| Frozen soil is an important forcing factor of the land surface. Freezing andthawing process has a great effect on the local energy and hydrological cycle, and inturn affects weather and climate. The Qinghai-Tibet Plateau has a large area offrozen soil, which affects the energy and water cycle on the regional and surroundingareas, and plays a certain role in the formation of the plateau climate and climatechange. This thesis studied the characteristic of land surface energy and water duringfreezing and thawing at a station of the Qinghai-Tibet Plateau; verified the simulationperformance of land surface model CLM3.5in the Qinghai-Tibet Plateau; introduceda new soil thermal conductivity parameterization scheme to regional climate modelRegCM4which coupled with CLM3.5; and carried out single point and regionalsimulation experiment of the Qinghai-Tibet Plateau both with and without freezingand thawing process. The full text of research work is divided into the following fouraspects:First, observational data of Ruoergai Station located in the eastern Qinghai-TibetPlateau in2010were analyzed. An initial understanding of the characteristics of theland surface energy and water during freezing and thawing was got. According to thedaily maximum and minimum temperature of the surface soil, the year was dividedinto four stages such as completely frozen, the ablation process, completely ablationand the freezing process. Analysis showed that the diurnal variation of soiltemperature at20cm and above layer is like a sine curve and soil temperature andsoil moisture of each layer show a significant annual change. In the ablation processstage, soil moisture at80cm and above layer surges, and soil temperature increasesrapidly; in the freezing process stage, soil moisture at80cm and above layer dropssuddenly, and soil temperature reduces quickly. There’s no freezing and thawing process occurring at160cm layer. Diurnal variation and annual variation of the landsurface energy are both very expressive. The annual change of net radiation andlatent heat flux shows a single peak, while the change of sensible heat flux showsdouble peaks, but the change of surface soil heat flux is much more complex. Duringcompletely frozen and freezing process period, heat is transferred from soil toatmosphere from the daily average; while during completely ablation and ablationprocess period, heat is transferred from atmosphere to soil from the daily average.Freezing and thawing process has a definite influence on the surface energy flux. Theaverage value of the Bowen ratio is larger during freezing and thawing than that incompletely ablation. Energy imbalance phenomenon is appeared throughout theyear, and which is more significant during freezing and thawing than in completelyablation.Secondly, using observational data of Ruoergai Station, Community Land Model(CLM) version3.5and3.0were respectively employed to do a single point simulationexperiment. The comparison of observational data and simulation verifies theapplicability of the model in the seasonal frozen soil area of the plateau. The resultindicates that CLM3.5do well in simulating the changes of soil temperature, soilmoisture and surface energy flux during freezing and thawing. As a newparameterization scheme of unfrozen water was added to CLM3.5, the model cansimulate the unfrozen water remaining in the frozen soil, which makes the simulationof soil moisture closer to the observation and better than the simulation of CLM3.0.Meanwhile, the simulation of soil ice content is reduced compared to CLM3.0. In thesimulation of soil temperature, CLM3.5also has some improvements compared toCLM3.0. The diurnal variation of the shallow soil temperature is reduced duringfreezing and thawing. Because there’s a big difference between water and ice in thethermodynamic properties, the simulated thermal capacity of permafrost isincreased while the simulated thermal conductivity is decreased, which leads to theimprovements of the soil temperature simulation. Nonetheless, there are still someproblems in the simulation. For example, the simulated time when the soil moisturedecreases and increases during freezing and thawing is in advance compared with observation; the simulated freeze-thaw rate is too large which leads to the freezingand thawing process completed faster than the real situation; the simulated freezedepth is much deeper than observation.Thirdly, the cause of the above problem was found. The thermal conductivity ofthe soil matrix parameterization scheme is too large in the plateau region. Due to thehigh thermal conductivity of the ice, permafrost thermal conductivity calculateddeviation is further increased. Therefore, the simulation deviation is larger duringfreezing and thawing period than during completely ablation period. So anothersingle point simulation experiment of Ruoergai Station was taken which replaced theoriginal thermal conductivity of the soil matrix parameterization scheme withJohansen scheme, and the result showed that the new simulation was better thanthe old one. For example, the simulated freeze depth is decreased; the simulatedtime when the soil moisture decreases and increases are more accurate; the low biasof the simulated soil temperature is also improved. Then, a regional simulation of theQinghai-Tibet Plateau with the Regional Climate Model (RegCM) version4was taken,which also replaced the original parameterization scheme with Johansen scheme.The result shows that the simulated soil temperature below80cm changessignificantly, soil temperature rises in winter but decreases in summer. So the yearlyrange of soil temperature is reduced, which is consistent with the observation sitedata. However, taking the overall cold bias exists in the simulated air temperatureinto account, the improvement in parameterization scheme can reduce thesimulation bias of soil temperature in winter but may increase the bias in summer.Finally, single point simulation of Ruoergai Station and regional simulation of theQinghai-Tibet Plateau both with and without freezing and thawing process wascarried out by CLM3.5and RegCM4. The result indicates that the freezing andthawing process is a buffer for the seasonal changes in soil temperature. Largeamounts of phase change energy released during freezing period makes soiltemperature not too low, which slows the cooling effect of the land surface from theair temperature decreasing; large amounts of phase change energy absorbed by soilduring thawing period makes soil temperature not too high, which also slows the heating effect of the land surface from the air temperature increasing. Thefreeze-thaw process strengths the energy exchange between the soil andatmosphere. The releasing of phase change energy increases the energy transmissionfrom soil to atmosphere, so surface upward radiation, sensible heat flux and latentheat flux are all increased; the absorbing of phase change energy also increases theenergy transmission from atmosphere to soil, so sensible heat flux and latent flux areboth increased. The freezing and thawing process of the plateau region also havesome impact on the situation in the high and low altitude. The freezing processincreases the plateau surface heat source in winter, which rises the air temperaturefrom near the ground to altitude350hPa, as the plateau is in control of the cold highin winter, the rising of air temperature lead to the reducing of the geopotentialheight. The ablation process reduces the plateau surface heat source in summer,which decreases the air temperature from near the ground to altitude200hPa, as theplateau is in control of South Asia High in summer, the increasing of the geopotentialheight in the high level of troposphere is on favorable to the high. So the freeze-thawprocess may make some contributions to the maintenance of South Asia High. Due toglobal warming, sizable part of permafrost of the Qinghai-Tibet Plateau is degradingto seasonal frozen soil, and the active layer thickness is increasing, the regionalclimatic effects of the freeze-thaw process are also increasing, which may acceleratethe degradation of permafrost. |
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