【作者】 沈杭锋；

【导师】 翟国庆； 徐世浙；

【作者基本信息】 浙江大学 ， 地球探测与信息技术， 2011， 博士

【摘要】 我国位于世界著名的季风区,暴雨对我国的影响非常大,每年初夏,梅雨暴雨发生过程中,在低空切变线上往往会有中尺度对流系统、中尺度涡旋等生成。为了深入了解严重影响长江下游地区梅雨锋暴雨过程中边界层内中尺度扰动涡旋的影响因子,揭示涡旋的发生发展机制,为精细化预报提供科学依据,本文利用高精度实况数据和模式模拟的手段进行分析,研究结果发现,在低涡切变暴雨发生前,长江流域的上空会出现一个低涡或一条低空切变线,雨区基本位于切变线的南侧,并且大部分的低涡切变降水过程中都会在切变线南面出现一支低空急流；在500hPa高空,西风带大槽和短波槽会将北方冷空气顺势南下,影响梅雨锋降水；在200hPa高空,均伴有40m/s以上的高空急流。边界层过程和边界层内辐合线对边界层及大气是至关重要的,雷暴的新生与边界层内的辐合线密切相关,在合适的地形作用下,边界层内会出现局地辐合线,当雷暴出流形成的阵风锋与之相遇,会爆发新的雷暴过程,表明了边界层内物理过程和辐合线对于中尺度系统的发生发展具有重要影响和作用。研究还发现,梅雨锋暴雨中经常会有中尺度涡旋出现,这些涡旋大都出现在低层,有些甚至起源于边界层内,边界层中尺度扰动涡旋主要是由于切变或槽,或者气流汇合,又或者在地形影响下发展而来的,这些涡旋的尺度不大,基本都在100km左右,有些甚至更小；它们的生命史较短,只有几个小时；这类涡旋的暴雨基本位于涡旋的东南及南侧区域。为了区别广义上的涡旋,我们把这一类起源于边界层,尺度约为100km,并与暴雨密切相关的涡旋称之为边界层中尺度扰动涡旋(PMDV)。通过数值模拟可知中尺度数值模式WRF能够较好地模拟出梅雨锋暴雨过程和中尺度涡旋发生发展情况,为进一步的诊断分析提供了可靠的依据。同时,运用WRF-Var三维变分同化系统,同化高精度的再分析实况资料,是提高模拟效果,改进模拟质量的一种有效方法。边界层中尺度扰动涡旋出现的形式比较多样,有些是单个涡旋,通过发展壮大成熟；有些则是出现了相邻的两个涡旋,相互影响,相互发展,并且最终发生了合并；有些则是在切变线上出现一串涡旋,形成涡旋簇。但是这几个中尺度涡旋过程都有一些共同特征,虽然涡旋最早出现的高度不一致,但都是在边界层内某一层高度的切变线上通过小扰动形成了涡旋,然后向上发展；扰动涡旋在形成时的尺度都比较小,涡旋直径基本在50-100km左右。边界层中尺度扰动涡旋过程在发生前均有来自南面的暖湿气流,水汽被输送到切变线附近。边界层中尺度扰动涡旋在发生前在切变线的南侧(东南侧)均有充足的不稳定能量储藏和积聚。锋生在涡旋形成之前,就有表现出来,主要出现在风速辐合和风向切变处,随后在切变线上生成的边界层中尺度涡旋基本都与锋生中心相重合,在涡旋生成之后,锋生和涡旋都进一步加强,两者相伴发展。边界层风场的切变和汇合是导致涡旋生成的主要动力条件,尤其是切变线北侧(左侧)的北风加大并南压,这样不仅将北方的干冷空气带入梅雨锋,造成降水,而且北风的锲入和挤压,使得切变线上风速梯度增强,切变更加显著,从而使得涡旋得以形成和发展。边界层中尺度扰动涡旋在形成时与MCS系统有密切关系,MCS在边界层涡旋形成之前就会发生,MCS的对流运动促进了涡旋的发生；当中尺度涡旋形成时,边界层内有气流的汇合,特别是偏北风气流的加大南压,由此造成了边界层内的中尺度强辐合中心,因此可以认为,中尺度涡旋是由其南侧暖区边界层内偏南气流与梅雨锋切变线北面带有冷空气的偏北气流相遇发展起来,并伴有一个强风速梯度的存在.涡旋在生成发展时,边界层内有一个相当位温高值中心,沿着涡旋伸展方向向上凸起,从而形成了一个伸向空间的高能舌；而在对流层中层,往往会出现一个冷中心,由此造成了在空间结构上的“上冷下暖”现象,涡旋生成处的空间上往往出现静力不稳定,不稳定中心一般都出现在边界层内,随着辐合和气流加强,涡旋生成过程中,边界层内强不稳定逐步释放,激发对流,在上升气流作用下,对流更加旺盛,造成涡旋区域降水和暴雨的发生,形成短时暴雨。更多还原

