2006年川渝地区夏季干旱的成因分析

利用NCEP/NCAR再分析月平均资料、全国160站降水资料、向外长波辐射OLR（outgoinglongwave radiation）资料和所计算的热源资料,分析了2006年夏季东亚大气环流的异常特征,并研究了热力异常与川渝地区夏季降水的关系。结果表明,2006年夏季由南向北的水汽输送较常年偏弱;西太洋副热带高压较常年异常偏强,脊线位置明显偏北,川渝地区受高压系统影响盛行下沉气流,中高纬环流场则表现为乌拉尔山地区和东北亚区域无明显阻塞高压形势,冷空气活动比常年弱;南亚高压比常年偏北偏强,持续控制川渝地区;2006年夏季青藏高原热源偏弱,热带西太平洋暖池区热源偏强,是引起西太平洋副热带高压偏北偏强的重要原因之一。川渝地区夏季降水与西太平洋副热带高压的异常变化有密切关系,川渝地区夏季干旱年,西太平洋副热带高压偏北,并且引起西太平洋副热带高压偏北的原因与2006年类似。     

2013年夏季浙江省高温干旱环流异常分析

利用浙江省10个站的温度和降水资料、国家气候中心160站的降水资料、NOAA的ERSST和OLR资料及NCEP再分析资料,对2013年7至8月发生在浙江省的持续高温干旱天气过程的基本特征和环流异常进行了分析。结果表明:浙江省各站的高温日数、高温持续日数、持续无降水日数、干旱指数都远超历史同期纪录,降水较常年偏少3成以上。南亚高压强度偏强,面积偏大,东侧脊点位置偏东;西太平洋副高强度偏强,西侧脊点位置偏西;东亚季风异常偏强及其造成的水汽辐合偏弱,是高温干旱天气产生的重要原因。其中7月底西太平洋副高的异常西伸及8月异常持续偏西对浙江8月持续高温干旱的发生起了决定性作用,这与同期热带西太平洋暖池区增暖、菲律宾周围对流活动加强有关,后者通过EAP/PJ遥相关型对前者产生影响。     

An Investigation of the Formation of the Heat Wave in Southern China in Summer 2013 and the Relevant Abnormal Subtropical High Activities

In the summer of 2013, an unprecedented heat wave was experienced over a vast area of southern China. The great areal extent, duration, and strength of this high temperature are very rare. For the 2013 hot spell, the major and direct influence mostly came from the anomaly of the western Pacific subtropical high （WPSH）. The abnormally strong and stable WPSH was associated with specific surrounding circulations. The eastward extension of a stronger Qinghai-Xizang high favored the westward extension of the WPSH. The weaker cold air activity from the polar region led to the northward shift of the WPSH and helped it to remain stable. In the tropics, the western segment of the ITCZ was abnormally strong in the period, and supported the maintenance of the WPSH from the south. In addition, the interdecadal variation of the WPSH provided a decadal background for the anomaly variation of the WPSH that summer.     

夏季东亚西风急流扰动异常与副热带高压关系研究

利用1979—2003年NCEP/NCAR月平均再分析资料, 探讨夏季 (6—8月) 200 hPa东亚西风急流扰动异常与南亚高压和西太平洋副热带高压的关系。研究指出:夏季200 hPa东亚西风急流扰动动能加强 (减弱), 东亚西风急流位置偏南 (偏北)、强度偏强 (偏弱); 东亚西风急流扰动动能强弱不仅与北半球西风急流强弱和沿急流的定常扰动有关, 而且还与东亚地区高、中、低纬南北向的扰动波列有关, 亚洲地区是北半球中纬度环球带状波列异常最大的区域。夏季200 hPa东亚西风急流扰动动能加强 (减弱), 南亚高压的特征为位置偏东 (偏西)、强度加强 (减弱); 西太平洋副热带高压的特征为位置偏南 (偏北)。东亚环流特别是500 hPa西太平洋副热带高压对东亚西风带扰动异常的响应由高空东亚西风急流南侧的散度场及其对流层中下层热带和副热带地区的垂直速度距平场变化完成。     

