Tibetan Plateau And Numerical Simulation Of Climate Dynamics

This work takes the effects of Sensible Heat driven'Air Pump'(SHAP) over the Tibetan Plateau (TP) on Asian Summer Monsoon as the research mainline. According to the observational analysis, here, with sensitivity experiments from Atmospheric General Circulation Model (AGCM) and Linear Barotropic Model (LBM) ,we deploy the numerical simulation study on the climate dynamics of TP.At the beginning of this work, the performance of Spectral Atmosphere Model of IAP LASG (SAMIL) has been improved greatly by inducing and modifying some physical processes. This thesis studys the climatic dynamical effects of TP on Asian Summer Monsoon (ASM) from the following two aspects: on the one hand, with the improved SAMIL model, the onset predictability of ASM has been investigated by the application of high-quality remote sensing products in numerical experiments, and focus on the effects of TP thermal condition. On the other hand, Intergovermental Panel for global Climate Change (IPCC) Fourth Assessment reported warming of more than 4oC will likely occur over TP by the end of 21st century, the numerical experiments have been used to understand the possible dynamical relationship between the warming TP and the interdecadal change of East Asian Summer Monsoon (EASM) rainfall. Basing on the classical theory of Rossby wave dispersion, thermal adaption, Sverdrup balance, the theory of the atmospheric response to convective heating and the results from a linear baroclinic model, this thesis reveals the mechanisms of climatic dynamical effects of TP on interdecadal change of EASM rainfall. Additionally, there is some discussion on the possible factors of interdecadal change of EASM rainfall. The conclusions this thesis draws can be list as following:1. The soil moisture remote sensing products of AMSR-E have been evaluated by five gauged stations from CAMP-Tibet datesets, comparing with reanalysis from NCEP and EAR40, remote sensing products of AMSR-E have the ability to catch the variability of soil moisture over TP, especially on the short timescale; The application of remote sensing products presents that reasonable soil moisture will improve model's performance of land surface flux, thereafter, the onset predictability of ASM can benefit a lot from it with numerical model.2. The surface of TP is warming up in recent decades. Basing on the analysis from in-situ surface temperature (TS), TP is wamring up with the speed of 0.31℃/10 from 1960 to 2003. Furthermore, there is a significant relationship between TS over TP and rainfall trend of EASM (well known as the trend of'Southern China flood and Northern China drought') on the interdecadal timescale. Therefore, observationaly the warming TP might contribute to occurring frequently of'Southern China flood and Northern China drought'in recent decades.3. Results from numerical experiments confirm that warming TP is one of possible reasons to favor the interdecadal change of'Southern China flood and Northern China'. Warming TP can echance the effect of SHAP, and then uplift the South Asian Hight on upper level and strengthen the Somalia lower level jet over Arabian Sea due to enhanced land-sea thermal contrast. Rossby waves can be given rise at both upper and lower levels. Rossby wave energy both disperse eastward and is traped within westly jet at upper and lower levels separately. Due to the barotropic character of Rossby wave propagation in mid-latitude region, the response of lower level is as same as upper level in mid-latitude region. Therefore, there is anticyclonic anomaly center to the east of Japan at the level of 850hPa. Due to the easterly vertical shear of basic state over north Indian Ocean, the Rossby wave energy can't influence the circulation on the upper level. So there is also anticyclonic anomaly around South China Sea when the energy propagates there. Thereafter, the two anticyclonic anomalies over northwest Pacific interact with each other and orient southwest-northeast eventually. The moisture is transported towards East Asian subtropical front and releases latent heating. As same as the response of atmosphere to imposed heating and the corresponding cooling, according to the Sverdrup balance, there is baroclinic structure over northwest Pacific, the lower anticyclonic anomaly further enhances moisture transport and strengthens North-West Pacific Subtropical High (NWPSH), then causes the diabatic cooling over ocean. Evently,'Meiyu'('Changma'in Korean and'Baiu'in Japan) front has been echanced, and northern China and northwest Pacific regions under the control of NWPSH become dry due to the subsidence anmoaly.4. Internal forcing of'Meiyu'front in China was also studied in this thesis. Results from numerical experiment suggest that the latent heating released from'Meiyu'front (China part) can firstly echances the'Changma'(Korean part) and'Baiu'(Japan part) due to the barotropic character of Rossby wave train in upper troposphere. Secondly, the phase speed of Rossby wave induced by Meiyu heating propagates northwestward, which exactly causes the subsidence over North China. In the lower troposphere, the cyclonic anomaly over'Meiyu'front (China part) has been modified both by mean flow and cyclonic anomaly on its northeast, then tilts southwest-northeast. Therefore, the southwesterly wind can transport more warmer and moister flow to'Changma'and'Baiu'regions and strengthen NWPSH simultaneously. Meanwhile, the northeasterly wind on the other side of cyclonic anomaly can weaken the monsoon flow at the lower level over North China. As a result, in terms of both linear and non-linear responses, the internal forcing from'Meiyu'front itself can also casue the'Sandwich pattern'of EASM rainfall.5. According to the physical mechanism proposed in this thesis, if the warming factors (e.g. green house gas, aerosol etc.) are still out of control, TP is also warming up, under the external and internal forcing,'Meiyu/Changma/Baiu'front will get more rainfall during the summer monsoon season, and the structure of'Southern China flood and Northern China drought'will be maintained.As the summary of this thesis, the innovation points can be listed as following:1. Developing SAMIL AGCM by improving its physical package and performing the numerical experiments to study climatic dynamical effects of TP.2. Evaluating the ASMR-E soil moisture products over TP and using them to investigate the onset predictability of ASM.3. Pointing out that warming TP is one of the possible reasons for the interdecadal change of EASM rainfall in recent 50 years.4. The internal forcing coming from diabatic heating of'Meiyu'front (China part) can also favor the interdecadal change of EASM rainfall.5. According to the prediction from IPCC AR4, TP will still warm up, therefore, the structure of'Southern China flood and Northern China drought'will also be maintained.By the numerical sensitive experiments, the climatic dynamical effects of TP become more and more clearly. As a result, the onset predictability of ASM, features of interdecadal changes of EASM and the climate change in the futher can all be better understood. Furthermore, results from this study also suggest that, in term of the climate change, the current big project named'South-to-North Water Diversion'will play a positive role in the future and our people will benefit a lot from this diversion.