| 19 Feb 2026
Impact of climate forcing time step on the modelled ice-sheet firn layer
Abstract. The firn layer regulates how an ice-sheet responds to climate change by modifying how changes in surface temperature, snow accumulation and ablation affect the ice-sheet mass balance. Firn properties are often simulated with a firn densification model. Firn models are forced with surface mass balance components, surface energy balance components and/or meteorological variables. In literature, a variety of climate forcing time steps have been used to force such firn models, ranging from 3-hours to 1-day, 1-month, or even annual. To investigate the impact of these different time steps, we force the firn densification model IMAU-FDM with different climate forcing time steps at the surface for the Antarctic Peninsula and southern Greenland Ice Sheet. We show that the modelled firn layer contains more pore space for larger forcing time steps, and that locations with limited firn pore space due to seasonal melt are most sensitive. The climate forcing time step impacts the creation of pore space by snowfall and the depletion of pore space by snowmelt and firn densification. The key in causing the differences in firn pore space is the presence or absence of a diurnal cycle in the input data. A climate forcing time step greater than a day allows for a non-physical coexistence of snowmelt and sub-zero surface temperatures, leading to immediate shallow refreezing of meltwater. Subsequent melting removes refrozen ice rather than porous firn, reducing the amount of firn air that is lost through melting. Therefore, the decoupled temperature and snowmelt in the upper layers give more firn air with a climate forcing time step larger than a day. We also found that firn density parameterizations can become unsuitable when applied outside the physical conditions or climate forcing time step on which they are based. These parameterizations lead to unrealistic firn densification and accumulation behavior in the model. We argue that (1) firn models require a timestep small enough to capture at least the diurnal cycle, (2) use of parameterizations should be critically assessed and used consistently with the way they were originally developed, and (3) the forcing time step should be considered when interpreting firn model output.
Competing interests: At least one of the (co-)authors is a member of the editorial board of The Cryosphere.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.
Overview
As firn models and climate forcing datasets become more widely available, firn modeling is becoming an increasingly important avenue of cryospheric research. My initial impression was that this manuscript would primarily be of interest to researchers focused on the technical details of model application. However, the paper ultimately contains broader and more important implications for how firn processes are represented (or misrepresented) in models. I therefore consider this an important contribution that will be valuable to the broader cryospheric science community.
Major Comments
1. Table 1 in the Introduction
Table 1 does not add much in its current form. The list of papers and their time steps does not appear to be exhaustive, which makes the table potentially misleading. Simply referencing some examples in the text may be sufficient. Alternatively, the introduction could include a more informative table or figure that explicitly links model time-step choices to processes, firn model classes, or data availability constraints, etc.
2. Conceptual Overview (Figure 4)
While a conceptual overview figure is a valuable addition to the paper, Figure 4 requires substantial revision. In its current form, the schematic is difficult to interpret and appears inconsistent with the physical processes described in the text (to be blunt, it looks like something generated by AI). A few examples of shortcomings: In Process 1, the depiction of large refreezing grains at depth versus smaller refreezing grains near the surface does not appear physically meaningful and is inconsistent with the description in the text; In Process 2, the distinction between the left and right panels is unclear, and the intended contrast is not adequately explained in either the caption or the manuscript; In Process 3, it is not clear how the illustrated elements relate to time stepping at all. Given the importance of this figure for synthesizing the paper’s key ideas, a clearer and more physically grounded conceptual diagram is needed.
3. Formation of Ice Layers in the Model
The model description needs to explicitly explain how ice layers are formed in IMAU‑FDM. This information is essential for interpreting the results. For example, it is unclear whether daily forcing could artificially promote near‑surface ice slab formation due to the way meltwater refreezing and layer formation are implemented.
Without a clear description of how refreezing transitions into discrete ice layers, whether those layers are permeable or impermeable, and how they evolve over time, the reader cannot fully evaluate the paper’s conclusions regarding the impacts of model time step on firn structure and pore‑space evolution.
4. Snowfall
The language surrounding “Snowfall” and Process 3 is confusing and needs revision. The term snowfall typically refers to a precipitation rate or mass flux, whereas this section is primarily concerned with the initial (fresh) snow density parameterization. Referring to this process as “Snowfall” is therefore confusing because the effect arises from how fresh snow density is prescribed as a function of meteorological variables that depend on time step. This distinction is important because elsewhere in the manuscript (e.g., in comparisons between Greenland and Antarctica), snowfall clearly refers to accumulation magnitude. Clarifying this terminology, or renaming this section, would substantially improve readability and reduce potential confusion. In fact, I am not confident that I have correctly deciphered the sections concerning snowfall.
5. Organization
Section 3.1 (delta-FAC) is difficult to interpret when it is presented before the relevant physical context. At this stage in the manuscript, the reader does not yet understand the mechanisms responsible for the reported differences and so has little ability to absorb or evaluate the overall findings presented in this section. The manuscript would be more accessible if Section 3.2, describing the processes affected by time stepping, were presented first, followed by the resulting FAC differences. This structure would allow readers to interpret delta-FAC patterns in light of the governing processes.
Minor Line‑by‑Line Comments
Line 4. “In literature …” is awkward phrasing. Consider “In the literature” or “Prior studies …”.
Somewhere in the introduction it may be useful to note that, while time step is often constrained by spatial resolution and numerical stability in atmospheric or ice‑sheet models, this is generally not the case for purely one‑dimensional firn models. Clarifying this distinction would help contextualize the choice of forcing time steps.
Line 17. The term aliasing would be appropriate when referring to distortion or loss of the diurnal signal.
Line 41. “We want to …” is awkward and redundant; the intent is already clear and “want” can be removed.
Line 90. Does this statement assume that the entire firn column is at the melting point? How are ice layers handled in this context? This again highlights the need for a clearer description of ice‑layer formation in the model.
Figure 5. The two dashed line styles are nearly indistinguishable; these should be revised.
Figure 7. Adding a 1 m depth marker to the top panels would help the reader interpret the figure.