天津春季气候特征及2013年春季影响因子分析

利用1974-2012年天津12个气象站的降水、气温资料及NCEP／NCAR再分析资料等,采用经验函数正交分解（EOF）和相关分析等统计方法,对天津春季的气候特征及主要影响因子进行分析。结果表明：1）天津春季降水空间分布主要有全市一致型和南北反位相型2种分布型,其总方差贡献接近90％,2013年春季为全市一致型。2）东亚冬季风、北极涛动（AO）和西太平洋副高是天津春季气候的主要影响因子。3）2013年,东亚冬季风偏强,AO冬、春持续负位相,春季西太平洋副高偏弱、偏东,均指示天津2013年春季降水偏少、气温偏低的可能性大。     

THE POSITION VARIATION OF THE WEST PACIFIC SUBTROPICAL HIGH AND ITS POSSIBLE MECHANISM

Using NCEP/NCAR daily reanalysis data and SCSMEX data, an investigation is carried out of the relationship between the position variation of the west Pacific subtropical high (WPSH) and the apparent heating in June 1998 based on the complete vertical vorticity equation. It is found that the non-adiabatic heating plays an important role in the position variation of WPSH. In comparison with climatic mean status, the vertical change of non-adiabatic heating is stronger in the north side of WPSH in June 1998, but weaker in the south side of WPSH. The anomalous non-uniform heating induces anomalous cyclonic vorticity in South China, areas to the south of the Yangtze and its mid-lower valleys, but anomalous anticyclonic vorticity in the Indo-China Peninsula and South China Sea areas lead to the more southward position of WPSH than the mean.     

Anomalous Western Pacific Subtropical High during Late Summer in Weak La Nia Years: Contrast between 1981 and 2013

Both 1981 and 2013 were weak La Niña years with a similar sea surface temperature (SST) anomaly in the tropical Pacific, yet the western Pacific subtropical high (WPSH) during August exhibited an opposite anomaly in the two years. A comparison indicates that, in the absence of a strong SST anomaly in the tropics, the cold advection from Eurasian high latitudes and the convection of the western Pacific warm pool play important roles in influencing the strength and position of the WPSH in August. In August 1981, the spatial pattern of 500 hPa geopotential height was characterized by a meridional circulation with a strong ridge in the Ural Mountains and a deep trough in Siberia, which provided favorable conditions for cold air invading into the lower latitudes. Accordingly, the geopotential height to the north of the WPSH was reduced by the cold advection anomaly from high latitudes, resulting in an eastward retreat of the WPSH. Moreover, an anomalous cyclonic circulation in the subtropical western Pacific, excited by enhanced warm pool convection, also contributed to the eastward retreat of the WPSH. By contrast, the influence from high latitudes was relatively weak in August 2013 due to a zonal circulation pattern over Eurasia, and the anomalous anticyclonic circulation induced by suppressed warm pool convection also facilitated the westward extension of the WPSH. Therefore, the combined effects of the high latitude and tropical circulations may contribute a persistent anomaly of the WPSH in late summer, despite the tropical SST anomaly being weak.     

Influence of the Northeast Cold Vortex on Flooding in Northeast China in Summer 2013

