大气自由模与中高纬准双周振荡

用数值方法研究了自由模对强迫耗散系统解的轨线的影响。首先,用变分法得到实际大气自由模,并设计一族解析自由模——理想大气自由模;其次,以两类自由模分别作强迫场,初始场取1993年7月1目的500hPa流函数场,然后应用强迫耗散正压涡度方程的全球谱模式积分300d,得到：(1)以1992年12月27—28日500hPα的实际大气自由模作强迫场时,①α=γ=k≥0．02,强迫、耗散系数相等且足够大时．随着时间演变,大气总是接近自由模态;②强迫系数α=0．1,耗散系数γ=0．02时．中高纬出现局地性准双周振荡。(2)以理想大气自由模作强迫场时,固定耗散系数γ=0．01,强迫系数0．0095≤α≤0．011时,振荡周期与强迫系数呈正相关关系。     

热带大气中的准双周（10—20天）振荡

基于欧洲中期天气预报中心（ECMWF）的资料（1981—1988），本文对全球热带大气中的10—20天（准双周）振荡进行了比较系统的研究，包括其动能的分布及演变。扰动的结构和传播特征等。资料分析充分表明，热带大气10—20天振荡是热带大气中另一重要低频系统，其动能比30—60天振荡还要大，而其结构和活动又同30—60天振荡很不一样。例如热带大气10—20天振荡主要表现为纬向波数2—4；垂直结构以正压特征更显著；主要表现为向西传播；其经向风分量与纬向风分量同等重要。因此，对热带大气10—20天振荡值得更多注     

西北太平洋大气准双周振荡对热带气旋活动的影响

利用JRA逐日风场资料、 NOAA/NCEP的逐日OLR场资料以及美国联合台风预报中心的热带气旋 (TC) 数据, 通过对西北太平洋 (WNP) 上空10～20天大气准双周振荡 (QBWO) 不同位相的划分, 深入分析了QBWO对WNP区域生成TC的调制作用。研究结果表明: 在西北太平洋准双周尺度上, 对流与纬向风表现出沿热带地区向西偏北传播的特性。不同位相合成的季风槽位置和强度也发生相应的改变, 由此可见, QBWO是WNP上空季风槽季内变化的重要影响因子。当处于位相1、 4时, WNP生成TC的概率较低, 且登陆我国TC的数量也较少; 当处于位相2、 3时, WNP发生TC的概率较高, 特别是处于位相3时, 不仅TC发生概率最高, 而且登陆我国的TC数量也最多。沿热带地区西传的天气尺度波动 (周期10天以下) 在WNP通过季风槽的纬向风辐合作用, 易于转变为波数较大、 波长较短的热带低压 (TD) 型扰动, 这种扰动在季风槽区通过能量的转换有利于发展成为TC。     

夏季青藏高原大气热源准双周振荡及其传播途径

采用1979—2017年NCEP/NCAR逐日再分析资料估算大气热源,研究夏季青藏高原大气热源准双周振荡(Quasi-BiWeekly Oscillation,QBWO)的特征及传播途径。结果表明:青藏高原及其周边的大气热源QBWO的前两个主模态,即荷载中心在高原东南部的全区一致型和高原东南-西北反位相变化的偶极型,呈现了高原夏季大气热源QBWO自东向西传播过程中所处的两种不同状态。这主要是由于在中纬度地区对流层中上层,低频大气环流的活动表现为大的异常气旋和反气旋环流从我国东北经青藏高原至西亚的自东向西的传播,当移近高原时迅速增强,当西移离开高原时明显减弱。在此过程中,青藏高原及其周边、孟加拉湾以及印度半岛等地区的降水都发生了异常变化。     

多要素表征的东亚季风区准双周振荡特征

为评估不同要素对东亚季风区准双周振荡的表征能力,对大气向外长波辐射（OLR）、500 hPa位势涡度、850 hPa相对涡度、850 hPa风场和750 hPa比湿等要素的准双周振荡特征进行对比,发现各要素均能很好反映东亚季风区明显的准双周振荡时空特征。OLR及500 hPa位势涡度、850 hPa相对涡度、850 hPa纬向风表征的准双周振荡呈明显的西北向传播特征,500 hPa位势涡度、850 hPa相对涡度、850 hPa纬向风北传更强,北传速度更快。850 hPa经向风的准双周振荡呈明显西移特征,北传弱,北传速度最慢。而750 hPa比湿准双周振荡呈东南向传播。不同要素准双周振荡的强度略有差异,其中750 hPa比湿与其他要素的差异大。总体而言,750 hPa比湿不能较好地表现出东亚季风区准双周振荡活动特征,而其余要素能很好地表征东亚季风区大气准双周振荡,其中500 hPa位势涡度和850 hPa相对涡度准双周振荡特征一致性高。     

