Optimization and Validation of Soil Frozen-Thawing Parameterizations in Noah-MP

Water-heat transport in frozen soil influences the energy and water exchanges between land and atmosphere. However, frozen soil water-heat transport simulation biases remaining in land surface model (LSM) result in uncertainties of climate model performance. In this study, the biases of the Noah with multi-parameterization (Noah-MP) land surface model in simulating soil water-heat transport in freeze-thaw (FT) process were investigated. On this basis, three parameterizations that were effective in other LSMs were implemented into Noah-MP to optimize its performance. The results show that Noah-MP still exhibits biases in simulating soil temperature and moisture in FT process. Even with various combinations of current parametrizations, the soil water-heat coupling relationship simulated by Noah-MP does not align with observations. Nevertheless, the implementation of a virtual temperature parameterization, a liquid water-dependent soil permeability parameterization, and modifications to soil hydraulic and thermal properties through considering the influences of organic matter and gravel, led to improvements in soil moisture and temperature simulations during both completely frozen and thawing periods. These enhancements resulted in a reduction of bias by approximately 20%-50%, and the simulated soil water-heat coupling relationship with implemented parameterizations closely matches observations. Global simulations further validate the improvements brought about by the implemented frozen-thawing parameterizations in Noah-MP. Simulation biases in frozen soil water-heat transport can introduce uncertainties in climate modeling and predictions for cold regions. How to improve the frozen soil hydrothermal parameterization is still an urgent issue. In this study, to optimize the frozen-thawing parameterizations in Noah-MP, a virtual temperature parameterization was implemented to redefine the phase change criteria, a liquid water-dependent frozen soil permeability parameterization was adopted to recalculate the hydraulic conductivity, and influences of organic matter and gravel on soil hydraulic and thermal properties were considered in Noah-MP. Through these enhancements, simulations of soil moisture and temperature during completely frozen and thawing periods were effectively improved. The findings of this study underscore the importance of frozen-thawing parameterizations in land surface models for simulating water-heat transport in frozen soil, which provide a potential way to improve the climate simulations in cold regions. Noah-MP with current frozen soil parameterizations combinations still have biases in simulating water-heat transport in the FT processThe soil water-heat coupling relationship simulated by Noah-MP is not consistent with observation during soil thawing periodSimulations of soil moisture and temperature were effectively improved by optimized parameterizations

Automated summary

This study investigates the biases of the Noah-MP land surface model in simulating soil water-heat transport during freeze-thaw processes. The implementation of parameterizations, including a virtual temperature parameterization and a liquid water-dependent soil permeability parameterization, as well as modifications to soil hydraulic and thermal properties, improves the simulation of soil moisture and temperature. These findings highlight the importance of frozen-thawing parameterizations in land surface models and provide a potential way to improve climate simulations in cold regions.