Medium-range oscillations in the summer tropical easterlies at 200 hPa

By the use of space-time spectral analysis and band-pass filter, some of the features of the medium-range Oscillations in the summer tropical easterlies (10oS-20o) at 200 hPa are investigated based on a two-year (1980 and 1982) wind (u, v) data set for the period from May to September. Space-time power spectral analysis shows that the total energy of the westward moving waves was the largest and that of the standing waves and eastward moving waves was relatively small in the 200 hPa easterlies; the total energy of the eastward moving waves was at minimum at 10oN. Three kind of the medium-range oscillations with about 50 day, 25 day and quasi-biweekly periods were found in the easterlies, which all show a remarkable interannual variation and latitudinal differences in these two years. The wave energy of zonal wind is mainly associated with the planetary waves (1-3), which all may make important contributions to the 50 day and 25 day oscillations in different years or different latitudes. The quasi-biweekly oscillation is mainly related to the synoptic waves (4-6). In equatorial region, the 50 day oscillation was dominant with a eastward phase propagation in 1982 while the dominant oscillation in 1980 was of 25day period with a westward phase propagations in 1980. Both of them are of the mode of zonal wavenumber 1. Strong westward 50 day oscillation was found in 10oN-20oN in these two years. Regular propagations of the meridional wind 50 day oscillation were also found in the easterlies.The 50 day and 25 day oscillation of zonal wind all demonstrate southward phase propagation over the region of the South Asia monsoon and northward phase propagation near interational date line, where are the climatic mean position of the tropical upper-tropospheric easterly jet and the tropical upper tropospheric trough (TUTT), respectively.     

A study on the decreasing trend in tropical easterly jet stream (TEJ) and its impact on Indian summer monsoon rainfall

Using the NCEP/NCAR reanalysis wind and temperature data (1948–2011) and India Meteorological Department (IMD) rainfall data, a long-term trend in the tropical easterly jet stream and its effect on Indian summer monsoon rainfall has been explained in the present study. A decreasing trend in zonal wind speed at 100 mb (maximum decrease), 150 mb, and 200 mb (minimum) is observed. The upper-level (100, 150, and 200 mb) zonal wind speed has been correlated with the surface air temperature anomaly index (ATAI) in the month of May, which is taken as the difference in temperature anomaly over land (22.5°N–27.5°N, 80°E–90°E) and Ocean (5°S–0°S, 75°E–85°E). Significant high correlation is observed between May ATAI and tropical easterly jet stream (TEJ) which suggests that the decreasing land–sea temperature contrast could be one major reason behind the decreasing trend in TEJ. The analysis of spatial distribution of rainfall over India shows a decreasing trend in rainfall over Jammu and Kashmir, Arunachal Pradesh, central Indian region, and western coast of India. Increasing trend in rainfall is observed over south peninsular and northeastern part of India. From the spatial correlation analysis of zonal wind with gridded rainfall, it is observed that the correlation of rainfall is found to be high with the TEJ speed over the regions where the decreasing trend in rainfall is observed. Similarly, from the analysis of spatial correlation between rainfall and May ATAI, positive spatial correlation is observed between May ATAI and summer monsoon rainfall over the regions such as south peninsular India where the rainfall trend is positive, and negative correlation is observed over the places such as Jammu and Kashmir where negative rainfall trend is observed. The decreased land–sea temperature contrast in the pre-monsoon month could be one major reason behind the decreased trend in TEJ as well as the observed spatial variation in the summer monsoon rainfall trend. Thus, the study explained the long-term trend in TEJ and its relation with May month temperature over the Indian Ocean and land region and its effect on the trend and spatial distribution of Indian summer monsoon rainfall.     

基于动力学模态分解的200 hPa急流模态特征分析

提出基于动力学模态分解（Dynamic Mode Decomposition,DMD）的大气运动数据分析方法,目的是改进对大气运动特征的认识。首先,采用DMD方法对200 hPa急流运动流场进行模态分析,从中得到了急流天气系统运动变化过程中的主要模态和对应频率以及模态随时间衰减/增长等信息。这些模态是对流场演化特征的低维描述,反映了蕴含在流场中的动力学特征,可用于实现高维复杂流场的低维近似表示。其次,建立了200 hPa急流运动流场演化的动力学降阶模型,能够重构和预测急流运动流场的动态发展过程。结果表明：通过对前6阶主要模态所包含的流场信息进行对比分析,DMD方法成功捕捉到了200 hPa急流运动流场不同尺度的流动结构,直观地显示了不同频率流场之间的差别,表明了DMD方法在对复杂大气动力学系统进行模态分解时的优势。通过不同时刻,模态叠加的重构流场与真实流场的直观比较,表明DMD方法只由前面6阶模态就能基本包含原始流场的流动信息,从而实现流场的准确重构。     

