青藏高原融冻过程中能量和水分循环的模拟研究

采用与先进陆面模式耦合的区域气候模式WRF V3.1,利用NCEP/DOE再分析资料作为驱动场,对青藏高原东北部1998年5-7月进行模拟.首先,对模拟区降水、感热和潜热通量的空间分布进行了检验,结果表明WRF模式能够合理且较好的模拟出高原东北部降水、感热和潜热通量的空间分布状况.通过对融冻过程中土壤温、湿度及水文要素的分析发现,融冻过程首先影响土壤温、湿度,进而影响地气间能量和水分交换,最终影响整个局地水循环过程.冻结时,感热通量占主要地位,陆地系统净水分输入主要转化为蒸发和径流,水分盈余变化基本为零;融化后,潜热通量占主要地位,随着降水量增大和土壤下渗能力的增强,陆地系统水分盈余迅速增大,径流相应减小.但不同下垫面上略有差别,草地、灌木丛、稀疏植被和苔原上水循环要素的变化趋势与区域平均基本类似,水分盈余变化呈增大的趋势,沿水体周围草地的水分盈余随时间减小,水体上的水循环活动最为剧烈,水分盈余最为丰富.

春末夏初土壤状态对后期区域降水模拟的影响

欧洲中期天气预报中心40 a的再分析资料(ERA40)对中国范围土壤湿度的表现好于美国国家环境预测中心/国家大气研究中心(NCEP/NCAR)再分析资料,尤其是在春末夏初阶段,对因融雪和土壤融冻产生的土壤湿度变化有着更好的表现.针对这一特点,分别用这两套再分析资料土壤温、湿度分布作为初始场驱动大气模式MM5,以1998年为例研究春末夏初不同的土壤状况对后期环流以及区域降水的影响.结果表明:土壤温、湿度分布对高空要素场的模拟有一定影响,运用ERA40土壤状况可以在一定程度上减小200 hPa高空急流的模拟误差,减小500 hPa位势高度场模拟误差,减小高原东侧向北的低空急流模拟误差,改进850 hPa温度场分布.运用ERA40土壤状况模拟的降水在长江中下游地区、西北干旱区以及青藏高原主体东侧的模拟不论从空间分布特征还是模拟误差方面均好于运用NCEP/NCAR土壤状况的模拟结果,运用ERA40土壤状况的模拟对季风雨带的演变时间和位置模拟都更接近于观测.

Numerical study on freezing-thawing phase change heat transfer in biological tissue embedded with two cryoprobes

A two-dimensional model for freezing and thawing phase change heat transfer in biological tissue embedded with two cryoprobes was established.In this model,the blood vessels were considered as tree-like branched fractal network,and the effective flow rate and effective thermal conductivity of blood were obtained by fractal method.The temperature distribution and ice crystal growth process in biological tissue embedded with two cryoprobes during freezing-thawing process were numerically simulated.The results show that the growth velocity of ice crystal in freezing process from 200 to 400 s is more rapid than that from 400 to 600 s. Thawing process of frozen tissue occurs in the regions around cryoprobes tips and tissue boundary simultaneously,and the phase interfaces are close to each other until ice crystal melts completely.The distance of two cryoprobes has a more profound effect on the temperature distribution in freezing process at 400 s than at 800 s.

Research on soil salt transfer under freeze-thawing condition

Soil salt transformation plays an important role in the freeze-thawing process,which is also one of basic problems of cryopedology. The very special law is made up of the two time salt-moisture transfer under freeze-thawing condition. Based on the latest research at home and abroad,through the investigation of soil moisture-salt change in the freeze-thawing process,the conclusion is made that the soil water potential gradient is the main driving force of soil salt movement and the factors are of quantities. The research shows that,when freezing,temperature drops,salt and moisture move towards frozen layer. All make the salinity content of the frozen layer increase significantly. In the thawing process,salinity and moisture in the soil move up again with evaporation and makes the salt second migration.

A soil water and heat transfer model including changes in soil frost and thaw fronts

Freeze-thaw processes in soils,including changes in frost and thaw fronts(FTFs),are important physical processes.The movement of FTFs affects soil hydrothermal characteristics,as well as energy and water exchanges between the land surface and the atmosphere and hydrothermal processes in the land surface.This paper reduces the issue of soil freezing and thawing to a multiple moving-boundary problem and develops a soil water and heat transfer model which considers the effects of FTF on soil hydrothermal processes.A local adaptive variable-grid method is used to discretize the model.Sensitivity tests based on the hierarchical structure of the Community Land Model(CLM)show that multiple FTFs can be continuously tracked,which overcomes the difficulties of isotherms that cannot simultaneously simulate multiple FTFs in the same soil layer.The local adaptive variable-grid method is stable and offers computational efficiency several times greater than the high-resolution case.The simulated FTF depths,soil temperatures,and soil moisture values fit well with the observed data,which further demonstrates the potential application of this simulation to the land-surface process model.

Reconstruction of Soil Particle Composition During Freeze-Thaw Cycling： A Review

Studies conducted over several decades have shown that the freeze-thaw cycles are a process of energy input and output in soil, which help drive the formation of soil structure, through water expansion by crystallization and the movement of water and salts by thermal gradients. However, most of these studies are published in Russian or Chinese and are less accessible to international researchers. This review brought together a wide range of studies on the effects of freezing and thawing on soil structure. The following findings are summarized： i） soil structure after freeze-thaw cycles changes considerably and the changes are due to the mechanical fragmentation of soil coarse mineral particles and the aggregation of soil fine particles; ii） the particle size of soil becomes homogeneous and the variation in soil structure weakens as the number of freeze-thaw cycles increases; iii） in the freezing process of soil, an important principle in the variation of soil particle bonding is presented as： condensation →aggregation→ crystallization; iv） the freeze-thaw cycling process has a strong effect on soil structure by changing the granulometric composition of mineral particles and structures within the soil. The freeze-thaw cycling process strengthens particle bonding, which causes an overall increase in aggregate stability of soil, showing a process from destruction to reconstruction.