【作者】 金荣花；

【导师】 李维京；

【作者基本信息】 中国气象科学研究院 ， 气象学， 2012， 博士

【摘要】 为探讨东亚副热带西风急流对长江中下游梅雨的中期预报意义，本文采用1960～2009年50年NCEP/NCAR再分析日平均资料、长江中下游梅雨数据以及中国714站20～20时日累计雨量观测资料，系统分析了梅雨期东亚副热带西风急流垂直结构和水平流型的气候态特征，构建了针对梅雨季节东亚副热带西风急流客观定量表征方法和长序列东亚副热带西风急流特征指数日资料历史数据集，以日、候时间尺度资料统计了东亚副热带西风急流经向活动、强度变化、中心纬向突变以及形态特征与梅雨的关系，探讨了东亚副热带西风急流影响梅雨降水异常和相关大型环流系统活动的机理，研究了东亚副热带西风急流斜压波波包传播特征。结果表明：（1）梅雨期东亚副热带西风急流的气候态特征分析表明，梅雨期东亚副热带急流空间分布特征不同于整个夏季背景场，表现出位置偏南、强度偏强、覆盖广、变率大等特征。梅雨期东亚副热带急流处于冬季型向夏季型转换的过渡阶段，急流经向活动、中心纬向突变以及强度瞬变涡动等都具有其独特的中期演变特征。急流在季节性持续向北移动过程中出现两次位置稳定时段和三次季节性北跳，两次位置稳定阶段分别对应华南前汛期和长江中下游梅雨雨季，急流第二和第三次季节性北跳分别对应梅雨的开始和结束，急流第二次北跳时间为6月7日，比梅雨入梅日提前10天，对于梅雨开始具有先兆性指示意义；急流第三次北跳和西太平洋上空中心消失与梅雨结束有一定的关联。（2）利用1960～2009年梅雨季节东亚副热带西风急流位置、强度和中心逐日资料，讨论了东亚副热带西风急流中期变化与梅雨的关系。结果发现，急流位置与雨带位置呈正相关，与梅雨强度呈反相关，即位置指数偏北（南），雨带位置偏北（南），梅雨强度偏弱（强）。急流强度与梅雨强度呈正相关，即急流强度偏强（弱），梅雨强度偏强（弱），但强度不及位置与梅雨的相关性好。丰梅年区域性暴雨主要出现在纬向型和东北-西南型形态的东亚副热带西风急流的右后方。东亚副热带西风急流中心位置西跳日与出梅日有很好的相关关系，大多数年份两者日期差小于5天。急流位置指数周期变化主要呈单周和双周特征，西风急流强度指数变化主要呈单周变化特征，说明在相对稳定的急流带上，有急流核以更高的变化频率东传。（3）异常丰梅年和空梅年东亚副热带西风急流中期变化特征差异显著。合成分析显示，丰梅年相对于空梅年西风急流强度偏强，急流带狭窄，质量与动量集中。从逐日变化情况来看，丰梅年，东亚副热带西风急流前北跳一般先于入梅日，后北跳与出梅几乎同步；入梅后，东亚副热带西风急流位置围绕气候态经向平稳摆动，关键区（110o～130oE，30o～37.5oN）纬向风强度偏强，最大中心主频次在125oE附近，靠近中国大陆并位于下风方。空梅年，东亚副热带西风急流一般不发生前北跳，一次性北跳至40oN以北地区，或者发生前北跳，但没有建立准稳定形势，急流位置偏北，经向移动幅度较大，急流强度总体偏弱，急流中心位置主要出现在日本岛及西太平洋上空，远离中国大陆。影响梅雨异常的物理机制分析表明，异常丰梅年，200hPa我国东部地区上空急流轴线、散度零线和散度距平零线在37.5oN重合，高空辐散中心区与辐散距平中心区在长江中下游地区上空亦重合，高空强辐散流出，对应低层有强辐合流入以及自低层到高层深厚的垂直上升运动，为梅雨提供了良好的动力环境场；高低空急流耦合作用，有利于低空西南风加强，为持续性降水提供了良好的水汽输送条件。高空副热带锋区和典型陡直梅雨锋区，有利于高空急流质量和动量的维持，也利于深对流发展。空梅年，情形相反。（4）异常丰梅年东亚副热带西风急流与副高、季风涌关系密切。200hPa东亚副热带西风急流强度偏强，经向移动相对稳定，主体偏西，则500hPa副高强度偏强，经向移动相对平稳，主体偏西，季风涌强度偏强；空梅年，情形相反。丰、空梅年东亚副热带西风急流位置与副高位置有很好的正相关关系，东亚副热带西风急流强度与季风涌纬向强度指数有很好的正相关关系，而且相关性丰梅年比空梅年好。影响副高、季风涌活动的机理研究表明，丰梅年，东亚副热带西风急流强度偏强，副高脊线以北至西风急流轴以南从低层到高层整层为负涡度距平，有利于副高强度偏强。副高脊线附近伴随的次级环流，即副热带季风环流圈（STMC），以及高层为辐合和下沉运动距平，非常有利于副高的强度偏强。副热带季风环流圈（STMC）通过低空西南风急流向北的经向运动在柯氏力作用下将增强其西风风速，而其东风急流的北侧向南的经向运动将增强其东风风速。由此增强了低空西南风急流和高空东风急流，并通过质量一动量调整有利于季风环流圈的维持。丰梅年，深对流降水引起高层凝结潜热释放，使得南北温度梯度进一步加大和维持，从而急流强度增强和维持，再通过动力作用促使副高强度增强，西伸脊点偏西，季风涌加强。空梅年，情形相反。（5）梅雨异常年东亚副热带西风急流斜压波波包传播特征分析表明，丰梅年200hPa传播的波包大值中心比较偏西偏南，空梅年传播的波包大值中心相对比较偏东偏北，丰梅年斜压波波包大值带弱于空梅年，波包传播群速度大于相速度，急流波动具有明显的下游频散效应。丰、空梅年在北半球的0o～180oE范围内，三条斜压扰动波波包大值带的强度变化和不同配置显示了丰、空梅年的差异，也可用来分析梅雨前、梅雨期和梅雨后的阶段性特征。即当波包大值带位于40o～50oN之间，且130oE以西的波包大值带增强，并85o～125oE高空急流出口区波包大值中心偏南至38o～42oN附近，同时西太平洋波包大值带ē减弱,长江流域就进入了梅雨期；当波包大值带尤其是高空急流出口区波包大值带偏北偏弱，而孟加拉湾波包大值带ē显著增强时，长江流域梅雨结束。三条波包大值带的不同配置和维系可能反映了副热带高空急流、季风环流系统的维系的过程。更多还原

【Abstract】 By using a dataset during1960～2009including the reanalysis daily data provided by NCEP/NCAR,Meiyu data in the lower-middle reaches of the Yangtze River and24h (from20:00to the next20:00Beijing time) total precipitation data collected from714fiducial stations in China, this paper aims toinvestigate impact of the east Asian subtropical westerly jet (EASWJ)to middle-range forecasting of Meiyuin the lower-middle reaches of the Yangtze River. The vertical structure and spatial pattern of the EASWJare analyzed systematically in its climatic state. Objective and quantificational methods have also beenfound to characterize the EASWJ and as a result long time series EASWJ dataset has been built. Besides,influence mechanism of the EASWJ on abnormal Meiyu and activity of corresponding large scalecirculation systems is discussed, by analyzing relationship between Meiyu and meridional activities,intensity variation, zonal abrupt change of center and morphological characteristics of the EASWJ withdaily and Pentad data. At last, the EASWJ is studied in an aspect of its propagation characteristics ofbaroclinic wave energy. The major results are as follows:(1) Study on climatic character of the EASWJ shows that its space distribution during Meiyu period isdifferent from mean character in the whole summer, featuring with its southward location, strongerintensity, wide coverage and big variable rate. The Meiyu period is an important stage for the EASWJtransiting from winter to summer, when the EASWJ has unique middle-range evolution character inaspects of its meridional activity, zonal abrupt change of jet center and transient eddy of jet intensity.Generally, there