兰州大学机构库 >大气科学学院
利用分段积分方法研究CP-El Niño对中国一次西南干旱事件的影响
Alternative TitleModeling the impacts of CP-El Niño on a drought event in southwestern China with a piecewise-integration method
甘茹蕙
Thesis Advisor杨毅
2019-05-20
Degree Grantor兰州大学
Place of Conferral兰州
Degree Name硕士
Degree Discipline气象学
Keyword气候归因 模式误差 西南干旱 分段积分 中部型厄尔尼诺
Abstract2009/2010年我国西南地区发生了近50年来最严重的干旱事件,造成了巨大的经济损失。基于数值模式进行敏感性试验是研究这类归因问题常用的方法,在传统的敏感性研究中,模式误差会在长期积分过程中不断累积,“已知”状态和“未知”状态的模拟都存在“气候漂移”现象,导致敏感性试验研究结果的可信度降低。分段积分方法将连续积分划分成一系列有序的短期模拟,以此来降低模式误差,对于“已知”状态的模拟,是在每一个短期积分结束后用分析资料代替模式场,对于“未知”状态则是用分析资料加上“扰动场”(即:每个子区间结束时刻“未知”状态与“已知”状态的差)的和来代替模式场。本文主要基于分段积分方法设计敏感性试验,研究CP-El Niño(Central Pacific El Niño,中部型厄尔尼诺)对此次西南干旱的影响。 首先,基于WRF(Weather Research and Forecasting)模式设计两组理想试验,探究分段积分方法在海温气候效应研究中的适用性。结果表明,分段积分方法在“已知”状态和“未知”状态模拟中都可以有效地减小模式误差的累积,提高模式中直接替换变量(如U、V、T、Q)以及诊断变量(如降水、2 m温度)的模拟准确度。分段积分对于“未知”状态的模拟效果不及“已知”状态的模拟效果,主要是因为“未知”状态模拟中没有准确的观测资料,在对模式场进行替换的过程中会引入比“已知”状态模拟更多的误差。两组理想试验对于“变化场”(海温气候效应)的模拟也有一定的改进,但模式误差较大的试验(理想试验Ⅰ)的改进效果比模式误差较小的试验(理想试验Ⅱ)差,主要原因是当模式误差远大于海温的气候效应时,连续积分模拟结果不具代表性,分段积分虽有改进但仍有可能出现模式误差掩盖外强迫气候效应的情况。对于不同的变量,分段积分的改进程度也不相同,本研究中变量T、Q的改进率较高,而变量U、V的改进率较低。总体而言,分段积分比传统的连续积分更适用于海温气候效应的敏感性研究。 基于理想试验结果,本文以CP-El Niño对2009/2010年我国西南地区特大干旱的影响为研究对象,基于WRF模式使用分段积分方法设计真实个例研究。结果表明,分段积分方法的模式误差显著小于连续积分的模式误差,连续积分敏感性试验中CP-El Niño造成西南地区干旱的强度过强,与真实情况明显不符,而分段积分模拟的干旱强度和范围均与观测比较一致,此外分段积分结果表明CP-El Niño会造成西南地区冬季温度升高,这与观测结果较为一致。之后基于分段积分模拟的数据进行干旱机制研究,CP-El Niño造成2009/2010年西南地区严重干旱的的主要原因是:CP-El Niño使得菲律宾地区出现异常的反气旋,该反气旋削弱了从南海到中国西南的水汽传输,造成该地区水汽匮乏,此外海温异常造成西南出现的异常下沉运动、高空温度异常偏高分别是使得干旱生成并得以维持的动力及热力原因。通过对近30年多个El Niño事件合成,CP-El Niño发生时西南地区均发生不同程度的干旱事件,而EP-El Niño(Eastern Pacific El Niño,东部型厄尔尼诺)发生时西南降水有所增多,后者也通过分段积分的方法也得到的了验证。
Other AbstractA drought event spanning from the autumn of 2009 to the spring of 2010 occurred in southwestern China (SWC). This drought was the most serious in the past 50 years and led to a huge economic losses. Model-based sensitivity experiment is a widely used method for studying climate change attributions. In traditional climate sensitivity studies, "climate drift" occurs because of the accumulation of model errors during long-term integrations, both in "known" and "unknown" states. The credibility of traditional sensitivity experiments is relatively low. A piecewise-integration method, dividing the long-term continuous simulation into a series of sequential short-term simulations, is used to reduce the model errors. The model fields are updated at the end of each subinterval with analysis data for the "known" state run, and with the sum of the analysis data and "perturbation field" (the difference between "unknown" and "known" state) for the "unknown" state run. This paper conducts sensitivity experiments with the piecewise-integration method to evaluate the impacts of the CP-El Niño (Central Pacific El Niño) on the severe drought in SWC.First, two groups of ideal sensitivity experiments based on Weather Research and Forecasting model (WRF) are conducted to evaluate the ability of piecewise-integration method in simulating the climate effects of sea surface temperature. The results of ideal experiments show that piecewise-integration method can effectivity reduce the accumulation of model errors both in the simulation of "known" state and "unknown" state, and the simulation accuracy of directly updated variables (such as U, V, T, Q) and model diagnostic variables (such as precipitation and 2 m temperature) can obviously increase. The simulation effect of piecewise-integration for "known" state is better than "unknown" state. It is mainly because there is no accuracy observation data in the simulation of "unknown" state and more model errors will be introduced in the process of updating the model field. It can distinctly improve the simulation accuracy of "change field" (climate effects of sea surface temperature anomalies), but the improvement effect of ideal experiment with larger model errors (ideal experiment Ⅰ) is poor than ideal experiment with smaller model errors (ideal experiment Ⅱ). The major reason is that model errors are obviously greater than the climate effect of sea surface temperature anomalies in ideal experiment Ⅰ. In this situation, the simulation of continuous-integration is not credible, and the improvement of piecewise-integration is not outstanding. For different variables, the improvement degree of the piecewise-integration is also different, the improvement rate of the variables T and Q is higher, while the improvement rate of the variables U and V is lower in this study. In general, piecewise-integration method apply to study the climate effect of sea surface temperature anomalies.Based on the ideal experiments result, real case study using piecewise-integration method based on WRF model is designed to study the effect of CP-El Niño on the severe SWC drought in 2009/2010. The results show that model errors in piecewise-integration experiment are less than in continuous-integration experiment. The degree of drought over SWC in continuous-integration sensitivity experiment is too strong to be believed. However, the magnitude and the distribution of the drought simulated by piecewise-integration sensitivity experiment are consistent with the observation. In addition, the results of piecewise-integration show that CP-El Niño may lead to warmer temperature in SWC. Then, based on the piecewise-integration simulation, the mechanisms of severe drought caused by CP-El Niño are analyzed. The anomalous anticyclone circulation in the lower troposphere over the Philippines, which originated from the strong development of the CP-El Niño event, is shown to inhibit the water vapor transport from the South China Sea to SWC, leading to a lack of moisture in the study region. On the other hand, pronounced subsidence and warm temperatures in the lower troposphere is proved to be the dynamic factor and thermodynamic factor that emerged and maintained the drought. Through the synthesis of several El Niño events in recent 30 years, drought events with different degrees occurred over SWC in CP-El Niño years while the precipitation increased over SWC in EP-El Niño years. The latter is also verified through piecewise-integration method in this study.
Pages65
URL查看原文
Language中文
Document Type学位论文
Identifierhttp://ir.lzu.edu.cn/handle/262010/338306
Collection大气科学学院
Affiliation大气科学学院
Recommended Citation
GB/T 7714
甘茹蕙. 利用分段积分方法研究CP-El Niño对中国一次西南干旱事件的影响[D]. 兰州. 兰州大学,2019.
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