热带大气准双周振荡的数值试验研究

本文利用一个地表温度由热平衡方程所控制的P—σ混合坐标五层原始方程球带模式,不仅模拟出了在SST暖异常地区扰动的形成,且模拟出了扰动及与其相对应的降水和SST的准双周振荡现象,SST的振荡位相较降水超前1/3周期,研究认为这种振荡是由云——辐射相互作用所造成的。初始SST暖异常在中西太平洋比在东太平洋时引起的振荡明显,因此在扰动东移过程中振荡是衰减的。     

热带大气准双周振荡对西北太平洋地区热带气旋路径的影响

利用美国国家海洋和大气管理局（National Oceanic and Atmospheric Administration,简称NO-An）的逐日对外长波辐射（outgoing longwave radiation,简称OLR）场资料,欧洲中期天气预报中心（European Center for Medium—Range Weather Forecasting,简称ECMWF）逐日风场（850hPa）资料,以及美国联合台风预警中心（Joint Typhoon Warning Center,简称JTWC）的热带气旋（tropicalcy—clone,简称TC）数据,参考Wheeler and Hendon（2004）提出的季节内振荡（Madden—Julianoscilla．tion,简称MJO）指数,通过多元EOF方法定义热带准双周振荡（quasi—biweekly oscillation,简称QBW）指数,诊断分析了西北太平洋地区QBW不同位相对于TC路径的影响。结果表明,TC主要生成在QSW对流湿位相中,集中位置随QBW向西北的传播而向西北移动。在QSW位相phasel中,南海上空盛行QBW反气旋性环流,西太副高西伸,其西南侧偏东南气流受QBW反气旋性环流东北侧气流抑制,生成在副高南侧的TC首先在副高南侧偏东气流的引导下移动至近海,在西南季风以及副高西侧偏南气流作用下顺时针北折,因此在140°E以西转折类路径的TC比例最高;而在phase3中,西太副高偏东,南海上空盛行QBW气旋性环流,西太副高西南侧气流强度受QBW气旋东北侧气流影响增强,季风槽偏东,140°E以东转折类的TC比例最高。本文还对TC个例中的QBW流场形势进行了分析,发现当QBW气旋或反气旋环流中心同TC中心一致时,热带气旋路径会发生突然的右折。     

提取准双周和准一月低频振荡的Lanczos滤波器及其应用

根据青藏高原大气热源季内振荡分析的实际需要,设计了适于提取准双周（10～20d）、准一月（20～40d）振荡的Lanczos滤波器（L.f.）。通过与Butterworth滤波器（B.f.）滤波效果的定量分析,确定了准双周、准一月L.f.窗宽参数l的临界值l0分别为24、46;当l≥l0时,L.f.滤波器性能全面优于B.f.。选用拉萨附近格点（90°E,30°N）的1950—2006年整层大气热源资料做了应用试验,结果表明,由l=121的L.f.得到的拉萨夏季准双周、准一月振荡分量质量可靠,可用于青藏高原大气热源季内振荡强度的年际差异和季内过程分析。     

江淮梅雨期降水南北反位相分布与大气准双周振荡

利用1954—2005年中国740站逐日降水资料和NCEP/NCAR再分析资料,分析江淮梅雨期降水的南北反位相分布(Anti-Phase Distribution:APD)和大气准双周振荡(Qusi-Biweekly Oscillation:QBWO)之间的关系,诊断结果表明:(1)1954—2005年江淮梅雨APD共有16年较为显著。从1990年代开始,梅雨APD显著增强,并且表现出显著的2年和4～6年振荡周期。在降水较多的区域,降水的准双周振荡往往也较强。(2)梅雨期降水APD和中国东部地区降水的南北变动同属一个位相,中国东部地区的南涝北旱或者南旱北涝在很大程度上可以由江淮地区降水的分布类型来说明。(3)"南旱北涝"年,准双周滤波的整层水汽通量能够传播到30°N以北,同时存在强烈的水汽通量辐合从中高纬度向南传播到江淮流域。而在"南涝北旱"年,准双周的水汽输送所能到达的纬度明显偏南,来自中高纬度向南传播的水汽通量辐合也不显著。(4)"南旱北涝"年降水正位相,西北太平洋副热带高压(简称西太副高)脊线位于22°N以北,850 hPa低频反气旋的位置相对于"南涝北旱"年偏北,调节西太副高进入南海位置也偏北。南海和江淮上空的准双周垂直速度异常位置整体偏北,第3位相的上升运动和第7位相的下沉运动都位于30°N以北,南海的垂直速度异常也主要位于南海北部,而在"南涝北旱"年,准双周垂直速度异常的分布偏南。     

