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

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.