are two stable stages and three northward shift stages of jet position when the EASWJseasonally moves to the north continuously. The two stable stages of the EASWJ are corresponding to thepre-flood season in south China and Meiyu over the lower-middle reaches of the Yangtze River valleyrespectively. The second and third seasonal north shifts of the EASWJ are corresponding to beginning andending of the Meiyu respectively. The second north jump of the EASWJ on June7th is10days ahead ofMeiyu beginning day, which has foreboding sense to forecast the beginning of Meiyu; the third jet northjump and jet center disappearing above the west Pacific are related to the ending of Meiyu. (2) Using daily data of position, intensity and center of the EASWJ during Meiyu period from1960to2009, relationship between middle-range variation of the EASWJ and Meiyu is discussed. The resultshows daily evolution of the EASWJ during Meiyu period indicates a negative relationship between jetposition and Meiyu intensity, that is to say, Meiyu intensity is weaker (stronger) when index of jet positionis northward (southward). Jet intensity is positively related to Meiyu intensity, namely, Meiyu intensity isstronger (weaker) when jet intensity is stronger (weaker), but relation of jet intensity to Meiyu is not asclose as that to jet position. Regional rainstorm in heavy Meiyu years mainly appears right and behind theEASWJ with zonal and northeast-southwest pattern. West jump day of the EASWJ is closely related to theend date of Meiyu, difference between west jump day of the EASWJ and the end date of Meiyu is less than5days in most years. Position index of the EASWJ varies periodically in a timescale of one week ordouble week. Intensity index of the EASWJ varies periodically in one week. Period of jet position andintensity indicates that there are jet cores propagating to the east with higher frequency in the relativelystable jet band.(3) Middle-range variation of the EASWJ is remarkablly different in abnormal heavy Meiyu years fromthat in light Meiyu years. The composite analysis shows that the jet intensity is stronger, jet band is morenarrow and mass and momentum of the EASWJ are more centralized in heavy Meiyu years than that inlight Meiyu years. From perspective in daily evolution of the EASWJ, in heavy Meiyu years, the secondnorth jump day of the EASWJ is usually earlier than the beginning date of Meriyu, the third EASWJ northjump day and the ending date of Meiyu is almost synchronous; after the beginning day of Meiyu, theEASWJ swings stably around its climatic longitude, zonal wind is stronger in key region（110o～130oE，30o～37.5oN）, the jet centeris frequently around125oE which is near and in leeward side of Chinesemainland. In light Meiyu years, the EASWJ usually doesn’t happen the second north jump and moves tonorth of40oN directly; jet position is northward and moves with bigger amplitude along longitude, jetintensity is weaker, jet center is mainly present over the Japanese islands and west Pacific which is faraway from Chinese mainland.The physical mechanism of influence of the EASWJ on abnormal Meiyu shows that, in heavy Meiyu years,200hPa jet axis, zero line of divergence and zero line of divergence departure overlap at37.5oN over eastChina, divergence center in upper level also coincides with center of divergence departure over thelower-middle reaches of the Yangtze River. Strong divergent flow in upper levels cooperating with strongconvergent flow in lower levels and deep vertical ascending movement from ground level to upper levelscan provide favorable dynamic condition for Meiyu to happen; Coupling between upper and lower jetstreaks is in favor for lower southwesterly to strengthen, as a result southwest water vapor flux departureexists in the south of lower-middle reaches of the Yangtze River, which could provide favorable warmvapor transport condition for durative rainfall. Upper frontal zone and typical steep Meiyu frontal zone are helpful for deep convection to develop by favoring mass and momentum of upper jet to maintain. In lightMeiyu years, situation is opposite.