第二讲 OLR资料应用于热带大气环流系统的分析

引言 如上一讲所述,OLR能很好地反映热带大气中的大规模上升与下沉区。而热带大气中最主要的两大环流系统——热带辐合带(ITCZ)和副热带高压,分别对应于大规模的上升和下沉运动区。故OLR对这两个热带大气环流系统的分析很有用。本讲介绍如何用OLR分析ITCZ及副热带高压,以及用OLR     

夏季西北太平洋热带辐合带的强度变化特征及其对热带气旋的影响

利用卫星资料和再分析资料讨论夏季6—8月平均热带辐合带(Intertropical Convergence Zone,简称ITCZ)的强度变化特征及其对热带气旋的影响。首先利用120～150 °E,5～20 °N区域平均的对外长波辐射(Outgoing Long-wave Radiation,简称OLR)定义一个夏季ITCZ强度指数,再根据这个指数划分ITCZ强弱年并进行合成分析和相关分析。结果表明:ITCZ强弱的变化与高中低层大气环流和前冬海温场有显著的关系,ITCZ强年往往伴随有副热带高压的偏北、偏弱,对流层低层出现气旋性距平风场,高层出现反气旋性距平风场,同时整个太平洋前冬的SST的分布呈现类似La Ni?a的海温分布,而弱年正好相反。另一方面,ITCZ的对流强弱对热带气旋(Tropical Cyclone,简称TC)的发生和路径均有重要影响,在ITCZ强年,相应区域上的TC个数增多,同时TC路径更易于打转;而ITCZ弱年,TC个数减少,TC路径更易于向西北行。      

南海ITCZ异常变化及其对非移入性南海热带气旋(TC)活动的可能影响

利用美国NOAA提供的向外长波辐射（OLR）资料、NCEP/NCAR再分析资料以及上海台风所提供的热带气旋（TC）资料等,通过定义一个描写南海范围内（5°N~20°N,105°E~120°E）的热带辐合带（Intertropical Convergence Zone,简称ITCZ）强度指数,研究了南海ITCZ年际和年代际异常变化特征及其对非移入性南海TC[South China Sea-generated tropical cyclone（SCS-G TC）]活动的可能影响,并从异常强、弱南海ITCZ年份的大气环流背景和海表温度等变化特征来尝试揭示南海TC的活动规律。结果表明:在年际和年代际时间尺度上,南海ITCZ强度指数与南海TC的生成频数存在显著的负相关关系,长期趋势变化间的关系存在不同。南海ITCZ的强、弱显著地影响到南海TC的生成频数。强南海ITCZ年,南海TC频数偏多;弱南海ITCZ年,南海ITCZ频数偏少。强、弱南海ITCZ年对于南海TC的生成源地、TC的维持时间以及路径和强度的影响不显著。进一步分析表明,动力和环境条件方面,强、弱南海ITCZ年可能差异较大。异常偏强年,对流层低层出现气旋性环流,上层出现反气旋性环流;季风槽在南海区域偏强、位置偏南。与OLR表示的深对流区相配合,存在暖的海表温度和低层强烈的正涡度和强辐合,在高层存在相应的强的气流辐散,形成了极有利于南海TC发生发展的条件。弱南海ITCZ年则相反。另外,ITCZ强年,太平洋异常SST（Sea Surface Temperature）出现为La Ni?a特征,南海ITCZ区对流活跃,强度偏强。反之,ITCZ弱年则表现为El Ni?o特征,南海ITCZ关键区的对流强度偏弱。这些结果可为深刻认识南海TC的生成规律以及对南海TC的预报提供线索。     

南海地区赤道辐合带的动力学与能量学特征分析

本文利用三角形区域法,对南海地区ITCZ的21个个例分析计算了有关动力学特征量随高度的分布,从而得出了平均的结果,这与蒋全荣和余志豪(1984)的分析结论基本一致。分析还发现在平均和分类情况下,南海地区ITCZ的高层(约300hpa)可能有一个相对的干燥层或“干盖”存在。能量学特征的计算表明,南海地区的ITCZ低层具有大量水汽、潜热和全能量的水平辐合,然后垂直向上输送并在高层向副热带地区水平辐散开去。因此,它在热带大气环流能量、水汽等方面的传输过程中起着重要的作用。      

