Comparison of adjoint and nudging methods to initialise ice sheet model basal conditions

Mosbeux, Cyrille; Gillet-Chaulet, Fabien; Gagliardini, Olivier

doi:10.5194/gmd-9-2549-2016

Cited by 25 publications

(30 citation statements)

References 39 publications

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“…For the one type, ice-flow models are applied in a pseudo-transient way such that the actual surface elevation remains close to observations optimising the bed topography (van Pelt et al, 2013). For the other type, ice velocities are taken from observations and enter the mass-conservation equation, which is then directly solved for ice thickness (Morlighem et al, 2011;McNabb et al, 2012;Mosbeux et al, 2016).…”

Section: Introductionmentioning

confidence: 99%

Application of a two-step approach for mapping ice thickness to various glacier types on Svalbard

et al. 2017

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Abstract. The basal topography is largely unknown beneath most glaciers and ice caps, and many attempts have been made to estimate a thickness field from other more accessible information at the surface. Here, we present a two-step reconstruction approach for ice thickness that solves mass conservation over single or several connected drainage basins. The approach is applied to a variety of test geometries with abundant thickness measurements including marine-and landterminating glaciers as well as a 2400 km 2 ice cap on Svalbard. The input requirements are kept to a minimum for the first step. In this step, a geometrically controlled, non-local flux solution is converted into thickness values relying on the shallow ice approximation (SIA). In a second step, the thickness field is updated along fast-flowing glacier trunks on the basis of velocity observations. Both steps account for available thickness measurements. Each thickness field is presented together with an error-estimate map based on a formal propagation of input uncertainties. These error estimates point out that the thickness field is least constrained near ice divides or in other stagnant areas. Withholding a share of the thickness measurements, error estimates tend to overestimate mismatch values in a median sense. We also have to accept an aggregate uncertainty of at least 25 % in the reconstructed thickness field for glaciers with very sparse or no observations. For Vestfonna ice cap (VIC), a previous ice volume estimate based on the same measurement record as used here has to be corrected upward by 22 %. We also find that a 13 % area fraction of the ice cap is in fact grounded below sea level. The former 5 % estimate from a direct measurement interpolation exceeds an aggregate maximum range of 6-23 % as inferred from the error estimates here.

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Section: Introductionmentioning

confidence: 99%

Application of a two-step approach for mapping ice thickness to various glacier types on Svalbard

et al. 2017

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“…Observed velocities were used to invert for optimal basal friction parameters. While that study employed a simplified version of the Stokes equations and lacked time-dependence, other researchers have since undertaken the task of using adjoint sensitivities in a variety of icerelated applications with more complexity (Vieli and Payne, 2003;Larour et al, 2005;Khazendar et al, 2007;Waddington 20 et al, 2007;Joughin et al, 2009;Pattyn et al, 2008;Heimbach and Bugnion, 2009;Morlighem et al, 2010;Brinkerhoff et al, 2011;Goldberg and Sergienko, 2011;Gillet-Chaulet et al, 2012;Petra et al, 2012;Brinkerhoff and Johnson, 2013;Gagliardini et al, 2013;Goldberg and Heimbach, 2013;Morlighem et al, 2013;Larour et al, 2014;Perego et al, 2014;Isaac et al, 2015;Goldberg et al, 2015Goldberg et al, , 2016Mosbeux et al, 2016).…”

Section: Sensitivity Analysismentioning

confidence: 99%

SICOPOLIS-AD v1: an open-source adjoint modeling framework for ice sheet simulation enabled by the algorithmic differentiation tool OpenAD

