青藏高原气候动力学的数值模拟研究

【作者】 包庆；

【导师】 吴国雄； Bin WANG； 刘屹岷；

【作者基本信息】 中国科学院研究生院（大气物理研究所） ， 气象学， 2007， 博士

【摘要】 本论文以青藏高原地表面感热作用对亚洲季风的影响为研究主线,采用大气环流模式和线性斜压模式为研究工具,基于大量观测资料的事实分析,通过设计各种有效的敏感性数值试验,展开了青藏高原气候动力学数值模拟的研究。论文首先从发展SAMIL大气环流模式着手,通过改进部分模式物理过程参数化方案,较大程度上提高模式模拟亚洲气候的能力。关于青藏高原气候动力效应对亚洲季风的作用,本文主要探讨了以下两个方面:第一,使用改进了的SAMIL模式,采用高质量卫星遥感资料,展开了高原地表热力状况对亚洲夏季风爆发影响的敏感性数值试验,从而探讨影响亚洲季风爆发可预报性的影响因素;第二,针对青藏高原持续增暖的事实,和政府间气候变化专门委员会第四次评估报告(IPCC AR4)中高原将继续增暖的预测,理解近50年青藏高原持续增暖趋势与东亚夏季降水变化趋势的动力联系。根据大气环流模式高原增暖实验的模拟结果,基于经典Rossby波能量频散理论、热力适应理论、Sverdrup涡度平衡关系和大气对深对流加热响应理论,并结合了线性斜压模式数值试验结果,从理论上分析和解释了高原增暖对东亚夏季风趋势变化影响的动力机制。另外,论文还对东亚夏季降水变化趋势的其他可能成因进行了简要的研究和讨论。论文的主要结论可概括如下:1、通过与青藏高原地区CAMP-Tibet 5个观测站点资料进行比较,AMSR-E卫星遥感产品,相对再NCEP和ERA40两套再分析资料而言,能够更加准确得反映青藏高原地区土壤湿度场的变化趋势,特别是时间尺度较小的信号;应用该卫星遥感产品的数值试验则表明,青藏高原地区土壤湿度准确度的提高有助于提高SAMIL模式对该地区陆气通量的模拟能力,进而有助于改善亚洲夏季风爆发的模拟。2、长期站点资料的分析表明,在1960到2003年期间,青藏高原地表温度以平均0.31℃/10年的幅度增暖。在年代际变化时间尺度上,青藏高原的地表增暖趋势与东亚夏季降水变化趋势(“南涝北旱”趋势)显著相关。因此,推测:青藏高原增暖可能是近50年来造成东亚地区降水变化趋势的重要原因。3、数值试验结果一定程度上验证了青藏高原增暖确是造成东亚夏季降水变化趋势(我国“南涝北旱”变化趋势)的重要原因。其主要物理动力过程描述如下:青藏高原增暖,导致高原感热驱动“气泵效应”增强和海陆热力对比增大,从而北印度洋的低空印度季风气流增强;另一方面,在感热加热和对流引起的潜热加热相互作用下,对流层高层南亚高压加强、使得对流层高空的高原北侧西风急流和高原南侧东风急流增强。高低空环流异常激发出Rossby波能量分别沿急流向东频散,当到达西北太平洋地区时均表现为反气旋环流异常,异常中心分别位于我国南海和日本以东地区,这种异常反气旋环流使得东亚地区低空西南暖湿空气输送增强,副热带锋区降水增强,其下游的副高区的降水减弱。在Sverdrup涡度平衡关系下,梅雨锋区东部的对流层上层出现异常气旋性环流,相对应的对流层低层的反气旋环流也得以增强。这样,对流层低层的三个异常反气旋性环流相互叠加,使得西北太平洋地区的异常反环流系统得以稳定加强。由此,高原增暖后降水异常表现为:印度西北部季风降水增加,长江中下游、朝鲜半岛和日本地区副热带锋区降水增多;西太平洋副热带高压控制下的广大地区和孟加拉湾东北部,季风降水减;副热带锋区西北侧盛行下沉气流,降水减少。4、近一步的数值实验表明,我国梅雨锋产生的非绝热加热(这里称为东亚季风变化趋势的“内强迫因子”)所引起上升气流的下沉支在华北地区,该下沉气流加强了我国华北夏季降水的负异常。另外,梅雨锋降水的非绝热加热在对流层高层可以激发出定常Rossby波。该Rossby波能量沿高空西风急流波导向东传播,由于其正压结构的特征,引起对流层低层气旋性环流异常,从而呈现出西南-东北向倾斜的结构,该倾斜结构又在基本气流的影响下加大。这种低层西南-东北倾斜的气旋性环流异常一方面增强韩国的‘Changma’和日本的‘Baiu’降水,另一方面,由于异常气旋环流中心两侧的西南风和东北风分别加强了西北太平洋副热带高压,和减弱我国华北地区低空暖湿气流的输送,西太副高控制地区和我国华北地区降水即表现为偏干趋势。因此,梅雨锋降水自身的非绝热加热作用可以强迫出与东亚夏季降水变化趋势相一致的“三明治”结构降水异常分布型。5、根据本文提出的物理模型,作者继而强调:如果温室气体和气溶胶等影响因子不能得到有效控制,青藏高原的增暖趋势将短期内无法抑制,这样,在内外强迫因子相互作用下,我国“南涝北旱”的气候格局还将持续。本论文的创新成果概括为以下几点:1.发展SAMIL大气环流谱模式,完善物理过程参数化方案,提高模式模拟气候的性能,并用于亚洲气候数值模拟的动力学研究。2.评估AMSR-E卫星遥感土壤湿度产品在青藏高原地区的质量,并利用该遥感资料进行亚洲季风爆发的模拟和预测的研究。3.指出青藏高原增暖是近50年来东亚夏季降水变化趋势(我国“南涝北旱”趋势)的重要成因。4.梅雨锋降水自身的非绝热加热作用作为内强迫因子可以强迫出与东亚夏季降水变化趋势一致的“三明治”结构降水分布型。5.根据IPCC AR4预测青藏高原持续增暖的事实,我国“南涝北旱”的气候格局还将持续。本论文通过一系列敏感性数值试验,更深刻得理解了青藏高原对亚洲季风影响的气候动力学过程。以此为基础,可以更好地理解亚洲季风爆发的可预报性以及东亚夏季风变化趋势的特征和成因,从而更准确地预测未来东亚气候变化的趋势。另外,本论文的研究结果也表明目前我国投入巨大的正在实施的“南水北调”工程将会在未来气候变化中发挥积极作用,有助于削减气候变化对我国人民生产生活的负面影响。更多还原

【Abstract】 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.更多还原