Abstract
Abstract. Perennial snow, or firn, covers 80 % of the Greenland ice sheet and has the capacity to retain surface meltwater, influencing the ice sheet mass balance and contribution to sea-level rise. Multilayer firn models are traditionally used to simulate firn processes and estimate meltwater retention. We present, intercompare and evaluate outputs from nine firn models at four sites that represent the ice sheet's dry snow, percolation, ice slab and firn aquifer areas. The models are forced by mass and energy fluxes derived from automatic weather stations and compared to firn density, temperature and meltwater percolation depth observations. Models agree relatively well at the dry-snow site while elsewhere their meltwater infiltration schemes lead to marked differences in simulated firn characteristics. Models accounting for deep meltwater percolation overestimate percolation depth and firn temperature at the percolation and ice slab sites but accurately simulate recharge of the firn aquifer. Models using Darcy's law and bucket schemes compare favorably to observed firn temperature and meltwater percolation depth at the percolation site, but only the Darcy models accurately simulate firn temperature and percolation at the ice slab site. Despite good performance at certain locations, no single model currently simulates meltwater infiltration adequately at all sites. The model spread in estimated meltwater retention and runoff increases with increasing meltwater input. The highest runoff was calculated at the KAN_U site in 2012, when average total runoff across models (±2σ) was 353±610 mm w.e. (water equivalent), about 27±48 % of the surface meltwater input. We identify potential causes for the model spread and the mismatch with observations and provide recommendations for future model development and firn investigation.
Highlights
In response to higher air temperatures and increased surface melt, the Greenland ice sheet has been losing mass at an accelerating rate over recent decades and is responsible for about 20 % of observed global sea-level rise (Van den Broeke et al, 2016; IMBIE Team, 2020)
We present comparisons of firn model outputs and model deviations from observations for firn temperature, density and liquid water content at sites representing different firn and meltwater regimes: dry firn (Summit), the percolation zone (Dye-2), ice slabs (KAN_U), and a firn aquifer (FA)
Nine state-of-the-art firn models were forced with mass and energy fluxes calculated from weather station data at four sites representative of various climatic zones of the Greenland ice sheet
Summary
In response to higher air temperatures and increased surface melt, the Greenland ice sheet has been losing mass at an accelerating rate over recent decades and is responsible for about 20 % of observed global sea-level rise (Van den Broeke et al, 2016; IMBIE Team, 2020). Increased surface melt in the firn area of the Greenland ice sheet affects the firn structure (Machguth et al, 2016; Mikkelsen et al, 2016), density (de la Peña et al, 2015; Vandecrux et al, 2018), air content (van Angelen et al, 2013; Vandecrux et al, 2019) and temperature (Polashenski et al, 2014; Van den Broeke et al, 2016) These changing characteristics impact the firn’s meltwater storage capacity, through its ability to either refreeze meltwater (Pfeffer et al, 1991; Braithwaite et al, 1994; Harper et al, 2012) or retain liquid water in perennial firn aquifers (e.g., Forster et al, 2014; Miège et al, 2016). The depth of firn-to-ice transition as well as the mobility of gases through the firn before they are trapped in bubbles within glacial ice is necessary for the interpretation of ice cores and heavily depends on the fine coupling between the firn characteristics and surface conditions (e.g., Schwander et al, 1993)
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