Improved representation of the contemporary Greenland ice sheet firn layer by IMAU-FDM v1.2G

Abstract

Abstract. The firn layer that covers 90 % of the Greenland ice sheet (GrIS) plays an important role in determining the response of the ice sheet to climate change. Meltwater can percolate into the firn layer and refreeze at greater depths, thereby temporarily preventing mass loss. However, as global warming leads to increasing surface melt, more surface melt may refreeze in the firn layer, thereby reducing the capacity to buffer subsequent episodes of melt. This can lead to a tipping point in meltwater runoff. It is therefore important to study the evolution of the Greenland firn layer in the past, present and future. In this study, we present the latest version of our firn model, IMAU-FDM (Firn Densification Model) v1.2G, with an application to the GrIS. We improved the density of freshly fallen snow, the dry-snow densification rate and the firn's thermal conductivity using recently published parametrizations and by calibration to an extended set of observations of firn density, temperature and liquid water content at the GrIS. Overall, the updated model settings lead to higher firn air content and higher 10 m firn temperatures, owing to a lower density near the surface. The effect of the new model settings on the surface elevation change is investigated through three case studies located at Summit, KAN-U and FA-13. Most notably, the updated model shows greater inter- and intra-annual variability in elevation and an increased sensitivity to climate forcing.

Spatial Response of Greenland's Firn Layer to NAO Variability

“…The disadvantage is that it does not allow for standing water to occur, nor for the formation of impermeable ice layers. Nevertheless, the modeled percolation depth of meltwater is comparable to the few available measurements (Brils et al., 2021; Heilig et al., 2018). IMAU‐FDM's performance in simulating liquid water content is also comparable to other models with a more elaborate handling of water percolation (Vandecrux, Mottram, et al., 2020).…”

“…Dedicated firn models are also much cheaper to run than regional climate models. This enables performing numerous sensitivity tests to calibrate and evaluate their performance (Brils et al., 2021; Kuipers Munneke et al., 2015; Ligtenberg et al., 2011). As a final advantage, the computational efficiency allows stand‐alone firn models to be initialized to a proper equilibrium state.…”

“…In addition to the densification equation, the model implicitly solves the 1D heat equation via the Crank‐Nicolson scheme (Crank & Nicolson, 1947) and includes latent heat release by refreezing liquid water. The firn's simulated temperature has been evaluated against in situ measurements showing a root‐mean‐square error (RMSE) of 3.1°C, caused by a systematic warm bias in the dry interior and a cold bias at low and wet locations (Brils et al., 2021).…”

“…In this study, we use a dedicated firn model, IMAU-FDM version 1.2G. Details of IMAU-FDM parametrizations and evaluation for the GrIS can be found in Brils et al (2021). Here, we will briefly discuss its main characteristics.…”

Rapid Basal Channel Growth Beneath Greenland's Longest Floating Ice Shelf

“…Finally, to investigate the propagation of dynamic thinning to the grounded ice, SERAC time series derived from altimetry were partitioned into components due to surface processes as estimated by the IMAU‐FDM v1.2G firn densification model (Brils et al., 2022) and ice dynamics, as per (Csatho et al., 2014; Shekhar et al., 2021).…”