Logan¹,

Narayanan²,

Greve³

et al. 2019

Preprint

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We present a new capability of the ice sheet model SICOPOLIS that enables flexible adjoint code generation via source transformation using the open-source algorithmic differentiation (AD) tool OpenAD. The adjoint code enables efficient calculation of sensitivities of a scalar-valued objective function or quantity of interest (QoI) to a range of important, often spatially varying model input variables, including initial and boundary conditions, as well as model parameters. Compared to earlier work on adjoint code generation of SICOPOLIS, our work is based on several important advances: (i) it is embedded 5within the up-to-date trunk of the SICOPOLIS repository -accounting for one and a half decades of code development and improvements -and is readily available to the wider community; (ii) the AD tool used, OpenAD, is an open-source tool; (iii) the adjoint code developed is applicable to both Greenland and Antarctica, including grounded ice as well as floating ice shelves, and with an extended choice of thermodynamical representations. A number of code refactorization steps were required. They are discussed in detail in an Appendix as they hold lessons for application of AD to legacy codes at large. As an example 10 application, we examine the sensitivity of the total Antarctic Ice Sheet volume to changes in initial ice thickness, summer precipitation, and basal and surface temperatures across the ice sheet. Simulations of Antarctica with floating ice shelves show that over 100 years of simulation the sensitivity of total ice sheet volume to the initial ice thickness and precipitation is almost uniformly positive, while the sensitivities to surface and basal temperature are almost uniformly negative. Sensitivity to summer precipitation is largest on floating ice shelves from Queen Maud to Queen Mary Land. The largest sensitivity to initial 15 ice thickness is at outlet glaciers around Antarctica. Comparison between total ice sheet volume sensitivities to surface and basal temperature shows that surface temperature sensitivities are higher broadly across the floating ice shelves, while basal temperature sensitivities are highest at the grounding lines of floating ice shelves and outlet glaciers. A uniformly perturbed region of East Antarctica reveals that, among the four control variables tested here, total ice sheet volume is most sensitive to variations in summer precipitation as formulated in SICOPOLIS. Comparison between adjoint-and finite-difference-derived 20 1 https://doi.an outlet glacier or floating ice shelf that drains an ice sheet). A computationally costly method for deriving such sensitivities might use individually-made perturbations to the bottom melting rate at each location of the ice sheet's base, in particular at its margins that are in contact with ocean water. This means that the ice sheet dynamics must be integrated throughout time for each simulation experiment in which a point-wise perturbation has been applied in order to assemble a sensitivity map across the entire domain to this control v...

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“…by the SMB forcing and other boundary conditions that have a larger impact on ice sheet evolution than choices made concerning the stress balance approximations [31]. Recent attempts have been made to simulate the ice sheet by relying on the shallow shelf approximation in combination with extensive data assimilation techniques [20,34,35]. However, the potential consequences of using these approximations to simulate ice dynamics for past and future ice sheet evolution are not well known.…”

Section: Approximations To the Stress Balance Equationmentioning

confidence: 99%

“…The optimisation of the basal topography can also be directly integrated into the data assimilation procedure, either by applying the shallow shelf approximation [35] or a higherorder, non-depth-integrated ice flow model [81]. In the latter, assuming thermomechanical equilibrium with a specified, equilibrium climate forcing, this adjoint-based method combines the misfit with observed surface velocities and ice thickness change in the cost function to be minimised.…”

Section: Data Assimilation Techniquesmentioning

confidence: 99%

Recent Progress in Greenland Ice Sheet Modelling

Goelzer

Robinson

Seroussi

et al. 2017

Curr Clim Change Rep

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Purpose of Review This paper reviews the recent literature on numerical modelling of the dynamics of the Greenland ice sheet with the goal of providing an overview of advancements and to highlight important directions of future research. In particular, the review is focused on large-scale modelling of the ice sheet, including future projections, model parameterisations, paleo applications and coupling with models of other components of the Earth system. Recent Findings Data assimilation techniques have been used to improve the reliability of model simulations of the Greenland ice sheet dynamics, including more accurate initial states, more comprehensive use of remote sensing as well as paleo observations and inclusion of additional physical processes. Summary Modellers now leverage the increasing number of high-resolution satellite and air-borne data products to initialise ice sheet models for centennial time-scale simulations, needed for policy relevant sea-level projections. Modelling long-term past and future ice sheet evolution, which requires simplified but adequate representations of the interactions with the other components of the Earth system, has seen a steady improvement. Important developments are underway to include ice sheets in climate models that may lead to routine simulation of the fully coupled Greenland ice sheet-climate system in the coming years.

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Comparison of adjoint and nudging methods to initialise ice sheet model basal conditions

Cited by 25 publications

References 39 publications

Application of a two-step approach for mapping ice thickness to various glacier types on Svalbard

Application of a two-step approach for mapping ice thickness to various glacier types on Svalbard

SICOPOLIS-AD v1: an open-source adjoint modeling framework for ice sheet simulation enabled by the algorithmic differentiation tool OpenAD

Recent Progress in Greenland Ice Sheet Modelling

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