The role of the subtropical jet stream during heat wave events over north-central Greece

Summary The role of the subtropical jet stream (SJ) in the occurrence of heat waves in South Balkans and Greece is sought here. For this purpose ECMWF grid-point data is examined, concerning the Balkan heat wave of 5–9 July 1988, that cost human lifes, at least in Greece. For the city of Thessaloniki, Greece, a temperature budget is presented, as a function of time. It turns out that the most important heating mechanism is the adiabatic heating. Horizontal mass convergence at the maximum wind level (200 hPa) causes descent and adiabatic heating. The convergence occurs in association with the Hadley Cell, as well as with the right exit quadrant of an anticyclonically curved subtropical jet streak. As air parcels that exit the above jet streak slow down and turn anticyclonically, a strong ageostrophic wind current is established towards and to the right of the flow direction. This ageostrophic current converges above the northeastern Balkans. Downward ageostrophic motion emerges from the above area of horizontal convergence and heads towards the SSW, affecting the Balkans. From the above case study, it is concluded that intense heat waves are favoured in the South Balkans and Greece when the SJ is anticyclonically curved to the north of the Balkans and a jet streak is situated to the north west of the Balkans.     

The effect of the subtropical jet on the rainfall over southern China in January 2008

ABSTRACT The third precipitation episode of China＇s great snowstorms of 2008 was analyzed using station observations and ECMWF six-hourly data. The variation of the shape of the upper-level subtropical jet played an important role in the rainfall over south- ern China. With the eastward movement of the trough, the jet shape changed from two straight jets to a tilting jet over China and then it moved southward. With these variations, the south-north movement of ascending flow and precipitation area over southern China occurred.     

冬季东亚副热带急流与温带急流的比较分析：大尺度特征和瞬变扰动活动

使用ERA40再分析的月资料和逐日资料,从大尺度特征和瞬变扰动活动两个角度对冬季东亚副热带急流(EASJ)和东亚温带急流(EAPJ)进行了比较分析.结果表明,使用月资料分析的EASJ与EAPJ在高层风场上没有清晰的地理分界区,而使用逐日资料计算得到的冬季逐日急流发生数则以高原北部上空所处的纬度带为分界岭,存在两个急流中心集中区,分别对应于EASJ和EAPJ区域.通过分析东亚上空天气尺度瞬变扰动活动(STEA)表明,与强盛的冬季EASJ相伴随的是较弱的南支STEA,而与较弱的EAPJ相伴随的北支STEA却十分活跃,显示出EAPJ是与瞬变活动相伴而存的急流.进一步的诊断分析揭示了冬季东亚温带急流的两种主要异常模态,一种是EAPJ区域反气旋性/气旋性异常环流型,另一种是局地西风的减弱/增加.与第1种异常模态相关的北半球大尺度环流异常主要集中在欧亚中高纬地区,其形成受中高纬大气环流以及东亚上游大气环流异常的共同影响.当EAPJ局地西风减弱/增加时.EASJ东段至西太平洋上空的西风急流呈现出与之相反的变化型,北半球大气环流异常表现为大气遥相关的欧亚(EU)型.冬季EAPJ的两种异常模态还与东亚上空STEA异常密切联系,其中在北支STEA区域出现的瞬变异常以波列的形式沿STEA北支轴线传播到达东亚沿海上空,然后东传入洋面上空,而对于靠近STEA南支轴线的异常扰动活动则只能存在于东亚东部及其沿海上空的200 hPa层上.     

