【作者】 索渺清；

【导师】 丁一汇；

【作者基本信息】 中国气象科学研究院 ， 气象学， 2008， 博士

【摘要】 本文利用NCEP/NCAR再分析资料和全国站点降水资料,采用多种统计分析方法,首先从南支急流演变的角度,对冬半年副热带南支西风槽的定义进行讨论,然后从气候学上探讨其结构和演变特征,以及南支槽的传播和多尺度变率,最后就南支槽对中国天气的影响进行了研究,并以2007年11月14～17日云南德钦暴雪为例,探讨了南支槽与孟湾风暴结合下西南地区强烈天气产生的过程。全文主要结论有以下几个方面:(1)南支槽是冬半年副热带南支西风气流在高原南侧孟加拉湾地区产生的半永久性低压槽, 10月南支槽建立表明北半球大气环流由夏季型转变成冬季型,6月南支槽消失同时孟加拉湾槽建立是南亚夏季风爆发的重要标志之一。南支槽在700hPa表现最明显,其槽前干暖平流有利于昆明准静止锋的形成和维持;冬季行星尺度辐散环流下沉支抑制了南支槽前上升运动的发展;春季随着辐散环流减弱,东亚急流入口区南侧高层辐散使得槽前垂直运动向上迅速伸展。冬季水汽输送较弱,上升运动浅薄,无强对流活动,南支槽前降水较少,雨区主要位于高原东南侧昆明准静止锋至华南一带;春季南支槽水汽输送增大,同时副高外围暖湿水汽输送加强,上升运动发展和对流增强,南支槽造成的降水显著增加,因此春季是南支槽最活跃的时期。(2)冬半年北半球副热带西风急流下方存在3个南支波动,分别位于阿拉伯海、孟加拉湾和华南,其中孟加拉湾南支槽变率最小。从北非、阿拉伯海到孟加拉湾的“-+-”遥相关波列表明南支槽与北非槽呈正相关,与阿拉伯槽呈反相关。西风波动从北非东传到孟加拉湾的过程中,往往会在阿拉伯海有所停顿甚至加强,这种由西向东的传播过程约需20天,具有明显的低频振荡特征。源自北非的定常Rossby波沿着急流波导经阿拉伯海传播到孟加拉湾,可能是南支槽明显增幅的一个主要机制;另外,青藏高原东西两侧南下冷空气的活动是南支槽加深发展的又一重要因素。(3)南支槽季节内振荡以10～30天和30～60天为主,年际变化周期为准4年和准6年,年代际变化周期为10～20年,并在70年代末出现了由强到弱的年代际突变。南支槽低频振荡与我国冬半年南方降水紧密相关,南支槽在孟加拉湾加深或东移都有利于华南低涡产生,当南支槽加深与斯里兰卡低涡打通时,槽前从孟加拉湾带来的大量水汽与来自南印度洋的偏南越赤道水汽输送相结合,并在菲律宾低频反气旋作用下向北输送,华南低涡使得低层辐合加强,对流活跃,以至于冬半年我国低频降水主要集中在南方。南支槽异常偏强(弱)年,副热带高空西风急流异常偏强(弱),有利于(不利于)上游西风波动向下游传播。中高纬环流经向度偏大(小)有利(不利)于冷空气南下,南支西风波动偏强(弱),副热带高压带显著偏弱(强),槽前水汽输送和垂直上升运动偏强(弱),使得我国南方降水异常偏多(少)。南支槽年代际偏强(弱)阶段,北非急流偏强(弱)位置偏南(北)有利(不利)于西风波动向下游传播;同时东亚急流偏弱(强)位置偏北(南),有利于(不利于)槽前垂直运动的发展。中高纬环流经向度加大(减小)有利于(不利于)冷空气向南爆发。南支西风较常年偏强(弱),槽前水汽输送偏强(弱),我国东部上空异常偏南(北)风水汽输送使得多雨区偏北(南)。(4)南支冷空气沿高原南缘东进时受横断山脉阻挡有干冷空气堆积,北支冷空气从中高纬沿近地层南下在高原东侧形成干冷气团,南支槽前西南风在低层绕过横断山脉转向北上时变性成为暖湿气流,形成中间暖湿两侧干冷的结构,在高原东侧形成昆明准静止锋,高原西侧形成另一个弱锋面。冬季南支槽前上升运动使锋面垂直环流得到加强,春季,受华南槽和急流入口区南侧高层辐散的影响,沿锋面抬升的垂直运动迅速加强;南支槽前干平流输送使得锋前降水偏少,而锋后850hPa和700hPa湿平流在春季迅速加强有利于华南水汽辐合和降水。华南冷空气活动频繁,冬季冷空气来源于南支急流和中高纬低层,春季冷空气主要来源于南支急流。此外,南支槽影响中国天气有移动和静止两种方式,且以静止居多。移动性南支槽在东移的过程中不但输送水汽,槽前辐合上升也提供必要的动力条件。静止性南支槽对我国降水的影响主要是源源不断的水汽供应。(5)南支槽和孟加拉湾风暴相结合,加上北方冷空气和有利的地形作用造成德钦暴雪天气。等熵位涡分析表明冷空气从里海附近的对流层上层下传,沿南支西风到达孟加拉湾使南支槽加深有利于引导孟湾风暴北上。槽前西南风低空急流把风暴产生的云和水汽向东北方向输送,随着风暴北上,低空急流加强将水汽输送上高原,受地形抬升高层大气最先增湿,德钦位于水汽输送大值带中,湿层深厚并有大量水汽通量辐合,同时冷空气从高原南下使槽前上升运动迅速增强有利于暴雪的产生。倾斜涡度发展和条件性对称不稳定可能是暴雪形成的重要因子。更多还原

【Abstract】 Using NCEP/NCAR reanalysis data and the observational precipitation collected in China which are then treated with various statistical methods, study is performed firstly of the definitions of the wintertime southern branch trough in the subtropical westerlies (WEBTISR) in terms of evolution of the southern branch jet, then the climatology features as the structure, development, propagation and the multi-scale variability of the WEBTISR, and finally its impacts on the weather and climate all over China. Also, a severe snowstorm in Deqin of Yunnan Province on November 14th-17th, 2007 has been taken as an example to investigate the formation process of the strong weather in Southwest China under the combined action of the WEBTISR and the storm over the Bay of Bengal. And the main results of this paper are as follows:(1) The WEBTISR is a semi-permanent low pressure trough brought about by the wintertime southern branch of the subtropical westerlies over the Bay of Bengal on the south of Qinghai-Tibet Plateau. The fact that WEBTISR established in October indicates that the Northern-Hemispheric circulation transits from summer pattern to winter pattern, while its disappearance in June, together with the Bay of Bengal trough establishment, is one of the important indications of summer monsoon onset. The trough is noticeable at 700 hPa, while the dry and warm advection in its front is favorable for formation and development of the quasi-stationary front in Kunming. In winter, the descending branch of the wintertime divergence circulation inhibits development of the ascending motion in front of the WEBTISR. And in the spring, as the descending branch of the divergence