Initialization of thermal models in cold and warm permafrost

Cameron Ross,Greg Siemens,Ryley Beddoe,Melissa Lafreniȳre

doi:10.1139/as-2021-0013

Abstract

Equilibrium modelling, also known as spin-up, is a technique for initializing a stable thermal regime in ground temperature models for permafrost regions. The results act as a baseline for subsequent transient analyses of ground temperature response to climate change or infrastructure. In practice, spin-up procedures are often loosely described or neglected, and the criteria by which a stable thermal regime is evaluated are rarely defined or presented explicitly. In this paper, model results show that no single criterion based on thresholds of inter-cycle temperature change can be used to identify a stable thermal regime in all spin-up scenarios. Results from simulations using a wide range of initialization temperatures and conditions show the number of spin-up cycles can range between 10 and 10 000, and a spin-up criterion as fine as 0.00001 °C/cycle is required to achieve a stable thermal regime suitable for deeper warm permafrost models. The implications of selected threshold criteria are examined in follow-up transient analyses and show that warm permafrost models can be highly sensitive to initial temperature profiles based on the criterion used. The results alert scientists and engineers to the importance of initialization on site-specific and regional permafrost models for transient ground temperature analyses.

Highlights

Climate change, as well as anthropogenic factors, can significantly alter the thermal regime of permafrost (Nelson et al, 2002)
The model domain is initialized at some starting temperature(s) (To) and subject to one Manuscript submitted to Arctic Science for consideration as an Article 80 or more years of cyclical forcing at the surface boundary for a set number of cycles or until the model domain reaches a declared state of thermal equilibrium
Manuscript submitted to Arctic Science for consideration as an Article 511 mean annual ground temperature (MAGT) gradient or two distinct initialization temperature zones (e.g. Bense et al, 2012; Langford et al, 2020) to initialize a warm permafrost model would likely reduce the required number of spin-up cycles

Summary

Introduction

As well as anthropogenic factors, can significantly alter the thermal regime of permafrost (Nelson et al, 2002). There has been increased access to reliable, long-term climate change projection data, along with advances in implementing land-climate interaction physics used to apply these projections within models (Koven et al, 2013; Melton et al, 2019; Burke et al, 2020). These advances, combined with increased computing capability, have prompted use of longer-duration simulations (Riseborough et al, 2008) and, at times, deeper model domains. The model domain is initialized at some starting temperature(s) (To) and subject to one Manuscript submitted to Arctic Science for consideration as an Article 80 or more years of cyclical forcing at the surface boundary for a set number of cycles or until the model domain reaches a declared state of thermal equilibrium. Lamontagne-Hallé et al (2020) describe this approach as ‘dynamic equilibrium spin-up’ to differentiate it from a steady-state initialization in which constant boundary conditions (such as mean annual air or surface temperature) are applied to obtain an equilibrium (e.g. Bense et al, 2012; Ednie et al, 2008; Kong et al, 2019)

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