对流层上层东半球副热带西风急流与副热带(南亚)高压的关系

用NCEP/NCAR月平均再分析资料对南亚高压和对流层上层西风急流的季节变化及盛夏两类型态进行对比。结果表明,南亚高压和西风急流中心都有从冬到夏的西移北进和从夏到冬的东退南撤,急流中心位于南亚高压中心北侧。东亚夏季风盛行期间南亚高压中心的北移提前于西风急流中心的北移,二者的强度呈反相的季节变化。一般情况下,伊朗高压对应西部急流型,青藏高压对应东部急流型。典型东、西部急流年份中高纬气温及高度场的差异表明气压梯度力强弱对比是急流东西型变化的主要原因,南亚高压的位置基本上决定了急流中心的型态,但由于南亚高压具有"趋热性",而急流的移动符合热成风的规律,因而二者的热力影响机制有所不同。     

The impacts of the East Asian subtropical westerly jet on weather extremes over China in early and late summer

Summer weather extremes (e.g., heavy rainfall, heat waves) in China have been linked to anomalies of summer monsoon circulations. The East Asian subtropical westerly jet (EASWJ), an important component of the summer monsoon circulations, was investigated to elucidate the dynamical linkages between its intraseasonal variations and local weather extremes. Based on EOF analysis, the dominant mode of the EASWJ in early summer is characterized by anomalous westerlies centered over North China and anomalous easterlies centered over the south of Japan. This mode is conducive to the occurrence of precipitation extremes over Central and North China and humid heat extremes over most areas of China except Northwest and Northeast China. The centers of the dominant mode of the EASWJ in late summer extend more to the west and north than in early summer, and induce anomalous weather extremes in the corresponding areas. The dominant mode of the EASWJ in late summer is characterized by anomalous westerlies centered over the south of Lake Baikal and anomalous easterlies centered over Central China, which is favorable for the occurrence of precipitation extremes over northern and southern China and humid heat extremes over most areas of China except parts of southern China and northern Xinjiang Province. The variability of the EASWJ can influence precipitation and humid heat extremes by driving anomalous vertical motion and water vapor transport over the corresponding areas in early and late summer.摘要东亚副热带西风急流是影响中国极端天气的重要原因之一, 然而之前的研究主要关注整个夏季急流的变率, 对其早夏和晚夏变率的区别及其对极端天气的影响关注较少. 本文研究了早夏和晚夏东亚副热带西风急流季节内变化特征的区别, 以及这种区别带来的极端天气的差异及其可能的动力学机制. 研究结果表明, 相比于早夏, 晚夏急流季节内变化中心位置偏西偏北, 通过改变垂直运动和水汽输送可以影响极端降水和湿热浪在相应区域的发生概率.     

夏季东亚副热带西风急流的季节内变化及其与我国东部降水的关系

利用ERA-interim再分析资料和中国降水观测数据,分析了夏季东亚副热带西风急流的季节内变化以及对我国降水的可能影响,结果表明:亚洲地区夏季200 hPa纬向风在空间上主要表现为南北振荡和东西振荡特征,在时间上具有10～40 d的周期;在低频尺度(10～40 d)上,纬向风异常由西北先向东传播,到达东亚地区后再向南传播;伴随低频纬向风季节内演变,高原以北的急流中心向东和东南移动,急流轴也呈现南北振荡的特征;降水异常对200 hPa风场低频振荡有显著响应,在东亚地区,低频纬向风与低频降水南移,降水在黄淮地区出现,并逐渐移至长江及其以南地区;急流附近的低频西(东)风异常在其南侧形成负(正)切变涡度,对应我国东部地区位势高度的升高(降低),使得南亚高压东伸(西退),从而使我国东部雨带的位置发生显著变化。     

On the association between synoptic circulation and wildfires in the Eastern Mediterranean

In the present paper cluster analysis of 2-month air mass back-trajectories for three contrasting fire and non-fire events is conducted (high, low, and zero burnt area). The large fire event displays an air mass history dissimilar to other events whereby a 39-day period of warm and dry chiefly northerly anticyclonic conditions is evident, before a week of warmer predominantly southwesterly cyclonic activity, immediately prior to ignition. The pressure level of these anticyclonic air masses is above 800 hPa for more than 75 % of the trajectory length; this region is above the principal moisture transport regime of 800 hPa altitude. Analysis of variance on the mean rate of change of potential temperature identified weak statistically significant differences between two air mass pairs regarding the large fire: anticyclonic and cyclonic air masses in both cases (p?=?0.038 and p?=?0.020). Such regularity of type and occurrence, approach pressure levels and statistically significant differences are not evident for the small and non-fire event air masses. Such understanding is expected to permit appropriate steps to be undertaken including superior prediction and improved suppression strategy.     

