A Simulation and Validation of CLM during Freeze-Thaw on the Tibetan Plateau

Xuewei Fang,Yan Chang,Shihua Lyu,Siqiong Luo,Yu Zhang,Boli Chen,Di Ma

doi:10.1155/2016/9476098

Abstract

The applicability of a new soil hydraulic property of frozen soil scheme applied in Community Land Model 4.5 (CLM4.5), in conjunction with an impedance factor for the presence of soil ice, was validated through two offline numerical simulations conducted at Madoi (GS) and Zoige (ZS) on the Tibetan Plateau (TP). Sensitivity analysis was conducted via replacing the new soil hydraulic property scheme in CLM4.5 by the old one, using default CLM4.5 runs as reference. Results indicated that the new parameterization scheme ameliorated the surface dry biases at ZS but enlarged the wet biases which existed at GS site due to ignoring the gravel effect. The wetter surface condition in CLM4.5 also leads to a warmer surface soil temperature because of the greater heat capacity of liquid water. In addition, the combined impact of new soil hydraulic property schemes and the ice impedance function on the simulated soil moisture lead to the more reasonable simulation of the starting dates of freeze-thaw cycle, especially at the thawing stage. The improvements also lead to the more reasonable turbulent fluxes simulations. Meanwhile, the decreased snow cover fraction in CLM4.5 resulted in a lower albedo, which tended to increase net surface radiation compared to previous versions. Further optimizing is needed to take the gravel into account in the numerical description of thermal-hydrological interactions.

Highlights

Land surfaces, by affecting water and energy flows between the ground and the atmosphere, have a significant impact on weather forecasting as well as climate change
In recent years, due to the complexity of land surface processes and the limitation of observations, the land surface models have become the major tool for landatmosphere interaction studies [6, 8,9,10,11]
Based on the significant impact of the soil mechanical components on the hydrothermal properties [6], this study mainly aims to assess the performance of the modified scheme in CLM4.5 during freeze-thaw cycle on the Tibetan Plateau (TP) at two sites with entirely different soil textures via sensitivity analysis, using the default CLM4.5 runs as reference

Summary

Introduction

By affecting water and energy flows between the ground and the atmosphere, have a significant impact on weather forecasting as well as climate change. The soil freeze-thaw process is an important parameter in the land surface and atmospheric modeling. Owing to the great gap of thermal conductivity and thermal capacity between liquid water and ice, the accuracy of simulated soil moisture can directly affect the calculation of thermal properties and thermal conductivities during freezing-thawing period. The phase transition of soil moisture spends much of the energy exchanged between the atmosphere and the soil. The energy liberation and the energy absorption during the period change the distribution characteristics of ground temperature and the pattern of energy. The soil temperature and soil moisture simulations should be considered prior to the assessment of land surface model performance

Objectives

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Results