【作者】 朱坚；

【导师】 张耀存；

【作者基本信息】 南京大学 ， 气象学， 2011， 博士

【摘要】 本文利用中国气象局整编的1960-2004年的逐日降水和温度资料以及NCEP/NCAR逐日再分析数据集,揭示出长江以南地区(以下简称江南地区)春季降水的特征,分析了长江以南地区春季发生连阴雨的典型环流特征,并通过数值试验探讨了青藏高原附近地区爬坡流和绕流等地形动力作用对长江以南地区春季降水的影响。主要结论如下：1长江以南地区春季降水的气候特征比较不同持续时间的降水分布发现,江南地区以发生4天及以上的连续降水为主,其中,频数较多的站点有55个,在空间上集中在22.5°-30°N、105°-120°E的区域范围内,而时间上主要集中在13-27候。按照本文对江南地区连阴雨的定义,45年间江南地区春季共发生152次连阴雨,气候平均态上,分别在江南的东北和西南地区存在两个连阴雨降水强度大值中心。对连阴雨日和强度的线性趋势分析表明,江南地区春季连阴雨日数在1960-2004年间呈下降趋势,尤其是1995年后,但总降水量线性趋势不显著。分析持续性降水雨日数和强度的时空异常分布发现,降水雨日数异常的主要空间模态为全场一致型和南北反相变化型,强度异常主要的空间模态为全场一致型和东北异常型,两者的时间序列均包含了年际和年代际变化信号。2长江以南地区春季连阴雨的环流特征通过比较单日降水和连阴雨发生前后以及发生期间的环流特征发现,当有一个冷槽过境或者冷空气突然爆发,江南地区的中高空出现冷暖空气相遇,产生降水,但此类过程持续时间较短,江南地区上空很快受下沉冷空气控制,水汽含量骤减,降水随即停止。而与单日降水明显不同的是,连阴雨发生期间,连续稳定的低层较强暖湿空气与稍弱的冷空气在南岭、武夷山脉附近对峙,这也是降水之所以能持续在江南地区的重要原因之一。从气候和天气过程分析的角度,进一步探讨冷暖空气对峙于南岭一带的原因。通过连阴雨和连晴、江南春季降水雨日多年和少年的对比合成分析,从各层风场和温度差异配置发现,发生连阴雨时,高空存在连续的西风急流中心带,此时高原南侧有较强的低空西风绕流,遇到云贵高原后,往南绕行至中南半岛,与南海副高西端较强的南风相汇,共同作用于江南地区,使得南岭山脉地形南侧低层存在持续并且比较活跃的暖湿偏南风,同时,在南岭山脉北侧至30°N附近,低层有较活跃的偏北冷空气,冷暖空气交汇于山脉附近,并且暖湿南风较强,在中高层继续抬升至35°N附近,与偏北下沉气流相遇。分析位势高度场发现,当江南地区发生连阴雨时,在100hPa存在弱的极涡,较浅的东亚大槽,并且30°N以南转变成弱的高压脊,这也是中高空北风仅到35°N附近就下沉,使得江南地区中高空能够被偏南的上升气流控制的原因之一。850hPa南海副高较强,西端位置在南海上空,近地层西伯利亚冷高压较强,导致近地面低空在南岭南北两侧存在比较活跃冷暖空气。3青藏高原影响长江以南地区春季降水的数值试验青藏高原大地形的存在,使得西风气流遇到高原时发生爬坡流和绕流两个分支,同时,云贵高原作为青藏高原最东南端的大地形,虽然比青藏高原范围小,但是由于其位置偏南,对江南地区春季降水也起到一定的作用。通过控制试验和爬坡流试验、控制试验和绕流试验的降水差值分析,发现爬坡流和绕流均增加了青藏高原下游地区的春季降水,主要的降水差异中心位于湖南省一带,但绕流影响的范围更广,主要表现在绕流影响到长江以南地区降水。在风场差异上,爬坡流对于低层风场,尤其是高原东侧地区的影响甚微,但绕流则增强了高原东侧地区低层风场的气旋性气流,范围较广,覆盖了长江以南地区,并且高原南侧以及高原东南侧的低层风场受绕流的影响也较显著,而高原东南侧地区正是江南地区水汽输送的关键区之一。进一步分析江南地区局地及临近地区的地形作用发现,南岭山脉使得低层冷暖空气在此交汇,暖湿气流抬升,有利于降水产生,对江南地区降水有着直接作用。而云贵高原对江南地区的影响并不是由于地形的抬升作用,而是阻挡了高原南侧绕流,使其进一步绕行至中南半岛,加强南海副高西端的转向气流,将更多的水汽输送至江南地区。更多还原

