ENSO预测的目标观测敏感区在热带太平洋海温的多模式集合预报中的应用

本文将ENSO预测的目标观测敏感区与多模式集合预报方法相结合, 提出了一种能够有效提高预报技巧且又具有较小计算成本的多模式集合预报方法。该方法在目标观测敏感区内采用模式不等权的多模式超级集合预报方法(SUP), 而在其他区域采用相对简单的等权的多模式消除偏差集合平均方法(BREM)。利用CMIP5中15个气候系统模式的工业革命前参照试验(pi-Control)数据, 针对热带太平洋海温的长期演变开展了理想预报试验。将新集合预报方法与现有的多模式集合预报方法进行了比较。结果表明, 在所考察的预报期内(即1～20年), 新集合预报方法与整个热带太平洋区域使用SUP方法具有相当的预报技巧, 但前者的计算成本明显小于后者, 计算时间仅为后者的1/4。可见, 新方法是一个具有较高预报技巧且计算成本较小的多模式集合预报方法。同时, 其较高的预报技巧强调了热带太平洋SST预测对ENSO目标观测敏感区内的模式误差也是极端敏感的, 也正因如此, 多模式集合预报方法才能够有效过滤模式误差的影响, 具有较高的预报技巧。     

条件非线性最优扰动在长江中下游地区冬季暴雨中的应用研究

为了提高长江中下游地区高影响天气的数值预报,利用条件非线性最优扰动(CNOP)方法,对一次长江中下游地区冬季降水个例(高影响天气事件)进行目标观测研究,并通过观测系统模拟试验(OSSE)检验了该方法确定敏感区的有效性和可行性。试验结果表明,CNOP方法可有效识别对应于高影响天气事件的敏感区。通过对敏感区进行初始场修正后,可明显改善验证区内24 h累积降水预报误差和总能量预报误差。进一步分析发现,通过改善敏感区内的初始场信息(如水汽通量场和低层冷空气活动等),使得数值模式不仅能更真实刻画该天气系统的初始结构,还能更好模拟出该天气系统随时间的演变特征,因而减少了验证区内对该天气系统的预报误差。这一结果表明可以把CNOP方法应用于长江中下游地区高影响天气事件的目标观测研究或实践中。      

条件非线性最优扰动在可预报性问题研究中的应用

本文总结了近年来条件非线性最优扰动方法的应用研究的主要进展.主要包括四个方面:(1)将条件非线性最优扰动(CNOP)方法拓展到既考虑初始扰动又考虑模式参数扰动,形成了拓展的CNOP方法.拓展的CNOP方法不仅能够应用于研究分别由初始误差和模式参数误差导致的可预报性问题,而且能够用于探讨初始误差和模式参数误差同时存在的情形;(2)将拓展的CNOP方法分别应用于ENSO和黑潮可预报性研究,考察了初始误差和模式参数误差对其可预报性的影响,揭示了初始误差在导致ENSO和黑潮大弯曲路径预报不确定性中的重要作用;(3)考察了阻塞事件发生的最优前期征兆(OPR)以及导致其预报不确定性的最优增长初始误差(OGR),揭示了OPR和OGR空间模态及其演变机制的相似性及其局地性特征;(4)研究了台风预报的目标观测问题,用CNOP方法确定了台风预报的目标观测敏感区,通过观测系统模拟试验(OSSEs)和/或观测系统试验(OSEs),表明了CNOP敏感区在改进台风预报中的有效性.具体地,台风OGR的主要误差分布在某一特定区域,空间分布具有明显的局地性特征,在台风OGR的局地性区域增加观测,有效改进了台风的预报技巧,该区域代表了台风预报的初值敏感区.事实上,上述El Ni(n)o事件、黑潮路径变异以及阻塞事件的OGR的空间分布也具有明显的局地性特征,这些事件的OGR刻画的局地性区域可能也代表了初值敏感区.     