circulation weakened, divergence field is strengthened on the south of the entrance region of the East Asian jet stream, which makes a rapidly upward extension of the vertical movement. In detail, due to the weak moisture transportation, limited upward motion and no strong convection in winter, there is less rainfall in front of the study trough, with the rainfall mainly over regions from the Kunming quasi-stationary front on the southwest of the Plateau to South China, as opposed to the situation in spring. Consequently, the WEBTISR is most significant in spring. (2) There are three southern disturbances under the wintertime subtropical westerlies jet in the Northern Hemisphere, located in the Arabian Sea, the Bay of Bengal and South China, respectively, in which the WEBTISR over the Bay of Bengal has the smallest variability. A teleconnection wave train with negative, positive and negative centers, migrated from North Africa to the Bay of Bengal via Arabian Sea, suggests that the WEBTISR is positive related with the trough over North Africa, but negative related with the trough over Arabian Sea. During the eastward process from North Africa to the Bay of Bengal, the westerlies waves has always stalled or even strengthened in the Arabian Sea. The eastward process costs about 20 days, marked by characteristics of low-frequency oscillations. It is likely to be a major mechanism of WEBTISR strengthening that the stationary Rossby wave originated in North Africa propagates from the Arabian Sea to the Bay of Bengal along the jet waveguide. Besides, the activity of the southward cold air on the west and east of the Plateau is another important factor for the deepening and development of the study trough.(3) The WEBTISR is marked by seasonal periods at 10-30 and 30-60 days, inter-annual periods at quasi-4 and quasi-6 yrs, as well as inter-decadal periods at 10-20 yrs, with an abrupt strong-to-weak change in the late 1970s on the inter-decadal scale. A good correlation between the low-frequency WEBTISR and the rainfall in South China. Till the WEBTISR is joined with the Sri Lanka low vortex, a great number of water vapors from the Bay of Bengal in front of the trough unite with the cross-equatorial transportation of water vapor from the South Indian Ocean. And then the moisture transports northward under the impacts of low-frequency anti-cyclone in Philippines, as in coactions with the South China low vortex, resulting in increased convergence at the lower level and active convection, which is responsible for the fact that the low-frequency rainfall in winter-half year is mainly concentrated in the South China. In the years with stronger(weaker) WEBTISR, the upper-level subtropical westerly jet is extraordinary increased (decreased), which is favorable (unfavorable) to the upstream-to-downstream propagation of the westerlies waves. The increased/decreased meridional circulation the mid-high latitudes is favorable/unfavorable to the southward transmission of cold air, resulting in the stronger/weaker southern branch of the westerlies waves, then the subtropical high is significantly weakened/strengthened along with the enhanced/reduced water vapor transmission and vertical ascension in front of the trough, so that the rainfall is plentiful/scarce in South China. During the period of the increased/decreased WEBTISR on an inter-decadal scale, the North Africa jet is enhanced/reduced and southward/northward than normal, as advantageous/disadvantageous to the upstream-to-downstream propagation of the westerlies waves. Meanwhile, the East Asia jet is reduced/enhanced and northward/southward than normal, which is prosperous/unprosperous to the development of the vertical ascension in front of the trough. The increased (decreased) meridional circulation the mid-high latitude is favorable (unfavorable) to the southward transmission of cold air. The southern branch of the westerlies is stronger (weaker) than the normal years, water vapor transmission is increased (decreased) in front of the trough, and the water vapor is transmitted by winds which are greatly southward (northward) than normal in East China, all these making the precipitation belt northward/southward than normal.