The Community Firn Model (CFM) v1.0

Abstract

Abstract. Models that simulate evolution of polar firn are important for several applications in glaciology, including converting ice-sheet elevation-change measurements to mass change and interpreting climate records in ice cores. We have developed the Community Firn Model (CFM), an open-source, modular model framework designed to simulate numerous physical processes in firn. The modules include firn densification, heat transport, meltwater percolation and refreezing, water-isotope diffusion, and firn-air diffusion. The CFM is designed so that new modules can be added with ease. In this paper, we first describe the CFM and its modules. We then demonstrate the CFM's usefulness in two model applications that utilize two of its novel aspects. The CFM currently has the ability to run any of 13 previously published firn-densification models, and in the first application we compare those models' results when they are forced with regional climate model outputs for Summit, Greenland. The results show that the models do not agree well (spread greater than 10 %) when predicting depth-integrated porosity, firn age, or trend in surface-elevation change trend. In the second application, we show that the CFM's coupled firn-air and firn-densification models can simulate noble-gas records from an ice core better than a firn-air model alone.

High-frequency climate variability in the Holocene from a coastal-dome ice core in east-central Greenland

“…a very small fraction of the total annual ice volume (< 1 cm for 45 cm ice per year). The AMR exhibited some centennial variability with a few distinct periods of increased melt but did not demonstrate a long-term trend over the last 2.1 kyr (Taranczewski et al, 2019). Furthermore, during brief periods in which there is a ∼ 1 % increase in AMR (i.e., 1850-1700 b2k, 200 b2k-present), increases in melt layer occurrence would likely serve to decrease the annual amplitude, which we do not observe.…”

“…Since diffusion corrections assume constant annual accumulation rates, diffusion correction can over-or underestimate the summer and winter signals. We utilize the Community Firn Model (CFM) (Stevens et al, 2020) to test an extreme case of shifts in seasonality of accumulation, from constant accumulation to summer-fall weighted accumulation, as given by MAR (Fettweis et al, 2017). We cannot rule out that centennial variations in summer and winter isotope values could be the result of seasonality of accumulation and the associated diffusional effects.…”

“…We use the Community Firn Model (CFM) (Stevens et al, 2020) to test the effects of seasonally biased accumulation on firn diffusion in the ice core. We test five 490-year scenarios for isotope evolution, based upon temperature and accumulation fields from 1958 to 1978 provided by the Modèle Atmosphérique Régional (MAR; version 3.9 with monthly ERA forcing) (Fettweis et al, 2017) (Fig.…”

“…The density of melt layers at RECAP was measured by Taranczewski et al (2019), determining a high-resolution record for the last 2.1 kyr. The ratio of snow water equivalent of a melt layer to the respective annual layer is characterized as the annual melt ratio (AMR), which over the last 2.1 kyr has an average value of less than 2 % and is therefore Figure 9.…”

High-frequency climate oscillations in the Holocene from a coastal-dome ice core in east central Greenland

“…We use the Community Firn Model (CFM) (Stevens et al, 2020) to test five 490-year scenarios for isotope evolution, based upon temperature and accumulation fields from 1958-1978 provided by the Modèle Atmosphérique Régional (MAR; version 3.9 with monthly ERA forcing) (Fettweis et al, 2017) and a constant amplitude (4‰) sine wave to represent the annual isotopic variability (Fig. 5).…”