Severe flooding occurred in Northeast China (NEC) in summer 2013. Compared with the rainfall climatology of the region, the rainy season began earlier in 2013 and two main rainy periods occurred from late June to early July and from mid July to early August, respectively. During the summer season of 2013, the western Pacific subtropical high (WPSH) was located farther westward, which strengthened the southerly winds on its west side in the lower troposphere. Under this circulation pattern, more water vapor was transported to North China and NEC. Another moisture transport pathway to NEC was traced to the cross-equatorial flow over the Bay of Bengal. In mid–high latitudes in summer 2013, the Northeast Cold Vortex (NECV) was much stronger and remained stable over NEC. Thus, the cold air flow from its northwest side frequently met with the warm and wet air from the south to form stronger moisture convergence at lower levels in the troposphere, resulting in increased precipitation over the region. Correlation analysis indicated that the NECV played a more direct role than the WPSH. Synoptic analyses of the two heaviest flood cases on 2 and 16 July confirmed this conclusion. The four wettest summers in NEC before 2000 were also analyzed and the results were consistent with the conclusion that both the WPSH and the NECV led to the intense rainfall in NEC, but the NECV had a more direct role.     

2007年6—7月西太平洋副热带高压的异常特征及其成因分析

采用NCEP资料以及NOAA卫星观测的OLR场资料,分析了2007年6—7月淮河流域暴雨及江南、华南异常高温期间西太平洋副热带高压的异常特征,其表现为强度强且南北位置相对稳定,东西方向进退明显于南北方向的移动。通过分析经圈平面上的垂直环流及风矢量场的特征表明,副高南北两侧都存在一定强度的上升气流,北侧上升气流对西太平洋副高中心区700 hPa以上高度的下沉运动有激发作用,而南侧上升气流对西太平洋副高中心区的下沉运动作用不大。中高纬度阻塞高压双阻型的建立,有利于西太平洋副高的加强和发展。强劲而稳定少动的中纬度西风急流和热带地区异常活跃的对流均有利于西太平洋副高的相对稳定。     

季节预测模式对东亚夏季环流的预测能力及其对热带海洋的响应分析

东亚夏季环流变化对中国夏季降水的年际变化有重要影响,因此需要进一步理解季节预测模式对东亚夏季环流的预测能力。利用1991~2013年美国国家环境预测中心（NCEP）、中国气象局国家气候中心（NCC）和日本东京气候中心（TCC）的三个季节预测模式（CFS V2、BCC_CSM V2和MRI-CGCM）以及NCEP/NCAR再分析资料,定量评估了模式对东亚夏季风（EASM）和夏季西太平洋副热带高压（WPSH）强度的预测能力。在此基础上,分析了模式预测的EASM和WPSH对热带海温异常的响应能力,以及ENSO事件对EASM和WPSH预测的影响,阐述了预测误差产生的原因。结果表明:整体而言,三个模式对EASM和WPSH的预测技巧较高,但TCC模式对WPSH的预测技巧相对较低。三个模式预测的850 hPa风场在西北太平洋存在一个异常气旋,使得预测的EASM偏强和WPSH偏弱。同时,二者的年际变率整体比观测小。三个模式预测的EASM和WPSH对热带海洋海温异常的响应随季节演变特征与观测比较接近,但NCEP模式和TCC模式预测的EASM对前期热带太平洋和前期、同期热带印度洋的海温异常响应要强于观测,NCC模式预测的EASM对前期和同期的热带太平洋的海温异常响应明显比观测强。此外,三个模式预测的WPSH对前期和同期的热带太平洋、热带印度洋和热带大西洋的海温异常响应明显强于观测。三个模式预测的EASM和WPSH在ENSO年的平均绝对误差（MAE）整体而言要比正常年的小很多,NCEP模式和NCC模式预测的EASM和WPSH的MAE在La Ni?a年和El Ni?o年差别不大,而TCC模式预测的EASM和WPSH的MAE在El Ni?o年比在La Ni?a年大很多,表明ENSO事件是东亚夏季环流重要的可预报源。     

Anomalous Western Pacific Subtropical High during El Niño Developing Summer in Comparison with Decaying Summer