Dissipation process of summer tropical easterly waves in Western North Pacific

Tropical easterly waves are common features in the trade wind zones and they are important sources of tropical cyclogenesis. Despite numerous studies have analyzed the genesis and maintenance of easterly waves in the Western North Pacific, few had examined their dissipation processes. Focusing on tropical easterly waves during May-September of 1979–2017, this study shows that most of the easterly waves (∼70 %) eventually dissipate when encountering the monsoon trough and associated westerlies, while 22 % were carried northward by the monsoonal southwesterly flows and became recurving disturbances. Less than 10 % of easterly waves propagate across the South China Sea against the prevailing monsoon westerlies and into the Indochina peninsula. The vorticity budget analysis illustrates that total vortex stretching in the lower troposphere is the key factor in propelling the small number of easterly waves westward, suggesting that stronger and more convectively active easterly waves tend to move further into the developed monsoon trough. This echoes the previous observation that tropical disturbances alone have a limited probability in developing into a typhoon, for those disturbances or easterly waves almost always need to interact with the monsoon trough or a monsoon gyre, as well as other intraseasonal features to sustain the organized convection and rotation.     

夏季东亚高空西风急流气候特征分析

利用NCEP/NCAR全球再分析风场资料定义了西风急流强度指数和位置指数,然后利用EOF方法对西风急流进行了进一步的分析,分析了高空西风急流的空间分布特征,从强度和位置两方面分析了西风急流与东亚环流及其与海温的关系。分析表明： EOF第一模态反映了东亚高空急流的位置指数,第二模态反映了高空急流的强度指数。东亚高空急流与对流层大气环流包括南亚高压,西太平洋副热带高压,东亚夏季风存在着密切关系,其气候变化与热带副热带东太平洋、印度洋海温密切相关。     

东风急流对孟加拉湾热带气旋Nargis初始涡旋形成的影响

利用NCEP-CFSR(National Centers for Environmental Prediction Climate Forecast System Reanalysis)再分析资料和WRF模式,研究了2008年4月孟加拉湾热带气旋Nargis的初始涡旋的形成过程。结果表明:受到印度洋赤道西风急流爆发及其伴随的东传MJO事件的影响,Nargis的初始扰动生成于苏门答腊岛北部地区。另外,源于中纬度地区经南海进入孟加拉湾的东风急流(4月22—25日)对Nargis初始扰动发展到热带低压起到了重要的作用。东风急流及其携带的冷空气使得孟加拉湾东部海洋向大气输送的感热通量迅速增加,低层大气的有效位能通过非绝热加热获得能量,并向总动能转化,从而近海表涡旋性环流得到增长,Nargis初始扰动向西北移动并最终发展为热带低压。数值试验结果进一步证实了东风急流对Nargis初始涡旋生成的作用,如果没有东风急流的出现,Nargis初始扰动将不能北上发展成为热带低压。     

5—10月全球热带和副热带200hPa多年平均环流的研究(二)——行星风系

北半球夏半年逐月200 hPa主要风系的分析表明: (1)北半球夏季副热带急流的两个中心分别位于亚洲和北美上空.纬度在45°N附近.另外在中东太平洋和中东大西洋热带地区存在高空西风.盛夏前者成为明显的独立风系. (2)北半球夏季高空东风急流具有三个中心,分别位于也门.印度半岛南部和马来西亚上空.前两个中心的变化明显地与南亚季风活动有关,最后一个中心则稳定少变. (3)南半球冬季最强的西风带位于大洋洲上空,最大值在大洋洲东岸. (4)200 hPa全球有四个跨赤道的质量交换区.其中沿100°E和印度洋的质量交换最强,它们与亚洲季风活动密切有关.     