Modulation of tropical cyclogenesis in the western North Pacific by the quasi-biweekly oscillation

The quasi-biweekly oscillation(QBWO) is the second most dominant intraseasonal mode over the western North Pacific(WNP) during boreal summer. In this study, the modulation of WNP tropical cyclogenesis(TCG) by the QBWO and its association with large-scale patterns are investigated. A strong modulation of WNP TCG events by the QBWO is found.More TCG events occur during the QBWO's convectively active phase. Based on the genesis potential index(GPI), we further evaluate the role of environmental factors in affecting WNP TCG. The positive GPI anomalies associated with the QBWO correspond well with TCG counts and locations. A large positive GPI anomaly is spatially correlated with WNP TCG events during a life cycle of the QBWO. The low-level relative vorticity and mid-level relative humidity appear to be two dominant contributors to the QBWO-composited GPI anomalies during the QBWO's active phase, followed by the nonlinear and potential intensity terms. These positive contributions to the GPI anomalies are partly offset by the negative contribution from the vertical wind shear. During the QBWO's inactive phase, the mid-level relative humidity appears to be the largest contributor, while weak contributions are also made by the nonlinear and low-level relative vorticity terms.Meanwhile, these positive contributions are partly cancelled out by the negative contribution from the potential intensity.The contributions of these environmental factors to the GPI anomalies associated with the QBWO are similar in all five flow patterns—the monsoon shear line, monsoon confluence region, monsoon gyre, easterly wave, and Rossby wave energy dispersion associated with a preexisting TC. Further analyses show that the QBWO strongly modulates the synoptic-scale wave trains(SSWs) over the WNP, with larger amplitude SSWs during the QBWO's active phase. This implies a possible enhanced(weakened) relationship between TCG and SSWs during the active(inactive) phase. This study improves our understanding of the modulation of WNP TCG by the QBWO and thus helps with efforts to improve the intraseasonal prediction of WNP TCG.     

梅雨准双周振荡的年际变化及其前期强信号分析

利用1954—2005年中国740站逐日降水资料和NCEP/NCAR再分析资料,分析了江淮梅雨准双周振荡的年际变化,并讨论了异常年的海气背景特征。将滤波后的每个降水周期划分为8个位相,其中第3位相对应准双周降水正距平最大,而第7位相对应准双周降水负距平最大,在此基础上的诊断结果表明：1）振荡强弱偏差场,在梅雨降水偏多的第3位相,低层大气中类似Rossby波列从南海经日本海、鄂霍次克海和阿留申群岛延伸至阿拉斯加地区,其中南海上空的反气旋和日本上空的气旋相互配合,造成冷暖空气在江淮地区交汇,有利于准双周降水正距平的产生。在降水偏少的第7位相,波列位相基本反向,传播路径偏南。2）第3位相时高层环流显示,鄂霍次克海地区高度场正异常明显,和我国东北地区的负距平共同作用为梅雨降水提供了足够的干冷空气。第7位相分布相反。3）前期3月至同期的黑潮及其延伸区海表温度和梅雨的准双周振荡强强度之间呈很好的正相关关系。黑潮及其延伸区持续的正海温异常,通过影响西太平洋副热带高压的位置和强度的变化,进而增强梅雨降水的准双周振荡     

越赤道气流准双周振荡对西北太平洋台风路径的调制作用

利用美国联合台风预警中心（Joint Typhoon Warning Center,JTWC）热带气旋（tropicalcy—clone,TC）数据、NCEP／NCAR再分析风场资料,研究了越赤道气流准双周振荡对西北太平洋台风路径的调制作用。将西北太平洋台风路径划分为：西行路径、西北行路径、转向登陆中国路径、转向中日之间路径、转向登陆日本路径、转向日本以东路径和140°E以东路径。利用超前滞后回归方法,合成分析了6—10月不同路径台风对应的越赤道气流准双周振荡的低频环流演变过程。结果表明,925hPa越赤道气流及与其相联系的经向风存在明显10～20d准双周振荡现象,且对西北太平洋台风路径预报具有一定的指示作用。在西太平洋赤道地区,低频越赤道气流强度、演变特征影响着西北太平洋低频气旋的位置和移动方向,调节风场强辐合带与季风槽的位置与强度,继而对台风生成位置、移动路径产生重要的影响。初步认为,强向北低频越赤道气流分量有利于北侧低频气旋加强和向北传播,继而使得强辐合带、季风槽位置偏北,台风易于在此区域生成且沿着强辐合带位置移动。而弱向北低频分量或向南低频分量则不利于台风转向移动。     

