【作者】 丛春华；

【导师】 陈联寿；

【作者基本信息】 中国气象科学研究院 ， 气象学， 2011， 博士

【摘要】 热带气旋(TC)登陆前后往往引发严重的暴雨及洪涝灾害,故热带气旋暴雨研究长期受到人们的关注。热带气旋降雨区可分六块(陈联寿,2010),分别为TC核心区降雨(Core region rain)、螺旋雨带(Envelop rain or spiral rain band)、TC内部中小尺度系统降雨(Meso or micro system rain)、不稳定降雨(Unstable rain)、台前飑线降雨(Squall line rain)和TC远距离降雨(Remote rain )。正是这第六块降雨出现在TC环流的千里之外,常受到TC暴雨预报的忽视。这块降雨虽然与TC相隔遥远,但它的出现和TC之间有密切的物理联系,故称之为热带气旋远距离降雨(Tropical cyclone Remote Precipitation—TRP).这块降雨(TRP)形成的机理正是本文研究的主题。并非所有的TC都能产生TRP现象,只有一小部分TC能产生,这是一种小概率事件。TRP不仅发生在中国,也发生在日本和美国。美国科学家(Cote,2007)将此类降雨称为Predecessor Rain Events ahead of tropical cyclones (PREs)。本文系统地研究了台风的TRP现象,所得结果对实际预报有参考价值。本文对1971年～2006年TRP的统计结果表明,这种现象具有明显的时空(spatial and temporal)分布特征。TRP遍及我国大陆27个省(市、自治区),其中环渤海地区和川陕交界处是TRP事件发生的高频区。大部分TRP发生在6～9月,其中7～8月最多。TRP事件具有范围大,降雨强和持续时间长等特点。36年间共有169个TC引发了TRP,占总TC的14.7%,其中有相当一部分的TRP可持续2日以上。引发TRP的TC存在三条优势路径,一是进入南海北部或登陆华南类(56.8%),二是登陆闽浙沿海(24.3%)类,三是近海转向(13%)类。TRP大多发生在TC强度减弱的阶段。本文用合成分析(composite data analysis)分别对环渤海地区和川陕地区有无TRP的大尺度环流背景和水汽输送特征进行对比分析。结果显示,有利于TRP发生的大气环流背景是近海TC能将水汽输送到上述两块遥远的雨区,并存在TC与中纬度槽的相互作用,高层急流相关的辐散场激发上升运动。如热带气旋北侧并无西风槽而代之为高压和偏北气流阻断,这对发生TRP不利。采用TC bogus技术,设计了去除TC和减弱TC强度的敏感性试验来研究TC对TRP的影响。试验结果显示,TC对TRP的发生发展至关重要,去除TC后,TRP降雨量明显减弱。TC不仅向TRP输送水汽,同时还通过其东侧低空急流的强弱影响TRP区的动力辐合和垂直运动。减弱TC强度,TRP区域水汽辐合、水汽的水平输送和垂直输送均变弱,TRP降雨量也减弱明显用Legendre滤波方法改变西风槽的强度,获得两组弱西风槽和两组强西风槽来研究其对TRP的影响。结果表明, TRP的强度和落区对西风槽强度的变化非常敏感。强西风槽更有利于TC东侧低层水汽向TRP区的输送和辐合上升。减弱西风槽强度,TRP降雨明显减弱,而增强西风槽的强度,TRP降雨强度显著增加。TRP的降雨强度与西风槽强度存在正相关。地形对TRP起到一定的增幅作用。去除地形,TRP雨量减弱,提高地形高度,TRP雨量变化不大,但中心位置发生位移。地形的作用主要体现在迎风坡的抬升、地形绕流后辐合及狭管效应等方面。地形对TRP的影响具有局地性,使TRP降雨局地性增强。三个TRP个例诊断分析对比表明,TC与西风带系统相互作用过程中不仅向TRP区输送水汽,同时还影响着TRP区的动力强迫场,激发TRP发生发展。综合研究结果表明,TRP是TC和西风槽(包括中纬度槽和涡、弱冷空气、切变线等低值系统)相互作用的结果,地形对TRP有一定的增幅作用。本文最后得出的TRP事件概念模型(conceptual model)对业务预报有一定的参考价值。更多还原

【Abstract】 Tropical cyclones may produce precipitation or torrential rainfall in an area in front of and away from the cyclone. We call this phenomenon as the Tropical-cyclone Remote Precipitation (TRP). The statistics in this paper show that TRP is a small probability event. It is difficult to predict this phenomenon in operational circumstance. The TRP can also be found in Japan and the United States. American scientists call this as Predecessor Rain Events ahead of tropical cyclone (PREs).The statistical analysis shows that 14.7% tropical cyclones could produce TRP events in 1971-2006 years. Majority of the events would last more than two days. A number of TRPs have the property of wide distribution and high rainfall rate. High frequency of TRPs occurs in July and August. Statistical analysis also shows that there are two areas with the high frequency of the TRPs occurring, i.e, encircling Bo Hai Sea ( regionⅠ) and adjacent area between Sichan and Shanci provinces ( regionⅡ).Diagnostic study with inter-comparison between the two group data of TRPs and Non-TRPs indicates that the prominent differences in lower level either in regionⅠorⅡare typhoon interact with a westerly trough in TRP data group and without a westerly trough in Non-TRP data group but with north-west dry flow instead. On the other hand, the inter-comparison shows tropical cyclones with TRP have strong south-east wet flow channel connected and Non-TRP cyclones without such channel. Prominent differences are also appeared in the upper level. In regionⅠ, jet stream is located to the north of the remote rainfall region, it incurs the divergence and ascending motion for TRP. On the other hand, the upper level jet stream covers the region and induces the descending motion in the same region for Non-TRP. Similar results are showed in the regionⅡ. The composite data analysis of moisture fluxes budget in regionⅠandⅡshows that tropical cyclones with TRP can transport moisture to reach the regionⅠandⅡand moisture transport for the cyclones without TRP is restricted only in a local area. The moisture increases remarkably for the area of TRP. On the contrary, moisture decreases obviously in the same area without TRP. Moisture transport of a tropical cyclone plays an important role for TRP.A numerical experiment with WRF/ARW model 3.1 version is conducted. Double nested domain is adopted with horizontal resolutions of 30km in outer region and 10km in inner region respectively. Vertical resolution is 27 levels. The initial and boundary condition are provided by the NCEP reanalysis data of“global final”with 1°×1°grid space. Typhoon Matsa (0509) producing typical TRP is selected for numerical experiment. Control experiment is conducted. The results fit with the observation well. Then three sensitive experiments are conducted and the results are as follows: (1) TRP will remarkably decrease if tropical cyclone is removed or its intensity is decreased. (2) Westerly trough is an important factor to TRP. The simulation shows that the strength of westerly trough has a direct proportion to the rain rate of TRP. (3) the simulation shows that topographic effect also plays a significant role in TRP. The remote rainfall would decrease 20% precipitation if the topography is removed.The study shows that the mechanism of the TRP is related to the interaction between the tropical cyclone and the mid-latitude trough, the ground topography would play an important role to increase the rainfall. The moisture transport of an offshore tropical cyclone is the most primary condition for the TRPs.更多还原