条件非线性最优扰动方法在黑潮目标观测研究中的应用

对近年来利用条件非线性最优扰动（Conditional Nonlinear Optimal Perturbation,CNOP）方法开展的黑潮目标观测研究进行了总结,主要包括日本南部黑潮路径变异的目标观测研究、黑潮延伸体模态转变的目标观测研究和源区黑潮流量变化的目标观测研究。通过计算这些事件的CNOP型扰动,发现这些事件的CNOP型扰动具有局地特征,可以作为实施目标观测的敏感区。理想回报试验结果表明,如果在由CNOP方法识别的敏感区内实施目标观测,则会大幅度提高上述事件的预报技巧。     

用一个简单海气耦合模式对20世纪90年代厄尔尼诺事件的预报试验

使用一个动力海洋与统计大气耦合的热带太平洋模式,对２０ 世纪９０ 年代的厄尔尼诺事件进行了数值模拟和预报。结果表明,模式能较好地再现３ 次暖事件的发展和演化过程,也能模拟１９９５／１９９６ 年的冷海温过程。预报试验的结果指出,２０ 世纪 ９０ 年代发生的厄尔尼诺事件具有其特殊性,很难在发展阶段作出准确预报,而一旦形成后,模式仍可对这类事件的成熟位相阶段作出大致提前 ６ 个月的有效预报。文中还用该模式对 １９９７ 年的热带太平洋海表温度作了预报试验,预测 １９９７ 年年底可能发生一次新的厄尔尼诺事件。关键词：２０ 世纪９０ 年代厄尔尼诺,简单海气耦合模式,预报试验。     

两类厄尔尼诺事件发展年秋季印度洋海温异常特征对比

基于1951—2010年逐月海气多要素观测资料,对比分析了两类厄尔尼诺事件发展年秋季印度洋的海温异常及大气响应特征,探讨了印度洋偶极子的发生与两类厄尔尼诺事件特征的可能联系。结果表明,两类厄尔尼诺事件的发展年均会出现印度洋偶极子,但出现的概率不同:大多数东部型厄尔尼诺事件都会伴有正位相印度洋偶极子发生;而仅一半的中部型厄尔尼诺事件期间会出现正位相印度洋偶极子的异常海温型,且强度较弱。从印度洋偶极子与两类厄尔尼诺事件的物理联系上看,东部型厄尔尼诺事件期间,印度洋偶极子的发生与其强度联系密切:印度洋偶极子发生在东部型厄尔尼诺事件较强期间,两者通过海洋大陆的异常强下沉运动及大范围负异常降水相联系;东部型厄尔尼诺事件偏弱时并无印度洋偶极子出现,海洋大陆异常下沉运动及负异常降水很弱。然而,中部型厄尔尼诺事件期间印度洋偶极子的发生与其强度并无显著的关系,而与太平洋高海温区的位置存在一定的可能联系:在有印度洋偶极子发生的中部型厄尔尼诺事件发展年秋季,热带太平洋异常高海温区的位置相对偏东,海洋大陆出现显著下沉运动和大范围负异常降水,热带东印度洋为大范围强异常东风控制;但无印度洋偶极子发生的中部型厄尔尼诺事件时,热带太平洋高海温区位置相对偏西,极弱的海洋大陆下沉支对热带印度洋异常海温作用非常有限。     

Lorenz 96模式中三种目标观测方法的有效性比较

目标观测是有效提升观测效能和观测质量的一种观测策略,其核心部分是敏感区的识别。本文在Lorenz-96模式上比较了奇异向量法(SVs)、集合变换卡尔曼滤波法(ETKF)和条件非线性最优扰动法(CNOP)识别敏感区的优劣,并尝试揭示ETKF方法性能不稳定的原因与机制。试验结果表明:在312h内的不同预报时刻,CNOP方法识别的敏感区范围较小且对预报效果的提升率最高;SVs方法识别的敏感区对72h内的预报有较好的改进,但72h后改进程度急剧下降,到120h后基本失效;ETKF方法识别的敏感区在72h内不如其他方法的效果好。此外,在ETKF方法识别的敏感区与随机选取的敏感区对比中发现,由于ETKF方法操作时采用顺序观测资料处理方案搜寻敏感区,本质上忽略了观测资料间的相关性,导致ETKF方法识别出的敏感区并不一定是全局信号方差最大的区域,对预报效果的改善有限,这也说明了如何优化敏感区搜寻方案是提高ETKF方法效能的关键。     

