Thaw processes in ice-rich permafrost landscapes represented with laterally coupled tiles in a land surface model

Abstract

Abstract. Earth system models (ESMs) are our primary tool for projecting future climate change, but their ability to represent small-scale land surface processes is currently limited. This is especially true for permafrost landscapes in which melting of excess ground ice and subsequent subsidence affect lateral processes which can substantially alter soil conditions and fluxes of heat, water, and carbon to the atmosphere. Here we demonstrate that dynamically changing microtopography and related lateral fluxes of snow, water, and heat can be represented through a tiling approach suitable for implementation in large-scale models, and we investigate which of these lateral processes are important to reproduce observed landscape evolution. Combining existing methods for representing excess ground ice, snow redistribution, and lateral water and energy fluxes in two coupled tiles, we show that the model approach can simulate observed degradation processes in two very different permafrost landscapes. We are able to simulate the transition from low-centered to high-centered polygons, when applied to polygonal tundra in the cold, continuous permafrost zone, which results in (i) a more realistic representation of soil conditions through drying of elevated features and wetting of lowered features with related changes in energy fluxes, (ii) up to 2 ∘C reduced average permafrost temperatures in the current (2000–2009) climate, (iii) delayed permafrost degradation in the future RCP4.5 scenario by several decades, and (iv) more rapid degradation through snow and soil water feedback mechanisms once subsidence starts. Applied to peat plateaus in the sporadic permafrost zone, the same two-tile system can represent an elevated peat plateau underlain by permafrost in a surrounding permafrost-free fen and its degradation in the future following a moderate warming scenario. These results demonstrate the importance of representing lateral fluxes to realistically simulate both the current permafrost state and its degradation trajectories as the climate continues to warm. Implementing laterally coupled tiles in ESMs could improve the representation of a range of permafrost processes, which is likely to impact the simulated magnitude and timing of the permafrost–carbon feedback.

Soil moisture and hydrology projections of the permafrost region – a model intercomparison

“…However, relevant soil column processes related to thermokarst by thawing of excess ground ice (Lee et al, 2014) are limited in these simulations despite their significant occurrence in the permafrost region (Olefeldt et al, 2016). As permafrost thaws, ground ice melts, potentially reducing the volume of the soil column and changing the hydrological properties of the soil (Aas et al, 2019;Figure 6. Runoff anomaly comparison between gauge data and models simulations for the period 1970-1999.…”

“…These lines of evidence may suggest that permafrost thaw may not dry the Arctic as fast as simulated by land models but rather maintain or enhance soil water saturation depending on the water balance of the modeled cell column. Recent efforts have been made to address the high subgrid heterogeneity of fine-scale mechanisms including soil subsidence (Aas et al, 2019), hillslope hydrology, talik and thermokarst development (Jafarov et al, 2018), ice wedge degradation (Abolt et al, 2018;Liljedahl et al, 2016;Nitzbon et al, 2019), vertical and lateral heat transfer on permafrost thaw and groundwater flow (Kurylyk et al, 2016), and lateral water fluxes (Nitzbon et al, 2019). These processes are known to have a major role on surface and subsurface hydrology, and their implementation in large-scale models is needed.…”

The ERA5-Land Soil-Temperature Bias in Permafrost Regions

“…Reanalysis, assimilating a broad range of observations into fully coupled process-based models (land-atmosphere-ocean-sea ice, and often biogeochemical components), is a valuable source of data for permafrost science. It has been successfully used in analyzing and simulating various permafrost phenomena, such as spatial distribution (e.g., Cao et al, 2019b;Fiddes et al, 2015;Slater and Lawrence, 2013), thermal state (e.g., Guo and Wang, 2017;Koven et al, 2013), active layer thickness (e.g., Tao et al, 2018;Qin et al, 2017), ground ice loss (e.g., Aas et al, 2019), and carbon release (e.g., Koven et al, 2015) at different scales. However, such applications are mostly restricted to using atmospheric variables as model forcing.…”

Projecting circum-Arctic excess ground ice melt with a sub-grid representation in the Community Land Model

“…A two-tile approach allowing lateral heat exchange between two land elements demonstrated that maintaining thermokarst ponds requires the heat loss from water to the surrounding land . A similar tiling approach has been applied to projecting the landscape changes due to permafrost thaw for ice-wedge polygons and peat plateaus with different features of ice melting and surface subsidence (Aas et al, 2019;Nitzbon et al, 2019).…”

“…This would allow for the representation of small-scale permafrost features within a large-scale landunit with a given excess ice content. An example of how this could work is given by Aas et al (2019) who simulated both polygonal tundra and peat plateaus with a two-tile interactive setup. This is also similar to the recent representation of hillslope hydrology by 400 Swenson et al (2019), where sub-grid tiles (on the column level in CLM) were used to represent different elements in a representative hillslope.…”