Optimizing CLM4.5 simulations of the active layer: The role of gravel in Tibetan Plateau permafrost

Gravel is widely distributed in soils of the Tibetan Plateau (TP), where permafrost also occurs over approximately half of the total area. Gravel has different thermal and hydrological properties to those of fine-grained soils, which may have a considerable impact on hydrothermal transport within TP soils. However, few land models consider gravel. Here, we incorporated the thermal and hydraulic properties of gravel into the Community Land Model and explored the effect of differing gravel contents on land surface processes. We found that all parameters affecting soil hydrothermal transport were sensitive to gravel content. Soil thermal and hydraulic conductivities increased with increasing gravel content, ultimately leading to variations in soil temperature, soil water content, and radiation fluxes with changing gravel content. Soil containing gravel exhibited higher infiltration and greater total water storage capacities. We also compared different model schemes in an attempt to improve the simulation of active layers in permafrost regions with high gravel content on the TP. The scheme in which both gravel and freeze-thaw parameterizations were applied (i.e., SP3) improved both the latent heat flux and ground heat flux transfer. The SP3 scheme performed best in terms of soil temperature and soil moisture simulations at both the Nagqu and Madoi field stations, demonstrating a particular ability to better characterize soil moisture processes as a function of temperature during soil freezing and thawing.