The anomalous behavior of the western Pacific subtropical high (WPSH) in El Niño developing summer is studied based on the composite results of eight major El Niño events during 1979-2013. It is shown that the WPSH tends to retreat eastwards with weak intensity during the developing summer. The anomaly exhibits an intraseasonal variation with a weaker anomaly in June and July and a stronger anomaly in August, indicating that different underlying physical mechanisms may be responsible for the anomalous WPSH during early and late summer periods. In June and July, owing to the cold advection anomaly characterized as a weak northerly anomaly from high latitudes, geopotential height in East Asia is reduced and the WPSH tends to retreat eastwards slightly. By contrast, enhanced convection over the warm pool in August makes the atmosphere more sensitive to El Niño forcing. Consequently, a cyclonic anomaly in the western Pacific is induced, which is consistent with the seasonal march of atmospheric circulation from July to August. Accordingly, geopotential height in the western Pacific is reduced significantly, and the WPSH tends to retreat eastwards remarkably in August. Different from the developing summer, geopotential height in the decaying summer over East Asia and the western Pacific tends to enhance and extend northwards from June to August consistently, reaching the maximum anomaly in August. Therefore, the seasonal march plays an important role in the WPSH anomaly for both the developing and decaying summer.     

2013年长江中下游地区夏季高温事件的环流特征及成因

采用ERA-interim和NCEP CFSR逐日再分析资料以及长江中下游29个测站的逐日温度资料,分析了2013年7月23日-8月14日长江中下游地区夏季异常高温的特点、环流特征及成因。研究结果表明,2013年夏季高温期间西太平洋副热带高压较往年异常偏强,西太平洋副热带高压控制区内大范围异常下沉运动产生的大气绝热加热是高温形成的主要原因。同时,与强大西太平洋副热带高压相联系的异常强大反气旋环流使得长江中下游地区上空的水汽向东北方向大量输出,导致了该区域水汽含量的减少,致使到达地面的太阳短波辐射增加,这是高温形成的又一原因。进一步分析表明,2013年夏季海洋性大陆地区大气热源异常偏强,该地区大气热源的异常增强可能是导致2013年夏季西太平洋副热带高压异常偏强的主要原因。     

新疆一次罕见的暖区暴雨过程特征分析

2018年7月31日新疆出现了一次罕见的暴雨过程,最大暴雨中心位于新疆东部哈密市,哈密市伊州区沁城乡小堡村过程累计雨量115.5 mm,连续2个时次小时雨强29.2 mm。对比哈密市建站以来6个国家气象站历年资料发现,此次暴雨特征与以往冷锋暴雨明显不同,暴雨降水时段更为集中,强度比冷锋暴雨更强,暴雨云团为典型的中尺度云团,范围小、呈块状。由于500 h Pa西太平洋副热带高压偏西偏北,高空无低值系统和冷空气入侵,哈密市主要受西太平洋副热带高压外围偏南暖湿气流的影响,此次暴雨结合了南方副热带高压型和偏南风风速辐合型暖区暴雨的典型特征,具有暖区暴雨的热力结构和风场配置,是一次罕见的暖区暴雨过程。暴雨区偏南风风速辐合和暴雨区山区地形的抬升效应是本次暴雨重要的触发机制,700 hPa孟加拉湾和日本海南部两支水汽在华北平原汇合沿东南输送路径至暴雨区且有强烈水汽辐合。暴雨发生在云顶亮温(TBB)等值线密集区梯度最大处,越接近TBB低值中心梯度处的暴雨强度越强。     

Physical Mechanism of Phased Variation of 2020 Extremely Heavy Meiyu in Middle and Lower Reaches of Yangtze River