Moisture structure of the quasi-biweekly mode revealed by AIRS in western Pacific

Using Atmospheric Infrared Sounder (AIRS) humidity profiles, rainfall from the Tropical Rainfall Mea- suring Mission (TRMM) Global Precipitation Index (GPI), Quick Scatterometer (QSCAT) satellite-observed surface winds, and SST from the Advanced Microwave Scanning Radiometer for NASA's Earth Observing System (AMSR E), we analyzed the structure of the summer quasi-biweekly mode (QBM) over the western Pacific in 2003-2004. We find that the signal of 10-20-day oscillations in the western Pacific originates fro...     

Recent Progress in the Impact of the Tibetan Plateau on Climate in China

Studies of the impacts of the Tibetan Plateau （TP） on climate in China in the last four years are reviewed. It is reported that temperature and precipitation over the TP have increased during recent decades. From satellite data analysis, it is demonstrated that most of the precipitation over the TP is from deep convection clouds. Moreover, the huge TP mechanical forcing and extraordinary elevated thermal forcing impose remarkable impacts upon local circulation and global climate. In winter and spring, stream flow is deflected by a large obstacle and appears as an asymmetric dipole, making East Asia much colder than mid Asia in winter and forming persistent rainfall in late winter and early spring over South China. In late spring, TP heating contributes to the establishment and intensification of the South Asian high and the abrupt seasonal transition of the surrounding circulations. In summer, TP heating in conjunction with the TP air pump cause the deviating stream field to resemble a cyclonic spiral, converging towards and rising over the TP. Therefore, the prominent Asian monsoon climate over East Asia and the dry climate over mid Asia in summer are forced by both TP local forcing and Eurasian continental forcing.
Due to the longer memory of snow and soil moisture, the TP thermal status both in summer and in late winter and spring can influence the variation of Eastern Asian summer rainfall. A combined index using both snow cover over the TP and the ENSO index in winter shows a better seasonal forecast.
On the other hand, strong sensible heating over the Tibetan Plateau in spring contributes significantly to anchor the earliest Asian monsoon being over the eastern Bay of Bengal （BOB） and the western Indochina peninsula. Qualitative prediction of the BOB monsoon onset was attempted by using the sign of meridional temperature gradient in March in the upper troposphere, or at 400 hPa over the TP. It is also demonstrated by a numerical experiment and theoretical study that the heating over the TP lea     

中国热带大气季节内振荡研究进展

热带大气季节内振荡(包括MJO)是大气环流的重要系统,它的活动及异常既对其他系统有一定的作用,也对长期天气和短期气候有明显影响。因此,热带大气季节内振荡一直是大气科学的前沿研究课题之一。文中对近5—10年中国学者的有关研究工作及其进展做了简要回顾和综合,主要包括:(1)热带大气季节内振荡特别是MJO的动力学机制;(2)热带大气季节内振荡以及MJO的数值模拟问题,特别是大气非绝热加热廓线对模式模拟MJO的重要作用;(3)热带大气季节内振荡和MJO,特别是在赤道西太平洋地区,与ENSO的相互作用关系;(4)热带大气季节内振荡(包括MJO)及其流场形势对西太平洋台风活动的重要影响,即MJO对西北太平洋台风生成数的调制作用,以及热带大气季节内低频气旋性(LFC)和反气旋性(LFAC)流场对西太平洋台风路径的影响;(5)热带大气季节内振荡(包括MJO)的活动及异常对东亚和南亚夏季风建立、活动异常的影响,以及它们与中国降水异常的密切关系。     

大气非绝热加热作用的研究进展与展望

大气非绝热加热与天气系统的发生发展有密切联系,与降水等天气过程密不可分,非绝热加热在大气运动中有着至关重要的作用。对非绝热加热的研究和理解,有助于改进数值预报模式,增强数值天气预报模式的预报能力。本文系统梳理了大气非绝热加热的基本内容,近几十年非绝热加热及其作用的研究成果,主要包括非绝热加热的概念及其表征、非绝热加热的时空分布特征、非绝热加热与季风、天气系统(如西太平洋副热带高压、热带气旋、温带气旋和急流)和降水之间的关系,以及非绝热加热在数值模式中的表征,进而指出有待于进一步研究的方面。     

