and in the UTM system. The uncertainty of the position of the source points is about 10% of the distance from the corresponding drill site. The ice-flow model also provides ice-core chronologies with an uncertainty of about 20%. Apart from the KCI core, the other four model chronologies are well in agreement with experimental results. Inclination angles of the boreholes KCC and KCI were measured in September 2016 (i.e. 3 and 11 years after drilling, respectively) to validate the modeled velocity field. The measurements were performed twice: downwards (from the top to the bottom of the borehole) and upwards. The uncertainty of the measurements is 1.9° and is estimated considering the diameter of the borehole and the size of the inclinometer probe. The uncertainty of the measurements is dominated by the potentially wrong (i.e. non-parallel to the borehole walls) positioning of the inclinometer probe inside the borehole. The quantity "Q orientation" indicates whether the inclinometer probe was likely correctly aligned with the borehole walls (Q orientation=1) or not (Q orientation=0)

Supplementing ice core time series at Colle Gnifetti with a 3D full stokes ice flow model using Elmer/Ice

The cold glacier saddle Colle Gnifetti (CG) is the unique drilling site in the European Alps offering ice core records substantially exceeding the instrumental period. However, the full exploitation of the unique potential of this site is hampered by depositional noise and, combined with a complex flow regime, upstream-effects. Here we present results from an ongoing new sophisticated flow modeling attempt, i.e. 3D full stokes with consideration of firn rheology, fully thermomechanically coupled, utilizing the finite element software Elmer/Ice. In view of our latest ice core drilled to bedrock in 2013, a major objective is to map source trajectories of existing ice core sites in order to evaluate potential upstream effects. An additional focus is to assist in finding a reliable age scale, especially targeting depths where annual layers can no more be counted. This includes the calculation of isochronous surfaces for intercomparison of different drilling sites within the CG multi core array. Previous numerical ice flow models of CG have been developed by W. Haeberli, S. Wagner, M. Lüthi (Lüthi & Funk 2000, 2001) and H. Konrad (Konrad et al. 2013). The level of accuracy of these previous works was mainly limited by the not well known bedrock topography and englacial temperatures. However, since the work of M. Lüthi several new temperature profiles are available (Hoelzle et al. 2011) and additional GPR measurements have been performed extensively (Bohleber 2011). Ongoing measurements will provide more precise information about the surface flow velocity, surface topography and their stationarity. In addition, two new ice cores have been drilled on CG, in 2005 and 2013. The 2013 drilling project employs a unique approach of combining multiple state-of-the-art methods in ice core analysis, for example new ultra-high resolution impurity analysis for detecting highly thinned annual layers as well as analysis of ice microstructure. All these new data sets together with the nowadays higher available computing power motivated a new model attempt at CG, the only way to evaluate potential upstream effects. Model input quantities comprise density profiles measured at the ice core sites, surface topography and GPR based bedrock topography. The model accuracy is limited especially by the latter, due to an uncertainty of typically 15%. Additional limitations arise from other model parameters, that are not directly constrained by measurement, for example the mechanical stress on the glacier boundaries. To achieve better constraints, the model input quantities are iteratively adjusted to provide the best fit between model derived and directly measured quantities. Here we present first results regarding the model validation based on comparison with empirical data, using for this purpose the measured surface velocities and borehole temperatures. Finally we discuss the next steps in building our model approach, which include comparing model results with ice core derived depth-dependent information like e.g. the observed layer thinning or the measured vertical age distribution as well as to use a flow law taking into account ice anisotropy, as observational evidence suggests. REFERENCES Bohleber, P. 2011: Ground-penetrating radar assisted ice core research: The challenge of Alpine glaciers and dielectric ice properties. Dissertation, Universität Heidelberg. Hoelzle, M., Darms, G., Lüthi, M. & Suter, S. 2011. Evidence of accelerated englacial warming in the Monte Rosa area, Switzerland/Italy. Cryosphere, 5(1), 231–243 (doi: 10.5194/tc-5-231-2011). Konrad, H., Bohleber, P., Wagenbach, D., Vincent, C. & Eisen, O. 2013: Determining the age distribution of Colle Gnifetti, Monte Rosa, Swiss Alps, by combining ice cores, ground-penetrating radar and a simple flow model. Journal of Glaciology, 59(213). Lüthi, M., & Funk, M. 2000: Dating of ice cores from a high Alpine glacier with a flow model for cold firn. Ann. Glaciol., 31:69–79. Lüthi, M., & Funk, M. 2001: Modelling heat flow in a cold, high altitude glacier: interpretation of measurements from Colle Gnifetti, Swiss Alps. J. Glaciol., 47:314–324