On the sensitive areas for targeted observations in ENSO forecasting

本研究分别从初始误差增长和粒子滤波同化的角度识别了 ENSO预测的目标观测敏感区.结果表明:与粒子滤波同化方法确定的敏感区相比,基于初始误差增长确定的敏感区位置稍微偏东,且在东南太平洋也有分布,但整体而言,这两种方法确定的敏感区存在大范围重合区域,是互为印证的.在实际目标观测中,如果考虑使用确定敏感区方法的不确定性,那么选择上述两种方法确定的敏感区的重合区域作为ENSO预测的目标观测敏感区将更为合理.     

台风“灿鸿”(1509)的适应性观测敏感区特征及其模拟观测同化研究

适应性观测可以改善资料同化和预报质量。本文利用集合卡尔曼变换适应性观测系统对2015年09号台风“灿鸿”进行了观测敏感区识别，并以第一目标时刻的观测敏感区为基础，运用观测系统模拟试验方法获取模拟的适应性观测资料。基于WRF中尺度同化和预报系统，开展了适应性观测敏感区模拟资料的同化和预报试验。研究发现，台风“灿鸿”（1509）的观测敏感区主要位于台风中心的东北侧及东南侧。同化敏感区内模拟观测资料比同化常规观测资料能更好地改善分析质量和高度、台风路径的预报质量，但对降水的预报改善较弱。     

基于集合卡尔曼变换的目标观测敏感区识别系统优化及影响试验

在基于集合卡尔曼变换(Ensemble Transform Kalman Filter,ETKF)方法的适应性观测系统的基础上,考虑湿度因子作用并增加对流层低层的大气运动信息,发展了更加适用于我国中尺度高影响天气系统敏感区识别的优化方案。针对环北京夏季暴雨和冬季降雪的高影响天气个例,分别设计4组试验进行观测敏感区识别试验,考察了优化方案目标观测敏感区识别质量,并对分析和预报结果进行了评估。结果表明:优化方案的目标观测敏感区识别效果最佳,对环北京夏季暴雨和冬季降雪天气的目标观测敏感区质量有明显改善,湿度因子可使最强观测敏感区更加集中,对夏季降水敏感区的影响比冬季降雪天气更加明显。低层大气信息的引入对最强观测敏感区的准确识别也具有重要的积极作用。目标观测敏感区的目标资料对分析和短期预报质量具有明显的正贡献。     

A New Approach for Classifying Two Types of El Nino Events

In recent decades, the typical El Nio events with the warmest SSTs in the tropical eastern Pacific have become less common, and a different of El Nio with the warmest SSTs in the central Pacific, which is flanked on the east and west by cooler SSTs, has become more fre-quent. The more recent type of El Nio was referred to as central Pacific El Nio, warm pool El Nio, or dateline El Nio, or the El Nio Modoki. Central Pacific El Nio links to a different tropical-to-extratropical teleconnection and exerts different impacts on climate, and several classification approaches have been proposed. In this study, a new classification approach is proposed, which is based on the linear combination (sum or difference) of the two leading Empirical Orthogonal Functions (EOFs) of tropical Pacific Ocean sea surface temperature anomaly (SSTA), and the typical El Ni o index (TENI) and the central El Nio index (CENI) are able to be derived by projecting the observed SSTA onto these combinations. This classification not only reflects the characteristics of non-orthogonality between the two types of events but also yields one period peaking at approximate two to seven years. In particular, this classification can distin-guish the different impacts of the two types of events on rainfall in the following summer in East China. The typical El Nio events tend to induce intensified rainfall in the Yangtze River valley, whereas the central Pacific El Nio tends to induce intensified rainfall in the Huaihe River valley. Thus, the present approach may be appropriate for studying the impact of different types of El Nio on the East Asian climate.     

