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

Abstract

<jats:p>Abstract. Perennial snow, or firn, covers 80 % of the Greenland ice sheet and has the capacity to retain part of the surface meltwater, buffering the ice sheet’s contribution to sea level. Multi-layer firn models are traditionally used to simulate the firn processes and estimate meltwater retention. We present the output from nine firn models, forced by weather-station-derived mass and energy fluxes at four sites representative of the dry snow, percolation, ice slab and firn aquifer areas. We compare the model outputs and evaluate them against in situ observations. Models that explicitly account for deep meltwater percolation overestimate percolation depth and consequently firn temperature at the percolation and ice slab sites although they accurately simulate the recharge of the firn aquifer. Models using Darcy's law and a bucket scheme compare favourably to observations at the percolation site but only the Darcy models accurately simulate firn temperature and thus meltwater percolation at the ice slab site. We find that Eulerian models, that transfer firn through fixed layers, smooth sharp gradients in firn temperature and density over time. From the model spread, we find that simulated densities (respectively temperature) have an uncertainty envelope of ±60 kg m−3 (resp. ±14 °C) in the dry snow area and up to ±280 kg m−3 (resp. ±15–18 °C) at warmer sites. </jats:p>

The Community Firn Model (CFM) v1.0

“…The Crocus model was developed for mountain snowpacks (Brun et al, 1992), but it has also been used to simulate firn densification and hydrology (e.g., Langen et al, 2017;Verjans et al, 2019). Its equations are also used for the subsurface scheme in the RCM MAR (Cullather et al, 2016), which is used to simulate the surface mass balance of the Greenland and Antarctic ice sheets (Fettweis et al, 2017;Agosta et al, 2019;Alexander et al, 2019).…”

“…We note that a similar enthalpy-based method was employed by Meyer and Hewitt (2017). The second computes latent heat release in the meltwater percolation scheme and separately uses the dry-firn heat diffusion scheme; details are provided in Verjans et al (2019).…”

“…The CFM has several meltwater percolation schemes to choose from, including two "tipping-bucket schemes", a Richards equation single-domain scheme, and a Richards equation dual-domain approach. The latter two and one of the bucket schemes are described in Verjans et al (2019). The second bucket scheme is similar to other bucket schemes that have been developed: at each time step, the volume of surface meltwater is allowed to percolate downward through the pore space.…”

“…The second grain-growth parameterization accounts for the effect of liquid water on grain metamorphism (Sect. 2.6; Verjans et al, 2019). It is taken from Katsushima et al (2009), who used equations from Tusima (1978) and Brun (1989) to simulate water flow in seasonal snowpacks.…”

“…The CFM been used in several studies to date, and we briefly mention those here. Verjans et al (2019) used the CFM to compare model outputs from three firn meltwater percolation schemes using HL, KM, and CRO. The firn meltwater Retention Model Intercomparison Project (RetMIP) (Vandecrux et al, 2020) compared results from nine firn densification and hydrology models, including the CFM.…”