CMIP6模式大气中南北半球际涛动的季节循环

利用1958—2014年47个CMIP6模式输出资料和NCEP/NCAR再分析资料,研究了模式大气中南北涛动(InterHemispheric Oscillation,IHO)的季节变化特征,且评估了CMIP6对IHO季节特征的模拟能力。结果表明:47个CMIP6模式都能模拟出IHO的季节演变特征,但模式间存在一定差异。通过比较,筛选出模拟IHO季节循环较好的16个模式,它们能成功模拟出半球大气质量的时间演变和空间结构。进一步分析表明,水汽对IHO季节变化有抵消作用且半球内部水汽质量变化可驱动越赤道质量流的产生;地表净短波辐射夏高冬低,其加热造成的水汽蒸发在水汽质量变化中起到重要作用;地表净长波辐射在春秋变化幅度较大,与大气质量逐月变化吻合。对比再分析资料表明,CMIP6模式模拟的半球大气质量的峰谷值变化有明显的月份偏差,且CMIP6模式模拟的地表气压异常值的偏差主要出现在北太平洋、欧亚大陆、南半球中纬度地区和两极极区,模拟的南北半球的蒸发和降水量、赤道风场、地表净长波和短波辐射通量等均存在明显的偏差。     

Modulation of the Intensity of Nascent Tibetan Plateau Vortices by Atmospheric Quasi-Biweekly Oscillation

The modulation of the intensity of nascent Tibetan Plateau vortices(ITPV) by atmospheric quasi-biweekly oscillation(QBWO) is investigated based on final operational global analysis data from the National Centers for Environmental Prediction. The spatial and temporal distributions of the ITPV show distinct features of 10–20-day QBWO. The average ITPV is much higher in the positive phases than in the negative phases, and the number of strong TPVs is much larger in the former,with a peak that appears in phase 3. In addition, the maximum centers of the ITPV stretch eastward in the positive phases,indicating periodic variations in the locations where strong TPVs are generated. The large-scale circulations and related thermodynamic fields are discussed to investigate the mechanism by which the 10–20-day QBWO modulates the ITPV. The atmospheric circulations and heating fields of the 10–20-day QBWO have a major impact on the ITPV. In the positive QBWO phases, the anomalous convergence at 500 hPa and divergence at 200 hPa are conducive to ascending motion. In addition, the convergence centers of the water vapor and the atmospheric unstable stratification are found in the positive QBWO phases and move eastward. Correspondingly, condensational latent heat is released and shifts eastward with the heating centers located at 400 hPa, which favors a higher ITPV by depressing the isobaric surface at 500 hPa. All of the dynamic and thermodynamic conditions in the positive QBWO phases are conducive to the generation of stronger TPVs and their eastward expansion.     

The Dynamical Influence of Land－Sea Contrast and Sea Surface Temperature on Intraseasonal Oscillation in Tropical Atmosphere

An equatorial β-plane model which includes realistic non-uniform land-sea contrast and the underlying surface temperature distribution is used to simulate the 30-60 day oscillation (LFO) processes in tropical atmosphere, with emphasis on its longitude-dependent evolution and convective seesaw between Indian and the western Pacific oceans.The model simulated the twice-amplification of the disturbances over Indian and the western Pacific oceans while they are travelling eastward. It reproduced the dipole structure caused by the out-of-phase oscillation of the active centres in these two areas and the periodical transition between the phases of LFO. It is suggested that the convective seesaw is the result of interaction of the internal dynamics of tropical atmosphere with the zonally non-uniform thermal forcing from underlying surface. The convective activities are suppressed over Indonesia mari-time continents whilst they are favoured over the Indian Ocean and western Pacific warm waters, so there formed two active oscillation centres. The feedback of convection with large-scale flow slows down the propagation of disturb-ances when they are intensifying over these two areas, therefore they manifest a kind of quasi-stationary component to favor the ‘dipole’ structure. Whereas the disturbances weaken and speed up over the eastern Pacific cold water re-gion due to the interaction of sensible heating and evaporation with perturbational wind. Therefore the two major centers just show out-of-phase oscillation during onecycle around the latitudinal beltBy introducing the SST anomalies in El Ni?o and La Ni?a years into the surface temperature, we also show that they have significant influence on LFO processes. In an anomalously warm year, the LFO disturbances dissipate more slowly over the central-eastern Pacific region and can travel farther eastward; whilst in an anomalously cold year, the opposite is true.