Relationships between the West Asian subtropical westerly jet and summer precipitation in northern Xinjiang

The relation between the spatial and temporal variations of the West Asian subtropical westerly jet (WASWJ) and the summer precipitation in northern Xinjiang has been explored using the NCEP/NCAR reanalysis data and the summer precipitation data at 43 stations in northern Xinjiang during 1961 to 2007. Results show that the position of the WASWJ is more important than its strength in influencing the summer precipitation in northern Xinjiang. When the jet position is further south, the anomalous southwesterly flow crossing the Indian subcontinent along the southern foothill of the Tibetan Plateau is favorable for the southwestward warm and wet air penetrating from low latitudes into Central Asia and northern Xinjiang and more rainfall formation. Further analysis shows that the interannual variations of the jet position are well correlated with the Arctic Oscillation (AO). In the weak AO years, the middle to upper troposphere becomes colder than normal and results in an anomalous cyclonic circulation at 200 hPa over Western and Central Asia, which enhances the westerly wind over middle and low latitudes and leads to the WASWJ located further south.     

Interesting cases of the interaction between a pair of tropical cyclones and the polar front

Studied is the evolution of the family of tropical cyclones in the Pacific Ocean in 2009. Analyzed is an unusual behavior of real simultaneously existing tropical cyclones (TCs). Investigated is a mechanism of the interaction between a pair of tropical cyclones of various intensities. Considered are the cases of triple interaction between a pair of TCs and the polar front, as well as the transformation and regeneration of TCs due to their entry to the cold front area. The dynamics of groups of real tropical cyclones is compared with the behavior of ideal cyclonic vortices in the experiments with the numerical model. Proposed are the variants of explaining the disappearance and formation of vortices, as well as of loops, zigzags, and sharp turns during their movement.     

东亚副热带急流与东北夏季降水异常的关系

利用东北地区88个气象站点观测的7、8月(夏季)降水量和NCEP/NCAR再分析资料,分析了东北地区夏季降水与同期东亚副热带西风急流之间的关系,发现东北地区夏季降水异常偏多年,位于青藏高原上空200 hPa西风急流中心强度偏强,东北地区上空急流轴向东北方向倾斜;东北地区夏季降水异常偏少年,青藏高原上空200 hPa西风...     

东亚副热带西风急流变化多模式模拟结果分析

利用政府间气候变化委员会第四次评估报告(IPCC AR4)中等强度温室气体排放情景试验(SRES A1B)多个耦合模式的输出结果,通过多模式集合的方法分析全球变暖背景下东亚副热带西风急流的变化特征,发现在多模式集合平均结果中,随着温室气体增多、温室效应进一步加剧,冬季急流强度增强,急流位置向北移动,夏季急流强度也呈现出增强的趋势,位置却向南移动。在全球变暖背景下,冬季北半球从低纬到中高纬度对流层中高层温度将增加,但低纬增温强、高纬增温弱,导致副热带地区温度梯度增加,夏季增温幅度比冬季大,且强增温区向北移,造成急流轴北侧区域温度梯度增大,并通过热成风关系使得急流区纬向风增强。     

夏季青藏高原移动性对流系统与中国东部降水的相关关系

利用国际卫星云气候计划提供的1985-2002年共18年的MCSs路径跟踪资料、 NCEP/NCAR逐月再分析资料和中国138个地面常规观测站资料,分析了夏季起源于青藏高原地区的移动性MCSs的主要时空分布特征,探讨了青藏高原MCSs与中国降水的关系.通过对MCSs爆发异常强弱年高度和风差值场的分析,概括出青藏高原MCSs影响中国降水的可能机制.结果表明:夏季青藏高原移动性MCSs主要生成于青藏高原东南部,其爆发时间具有明显的日变化特征,它们能够传播到我国中东部及南亚许多地区;夏季MCSs对我国降水具有重要影响,它们与中国夏季降水的相关系数分布以4条正、负相间的东西向分布带的形势存在,从南到北依次为"- - ",这与我国夏季降水带的变化形势非常一致;南亚高压、西太平洋副热带高压和东北冷涡的强度、位置变化与高原MCSs生成的多少密切相关,并通过它们对我国夏季降水带的分布造成重要影响.     

