| The thermal and dynamic effects of the Tibetan Plateau(TP)play an important role in the Asian monsoon system(ASM),and how TP thermal and dynamic forcing anomalies impact the atmospheric circulation and climate and weather in East Asia has been always a hot issue.The seasonal frozen ground and permafrost are widely distributed over the TP,and its seasonal changes affect the energy and water exchange between land surface and atmosphere.Surface diabatic heating is related to land surface processes.As one of the most prominent features of TP land surface process,freeze-thaw(FT)process should inevitably affect the TP surface diabatic heating changes,and it will also affect atmospheric circulation and weather and climate in East Asia.This thesis first analyzed the characteristics of soil water and heat transport during the FT process using site observation,combined with numerical model experiments,focused on the quantitative changes of soil temperature and moisture in different periods caused by the FT process.Based on it,the deviation characteristics of surface diabatic heating from reanalysis dataset were analyzed,and the characteristics of surface diabatic heating during different periods were compared.Changes in land surface diabatic heating induced by FT process were quantitatively calculated,the possible impacts of TP surface diabatic heating on general circulation in East Asia were also discussed.Next,the coupling relationship between soil moisture changes caused by the thawing process of TP frozen soil in spring and local summer precipitation was analyzed,and the effects of the FT process on the coupling relationship between soil moisture and precipitation over TP were analyzed.In addition,the seasonal persistence of soil moisture anomalies during the FT process from preceding autumn to spring were analyzed,and impact of seasonally persistent soil moisture anomalies on summer precipitation in Eastern China(EC)and related the mechanism were explored.Finally,the dynamic mechanism of the effects of TP thermal forcing anomalies on the surrounding atmospheric circulation related to the FT process was analyzed.The main research contents and conclusions are as follows:1.The results reveal the"water storage"effect of the FT process.The defined water storage index(SI)is 0.95 in shallow soil,regional average of SI in deep soil is larger than that in shallow soil.When the soil is without FT process,the soil moisture exists in liquid form,which will increase the evaporation of the ground surface,resulting in loss of soil moisture about 10%after soil thawing,and its magnitude is comparable to the interannual anomaly of soil moisture in spring.The extent of water storage effect of the FT process is significantly inversely related to the soil moisture content before soil freezing and the soil temperature after soil frozen,that is,when the soil is wet and the soil temperature is high,soil moisture is more easily lost.No soil FT process caused the soil temperature to be lower by-1.02°C during FT period and the soil temperature to be higher by 0.91°C after thawing period,indicating that the FT process buffered the seasonal changes in soil temperature.Impact of FT process on soil temperature is significantly positive correlated with the soil moisture and ice content during soil frozen.The FT process causes anomalous changes in soil temperature and moisture by changing soil hydrothermal properties(heat conductivity,heat capacity,hydraulic conductivity)and causing energy changes in phase change.The anomalous change of soil temperature is mainly affected by the release or absorption of energy during water phase change,and the anomalous transport of soil heat caused by changes in soil thermal conductivity and heat capacity benefit for mitigating the anomalous soil temperature.Impacts of soil FT process on soil moisture is mainly related to the anomalous changes of soil hydraulic conductivity and vertical water flux transport caused by the ice content anomalies;the migration of soil moisture to the frozen front is an important factor for the anomalous change of deep soil moisture.2.Results show that the reanalysis dataset has large biases in describing land surface diabatic heating over TP during FT process,especially during the thawing period in spring.Compared with the biases characteristics of different reanalysis dataset,ERA-Interim biases are smallest(approximately 5 W/m~2);the land surface diabatic heating biases of the reanalysis dataset in the western TP are larger than that in eastern TP.In FT process,the land surface diabatic heating during freezing and non-freezing periods can differ by more than 10 W/m~2;without soil FT process,the surface sensible heat during the FT period decreases by-2.63 W/m~2,and the surface latent heat increases by 2.92 W/m~2;the surface sensible heat during after thawing period increases by 4.72 W/m~2,and the surface latent heat decreases by 1.07 