Validation of frictions laws from historical simulations&#160;of Upernavik Isstr&#248;m
Authors: Jager Eliot , Gillet-Chaulet Fabien , Mouginot Jérémie 
Published: March 2022

&lt;p&gt;Lack of observations was one of the main limitations for improving ice flow model prediction in glaciology. However, over the past few years, the amount of satellite observations has increased at a phenomenal rate,. which should allow us to better validate the models and their parameterizations. While inverse methods are now fairly common in glaciology to infer uncertain parameters from observed surface velocities acquired at a given date, a validation of these methods in transient simulation has to be made, particularly during key events such as important glacier retreat). Moreover, the choice of friction law in glaciological models remains an open question and validation of it at global scale has to be made.&lt;/p&gt;&lt;p&gt;Here, we investigate the capabilities of an ice flow model (Elmer/Ice) using two different friction laws to reconstruct observations of velocity and elevation changes&amp;#160; at Upernavik Isstr&amp;#248;m in Greenland between 1985 and 2020. The particularity of Upernavik Isstr&amp;#248;m during this period is the different behaviour of its 3 main glaciers : two of them are subject to important front position variations but with different timings, while the front position of the third one remains constant. First, we create two ensembles of simulation, which are evolving by prescribing the front position (Wood et al. 2021). The only difference between these two ensembles is the choice of the friction laws either a Weertman law or Regularised-Coulomb law similar to that of Joughin et al. (2019). Each ensemble represents the different possibilities of initial states given uncertain parameters (e.g. friction field, initial surface elevation, ice viscosity) and forcings (from two surface mass balance models : RACMO and MAR) sampled using a Latin hypercube. Second, we applied multiple metrics (scoring methods) to evaluate the accuracy and the precision of each ensemble against observations of surface velocity, elevation and ice discharge. Finally, we used Sobol indices which estimate the sensitivity of the transient simulations to the input parameters, and allow us to see which ones have the largest impact&amp;#160; in our study case.&lt;/p&gt;&lt;p&gt;In terms of global performance, we find that using a regularised-Coulomb law improves our ability to reproduce observations compared to a Weertman law. With the Sobol sensitivity analysis of the regularised-Coulomb ensemble, we show that the modeled response to the glacier retreats is very sensitive to the ice rheology and the initial friction fields. In contrast, the model is only weakly sensitive to the surface mass balance or the initial surface elevation.&lt;/p&gt;&lt;p&gt;Finally, we summarize and conclude on the methods developed here to evaluate the physical laws and parameters in the model that are best suited to reproduce the recent evolution of a Greenlandic glacier. By using scoring methods calculated on the observation of the recent past, we hope to be able to produce a more realistic and accurate projection of the future evolution of these glaciers, in which the uncertainties related to the initial state and the uncertain parameters are also better quantified.&lt;/p&gt;