地形作用下低空急流的演变与强降水对流风暴系统的相互作用

利用雷达、卫星、风廓线雷达和地面加密区域自动气象站等观测资料,分析了2016年入梅后发生在鄂东地区一次极端强降水事件的中尺度对流系统发生发展过程、结构演变及其传播特征,旨在揭示造成强降水过程中的3个中尺度对流系统(MCS)的触发、发展、维持机理以及它们之间内在的中尺度动力学关系,尤其是地形作用下的低空急流的演变与强降水对流风暴系统相互作用过程。研究表明:(1)与大多数梅雨锋上的强降水带与低空切变线平行分布不同,此次极端强降水雨带呈倾斜的"n"字形,其中两条主雨带近乎与低空切变线垂直;此次极端强降水分别由大别山迎风坡上西北—东南向MCS、湖北中东部平原地区西北—东南向MCS和桐柏—大洪山东侧东北—西南向MCS造成。3个MCS移动缓慢,都具有后向传播的特征。(2)大别山迎风坡上MCS初始雷暴是低空急流下边界不断向下扩展过程中在地形抬升作用下触发的,而湖北中东部平原地区的MCS和桐柏—大洪山东侧MCS的触发、发展、加强都与大别山迎风坡上MCS形成的冷池加速推进形成的出流边界与环境气流形成的强烈辐合抬升作用有关。(3)垂直于大别山的边界层西南急流对山坡上的对流冷池产生的顶托作用不仅平衡了冷池密...     

江淮梅雨期降水异常分布型及其与东亚副热带高空西风急流的关系

利用1980—2010年的NCEP/NCAR再分析资料与江淮地区44个站逐日降水资料,分析了江淮地区1980—2010年梅雨期(6月16日—7月15日)降水的基本空间分布型及其与东亚副热带西风急流的关系。结果表明,江淮梅雨降水的第一分布型为"南正(负)北负(正)",该型受副热带高空西风急流位置的影响,急流位置偏南(北),则雨带位于江淮南(北)部地区;第二分布型为"中间负(正),两边正(负)",该型受副热带高空西风急流强度的影响,急流强度异常偏弱(强),则雨带位于江淮地区西北、东南部(中部)。合成分析表明,高空急流位置异常偏南时,500 h Pa副高偏弱、偏南,850 h Pa江淮南部地区为水汽、风场辐合区,高低空配置有利于降水呈"南正北负"的分布型。高空急流强度异常偏弱时,从我国江淮中东部地区至日本南部,500 h Pa上无明显垂直运动,850 h Pa上有水汽和风场的辐散区,高低空配置有利于降水呈"中间负,两边正"的分布型。     

阿拉伯海低压形成和结构的个例分析

利用1979年夏季季风试验(Summer MONEX)得到的资料,对一个阿拉伯海低压的形成和结构进行了分析,得到了以下重要结果: (1)低压的形成和发展与低空急流的活动,尤其是其中的最大风速中心有很密切的关系。低压是在东西两支急流间的强正涡度区中形成和发展的。 (2)低压的结构有明显的不对称性。强烈的质量辐合、水汽辐合和上升运动等位于低压东侧和南侧。 (3)低压本身最强的环流位于对流层中层(700—500毫巴)。在风场、湿度场、垂直运动场等分布上有类似中层气旋之处,但在其他方面也有许多差别。 (4)低压是从一个十分暖湿的中层热带扰动发展而来。干空气的侵入可能是造成低压减弱的一个重要因子。 (5)暖心的建立是从上层向下层进行的,这说明积云对流的重要性。 (6)最后给出了低压的三维气流结构。     