Responses of Cloud-Radiative Forcing to Strong El Ni?o Events over the Western Pacific Warm Pool as Simulated by CAMS-CSM

Cloud-radiative forcing(CRF) at the top of the atmosphere(TOA) over the western Pacific warm pool(WP) shows unique characteristics in response to El Ni?o events. In this region, the responses of CRF to El Ni?o events have been a useful metric for evaluating climate models. Satellite data are used to analyze the CRF anomalies to El Ni?o events simulated by the new and old versions of the Climate System Model of the Chinese Academy of Meteorological Sciences(CAMS-CSM), which has participated in the Atmospheric Model Intercomparison Project(AMIP). Here, simulations for super El Ni?o years, El Ni?o years, and normal years are compared with observations. The results show that the mean values of both longwave CRF(LWCRF) and shortwave CRF(SWCRF) in CAMS-CSM are weaker than the observations for each category of El Ni?o events. Compared with the old version of CAMS-CSM, the decrease in LWCRF during El Ni?o events is well simulated by the new version of CAMS-CSM. However, both new and old models cannot reproduce the anomalous SWCRF in El Ni?o events. The biases in the CRF response to El Ni?o events are attributed to the biases in the cloud vertical structure because of a weaker crash of the Walker circulation in CAMS-CSM. Due to the modification of the conversion rate from cloud droplets to raindrops in the cumulus convection scheme, the new version of CAMS-CSM has better CRF skills in normal years, but biases in El Ni?o events still exist in the new version. Improving the response of the Walker circulation to El Ni?o events is key to higher skills in simulating the cloud radiative responses.     

Different Summer Rainfall Anomaly Patterns in Northeast China Associated with Two Kinds of El Niño Events

The relationship between summer rainfall anomalies in northeast China and two types of El Ni?o events is investigated by using observation data and an AGCM. It is shown that, for different types of El Ni?o events, there is different rainfall anomaly pattern in the following summer. In the following year of a typical El Ni?o event, there are remarkable positive rainfall anomalies in the central-western region of northeast China, whereas the pattern of more rainfall in the south end and less rainfall in the north end of northeast China easily appears in an El Ni?o Modoki event. The reason for the distinct differences is that, associated with the different sea surface temperature anomalies (SSTA) along the equatorial Pacific, the large-scale circulation anomalies along east coast of East Asia shift northward in the following summer of El Ni?o Modoki events. Influenced by the anomalous anticyclone in Philippine Sea, southwesterly anomalies over eastern China strengthens summer monsoon and bring more water vapor to Northeast China. Meanwhile, convergence and updraft is strengthened by the anomalous cyclone right in Northeast China in typical El Ni?o events. These moisture and atmospheric circulation conditions are favorable for enhanced precipitation. However, because of the northward shift, the anomalous anticyclone which is in Philippine Sea in typical El Ni?o cases shifts to the south of Japan in Modoki years, and the anomalous cyclone which is in the Northeast China in typical El Ni?o cases shifts to the north of Northeast China, leading to the “dipole pattern” of rainfall anomalies. According to the results of numerical experiments, we further conform that the tropical SSTA in different types of El Ni?o event can give rise to observed rainfall anomaly patterns in Northeast China.     