W/m~2.The land surface diabatic heating anomalies by±5W/m~2 over TP has significant impacts on summer precipitation in India,Eastern China(EC),and the Indo-China Peninsula,causing significant changes in the TP monsoon circulation,the westerly jet on the north side of TP,and the atmospheric circulation in EC.3.Results reveal that there is a significant coupling relationship between spring soil moisture anomalies caused by soil thawing and summer precipitation over TP.The characteristics of the soil moisture-precipitation coupling relationship over TP have large spatial differences.In the central and eastern part of TP,there is a positive coupling relationship between spring soil moisture and summer precipitation,while the coupling relationship in the western TP is negative.In general,the strength of coupling between spring soil moisture and summer precipitation increases with increasing soil moisture;when the altitude is greater than 3 km,the coupling strength between spring soil moisture and summer precipitation decreases with the increasing altitude.The spatial relationship between soil moisture and precipitation in the eastern TP is consistent with the change of surface sensible heat flux in spring;the coupling relationship in the western TP is consistent with the changes of spring soil moisture;the spatial difference in coupling between soil moisture and precipitation is related to the changes in soil moisture and surface diabatic heating caused by the FT process.The mechanism of the influence of spring soil moisture anomalies on the local subsequent summer precipitation is mainly caused by the land surface diabatic heating anomalies,which affects the atmospheric circulation around the TP and changes the water vapor transport from the south side of TP to the interior regions.4.Results reveal the seasonal persistence characteristics of the soil moisture anomalies over TP.Soil moisture anomalies in the preceding autumn and winter can persist to the spring through the soil FT process,soil moisture anomalies in the preceding autumn are stored in the frozen soil,and released along with soil freezing in spring,which could cause the diabatic heating anomalies,further significantly affect summer precipitation in EC in summer.When the spring soil moisture in the eastern TP increased,summer precipitation in southern China and the Yellow River Basin was less,while summer precipitation in the Yangtze River Basin and the northeast China was more.When soil moisture during preceding winter or preceding autumn is increased,the pattern of summer precipitation anomalies is similar,which implies climatic effects of soil moisture anomalies over TP in the preceding autumn and winter are similar to those in spring,and can be used as factors for seasonal prediction of summer precipitation in EC.When the spring soil moisture in eastern TP is increased,the land surface diabatic heating weakens,which causes the atmosphere cold anomalies at the north side of TP in summer,which increases the meridional temperature gradient and accelerates the westerly wind at the north side of TP,and weaken the propagation of Rossby wave.The atmospheric circulation anomalies show an anomalous anticyclone near region from the northeast China to Japan.The anomalous low-level wind field affects the water vapor transport,which causes the summer precipitation anomalies in EC.5.The results deepen understanding of mechanism linked to the impacts of TP thermal forcing anomalies related to the FT process on the surrounding atmospheric circulation.The numerical experimental results of adding“TP heat source”type thermal forcing in the baroclinic channel model without terrain show that the model can reproduce the monsoon circulation pattern,i.e.,meridional temperature gradient inversion at the south side of TP and the appearance of the east wind,enhanced meridional temperature gradient and accelerated westerly wind at the north side of TP,which indicate the dominant role of TP thermal forcing in the activity of the ASM,this also means the important impacts of soil FT process on atmospheric circulation in East Asia by TP thermal forcing.Eddy heat flux and momentum flux transport play a positive feedback role in the process of zonal wind responding to the thermal forcing of TP.During the development of easterly,the eddy momentum flux firstly lead to the formation of easterly,and combination of eddy momentum and heat fluxed contribute to the easterly acceleration;during the development of the westerly,the eddy momentum flux dominates the westerly wind speed,while the contribution of the eddy heat flux is small.The model results further verified that the TP is an important negative vorticity source,the TP thermal forcing can excite the stationary Rossby wave,and affects the downstream atmospheric circulation through the anomalous transport of stationary vortices. |
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