The extremely heavy Meiyu in the middle and lower reaches of the Yangtze River in 2020 features early beginning, extremely late retreat, long duration, and a dramatic north-south swing rain belt. It can be divided into three phases. The key point of the extremely heavy Meiyu is the long duration of precipitation. The physical mechanism of the phased variation is researched here by analyzing the phased evolution of atmospheric circulation, the thermal effect of Tibetan Plateau, the sea surface temperature anomalies (SSTA), and tropical convection. The results show that: (1) Throughout the whole Meiyu season, the western Pacific subtropical high (WPSH) is stronger and westward, the South Asian high (SAH) is stronger and eastward, and blocking highs are very active with different patterns at different stages; they all form flat mid-latitude westerlies with fluctuation interacting with WPSH and SAH, causing their ridges and the rain belt to swing drastically from north to south or vice versa. (2) The higher temperatures in the upper and middle atmosphere in the eastern and southern Tibetan Plateau and the middle and lower reaches of the Yangtze River, which are produced by the warm advection transport, the heat sources in Tibetan Plateau, and the latent heat of condensation of Meiyu, contribute greatly to the stronger and westward WPSH and the stronger and eastward SAH. The dry-cold air brought by the fluctuating westerlies converges with the warm-humid air over Tibetan Plateau, resulting in precipitation, which in turn enhances the heat source of Tibetan Plateau and regulates the swings of WPSH and SAH. (3) Different from climatological analysis, real-time SSTA in the Indian Ocean has no obviously direct effect on WPSH and Meiyu. The anomalous distribution and phased evolution process of real-time SSTA in South China Sea and the tropical western Pacific affect WPSH and Meiyu significantly through tropical convection and heat sources. The maintenance of strong positive SSTA in the western equatorial Pacific is a critical reason for the prolonged Meiyu season. Both the onset and the retreat of Meiyu in 2020 are closely related to the intensified positive SSTA and corresponding typhoons on the ocean east of the Philippines.     

Changed Relationships Between the East Asian Summer Monsoon Circulations and the Summer Rainfall in Eastern China

In previous statistical forecast models, prediction of summer precipitation along the Yangtze River valley and in North China relies heavily on its close relationships with the western Pacific subtropical high (WPSH), the blocking high in higher latitudes, and the East Asian summer monsoon (EASM). These relationships were stable before the 1990s but have changed remarkably in the recent two decades. Before the 1990s, precipitation along the Yangtze River had a significant positive correlation with the intensity of the WPSH, but the correlation weakened rapidly after 1990, and the correlation between summer rainfall in North China and the WPSH also changed from weak negative to significantly positive. The changed relationships present a big challenge to the application of traditional statistical seasonal prediction models. Our study indicates that the change could be attributed to expansion of the WPSH after around 1990. Owing to global warming, increased sea surface temperatures in the western Pacific rendered the WPSH stronger and further westward. Under this condition, more moisture was transported from southern to northern China, leading to divergence and reduced (increased) rainfall over the Yangtze River (North China). On the other hand, when the WPSH was weaker, it stayed close to its climatological position (rather than more eastward), and the circulations showed an asymmetrical feature between the stronger and weaker WPSH cases owing to the decadal enhancement of the WPSH. Composite analysis reveals that the maximum difference in the moisture transport before and after 1990 appeared over the western Pacific. This asymmetric influence is possibly the reason why the previous relationships between monsoon circulations and summer rainfall have now changed.     

东亚夏季风的强度与前期西太平洋暖池热含量异常的关系

本文利用日本气象厅提供的历史海温资料、Hadley海温资料以及NCEP/NCAR再分析资料(1951~2010年)等探讨了东亚夏季风的强度与前期暖池热含量异常的关系。结果表明,西太平洋暖池热含量可以作为东亚夏季风强度的前期预测因子,两者正相关关系显著。本文选取相关系数更大、持续性更好的前期冬季暖池关键区(-5.5°~5.5°N;157.5°~170.5°E)热含量来进行预报。将暖池热含量指数和东亚夏季风指数均回归到夏季大气环流场上,发现在暖水年次年夏季西太副高偏弱、位置偏北,菲律宾以东以北洋面为气旋性环流,对流上升运动增强,赤道西太平洋地区为显著的西风距平,日本岛以东洋面为反气旋环流,对流下沉运动增强,日本岛以南、黄海至我国中东部地区为显著的东风距平,且前期2月西风带位置偏北,引起夏季海陆热力差异较大,最终导致东亚夏季风强度异常偏高;冷水年则相反。综上所述,当前期冬季西太平洋暖池热含量异常偏高(低)时,会造成次年东亚夏季风强度偏强(弱)。      