影响华东地区热带气旋年频数与热带对流场的关系

利用由美国NOAA系列卫星观测的OLR资料研究了影响华东地区热带气旋年频数（记为ATC）与热带对流场的关系，结果表明两者存在密切的联系：（1）当华东地区热带气旋年频数ATC为异常偏多（少）时，ITCZ位置累积距平为正（负）值，即ITCZ位置异常偏北（南）；（2）利用OLR设计了Walker综合指数WI，该指数的强弱可以反映ATC的异常情况，即当ATC为异常偏多（少）时，walker环流圈异常强（弱）；（3）OLR距平累积量场能较好的表征ATC异常年份所对应的低纬度及中高纬地区大气环流的配置，其可以作为预测影响华东地区热带气旋年频数的强信号。     

南太平洋辐合带季节循环及与地形和非绝热加热变化的联系

利用1981—2015年NCEP/NCAR月平均资料、NOAA的逐月CMAP（CPC（Climate Prediction Center）Merged Analysis of Precipitation）降水资料以及GODAS的月平均洋流资料和SODA的月平均风应力资料,定义了南太平洋辐合带（SPCZ）的关键区域,对南太平洋辐合带的季节变化特征及南太平洋辐合带的形成和维持原因进行了分析。结果表明,在南太平洋辐合带,4月存在由东西风切变型辐合带向东风辐合型辐合带转变的现象,而12月则存在相反的转换。在对流层低层,南太平洋辐合带区域的向上伸展高度和辐合在北半球冬季较其他季节明显高和强。引起南太平洋辐合带形成与维持的原因有2个方面:一是地形作用。由于地形的阻挡,造成等位涡线发生沿澳大利亚地形的绕行,利于澳大利亚地区反气旋性环流和南太平洋辐合带区域气旋性环流的形成与维持;同时,在地形和科里奥利力共同作用下,还易使暖海水在南太平洋辐合带区域汇聚,形成高海表温度区,从而加热大气,利于南太平洋辐合带的形成与维持。二是非绝热加热作用。南太平洋辐合带区域范围内的热源作用可以使其上方的大气受到加热,并产生加热强迫纬向梯度,驱动低层大气产生辐合。这些结果对深刻认识全球环流特别是南半球热带环流变化有重要意义。      

北半球夏季西太平洋热带地区的环流特征及其与台风发生的关系

关于热带地区的环流特征,陶诗言等曾将东亚热带地区的环流分成经向和纬向两种环流型;大气所热带气象研究组根据月平均资料将西太平洋热带地区的环流分成辐合带强和弱两种类型。近年来,飞机报告的数量和质量都有提高,并出现了从地球静止卫星推导的风。这两种新的资料来源,使得热带太平洋上对流层上层环流的分析质量有了显著的提高。本文用这些新资料,分析了76年6-9月西太平洋热带地区这两种特征相反的环流型,并讨论了这两种流型的特征与台风发生的关系。     

初论西风带和热带辐合带环流系统的相互作用

探讨中高纬和低纬环流系统之间的相互作用,对指导天气预报实践和研究大气环流理论都具有重要的意义。本文是基于近几年来在低纬环流和我国北方盛夏暴雨方面一系列分析研究工作,对西风带和热带辐合带环流系统之间相互作用的一个初步总结。 首先,根据大范围环流分析和全球卫星云图,提出了一个连结中高纬西风带和热带辐合带的简要的总环流模型。其次,将西风带和热带辐合带环流系统之间相互作用的现象,列举出常见的有代表性的六个方面。最后强调指出,要想进一步解决中高纬地区夏季暴雨天气预报问题,应该特别重视低纬环流和天气过程的研究,同时,     

全球大气垂直环流间的相关分析

利用三维分解方案对全球大气环流进行分解,得到全球大气垂直环流分量,计算各大气垂直环流活动中心之间以及赤道中太平洋大气垂直环流与全球大气垂直环流的相关系数,分析表明,全球大气垂直环流活动中心的季节变化具有明显的一致性,南半球热带垂直环流和亚洲季风环流关系密切;赤道中太平洋地区纬向垂直环流与热带大气经向垂直环流有较强的同步变化特征,与青藏高原上空大气纬向环流关系密切     

