Explicitly modelling microtopography in permafrost landscapes in a land-surface model (JULES vn5.4_microtopography)

Abstract

Abstract. Microtopography can be a key driver of heterogeneity in the ground thermal and hydrological regime of permafrost landscapes. In turn, this heterogeneity can influence plant communities, methane fluxes and the initiation of abrupt thaw processes. Here we have implemented a two-tile representation of microtopography in JULES (the Joint UK Land Environment Simulator), where tiles are representative of repeating patterns of elevation difference. We evaluate the model against available spatially resolved observations at four sites, gauge the importance of explicitly representing microtopography for modelling methane emissions and quantify the relative importance of model processes and the model’s sensitivity its parameters. Tiles are coupled by lateral flows of water, heat and redistribution of snow. A surface water store is added to represent ponding. The model is parametrised using characteristic dimensions of landscape features at sites. Simulations are performed of two Siberian polygon sites, Samoylov and Kytalyk, and two Scandinavian palsa sites, Stordalen and Iškoras. The model represents the observed differences between greater snow depth in hollows vs raised areas well. The model also improves soil moisture for hollows vs the non-tiled configuration (‘standard JULES’) though the raised tile remains drier than observed. For the two palsa sites, it is found that drainage needs to be impeded from the lower tile, representing the non-permafrost mire, to achieve the observed soil saturation. This demonstrates the need for the landscape-scale drainage to be correctly modelled. Causes of moisture heterogeneity between tiles are decreased runoff from the low tile, differences in snowmelt, and high to low-tile water flow. Unsaturated flows between tiles are negligible, suggesting the adequacy of simpler water-table based models of lateral flow in wetland environments. The modelled differences in snow depths and soil moistures between tiles result in the lower tile soil temperatures being warmer for palsa sites. When comparing the soil temperatures for July at 20 cm depth, the difference in temperature between tiles, or ‘temperature splitting’, is smaller than observed (3.2 vs 5.5 °C). The mean temperature of the two tiles remains approximately unchanged (+0.4 °C) vs standard JULES, and lower than observations. Polygons display small (0.2 °C) to zero temperature splitting, in agreement with observations. Consequently, methane fluxes are near identical (+0 to 9 %) to those for standard JULES for polygons, though can be greater than standard JULES for palsa sites (+10 to 49 %). Through a sensitivity analysis we identify the parameters resulting in the greatest uncertainty in modelled temperature. We find that at the sites tested, varying the parameters can result in the modelled July temperature splitting being at most 0.9 or 3 °C larger than observed for palsa or polygon sites respectively. Varying the palsa elevation between 0.5 and 3 m has little effect on modelled soil temperatures, showing that having only two tiles can still be a valid representation of sites with a large variability of palsa elevations. Lateral conductive fluxes, while small, reduce the temperature splitting by ~1 °C, and correspond to the order of observed lateral degradation rates in peat plateau regions, indicating possible application in an area-based thaw model.

Long‐Term, High‐Resolution Permafrost Monitoring Reveals Coupled Energy Balance and Hydrogeologic Controls on Talik Dynamics Near Umiujaq (Nunavik, Québec, Canada)

“…Together, the variations in surficial conditions attributed to changes in topography, vegetation cover, and seasonal snow cover lead to distinct subsurface thermal and infiltration regimes on the top and sides of the studied permafrost mound (Jin et al., 2022; Perreault et al., 2021; Young et al., 2020). Similar topography, vegetation, and snow cover controls on the local distribution of permafrost have also been studied in northern Scandinavia and Siberia (Aas et al., 2019; Liljedahl et al., 2016; Martin et al., 2019, 2021; Nitzbon et al., 2019, 2020; Smith et al., 2022), in Alaska (Abolt et al., 2018; Farquharson et al., 2022; O’Connor et al., 2019) and in the Canadian High Arctic (Farquharson et al., 2019).…”

Representation of soil hydrology in permafrost regions may explain large part of inter-model spread in simulated Arctic and subarctic climate

“…For the treatment of the lateral movement of water the current generation of LSMs requires a number of new sub-modules that account for the moisture variability on the sub-meter scale, e.g. in the polygonal tundra (Cresto Aleina et al, 2013), but also for the lateral fluxes from high to low-lying areas along gradient slopes that act on the scales of tens to thousands of meters (Nitzbon et al, 2021;Smith et al, 2022). Given that this will require at least one additional layer of tiles and that the hydrological conditions may vary over short periods of time, which results in a comparatively fast changes in the tile fractions, this approach requires a flexibility in the model structure, which few of the present-day LSMs possess.…”

We Must Stop Fossil Fuel Emissions to Protect Permafrost Ecosystems

“…Though the divergence of model outputs is clearly problematic, the multi-model mean of this study has become the default reference for comparison (Turetsky et al, 2020;de Vrese and Brovkin, 2021;Natali et al, 2021). These models were state-of-the art seven to 10 years ago, but there have been considerable advances in permafrost domain modeling since then (Randers and Goluke, 2020;Shu et al, 2020;Smith et al, 2021;Wiltshire et al, 2021;Chadburn et al, 2022). This emphasizes the larger problem of the continued exclusion of permafrost in Earth system models.…”