PoLIM: an open source 2D higher-order thermomechanically coupled mountain glacier flow model

The worldwide glacier is retreating and is expected to continue shrinking in a warming climate. Understanding the dynamics of glaciers is essential for the knowledge of sea-level rise, water resources in high mountain and arid regions, and the potential glacier hazards. Over the past decades, various 3D higher-order and full-Stokes ice flow models including thermomechanical coupling have been developed, and some have opened their source codes. However, such 3D modeling requires detailed datasets about surface and bedrock topography, variable climatic conditions, and high computational cost. Due to difficulties in measuring glacier thickness, only a small minority of glaciers around the globe have ice thickness observations. It is also a challenge to downscale the climate data (e.g., air temperature, precipitation) to the glacier surface, particularly, in rugged high-mountain terrains. In contrast to 3D models, flowline models only require inputs along the longitudinal profile and are thus computationally efficient. They continue to be useful tools for simulating the evolution of glaciers and studying the particular phenomena related to glacier dynamics. In this study, we present a two-dimensional thermomechanically coupled ice flow model named PoLIM (Polythermal Land Ice Model). The velocity solver of PoLIM is developed based on the higher-order approximation (Blatter-Pattyn type). It includes three critical features for simulating the dynamics of mountain glaciers: 1) an enthalpy-based thermal model to describe the heat transfer, which is particularly convenient to simulate the polythermal structures; 2) a drainage model to simulate the water transport in the temperate ice layer driven by gravity; 3) a subglacial hydrology model to simulate the subglacial water pressure for the coupling with the basal sliding law. We verify PoLIM with several standard benchmark experiments (e.g., ISMIP-HOM, enthalpy, SHMIP) in the glacier modeling community. PoLIM shows a good performance and agrees well with these benchmark results, indicating its reliable and robust capability of simulating the thermomechanical behaviors of glaciers.