the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 Oct 2025
T-REX: The tile-based representation of lateral exchange processes in ICON-Land
Abstract. The vast majority of land-surface models uses a tiling-approach to capture the effects of subgrid-scale spatial heterogeneity in the land-surface properties. In most cases, however, the tiles, which represent patches with homogeneous characteristics at and below the surface, are treated independently of each other and the lateral exchange between them is not being taken into consideration. The present manuscript describes an approach for a tile-based representation of lateral exchange processes in heterogeneous landscapes that was recently implemented into ICON-Land, the land surface component of the ICON framework. The scheme captures the horizontal fluxes on a broad range of spatial scales and represents 5 lateral exchange processes, namely gravity-driven moisture fluxes and the corresponding convective heat transport, diffusive- and conductive fluxes of water and heat as well as the wind-driven redistribution of snow. In the approach, the relationships between any two tiles are determined by a set of characteristic connectivities which are treated as inherent properties of the pair of tiles – invariant in time and independent of the location – and derive from the internal logic underlying the definition of the tiles. The characteristic connectivities are used to calculate the spatio-geometric relationships between two tiles, such as the (geometrical) contact length or the characteristic center-to-center distance between two adjacent surface clusters. These, in turn, define gradients as well as the time-lag factors that govern the lateral transport in the model. In addition to a description of the model development, we present two example applications of the new scheme, which address the effect of sub-grid scale fluxes on the model's ability to capture the spatial variability in the state of the surface and sub-surface and the overall terrestrial water storage. Here, our results suggest that lateral exchange processes, especially on small horizontal scales, are highly relevant for the spatial variability in the soil temperatures and for the simulated extent of surface water bodies, while the effects on the grid-cell mean state and the turbulent exchange with the atmosphere appear to be largely negligible.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-4031', Anonymous Referee #1, 28 Nov 2025
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AC1: 'Reply on RC1', Philipp de Vrese, 25 Feb 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-4031/egusphere-2025-4031-AC1-supplement.pdf
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AC1: 'Reply on RC1', Philipp de Vrese, 25 Feb 2026
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RC2: 'Comment on egusphere-2025-4031', Anonymous Referee #2, 24 Jan 2026
The manuscript “T-REX: The Tile-Based Representation of Lateral Exchange Processes in ICON-Land” introduces significant extensions to the ICON-Land model. These extensions primarily utilize a tile-based representation to address lateral exchange processes in heterogeneous landscapes. The approach, named T-REX, effectively captures horizontal fluxes across various spatial scales and configurations. The manuscript is well written and well justified, and it offers an innovative contribution to the land surface modeling community. I believe that the broader Earth system and climate modeling communities would greatly benefit from a more detailed representation of sub-grid scale processes in land surface models. Therefore, I consider the contributions introduced in this manuscript to be highly valuable. Below, I present my main comments and questions, along with some minor fixes.
1. The scheme relies on fixed characteristic connectivities between tile types. It is unclear how these should be specified for real landscapes. In particular, equation 1 for the weighted connectivity c(t,s) is hard to understand from the text alone. The authors should consider clarifying this concept, as it is vital to the methods that follow. It would strengthen the paper to include an example or add an appendix including it.
2. The spatial relationship module assumes circular patch shapes to compute center-to-center distances, as shown in equation 3. This is a substantial simplification. How robust are the results to this assumption? Many natural patterns are not circular. I would encourage the authors to discuss this decision more extensively. Additionally, the authors mention that only two‐level nesting is allowed. Please discuss the implications of this decision. At a minimum, I consider that the limitations of the circular‐patch assumption should be stated clearly.
3. Both examples use highly idealized tiling configurations (i.e., a two‐tile upland/depression split, and uniform concentric circles everywhere in the Arctic). Even though the rationale for selecting these 2 configurations is clear (i.e., testing the effects of different lateral transport processes introduced by T-REX), the results may depend strongly on these assumptions. How sensitive are the findings to these choices? It would be helpful to discuss how one would apply T REX to a more complex tiling. Can the authors comment generally on their conclusions beyond the toy cases? Perhaps an additional test or sensitivity (even qualitative) could be suggested.
4. In the new tiling structure, many associated parameters are introduced (e.g., characteristic elevations, areas, contact fractions, connectivities, slopes, etc.). The manuscript should explicitly list the variables. Some guidance, or even just a table, with parameter definitions would improve clarity. This is especially important since the formulations presented in equations 3 to 5 can be confusing without context on the variables used.
5. One aspect that remains unclear is whether the snow redistribution scheme accounts for wind direction and speed. These are critical drivers of snow transport in natural systems, particularly in open tundra or alpine environments. Yet, the manuscript does not indicate whether wind forcing influences the redistribution pattern or magnitude. Additionally, the effects of vegetation, such as snow interception by shrubs or reduced wind exposure under forest canopies, are not addressed, despite their role in limiting wind-driven snow transport. These simplifications may be reasonable for a first implementation, but should be explicitly acknowledged. Furthermore, the manuscript lacks dedicated test cases or diagnostic outputs focused solely on snow redistribution. Without a targeted evaluation, it is difficult to assess the realism or the impact of this component relative to the other lateral processes included in T‑REX. The authors should consider highlighting this as a current limitation and outlining whether validation or further development is planned.
6. The study uses prescribed meteorological forcings (i.e., uncoupled simulation). The authors conclude that the grid-mean surface fluxes are hardly affected by the implementation of T‑REX. However, even small changes in moisture or temperature patterns can alter atmospheric response in a fully coupled simulation. The paper should acknowledge this limitation. It would strengthen the conclusions to note that “while our offline results show negligible change in mean FLUX/STATE, coupled simulations would be needed to assess any feedback on climate fully.”
7. Minor fixes:
• In the figure 5 caption, correct “sugrid-scale” and “subgrig-scale” to subgrid-scale, and remove the stray comma in “sum of, surface runoff”.
• In the figure 4 caption the phrase “layers fn the vertical grid” should be “layers in the vertical grid”.In summary, this manuscript presents a novel and promising extension to ICON-Land that enables the representation of lateral exchange processes within a tile-based framework. The technical implementation is well motivated and clearly described, and the examples effectively demonstrate T-REX's potential to improve subgrid-scale representation in land surface models. Addressing the points above, particularly those regarding the connectivity specification, the geometric assumptions, and the clarity of the snow redistribution component, will further enhance the impact and usability of the framework.
Citation: https://doi.org/10.5194/egusphere-2025-4031-RC2 -
AC2: 'Reply on RC2', Philipp de Vrese, 25 Feb 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-4031/egusphere-2025-4031-AC2-supplement.pdf
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AC2: 'Reply on RC2', Philipp de Vrese, 25 Feb 2026
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The publication represents a strong showing on the potential for subgrid lateral exchange in a land surface model to significantly impact earth system models. The methods are described in great detail, with a few somewhat confusing areas that are understandable given the complexity of the issue. The paper would benefit from a bit more discussion on how this simplified tile interaction scheme would compare to higher resolution/more robustly physics based solutions to the problem. Additionally, the wind-blown snow section appears limited and has significant questions relating to the methodology. Excluding the section on snow redistribution, the publication is in a very strong shape and this reviewer looks forward to seeing it published.
Additional Comments are available on the PDF attached.