The firn meltwater Retention Model Intercomparison Project (RetMIP): evaluation of nine firn models at four weather station sites on the Greenland ice sheet

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.

Downscaled surface mass balance in Antarctica: impacts of subsurface processes and large-scale atmospheric circulation

“…These results therefore emphasise the importance of adequate spin-up and assessment of the effects of snowpack spin-up in producing and using SMB in Antarctica. Vandecrux et al (2020b) found that the Fixed version smoothes the firn density profiles, when compared to the dynamical version, this is confirmed by our results. One of the criteria for the dynamical version is that it prefers to merge layers deeper than 5 m of w.eq., meaning that the top 5 m w.eq.…”

“…have a high vertical resolution, this makes it easier to detect changes in density. In areas such as the AP, Ronne-Filchner ice shelf, Ross ice shelf and in coastal areas of Dronning Maud Land where seasonal melt occurs (Zwally and Fiegles, 1994;Wille et al, 2019), meltwater can percolate into the firn and refreeze, creating ice lenses that change the density, but that cannot be detected if the subsurface scheme have layers with a fixed mass even if the vertical resolution is increased (Vandecrux et al, 2020b). Not only is there a difference between the models when evaluating density profiles, this study also shows the importance of spatial evaluation, here the three simulations follow the same pattern by over/underestimating the densities in the same areas (Fig.…”

Uncertainty in East Antarctic Firn Thickness Constrained Using a Model Ensemble Approach

“…This period is chosen to match the long-term altimetry record of Shepherd et al (2019), hence facilitating intercomparison of observed elevation changes and modelled firn thickness change experiments of this study. We limit our analysis to the EAIS because surface melt there is minor compared to the AP and WAIS, and FDM fidelity remains questionable for simulating wet firn compaction, water percolation and refreezing (Steger et al, 2017;Verjans et al, 2019;Vandecrux et al, 2020). Table 1: The nine firn densification models (FDM), three regional climatic models (RCM) and two surface density parameterisations ( ) used in this study.…”

Uncertainty in East Antarctic firn thickness constrained using a model ensemble approach

“…This period is chosen to match the long-term altimetry record of Shepherd et al (2019), hence facilitating intercomparison of observed elevation changes and modelled firn thickness change experiments of this study. We limit our analysis to the EAIS because surface melt there is minor compared to the AP and WAIS, and FDM fidelity remains questionable for simulating wet firn compaction, water percolation and refreezing (Steger et al, 2017;Verjans et al, 2019;Vandecrux et al, 2020). 1: The nine firn densification models (FDM), three regional climatic models (RCM) and two surface density parameterisations ( ) used in this study.…”