(4) The southern branch of the cold air along the southern edge of the plateau to the east when there are the Hengduan Mountains to block the accumulation of dry and cold air, and the northern branch of the cold air has composed the dry and cold air masses on the east of the plateau when it is moving southward along the surface layer from the mid-high latitudes. The southwesterly in front of the WEBTISR has transformed into warm and wet flows when it is bypassing the Hengduan Mountains to the north in lower level. They merge into a structure that is warm and wet in the center and dry and cold on the both sides, Kunming quasi-stationary front on the south of the Qinghai-Tibet plateau, as well as a weak front on the west of the plateau. In winter, the vertical circulation of the front is strengthened by the ascension in front of the study trough. In spring, under the impacts from the South China trough and the upper divergence on the south of the jet’s entrance region, the vertical motion along the front has enhanced markedly. The dry advection in front of the WEBTISR always makes decreased rainfall before the front, while the rapid increase in the wet advection at 850 hPa and 700 hPa behind the front is beneficial to the water vapor convergence and rainfall in South China. Cold air is active in South China. In winter, cold air is from the southern branch of the westerly jet and the lower levels of the mid-high latitudes. And in spring, cold air is basically from the southern branch of the westerly jet. The WEBTISR influencing rainfall in South China can be divided into mobile and static patterns, and the latter is the major means. Mobile trough transports moisture, and the ascending convergence in its front provides dynamic conditions which are necessary to the rainfall. And the static trough provides plenty of moisture for precipitation in China.(5) The severe snowstorm in Deqin was formed under the collaboration of the WEBTISR and storm over the Bay of Bengal, while combined with the north cold air and favorable topography there. Isentropic potential vorticity analysis indicates that cold air originates from the upper troposphere near the Caspian Sea, traveling along the southern branch of the westerlies to the Bay of Bengal, which deepens the trough and beneficial to leading the storm over Bay of Bengal to move northward. The southwest lower-level jet occurs in front of the trough, which transports the storm-induced cloud and moisture to the northeast. And then the uplifting topography humidifies first the upper atmosphere. Deqin is located in the belt of strong moisture transport, where the wet level is thick with convergence of a large number of water vapor flux therein. With the northward shift of the storm, the lower jet has strengthened but obstructed by the Plateau, resulting in convergence and shear over Deqin. The combination of the storm and the WEBTISR, and southward movements of cold air from the plateau make rapid enhancement of the ascension in front the trough. Generally, the slantwise vorticity development and conditional symmetric instability are the important factors for the heavy snow formation.更多还原