【Abstract】 Rainstorm has great impact to China for its locating in the world’s famous monsoon region. Every early summer, it comes into mei-yu season and will bring plenty of rainfalls. There are several mesoscale convective systems (MCS) or mesoscale vortexes generated on the shear line during the mei-yu rainstorm.In order to further understand the origin and development of the mesoscale disturbance vortex in planetary boundary layer (PBL) in lower reaches of Yangtze River, discover the trigger mechanism of the rainstorm and provide some scientific references for the refined short-range forecast, the advanced research WRF model is employed to simulate this kind of rainstorm processes in recent years. Results reveal that there’s a low level vortex or shear line over the reaches of Yangtze River before rainstorm happens. Generally, the rainfall, which always accompanies with low level jet, will fall to south of shear line. While there are short wave troughs and westerly zone at500hPa, there exists up-level jet with the wind value more than40/s at200hPa.Physics and mesoscale convergence line/zone in PBL are very important for atmosphere. There will produce a local convergence line under suitable terrain. It will generate a new gust front when the existing gust front and the local convergence line meet, which can intensify the upward movement and break out of strong convection.The analysis also indicates that there’re generally mesoscale vortexes that produced in low level during mei-yu rainstorm. However, some vortexes with the horizontal scale of about100km or smaller initial from PBL and most of them develop by shear or confluence. In order to distinguish from generalized vortexes, these vortexes, that the horizontal scale is about100km, generates in PBL initially and is responsible for the heavy rainfall, are called PBL mesoscale disturbance vortex (PMDV).The mesoscale numerical model WRF with WRF-Var3D assimilation and high resolution observation data can simulate the formation and development of mei-yu rainstorm and mesoscale vortexes well.There are kinds of types for formation of PMDV. Some are single, while some are pairs with interaction each other, and some even are cluster. Whatever, there are some common features for all of them. These vortexes are not at the same level, but they initial all in PBL and then develop upwards. The diameters of the vortexes are about50-100km, with warm wet flow from south and plenty of unstable energy to south of shear line before vortexes happen. There will turn up frontogenesis at the region of confluence or shear. What’s more, the vortex center will be superposition with the center of frontogenesis and they will develop with each other. The main dynamic conditions of vortex formation is that the confluence and shear of wind in PBL, especially the enhancement and aggression of north wind.There’s close relationship between PMDV and MCS when the PMDV happens. MCS existed before PMDV generates, will promote the formation of PMDV for its surface outflow will bring cold wind into PMDV. Of course, the PMDV will enhance the convective, convergence and rain in back. So it can be considered that the PMDV forms when the north jet to north of the shear line in the PBL encounters with the southwest flow in the PBL of the warm section south of the shear line. The mesoscale vortex is accompanied with a strong wind velocity gradient. At the stage of development of the vortex, there’s high equivalent potential temperature center in PBL, which extends upwards and form a high energy tongue. However, there will appear a cold center in middle of troposphere and it will cause static instability. The convective instability energy can be released with the strengthenment of the convergence and lower-level flow. So it can bring short-term storm and increase the precipitation of the region near vortex.更多还原