青藏高原非绝热加热异常和热带热力强迫对东亚季节内预测的协同影响

The subseasonal to seasonal (S2S) prediction is an emerging weather and climate forecasting science, targeting time scales from two weeks to a season. The sources of predictability for S2S are the hot spots and difficult issues for the international academic community and World Meteorological Organization, and also the urgent needs for our country. The intra-seasonal variability (low-frequency) of diabatic heating over Tibetan Plateau (TP) is active, which is not only affected by the low-frequency activity from mid- and high-latitude, but is also affected more significantly by the intra-seasonal thermal forcing of tropics. It is more likely to be an important source of predictability for intra-seasonal prediction. However, what is its intrinsic mechanism? To what extent does it affect the prediction skill? These issues have not been studied. Based on the Intra-seasonal Oscillation (ISO) of the atmospheric heat source over TP and tropical thermal forcing marked by MJO, the project will study the following contents through diagnostic analysis and numerical simulation: 1. The way how the intra-seasonal diabatic heating over TP is combined with the intra-seasonal thermal forcing of tropics in the two aspects of the strength and phase. 2. The significant region and possible mechanism of the above-mentioned joint influence on intra-seasonal variability of meteorological field in eastern China. 3. To evaluate the contribution of the joint influence of diabatic heating anomalies over the TP and tropical thermal forcing on intra-seasonal prediction skill. The results of the project will help to improve our understanding on the causes of intra-seasonal anomaly over East Asia, and to provide scientific basis for the prediction.

次季节—季节（S2S，简称季内）预测是新兴的天气和气候预测科学，它关注的时间尺度介于2周—1季度间，其可预报性来源是国际学术界和世界气象组织关注的热点与难点，也是国家的迫切需求。高原是非绝热加热季内变率（低频）的活跃区，它不仅受到中高纬低频活动的影响，更显著受到热带季内热力强迫的影响，它很可能是季内预测的一个重要的可预报性来源。然而，其内在机理是什么？在多大程度上影响预报技巧？这些问题尚鲜有研究。本项目拟以高原大气热源季内振荡与热带以MJO为代表的热力强迫的协同为切入点，通过诊断分析和数值模拟研究：1、高原季内非绝热加热和热带季内热力强迫在强度、位相两方面的协同（配置）方式及可能途径；2、上述协同对我国东部气象场季内变率影响的显著区及其可能机制；3、评估高原加热和热带热力强迫在季内尺度上的协同影响对季内尺度预测技巧的贡献。其成果将加深对我国东部季内变化成因的理解并为其预测提供科学依据。

本项目围绕着青藏高原大气热源季节内振荡与热带以MJO为标志的热力强迫对中国东部降水的协同作用展开一系列研究，得到的主要成果是：1）夏季青藏高原大气热源准双周振荡沿纬向传播有两条不同路径，一是自东向西的传播，另一条是自西向东的传播。高原大气热源的准双周振荡的低频活跃度对中国东部天气变化有一定指示意义，高原低频活跃度较大时，中国东部夏季存在“旱涝并存”和“旱涝急转”现象。2）青藏高原大气非绝热加热的季节内振荡通过调控EU型（Eurasian teleconnection wavetrain）涡度波列的强度和范围，对长江中下游以南的季节内降水异常产生放大效应。3）东亚梅雨区峰值降水与热带环流及北方高位涡冷空气输送的低频演变密切关联。4）揭示了MJO以及太平洋和印度洋对极端梅雨的联合影响，并通过评估气候模式业务预测和模拟性能，进一步揭示了极端梅雨的预测技巧和可预测性。5）影响东亚的气候态大气河（东亚AR）对长江中下游梅雨期降水的季节循环和气候季节内振荡（CISO）有密切的联系。通过评估ACCESS-S1耦合季节预报系统23年回报发现，影响东亚地区夏季风降水的ARs可能比降雨本身更具可预测性。6）揭示中国东部夏季降水季节预测的不确定性来源。一是与中高纬度影响密切关联的内部动力过程。二是超过20%的降水观测方差不能被再分析资料把握住。三是再分析资料和气候模式本身对夏季平均降水的模拟存在偏差。7）揭示了东亚夏季气温的次季节变率主模态的前期信号是来自副热带大西洋经欧洲至东亚的罗斯贝波遥相关，以及热带MJO。应用加拿大ECCC模式，研制了可实时应用的东亚夏季气温次季节变率主模态的动力统计延伸预测模型。该模型较持续性预测有较好的预测技巧，预测时效可达3周。（8）构建了基于MJO等次季节信号的梅汛期延伸期预测模型，该模型对未来10-25天的降水具有较好预报效果，可较准确地预报出梅汛期3/4左右的降水量级和降水发生时段。