2003年云南夏季罕见高温干旱的诊断研究

2003年夏季云南出现了历史上罕见的大范围异常高温干旱天气,是自1961年以来云南最严重的夏季高温干旱天气之一,影响范围、持续时间及危害程度仅次于1983年。是20世纪80年代中期云南进入气温偏暖期后的一次夏季极端气候事件,打破多项历史纪录。利用高度、OLR、TBB等资料对此次气候灾害过程进行了诊断分析。发现西太平洋副热带高压持续稳定且偏强偏西,季风低压偏弱偏西,云南TBB值偏高,降水云系较少,OLR值偏高,对流活动抑制,云南正好处于东、西两个垂直环流圈的下沉气流控制区是形成高温干旱的主要原因。     

东部和中部型El Nino事件对云南冬季降水影响的差异分析

利用1981-2017年云南125个站逐日降水观测资料和NCEP再分析资料,对比分析了两类El Nino事件期间云南冬季的降水差异,发现东部型El Nino有利于冬季云南大部地区降水显著偏多,而中部型El Nino的影响却不明显。进一步从两类事件相应的大气环流等方面分析了造成降水差异的物理原因。研究表明:(1)东部型El Nino年,Walker环流在西太平洋为异常强的下沉气流,西太平洋副热带高压偏强偏西,Hadley环流在20°N为异常上升气流,造成云南水汽输送和辐合加强,中高纬度巴尔喀什湖和贝加尔湖脊发展,冷空气沿高原东侧南下影响云南,冷暖空气在云南频繁交汇,导致云南冬季降水日数明显偏多,相应降水偏多;(2)中部型El Nino年,Walker环流在西太平洋下沉支强度较东部型弱,相应的西太平洋副热带高压较东部型偏弱偏东,Hadley环流在20°N为下沉气流,对云南区域水汽输送影响较弱,同时中高纬度环流不利于冷空气南下影响云南,除对应冬季云南东部降水日数偏少外,对其他地区降水的影响不明显。     

2003年7月西太平洋副热带高压变异及中国南方高温形成机理研究

运用全型垂直涡度倾向方程理论, 研究了空间非绝热加热在垂直方向的变化对2003年7月副高强度和位置变异的影响.结果表明: 副热带地区的非绝热加热对副高的强度和位置的变化有极其重要的影响.与气候平均状况相比较, 2003年7月副高北侧、西侧非绝热加热的垂直变化异常偏强, 而南侧、东侧较常年异常偏弱.这种异常的非绝热加热状况导致我国江南到副热带西太平洋地区呈现带状的异常反气旋性涡度制造, 使得7月副高偏强偏西.     

温州地区夏季高温气候特征与成因

利用1971-2010年温州地区夏季高温资料,建立温州地区高温过程时间序列,探讨温州地区的高温气候特征和主要天气系统。结果表明：夏季高温日分布具有明显的时间和空间特征,从高温日的年际变化看,温州地区夏季高温自2003年后出现了突增,危害性高温出现频率明显增加;副热带高压是影响温州地区夏季高温的主要天气系统,当本区处于副热带高压脊线附近且为偏西气流,低层有暖中心配合,且日照时数比较长时,将会出现危害性高温;当副热带高压减退温州地区处于西南（或偏西）气流中,850 hPa温度场上华南处于20 ℃（或以上）的暖区（舌）控制时,日照时数长,本区也易出现高温天气,但出现危害性或强危害性天气的概率较低,此类高温一般出现在6月下旬旬末到7月上中旬;台风等热带低值系统外围的偏北气流引起的强烈下沉增温作用,也会导致温州地区出现高温。