一种新的El Niño海气耦合指数

利用1980～2010 年月平均Hadley中心海表温度、美国全球海洋资料同化系统(GODAS)海洋温度和NCEP/NCAR 大气环流再分析资料,通过对2 个海洋要素(海表温度SST、上层热含量HC)和5 个大气要素(海平面气压SLP、850 hPa 风场、200 hPa 速度势和对外长波辐射OLR)的多变量经验正交函数展开(multivariate EOF,简称MV-EOF)探讨了热带太平洋的主要海气耦合特征。结果表明,MV-EOF 分析的前两个耦合模态分别很好地对应了传统型El Ni?o 和El Ni?o Modoki 的海气耦合特征:传统型El Ni?o 期间,伴随着赤道中东太平洋SST 的异常增温,HC、SLP、200 hPa 速度势等要素总体呈东西反相的“跷跷板”变化,低层850 hPa 赤道中太平洋出现较强西风距平,西北太平洋上空为反气旋性异常环流;El Ni?o Modoki 期间,SST 持续增温和HC 正异常中心均显著西移至中太平洋,低层SLP 和高空200 hPa 速度势均呈现纬向三极型异常分布,低层异常强西风向西移至暖池东部,西北太平洋上空呈现气旋性异常环流。两类El Ni?o 的海气耦合特征存在显著差异,较优的El Ni?o 指数应不仅可以客观描述和区分El Ni?o 现象本身,更要紧密联系两类事件所产生的大气响应。以往定量表征El Ni?o 年际变化的指标大多立足于SST 或SLP,本文选取HC 作为研究指标,定义了一组新的El Ni?o 指数HCEI 和HCEMI。较以往基于SST 的El Ni?o 指数,HCEI 和HCEMI 不仅能更清楚地表征和区分两类El Ni?o(如1993 年的传统型El Ni?o 和2006 年的El Ni?o Modoki),而且能更好地反映和区分两类El Ni?o 与大气间的海气耦合特征,为El Ni?o的监测和短期气候预测工作提供了一个新工具。     

The Impact of an Abnormal Zonal Vertical Circulation in Autumn of Super El Ni?o Years on Non-tropical-cyclone Heavy Rainfall over Hainan Island

This study reveals a significant positive connection between autumn non-tropical-cyclone heavy rainfall on Hainan Island and the intensity of Eastern Pacific (EP) El Ni?o events. That is, the amount of rainfall in super EP El Ni?o years is more than that in normal EP El Ni?o years. Comparing EP El Ni?o years of different intensities, the cooler sea surface temperature in the northwestern Pacific during super EP El Ni?o years stimulates a negative surface latent heat flux (LHF) anomaly and abnormal anticyclonic circulation at 850 hPa. Under these conditions, an abnormal zonal vertical circulation develops in the northern South China Sea once a positive LHF anomaly and abnormal cyclonic circulation (ACC) at 850 hPa occur in the Beibu Gulf. The abnormal zonal vertical circulation further strengthens the ascending motion over Hainan Island, as the critical factor that leads to excessive rainfall. Further analysis shows that the positive LHF anomaly, which can be attributed to the increased latent heat transfer which resulted from the increased surface wind speed, is an important trigger for the ACC. However, the ACC is also the supplier of favorable moisture conditions because it intensifies vapor convergence over Hainan Island and meridionally transports moisture from the South China Sea to northeastern Hainan Island, thereby generating heavy rainfall. This paper emphasizes that the impact of El Ni?o events, especially super El Ni?o events, on rainfall over Hainan Island cannot be ignored, even if the traditional view is that frequent rainfall occurs mainly in La Ni?a years.     