(4) In abnormal heavy Meiyu years, the EASWJ is closely related to west pacific subtropical high andmonsoon surge. If the EASWJ at200hPa is characterized with stronger intensity, comparably stablemeridional movement and westward main part, the west pacific subtropical high shows the same characterat500hPa and the monsoon surge is stronger. In light Meiyu years, situation is opposite. Position of theEASWJ is closely related to position of the west pacific subtropical high, intensity of the EASWJ isclosely related to zonal intensity index of monsoon surge in both heavy and light Meiyu years, andhowever, such relationship is closer in heavy Meiyu years than that in light Meiyu years.Study in influence mechanism on activity of the west pacific subtropical high and monsoon surge showsthat, in heavy Meiyu years, intensity of the EASW is stronger, negative vorticity departure exists in thewhole levels from bottom level to the upper levels expanding from north of ridge line of subtropical highto south of westerly jet axis, which is helpful for subtropical high to be stronger. Subtropical monsooncirculation (STMC) which is accompanying around ridge line of subtropical high and upper convergentdeparture are both helpful for subtropical high to be stronger. Ascending part of the STMC is in south ofthe EASWJ and north of southwest jet in lower level, while submerging part of the STMC is in north oftropical easterly. In that way, the STMC enhances westerly speed of lower southwesterly jet by Coriolisforce though its meridional movement to the north, easterly speed has also been enhanced thoughmeridional movement from north of easterly jet to the south. As a result, westerly jet in lower level andeasterly in upper level are enhanced, monsoon circulation is holden by means of mass-momentumadjustment. In heavy Meiyu years, latent heat of condensation releasing from deep convective precipitationleads to increasing and maintaining of north-south temperature gradient, thus jet intensity becomesstronger and keeps maintaining, this effect coordinating dynamic effect make intensity of subtropical highto be strengthened and west ridge point of subtropical high to be westward, so that monsoon surge can beenhanced also. In light Meiyu years, situation is opposite.(5) Analysis on propagating character of baroclinic wave packets of the EASW in abnormal Meiyu yearsshows that center of wave packets with great values is inclined to the westward and southward at200hPain heavy Meiyu years. However, center of wave packets with great values is inclined to the eastward andnorthward in light Meiyu years. Band of baroclinic wave packets with great values is weaker in heavyMeiyu years than that in light Meiyu years. In heavy Meiyu years, propagating speed of wave packets isbigger than phase velocity, indicating jet wave has development effect to downstream area. There are threedistinct great value zones of baroclinic disturbance wave packet persisting a period of2~7d. Intensityvariation and different collocation of the center of these three wave packets with great values couldrepresent the difference in heavy and light Meiyu years. This result could also be used to investigate stage characteristics of precipitation before Meiyu, during Meiyu and after Meiyu. That is, the Meiyu seasonbegins in the Yangtze River basin when band of wave packets with great values is located between40oNand50oN, band of wave packets with great values west of130oE enhances, center of wave packets withgreat values in exit region of85o～125oE moves to the south around38o～42oN, contemporary with bandē of wave packets with great values weakens in west Pacific. The Meiyu season ends in the YangtzeRiver basin when band of wave packets with great values especially band of wave packets with greatvalues in exit region is inclined to the northward and be weaker, band ē of wave packets with greatvalues enhances observably in the Bay of Bengal. Different collocation and maintenance of these threewave packets zone with great values may reflect the course of maintenance between subtropicalupper-level jet and monsoon circulation.更多还原