High-resolution firn on Antarctic ice shelves

Abstract

Firn provides pore space that can retain meltwater. On the Antarctic ice shelves, depletion of the firn air content (FAC) can lead to hydrofracturing, by allowing for accumulation of meltwater. To understand the behaviour of the firn layer, we model the firn densification, offering insights into the dynamics of the entire firn column across the Antarctic continent over multiple years. So far, firn modeling across the Antarctic continent is done on a relatively coarse grid, for example, at 27 km for the IMAU firn densification model (IMAU-FDM). The coarse resolution cannot capture FAC gradients in areas with complex topographies, overestimating FAC near the grounding lines. High-resolution FAC is needed to assess ice shelf vulnerability more accurately. This study pioneers the statistically downscaled forcing data to drive IMAU-FDM at a 2 km resolution. Around grounding lines with steep elevation gradients, high-resolution FAC shows the highest decreases (over 10%) on ice shelves and the highest increases (ranging from 5% to 30%) on grounded ice. These changes in FAC align with the changes in downscaled snowmelt and snowfall, the main drivers of the firn layer. FAC in flatter areas shows no clear change after the downscaling. Surprisingly, the absence of a daily cycle in the downscaled input data increased the FAC by over 20% in areas with high snowmelt and a shallow firn layer, compared to earlier simulations that included a daily cycle. Overall, we found that the high-resolution FAC aligns better with the observations in areas with complex topographies. As downscaling primarily enhances lower-resolution data through topographical adjustments, biases in the low- resolution input unrelated to elevation or lacking physics in the firn model remain unresolved at higher resolutions. Thus, an improvement of modeled firn resulting from the downscaled input is constrained to regions with complex topography. In these areas, the high-resolution FAC gives a more realistic insight into the ice shelf vulnerability regarding the firn layer.