厄尔尼诺/拉尼娜与重庆次年夏季涝/旱的非对称关系

本文详细分析了厄尔尼诺/拉尼娜与重庆夏季典型涝/旱年之间的不对称关系。结果表明:(1)厄尔尼诺和拉尼娜对重庆次年夏季降水有不对称影响。厄尔尼诺年的大气环流异常与重庆夏季典型涝年的特征一致;然而,拉尼娜年的大气环流异常与重庆夏季典型旱年的特征不一致。(2)从冬季到次年夏季,厄尔尼诺对重庆夏季典型涝年的影响主要是通过热带印度洋海温的‘接力效应’维持的。     

ENSO多样性研究进展

El Ni?o是热带中东太平洋异常偏暖的现象,发展过程具有显著的季节锁相特征。近年来,新形态事件更频繁发生引起了科学界广泛关注。学者们根据空间分布形态或爆发时间将ENSO事件分为两类,虽然选取标准不同,分类结果却有诸多相似点:中太平洋(Dateline、Modoki、CP、WP及SU型)El Ni?o事件发展至成熟时,正SSTA中心位于赤道太平洋中部;东太平洋(传统、EP、CT及SP型)El Ni?o发展至成熟时,正SSTA中心位于赤道东太平洋,低层西风异常更强,向东传输的距离也更远。研究结果显示,东太平洋El Ni?o比中太平洋El Ni?o持续时间更长,强度也更强;两类事件对全球气候的影响模态有很大的差异。近几十年,中太平洋El Ni?o出现频率有所增加,但其原因尚未清楚。关于两类事件生成发展和位相转换的动力原因,目前科学界普遍认为东太平洋El Ni?o是一个海盆尺度的海气耦合过程,其生消过程与温跃层的变化有紧密联系,但对中太平洋El Ni?o的动力机制尚未有统一的认识。      

NUIST地球系统模式模拟ENSO对西北太平洋热带气旋活动的影响

地球系统模式已经逐步成为研究热带气旋(TC)活动气候变化的重要工具之一,之前的研究发现南京信息工程大学地球系统模式(NESM)高分辨率版本可以较好地模拟全球海温分布及TC活动的气候特征.本研究进一步分析了NESM地球系统模式模拟西北太平洋TC活动的年际变化,并与1967～2016年观测的TC活动进行对比.NESM模式高...     

Contrasting Regional Responses of Indian Summer Monsoon Rainfall to Exhausted Spring and Concurrently Emerging Summer El Ni?o Events

The inverse relationship between the warm phase of the El Ni?o Southern Oscillation(ENSO) and the Indian Summer Monsoon Rainfall(ISMR) is well established. Yet, some El Ni?o events that occur in the early months of the year(boreal spring) transform into a neutral phase before the start of summer, whereas others begin in the boreal summer and persist in a positive phase throughout the summer monsoon season. This study investigates the distinct influences of an exhausted spring El Ni?o(springtime)...     

Distinctive MJO Activity during the Boreal Winter of the 2015/16 Super El Nino in Comparison with Other Super El Nino Events

Many previous studies have demonstrated that the boreal winters of super El Nino events are usually accompanied by severely suppressed Madden-Julian oscillation(MJO) activity over the western Pacific due to strong descending motion associated with a weakened Walker Circulation. However, the boreal winter of the 2015/16 super El Nino event is concurrent with enhanced MJO activity over the western Pacific despite its sea surface temperature anomaly(SSTA)magnitude over the Nino 3.4 region being comparable to the SSTA magnitudes of the two former super El Nino events(i.e.,1982/83 and 1997/98). This study suggests that the MJO enhanced over western Pacific during the 2015/16 super El Nino event is mainly related to its distinctive SSTA structure and associated background thermodynamic conditions. In comparison with the previous super El Nino events, the warming SSTA center of the 2015/16 super El Nino is located further westward, and a strong cold SSTA is not detected in the western Pacific. Accordingly, the low-level moisture and air temperature(as well as the moist static energy, MSE) tend to increase in the central-western Pacific. In contrast, the low-level moisture and MSE show negative anomalies over the western Pacific during the previous super El Nino events.As the MJO-related horizontal wind anomalies contribute to the further westward warm SST-induced positive moisture and MSE anomalies over the western tropical Pacific in the boreal winter of 2015/16, stronger moisture convergence and MSE advection are generated over the western Pacific and lead to the enhancement of MJO convection.