Hydrologic-land surface modelling of the Canadian sporadic-discontinuous permafrost: initialization and uncertainty propagation

Abstract

Permafrost thaw has been observed in recent decades in the Northern Hemisphere and is expected to accelerate with continued global warming. Predicting the future of permafrost requires proper representation of the interrelated surface/subsurface thermal and hydrologic regimes. Land surface models (LSMs) are well suited for such predictions, as they couple heat and water interactions across soil-vegetation-atmosphere interfaces and can be applied over large scales. LSMs, however, are challenged by the long-term thermal and hydraulic memories of permafrost and the paucity of historical records to represent permafrost dynamics under transient climate conditions. In this study, we address the challenge of model initialization by characterizing the impact of initial climate conditions and initial soil frozen and liquid water contents on the simulation length required to reach equilibrium. Further, we quantify how the uncertainty in model initialization propagates to simulated permafrost dynamics. Modelling experiments are conducted with the Modélisation Environmentale Communautaire – Surface and Hydrology (MESH) framework and its embedded Canadian Land Surface Scheme (CLASS). The study area is in the Liard River basin in the Northwest Territories of Canada with sporadic and discontinuous regions. Results show that uncertainty in model initialization controls various attributes of simulated permafrost, especially the active layer thickness, which could change by 0.5-1.5m depending on the initial condition chosen. The least number of spin-up cycles is achieved with near field capacity condition, but the number of cycles varies depending on the spin-up year climate. We advise an extended spin-up of 200-1000 cycles to ensure proper model initialization under different climatic conditions and initial soil moisture contents.

Advances in modelling large river basins in cold regions with Modélisation Environmentale Communautaire—Surface and Hydrology (MESH), the Canadian hydrological land surface scheme

“…Figure 3 highlights the performance of MESH in simulating permafrost for the three sites, after Elshamy et al (2020). Abdelhamed et al (2022) have further highlighted the role of initial moisture content, especially ice, and the choice of the spin‐up year meteorology on model initialization and subsequent simulation. This research has highlighted the sensitivity of hydrological response to permafrost simulations, but nevertheless, the need to initialize what is a transient permafrost state with a long (decadal or more) memory, limited ground observations and highly uncertain historical forcing data remains a major challenge.…”

“…This methodology resulted in a 50 m deep soil profile across the domain, accompanied by 50-100-year spin-up cycles. Figure3highlights the performance of MESH in simulating permafrost for the three sites, after Abdelhamed et al (2022). have further highlighted the role of initial moisture content, especially ice, and the choice of the spin-up year meteorology on model initialization and subsequent simulation.…”

Challenges in hydrologic-land surface modelling of permafrost signatures - Impacts of parameterization on model fidelity under the uncertainty of forcing

“…soil temperature and liquid/frozen contents) was achieved by spinning the first year of the climate record (i.e. Oct 1 st , 1979 -Sep 30 th , 1980) for 1000 cycles for each sampled set of parameters, as recommended by Abdelhamed et al (2021).…”

“…Further, initializing the model prognostic states is problematic. This is commonly achieved by spinning up the models to reach a set of states that are consistent with the 'transient' climate (Abdelhamed et al, 2021;Elshamy et al, 2020;Sapriza-Azuri et al, 2018), or based on realistic field observations (e.g. soil moisture and temperature), if available.…”