亚非季风区夏季降水与热带东风急流的关系

本文分析了亚非季风区(0—40°N,30°W—150°E)6—8月各月的平均降水分布与热带东风急流的关系,主要结果如下: 1)降水分布和降水量的逐日变化与热带高空东风急流的位置和强弱变化有密切关系,在急流不同部位降水特征不同,多雨区主要出现在急流入口区的右侧和出口区的左侧,对多年平均东风急流中垂直速度的计算表明,热带高空东风急流与降水分布的关系可以用急流的动力学机制来解释。 2)比较季风较弱的1972年(大部分地区降水偏少)和季风偏强的1975年(大部分地区降水偏多)发现,1975年东风急流较1972年强     

5—10月全球热带和副热带200hPa多年平均环流的研究(一)——行星尺度环流系统

本文用近13年200hPa高空风资料得到了夏半年(5—10月)逐月平均全球热带风场,发现如下一些主要结果: (1)在亚洲地区对流层上部反气旋存在两个中心;一个在青藏高原上空;一个在伊朗和阿富汗地区。两者的演变不同。伊朗高压的北推和撤退都比青藏高原上的早。北美墨西哥高压的变化在时间上与伊朗高压很相似。 (2)北太平洋中部高空槽(TUTT)出现在5—10月,8月份最强,位置最北;北大西洋中部高空槽出现在5—9月,7—8月最强。 (3)南半球冬季有三个主要的长波脊和长波槽,分别位于大陆以西和以东海区。     

Annual cycle of the mid-tropospheric easterly jet over West Africa

Summary The paper examines the annual cycle of the mid-tropospheric easterly jet (MTJ) over West Africa against the background of many reviews indicating different locations and characteristics of the jet and considering it as a summer feature. NCEP–NCAR reanalysis zonal wind datasets for the period 1971–2000 and upper air datasets over the region are used. The results exhibit realistic spatial structure of the easterly jet. The long-term mean of the datasets suggests that the jet over West Africa is not only a summer feature but can also be found in winter with the same order of magnitude in the wind velocity at the core. The jet axis is located at about lat. 2° N close to the Guinean Coast in winter and at lat. 14° N in summer. The meridional oscillation of the jet suggests that as it advances northward, it maintains an altitude of 700 hPa in winter and transits in mid-spring to 650 hPa and reaches 600 hPa in summer. In the retreat, it displaces to 650 hPa at the end of September rather sharply to reach 700 hPa in October. The jet’s core has been observed to have a northeast–southwest orientation from season to season, covering a longitude of 29° from its southernmost to the northernmost positions.     

200hPa面上副热带准常定反气旋系统的形成及准周期振荡

本文采用一个球坐标下的准地转斜压两层高截谱模式,模式中包括了纬向不对称热力强迫、经向热力强迫以及基本纬向风速分布,结果得到了一类稳定的平衡态,从而讨论了200 hPa面上副热带准常定反气旋系统形成、维持的一种可能动力机制;同时,还进行了稳定性分析,发现当热力强迫超过一定强度时平衡态失稳而引起振荡,并进一步用数值试验探讨了平衡态分叉到周期解的特点.     

平流层30 hPa月平均高度场的气候特征

采用经验正交函数分解(EOF)方法对NCEP／NCAR全球再分析资料中1958—1997年共40年,30hPa月平均高度和月平均风场进行了分析,讨论了40年平流层的主要特征向量和相应时间系数的变化特征,发现高度场的EOF第一模态具有很好的空间整体性和明显的季节变化。在平流层中,北半球冬季为一较大的环极冷低压,夏季为一以整个半球为规模的环极暖高压。并以1月和7月代表冬、夏季,选取1月和7月的40年资料作为时间序列,用EOF分析了冬、夏季不同的空问振荡型的特征,运用小波分析方法考察了其年际及年代际变化周期。并据此定义了不同的振荡型指数,为以后分析平流层要素场与气候变化的关系奠定了基础。     

冬季西北太平洋热带气旋的生成与热带东风扰动的关系

利用合成技术对1995—2006年冬季(11月—次年2月)生成在西北太平洋上的34个热带气旋(tropicalcyclone,TC)个例进行分析,研究冬季西北太平洋TC生成的大尺度环流特征及其生成机制,结果表明:冬季TC生成的大尺度环流特征型为东风波西传型;北半球冬季对流层低层出现的跨赤道气旋对是冬季北半球TC形成的重要特征;太平洋中部赤道混合Rossby重力波西北传,与强对流中心重合,性质转为"热带低压型扰动",为冬季热带气旋生成提供扰动源。对合成TC初始场的涡动扰动动能的收支分析表明,涡动有效位能和正压不稳定转换为TC形成提供了能量,这两种能量分别与积云对流加热和水平不均匀气流有关。正压不稳定能量转换为动能主要位于对流层中下层,而扰动有效位能的转换主要位于对流层中上层。低层热带东风波动从平均气流中获得正压不稳定能量,并与强积云对流耦合,热力和动力共同作用下形成TC。     

东亚500hPa环流与盆东伏旱的关系

为了认识东亚500hPa环流与盆东伏旱强度关系,我们利用1951 ～1980年500hPa的平均图资料,取110°E经向剖面,计算出各类旱年平均地转西风风速分量的时间演变图.采用对比分析方法,分析了伏旱同期和前期东亚上空500hPa环流演变特征,并运用分析结果建立了判别预报指标,可为制作长期预告参考.     