非对流云的观测事实及其对大气环流模式设计的意义

通过非对流云的气候资料分析和个例分析表明：(1)非对流云有季节变化,也随海陆分布的不同而变化,还与大气三圈环流及季风等密切相关。由于它们的相关性,在大气环流模式(GCM)中对非对流云的模拟必须与提高模式其他部分的模拟能力相辅相成。(2)产生于中高纬度大范围上升气流的非对流云,由赤道辐合带积云对流所致的高空赤道地区的卷云与卷层云和形成于副热带冷海水上空的层云与层积云是新一代大气环模式显式预报的3类主要非对流云。这3类非对流云均是大尺度的,GCM的网格能显示分辨,但在垂直方向如何提高GCM的分辨率问题仍是一个有待研究的问题。(3)在GCM中如何模拟冷海水上空的层云和赤道ITCZ所对应的大范围卷云和卷层云是十分困难和必要的。(4)通过对东亚及西太平洋区域非对流云系的个例分析,可以认为在新一代大气环流模式中,应显式预报行星大槽及赤道辐合带所对应的非对流云系。在模拟这些非对流云系时,应考虑它们的生消过程、平流过程与辐射过程。由于一段时间内大气环流模式尚难以分辨锋面与α中尺度的气旋,因此有必要在GCM中参数化这些系统,或采用更小的网格距。至于对非对流云所对应的降水参数化问题的研究,需要进一步的观测为基础。     

THE INTENSITY VARIATION OF THE SUMMER INTERTROPICAL CONVERGENCE ZONE IN WESTERN NORTH PACIFIC AND ITS IMPACT ON TROPICAL CYCLONES

Based on the satellite data from the National Oceanic and Atmospheric Administration and the NCEP/NCAR reanalysis data, the variation of the intensity of convection over the Intertropical Convergence Zone(ITCZ) in summer and its impacts on tropical cyclones are studied. In this paper, an intensity index of the ITCZ is proposed according to Outgoing Longwave Radiation(OLR) in the region of(5°–20°N, 120°–150°E) in the western North Pacific(WNP). Then strong and weak ITCZ years are classified and different variables during the strong/weak ITCZ years are analyzed. The composite results show that the ITCZ anomaly is connected to the general atmospheric circulation and SST distribution. In the strong ITCZ years, the subtropical anticyclone weakens and shifts northward. Besides, there is salient cyclonic anomaly at the low level and anticyclonic anomaly at the high level. SST patterns in the preceding winter resemble to those of La Nina. It could persist into the succeeding summer. However, it is opposite in the weak ITCZ years. The impact of the ITCZ anomaly on the tropical cyclone(TC) formation and track is also discussed. There are more TCs over the WNP(5°–20°N, 120°–150°E) in the strong ITCZ years and there is a significant increase in the northward recurving TCs. In the weak ITCZ years, fewer TCs occur and the frequency of the northwestward track is higher.     

Questions relative to ITCZ migrations over the tropical Atlantic ocean,sea surface temperature and Senegal River runoff

Summary The ITCZ (Intertropical Convergence Zone) is an important parameter for climatic studies in tropical areas, and meteorological satellite imagery provides an original way to follow its location. Using archive imagery covering the 1971–1987 period, we attempted to study further some of the relationships (suggested by former studies) between ITCZ locations (followed here over the Atlantic ocean at 28°W), and climate anomalies in the Sahel, an area affected by periodic drought for the last seventeen years. We also paid close attention to more frequently studied parameters, such as upper air data, wind at sea level, and sea surface temperature. As for relative drought estimates, we assumed that runoff from the Senegal River was representative of the sahelian area and we observed that its variations were consistent with the Lamb's rainfall index over the 1965–1987 period.Since the onset of the rainy season for West Africa responds to wind changes, we assessed the link between ITCZ and wind at sea level and found the timing of northward ITCZ migration to be highly correlated (r=0.84) with the date of zonal wind stress intensification.On a general point of view, the relationships we found between rainfall amount and ITCZ position anomalies (or SST anomalies) agree with known results of precedent works, though better fit is found with the seventies than the eighties. We think this discrepancy is due in part to the fact that the parameters studied were not identical and, perhaps to a possible change in climatic conditions (on a long term basis, the data show a continuous trend for less intense equatorial upwelling in the gulf of Guinea, and our time series covers a more recent period than referenced works).With a closer look on the first half of the year, it appears that typical (wet/dry) schemes of the ITCZ migration can be evidenced more clearly, than in reporting the northernmost ITCZ location, that we found to be a less significant index: in other words, a sooner (respectively later) northward ITCZ migration corresponds to dry (respectively wet) episodes during the rainy season in sahelian areas. Hence, we propose the speed of ITCZ northwards movement as a parameterization of this event.Moisture content of the lower troposphere revealed that steady anomalies of this parameter may last several years over sahelian areas. Taking into consideration the relative strength African tropical and easterly jets, some limited results were obtained, in regard of climatic anomalies.As first conclusions, moisture transportation over sahelian area (associated with larger negative SST anomalies) is more efficient for wetter rainy season, than the intensity of convective process linked to higher local SST in the equatorial Atlantic area. In joining moisture analysis and ITZ migration (1980–1987 period), wetter rainy seasons were observed each time that positive humidity anomalies coincided with a later northward ITCZ migration (or greater northward ITCZ speed).With 8 Figures     

