Development of physically based liquid water schemes for Greenland firn-densification models

Vincent Verjans,Michiel R. van den Broeke,Brice Noël,C. Max Stevens,Michael MacFerrin,Amber A. Leeson

doi:10.5194/tc-13-1819-2019

Abstract

Abstract. As surface melt is increasing on the Greenland Ice Sheet (GrIS), quantifying the retention capacity of the firn layer is critical to linking meltwater production to meltwater runoff. Firn-densification models have so far relied on empirical approaches to account for the percolation–refreezing process, and more physically based representations of liquid water flow might bring improvements to model performance. Here we implement three types of water percolation schemes into the Community Firn Model: the bucket approach, the Richards equation in a single domain and the Richards equation in a dual domain, which accounts for partitioning between matrix and fast preferential flow. We investigate their impact on firn densification at four locations on the GrIS and compare model results with observations. We find that for all of the flow schemes, significant discrepancies remain with respect to observed firn density, particularly the density variability in depth, and that inter-model differences are large (porosity of the upper 15 m firn varies by up to 47 %). The simple bucket scheme is as efficient in replicating observed density profiles as the single-domain Richards equation, and the most physically detailed dual-domain scheme does not necessarily reach best agreement with observed data. However, we find that the implementation of preferential flow simulates ice-layer formation more reliably and allows for deeper percolation. We also find that the firn model is more sensitive to the choice of densification scheme than to the choice of water percolation scheme. The disagreements with observations and the spread in model results demonstrate that progress towards an accurate description of water flow in firn is necessary. The numerous uncertainties about firn structure (e.g. grain size and shape, presence of ice layers) and about its hydraulic properties, as well as the one-dimensionality of firn models, render the implementation of physically based percolation schemes difficult. Additionally, the performance of firn models is still affected by the various effects affecting the densification process such as microstructural effects, wet snow metamorphism and temperature sensitivity when meltwater is present.

Highlights

Estimating the properties of the firn layer – and how it evolves under a warming climate – is a critical step in measuring the ice sheets’ contribution to sea level rise, yet it remains one of the key sources of uncertainty in present assessments (McMillan et al, 2016)
We describe and compare liquid water schemes of different levels of physical complexity from snow models, and we apply these in combination with firndensification models in order to evaluate the impact of the treatment of liquid water flow on modelled firn densification and temperature
We begin by comparing bucket method (BK), R1M and dual-permeability Richards equation scheme (DPM) in a base case parameterisation: BK wh02 ip810, R1M grLK ip810 and DPM grLK ip810 respectively

Summary

Introduction

Estimating the properties of the firn layer – and how it evolves under a warming climate – is a critical step in measuring the ice sheets’ contribution to sea level rise, yet it remains one of the key sources of uncertainty in present assessments (McMillan et al, 2016). V. Verjans et al.: Physically based liquid water schemes ing has become more widespread and intense on the Greenland Ice Sheet (GrIS), with annual total melt rates rising by 11.4 Gt yr−2 between 1991 and 2015 (van Angelen et al, 2014; van den Broeke et al, 2016). Verjans et al.: Physically based liquid water schemes ing has become more widespread and intense on the Greenland Ice Sheet (GrIS), with annual total melt rates rising by 11.4 Gt yr−2 between 1991 and 2015 (van Angelen et al, 2014; van den Broeke et al, 2016) This meltwater percolates into the firn layer, where it can refreeze, run off or remain liquid in temperate firn. As such, understanding physical processes in firn, including in particular the transport of liquid water, is becoming increasingly important in order to accurately constrain and predict the mass balance of the GrIS (van den Broeke et al, 2016)

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