Assessment of climate extremes in the Eastern Mediterranean

Summary Several seasonal and annual climate extreme indices have been calculated and their trends (over 1958 to 2000) analysed to identify possible changes in temperature- and precipitation-related climate extremes over the eastern Mediterranean region. The most significant temperature trends were revealed for summer, where both minimum and maximum temperature extremes show statistically significant warming trends. Increasing trends were also identified for an index of heatwave duration. Negative trends were found for the frequency of cold nights in winter and especially in summer. Precipitation indices highlighted more regional contrasts. The western part of the study region, which comprises the central Mediterranean and is represented by Italian stations, shows significant positive trends towards intense rainfall events and greater amounts of precipitation. In contrast, the eastern half showed negative trends in all precipitation indices indicating drier conditions in recent times. Significant positive trends were revealed for the index of maximum number of consecutive dry days, especially for stations in southern regions, particularly on the islands.Current affiliation: National Observatory of Athens, Athens, Greece.     

北太平洋海表温度与500hPa西太平洋副高和极涡相互作用的统计分析

本文采用斜交PROMAX因子分析方法分析了1954—1986年北太平洋逐月海表温度与500hPa西太平洋副高和极涡环流指数(7个因子)的相互关系,指出(1)500hPa西太平洋副高和极涡指数与北太平洋海温的相关值存在明显的年变化,以赤道太平洋区最敏感,(2)赤道东太平洋海表温度的变化与10个月前的极涡中心强度、同期和1—3个月前西太平洋副高面积、强度、位置变化有联系,(3)500hPa西太平洋副高面积和强度的变化受到前3—5个月赤道东太平洋和3个月前赤道中太平洋海表温度的影响,500hPa西太平洋副高位置与前3—5个月赤道东太平洋海表温度有联系.     

Anomalous gradient winds in the subtropical jet stream and interpretations of forecast failures

Summary Dramatic examples of forecast failures in global models of moderate resolution (i.e., T106) have been shown to occur during periods of the negative phase of the Pacific/North American (PNA) pattern in the Northern Hemisphere winter. Specifically, in these periods forecast skills at 500hPa as measured by the standard anomaly correlation index dropped to rather low values by days 4 and 5 of the forecasts. This paper examines systematically some of the factors that may have contributed to the failure of these model forecasts.In particular, strong winds approaching intensities on the order of 100ms^–1 south of Japan at the 200hPa level were degraded by the initialization and data assimilation procedures of the models. These observed winds were found to be supergradient in nature and representative of the anomalous solution of the gradient wind equation. Procedures such as the multivariate optimum interpolation (with its geostrophic constraints) and the normal-mode initialization including several vertical modes apparently were factors that led to the degradation of these strong winds in the initial model states. In this paper, an analysis of these factors is presented, and it is shown that uninitialized analyses (with no constraints) based on a simple successive correction procedure can retain the strong winds evident in the observations. Forecasts thus performed appear to retain wave trains, a characteristic feature of negative PNA initial states, leading to a significant improvement in forecast skill.     

Role of the subtropical westerly jet waveguide in a southern China heavy rainstorm in December 2013

An extreme rainstorm hit southern China during 13–17 December 2013, with a record-breaking daily rainfall rate, large spatial extent, and unusually long persistence. We examined what induced this heavy rainfall process, based on observed rainfall data and NCEP–NCAR reanalysis data through composite and diagnostic methods. The results showed that a Rossby waveguide within the subtropical westerly jet caused the event. The Rossby wave originated from strong cold air intrusion into the subtropical westerly jet over the eastern Mediterranean. With the enhancement and northward shift of the Middle East westerly jet, the Rossby wave propagated slowly eastward and deepened the India–Burma trough, which transported a large amount of moisture from the Bay of Bengal and South China Sea to southern China. Strong divergence in the upper troposphere, caused by the enhancement of the East Asian westerly jet, also favored the heavy rainfall process over Southeast China. In addition, the Rossby wave was associated with a negative-to-positive phase shift and enhancement of the North Atlantic Oscillation, but convergence in the eastern Mediterranean played the key role in the eastward propagation of the Rossby wave within the subtropical westerly jet.