IMPACT OF THE HEATINC OVER SOUTH ASIA UPON THE SUBTROPICAL HIGH OVER WEST-PACIFIC

A case is reported, during which the Subtropical High over the Western Pacific(hereafter, SHWP in abbreviation)shifted northwestward and met-yu at Chaniiang River valley ended. Several numerical experiments on SHWP activity influenced by the heating over south Asia monsoon area are carried out, and the statistic significance of the results is checked. The results indicate that the enhancement of positive heating over South Asia will motivate a wave-like series of anomaly centers, which propagate northeastward from the maximum heating center.so that a strong positive potential height anomaly center will set up from North China to Japan at Day 3 result in the enhancement of SHWP. Comparison of the influence upon SHWP by the heating over south Asia monsoonarea with that over ITCZ area south to SHWP is also carried out. It is pointed out that the heating over South Asiamonsoon area tends to favor SHWP northward movement while the heating over ITCZ area tends to thvor SHWP westward stretching. As for the time to begin to influence on SHWP, the heating over south Asia monsoon areafavors the enhancement of SHWP atter Day 3 while that over ITCZ south to SHWP effects after Day 5.     

辐合带台风形成与对流层中、低空急流的联系

本文的目的,是探讨我国近海辐合带中台风的形成与其两侧对流层中、低空急流的联系。11次实例的分析结果表示,多数(8/11)辐合带台风形成前,在其两侧或一侧有明显的、大尺度的对流层中、低空急流。辐合带扰动,是在这些中、低空急流加强和基本气流气旋性切变增大以后发展成台风的。扰动发展的动能,可能主要来自具有较强气旋性切变的基本气流动能。辐合带两侧明显的中、低空急流的出现和加强,可以作为台风形成的一个预报依据。     

华南后讯期降水的年际变化

本文研究了近30多年来(1951——1984)华南后讯期降水的年际变化。结果表明:(1)华南后讯期降水有3年周期变化和10年尺度的长期变化;(2)夏季西太平洋副热带高压轴线南北位置和西脊点西伸位置的年际变化与华南后讯期降水有密切联系。当副高位置偏北偏东时,华南后讯期降水偏多。反之,降水偏少。 利用1974—1983年的月平均OLR资料研究了热带辐合带对华南后讯期降水的影响。结果表明,华南后讯期降水多年和少年热带辐合带的位置和强度有显著差异。多雨年热带辐合带位置偏北强度偏弱,少雨年则相反,热带辐合带位置偏南强度偏强。      

Seasonal variation of the ITCZ and its characteristics over central Africa

We investigated the seasonal march of the Intertropical Convergence Zone (ITCZ) shown by the 22 coupled general circulation models of the 20th Century Climate in Coupled Models experiment in seven regions (Africa, Indian Ocean, western Pacific, central Pacific, eastern Pacific, South America, and Atlantic Ocean). Inter-model differences in the seasonal march of the ITCZ over Africa (10?C40°E) were significantly smaller than those over other regions. This finding indicates that the seasonal march of the ITCZ over Africa is insensitive to differences in model physics and resolution and suggests that the seasonal march of the African ITCZ is controlled by robust and simple mechanisms. Motivated by this result, we tried to understand the process of the seasonal march of the ITCZ over central Africa (15?C30°E) based on an analysis of ERA-40 data. The analysis results revealed the following features of the ITCZ in this region: (1) The ITCZ itself produces large convective available potential energy that generates deep convection. (2) The abundant water vapor within the ITCZ is maintained by horizontal moisture flux. (3) Outside but near the ITCZ, shallow convection exists and may act to pre-moisten deep convection in spring and autumn. (4) The seasonal change of the ITCZ is preceded by that of the vertical moist instability in the lower free atmosphere caused by the seasonal change in insolation.     