【Abstract】 Based on the daily precipitation and temperature data of China from the National Meteorological Information Center during the period of1960-2004and the atmospheric data from National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP/NCAR) reanalysis daily datasets, the features of spring rainfall over regions south to the Yangtze river (SYR), the typical circulation of spring persistent rainfall (SPR) and the mechanical effect of the climbing and detouring flow around the Tibetan Plateau (TP) have been investigated. The conclusions are as follows:1. The climatic features of spring rainfall over SYRComparing different-persistent-days rainfalls in spring, the dominant process is4-and-more-days persistent rainfall event over SYR. Moreover, significant stations are55stations within the area of22.5°-33°N,105°-120°E, and time period is concentrated in the period of13th-27th pentad. Based on the definition of SPR over SYR, during45years,152SPR processes have been picked out, and two persistent heavy rainfall centers are located over the northeastern and southwestern part of SYR.For the linear trend distribution of days of SPR, significant decreasing trend is found over the whole region of SYR, especially after1995. For the intensity of SPR, the trend is not significant. The dominant patterns of SPR days and intensity variations show homogeneous anomalies. The second patterns of SPR days and intensity variations are characterized by north-south dipole pattern and north anomaly pattern. Both of time serials of SPR days and intensity indicate the loading of these patterns varies on decadal and interannual timescales.2. The circulation features of SPRAccording to the one-day rain process, when a cold trough goes across or the cold air suddenly outbreaks, the cold and warm air meets at middle layer over SYR, which results in the precipitation. But, this process is so quick that will not persist long. However, during the SPR occurred, the continuous stable low-level warm air and cold air meet at lower layer over SYR, which is one of the important reasons that the rainfall are persistent over SYR.To furtherly investigate the reason for the warm and cold air sustaining over SYR, from climatic and synoptic point of view, based on composite analysis on the persistent sunny and SPR, the typical circulation features have been investigated. The coupled configuration of vertical winds and temperature differences and the global potential height field have been analyzed. In the upper levels, continuous belt of westerly jet center have been found. In the lower levels, the strong westerly flow located over the southern part of TP and encounters with the Yunnan-Guizhou Plateau (YGP), then the flow bypass to the Indo-China Peninsula. So, the strong southerly winds along the western part of South China Sea Subtropical High (SCSSH) and westerly winds around Indo-China Peninsula will join together and flow to SYR, which gives robust and sustained warm southwest winds along the southern part of Nanling Mountains. Meanwhile, stronger cold air at low level is found along the northern part of Nanling Mountains. The warm and cold air nearly intersects in the Nanling Mountains. And, the warm southerly wind is stronger than the northerlywind that continues rising until it reaches nearly35°N, where it meets with northerly winds. In the global geopotential height fields, when the SPR occurs, significant patterns are with a weak polar vortex at100hPa and weaker trough in East Asian. Moreover, a weak ridge of high pressure is located over south to30°N, which maybe is the reason why the north winds sinks near35°N, causing the controlling movement over SYR to be the southerly upper flow. On the low levels, under the coupled controlling of the stronger SCSH and Siberian cold high, robust warm and cold air have steadily located over the northern and southern part of Nanling Mountains.3. The simulations on the mechanical effect of TP on spring rainfall over SYR Because of TP, the westerly flow is divided into climbing and detouring flow crossing the TibetanPlateau. Meanwhile, the Yunnan-Guizhou Plateau (YGP) locates over the southern part of China. As part of TP, the range and height is not as large as TP, but due to its location, it will largely affect the SPR. The model simulations have been used in this section for analyzing the effect of TP and YGP during SPR.Both of the climbing and detouring flow will increase the spring precipitation over eastern China, especially around Hunan province, while under the detouring flow, the impact regions are broader. The detouring flow has enhanced the cyclonic flow over eastern China, and the affected range is much larger than climbing flow. The detouring flow significantly increases low-level winds, especially over the southeastern part of TP, which is one of the key areas of water vapor transferring.Furtherly analyzing the local terrain effect on the spring rainfall over SYR, it is found that the Nanling Mountains will directly make the low-level warm and cold air intersect, which is benefit for the rising of warm air and rainfall. However, the effect of YGP is not due to topographic lifting, but forcing the flow at the south side of the Plateau bypass to the Indo-China Peninsula, increasing the southwesterly winds around the western part of South China Sea High, which brings more water vapor into SYR.更多还原