MOPITT与MLS观测的CO浓度在200 hPa的差异及其原因分析

具有较优垂直分辨率和反演精度的MLS（Microwave Limb Sounder）数据与MOPITT（Measurements of Pollution in the Troposphere）数据在上对流层—下平流层（UT-LS）区域有一个交集,因而将MOPITT与MLS测量的200 hPa高度上CO数据进行对比分析。比较结果显示,两者在中低纬度分布较为接近,在非洲中西部、南美中北部和东南亚地区均有大范围高值中心区存在;MOPITT CO在浓度值上明显高于MLS CO,并且MOPITT CO浓度在低纬度存在约35 ppb（10-6）的全球性系统性偏差。通过CALIOP云层数据对MOPITT和MLS CO差异原因进行分析,表明CO高值区的形成与旺盛的对流有关。     

对流层及平流层低层全球动能的季节性急变

利用１９８０－１９８８年ＥＣＭＷＦ的资料，分析对流层５００ｈＰａ，３００ｈＰａ和平流层低层１００ｈＰａ全球范围的纬向平均动能和涡动动能的季节过渡，比较不同层次不同纬度带动能模态的季节性急变，得到如下结论：（１）北半球热带与温带各层次的ＫＺ都有季节性急变，发生在３－４月和１０月附近，而南半球仅在热带有急变。（２）北半球热带地区ＫＺ在６月还有一次季节性急变，出现有１００ｈＰａ和５００ｈＰａ．（３）１０     

Characteristics of Soliton with Dynamic Constraints on its Existence/Propagation in Tropical Easterly Wave

The study concerns the propagation of easterly wave (EW) at tropics as west-moving soliton more steady both in ionn and velocity as evidenced in the dynamic framework.Under the impact of different circulation patterns over the regions of western Pacific trade wind,South-Asia monsoon and their transition,such a soliton becomes tapering off during its westward movement and degrading to a common dispersive wave on the whole,followed by dismtegra-non when striking the South-China Sea monsoon segment,thereby indicating that the sea sector is inaccessible to the soliton When no monsoon trough is present over the South Asian monsoon area around 30癗 or the monsoon depression ]S shallow,it is likely to have west-travelling soliton,which suggests the incursion of the EW into the South-Asian monsoon region.     

秋、冬季500hPa月平均极涡变化与黑龙江省夏季温度的关系

利用经验正交函数（ＥＯＦ） 展开方法,研究了黑龙江省夏季温度时空分布的主要特征,找出北半球５００ ｈＰａ月平均图上秋、冬季（１０ ～１１ 月） 极涡位置和强度变化与次年黑龙江省夏季温度主要特征的关系,并用于制作夏季温度预报     

高层东风波引起的一次超级单体雹暴天气数值研究

利用WRF中尺度模式对2018年7月26日发生在浙中北的一次超级单体雹暴过程进行数值研究,结合自动站资料,天气雷达资料、NCEP再分析资料等分析冰雹天气发生的环流背景,冰雹云的雷达回波和流场结构特征,并探究冰雹形成的物理机制。结果表明:此次强对流天气是在高层东风波环流背景下,由地面辐合线触发的超级单体雹暴过程。雹暴发生在强的对流有效位能、上干下湿的层结和弱垂直风切变的环境场中。模拟试验成功地模拟出了雹暴云团的发展演变过程。0℃层位于5 km的高度,-20℃层位于8. 5 km的高度,且超过40 dBZ的强回波向上扩展至-20℃层以上,有利于冰雹的生长。雹云发展旺盛时呈现典型的低层辐合、高层辐散特征。雹云右侧出现悬垂回波和有界弱回波区。雹云中存在大量的过冷云水,冰雹粒子的核心生长区位于0℃层和-20℃层之间,主要由霰粒子转化而成,并通过不断碰并过冷云水和过冷雨水增长。