Impact of frozen soil processes on soil thermal characteristics at seasonal to decadal scales over the Tibetan Plateau and North China

Abstract

Abstract. Frozen soil processes are of great importance in\ncontrolling surface water and energy balances during the cold season and in\ncold regions. Over recent decades, considerable frozen soil degradation and\nsurface soil warming have been reported over the Tibetan Plateau and North\nChina, but most land surface models have difficulty in capturing the\nfreeze–thaw cycle, and few validations focus on the effects of frozen soil processes on soil thermal characteristics in these regions. This paper\naddresses these issues by introducing a physically more realistic and\ncomputationally more stable and efficient frozen soil module (FSM) into a\nland surface model – the third-generation Simplified Simple Biosphere Model (SSiB3-FSM). To overcome the difficulties in achieving stable numerical\nsolutions for frozen soil, a new semi-implicit scheme and a physics-based\nfreezing–thawing scheme were applied to solve the governing equations. The performance of this model as well as the effects of frozen soil process on\nthe soil temperature profile and soil thermal characteristics were investigated over the Tibetan Plateau and North China using observation\nsites from the China Meteorological Administration and models from 1981 to 2005. Results show that the SSiB3 model with the FSM produces a more realistic soil\ntemperature profile and its seasonal variation than that without FSM during\nthe freezing and thawing periods. The freezing process in soil delays the\nwinter cooling, while the thawing process delays the summer warming. The\ntime lag and amplitude damping of temperature become more pronounced with\nincreasing depth. These processes are well simulated in SSiB3-FSM. The\nfreeze–thaw processes could increase the simulated phase lag days and land memory at different soil depths as well as the soil memory change with the soil thickness. Furthermore, compared with observations, SSiB3-FSM produces\na realistic change in maximum frozen soil depth at decadal scales. This study shows that the soil thermal characteristics at seasonal to decadal scales\nover frozen ground can be greatly improved in SSiB3-FSM, and SSiB3-FSM can be used as an effective model for TP and NC simulation during cold season. Overall, this study could help understand the vertical soil thermal\ncharacteristics over the frozen ground and provide an important scientific\nbasis for land–atmosphere interactions.