ITCZ的季节内振荡及其与热带气旋发生阶段性的关系

利用中国气象局提供的热带气旋资料和NCEP/NCAR再分析等资料, 研究了热带辐合带(Intertropical Convergence Zone, 简称ITCZ)上对流强度的季节内振荡特征及其与热带气旋生成频数阶段性变化的关系, 并进一步研究了它与越赤道气流、 赤道西风和ITCZ北侧偏东风季节内振荡的关系。研究发现: (1) ITCZ对流强度的变化有明显的30～60 d振荡, 西太平洋 (5°N～20°N, 120°E～150°E) 范围内的热带气旋约有2/3发生在30～60 d振荡的活跃位相。(2) ITCZ季节内振荡在热带地区表现为向东传播的特征, 而在副热带地区 (25°N～35°N) 表现出清晰的西传特征。在ITCZ季节内振荡较强年, 振荡在由赤道传播至15°N左右时, 与北面向南传播的振荡在该纬度附近汇合, 对流强度增强, 使热带气旋在此期间频繁发生。而在弱年, 振荡由赤道一直向北传播至30°N附近, 15°N附近的ITCZ对流较弱, 热带气旋生成偏少。(3) 赤道西风、105°E～110°E越赤道气流和ITCZ北侧的偏东风气流本身也存在30～60 d振荡。这三支气流的30～60 d振荡与ITCZ的季节内强弱变化密切相关。然而, 相比之下偏东风气流的30～60 d振荡和ITCZ对流强弱的30～60 d振荡对应关系略差。     

用平均云量图分析太平洋热带辐合带

本文用月平均卫星云量图揭示了太平洋上热带辐合带的基本气候特征。西太平洋热带辐合带的位置和强度有明显的季节变化,并且有双辐合带结构。而东太平洋热带辐合带则始终稳定在5—8°N附近,季节变化很小。东西太平洋的这种差异可能与海洋热状况有关。同时,西太平洋热带辐合带又有明显的年际变动,它对台风活动有一定的影响。 1984年8月18日      

Intraseasonal and submonthly variability over the Eastern Amazon and Northeast Brazil during the autumn rainy season

Summary The precipitation variability over the eastern Amazon (EAM) and northeast Brazil (NEB) during the autumn rainy season (March to May) is diagnosed using raingauge-based weekly data from 1982 to 2001. Since precipitation in this region is remarkably modulated by the combined effects of the Pacific and Atlantic Sea Surface Temperature (SST) anomalies, two extreme and contrasting large-scale climatic scenarios are considered in this study. The unfavorable (UNFAV) scenario, defined by the simultaneous occurrences of the El Niño and northward SST gradient in the intertropical Atlantic and the favorable (FAV) scenario, by the simultaneous occurrence of the La Niña and southward SST gradient in the intertropical Atlantic. UNFAV (FAV) composites with unfiltered data show remarkable changes in both the Walker and the Hadley cells associated with the Atlantic Intertropical Convergence Zone (ITCZ) anomalously weakened (enhanced) thus with deficient (abundant) seasonal rainfall in most of the EAM/NEB. Precipitation variability is identified objectively through EOF analyses performed on the (30–70 day) intraseasonal and submonthly (蠄21 day) filtered weekly precipitation anomalies for 18 autumn seasons (1983 to 2000). The principal components (PC) of the first mode of each analysis show strong oscillations. In particular, the oscillations of the PC series during UNFAV and FAV years reveal that events with anomalously deficient and abundant precipitation over the EAM/NEB occur alternately, even under extreme climatic conditions in the tropical Pacific and Atlantic. Composites based on events with anomalously increased precipitation (objectively selected from the PC series) on intraseasonal and submonthly scales are analyzed separately for the UNFAV and FAV years. These analyses show that for both scenarios the more important rainfall-producing atmospheric mechanism over the EAM/NEB on intraseasonal time scales consists of the establishment of deep convective bands trigged by South Atlantic Convergence Zone events or persistent frontal systems over northeast Brazil. Such a regional pattern is embedded in a large-scale dynamic environment related to the propagation of the Madden-Julian Oscillation over tropical South America. On the other hand, the main rainfall-producing atmospheric mechanism over the EAM/NEB on submonthly time scales is the Atlantic ITCZ during FAV years, whilst weakened Atlantic ITCZ activity may be forced by meridionally elongated mid-latitude wave trains in the upper troposphere during UNFAV years.     

长江流域旱涝的OLR特征

卫星观测的OLR(射出长波辐射)资料已在大气环流变化及气候异常的研究中得到广泛的应用。本文试图利用美国气候分析中心所提供的9年NOAA卫星所观测的OLR资料,对长江流域的夏季雨量的估算进行试验并对旱年、涝年的广大低纬地区的OLR特征进行诊断分析,为卫星资料应用于长江流域的旱涝分析预报作一初步探索。