水汽输送影响登陆热带气旋维持和降水的数值试验

Bilis(0010)是一个登陆后长久维持的热带气旋,其维持期间与一条西南风低空急流水汽通道长时间保持联结.用中尺度模式MM5V3对其维持过程进行模拟,并通过多个敏感性试验,研究不同方向水汽输送对热带气旋在陆上维持和降水的影响.结果表明:外界持续的水汽输送有利于热带气旋气旋性环流在陆地上维持,减缓其登陆后的强度衰减.水汽输送通过影响热带气旋的热力结构来影响热带气旋强度,充沛的水汽供应有利于热带气旋暖心维持.外界水汽输送有助于热带气旋雨带中的强对流活动,使雨量加强.而对流活动过程中水汽凝结潜热释放是热带气旋获得能量继续维持的重要条件.结果还表明,南边界水汽输送对热带气旋维持和降水的影响最为明显,其次是东边界.来自北界和西界水汽的影响微弱.     

Convective Asymmetries Associated with Tropical Cyclone Landfall： β-Plane Simulations

The physical processes associated with changes in the convective structure of an idealized tropical cyclone (TC) during landfall on a beta-plane were studied using the fifth-generation Pennsylvania State University--National Center for Atmospheric Research Mesoscale Model, version 3 (MM5). The simulation results suggested that the suppression of moisture supply and increased friction acted to enhance the convection from the left and front quadrants of the TC to the front and right of the TC during different periods of landfall. When surface moisture flux was turned off, convection in other parts of the quadrant was clearly suppressed and the total rainfall was reduced. When surface friction was increased, precipitation showed a marked increase after the TC made landfall. Wetter air at low and intermediate levels, and drier air at high levels around the onshore side of the coastline led to a high value of convective available potential energy (CAPE). Consequently, convection was enhanced immediately downstream of this area when the surface moisture flux was cut off. When surface friction was increased, the physical process was similar prior to landfall. After landfall, increased convergence at the onshore side of the land resulted in enhanced convection in front of the TC. Consistent with previous findings, our results suggest that during landfall the TC structure changes from one of thermodynamic symmetry to asymmetry due to differential moisture flux between the land and sea surface. The asymmetry of the thermodynamic structure, which can be explained by the distribution of CAPE, causes an asymmetric rainfall structure.     

Structural Changes of a Tropical Cyclone during Landfall: β-plane Simulations

This study investigated the effects of landfall on the structure of a tropical cyclone (TC). Numerical simulations were performed using the Weather Research and Forecasting Model on a β-plane. Two landfall experiments, one with an east--west and another with a north--south oriented coastline, were performed. Similar to previous studies on an f-plane, large-scale flows in the low-to-middle troposphere were modified due to friction. A pair of counter rotating gyres was found, which was shown to be consistent with the slight deflection of the TC relative to the control experiment without land. Compared to previous f-plane simulations, because of the inherent asymmetries due to the β-gyres, the large-scale pattern of flows and convergences/divergences related to friction were found to depend on coastline orientations. On the other hand, regardless of the coastline orientation, convergences were found to be stronger to the left for both cases near landfall, as in previous f-plane simulations. Such a convergence pattern subsequently induced a change in convection and rainfall at the eyewall.     

登陆中国不同强度热带气旋的变化特征

根据《热带气旋等级》国家标准(2006),将热带气旋(TC)划分为热带低压(TD)、热带风暴(TS)、强热带风暴(STS)、台风(TY)、强台风(STY)、超强台风(SSTY)6个等级,利用中国气象局整编的1949—2006年共58年的《台风年鉴》和《热带气旋年鉴》资料,分析了登陆中国大陆、海南和台湾不同强度TC变化特征。结果表明：(1) 不同强度登陆TC频数存在年际和年代际变化,在长期趋势上,TD、TS登陆频数呈现显著的线性递减趋势,STY登陆频数呈现显著增加趋势。(2) 登陆TD、TS、STS存在6—8年的周期变化,TY具有准16年的周期变化。(3) 登陆TD、TS主要生成于南海东北部海面,登陆TY、STY、SSTY多生成于巴士海峡东南部海面和菲律宾以东洋面。(4) 在年代际变化上,南海生成的登陆TD、TS频数有减少趋势,TY、STY有增多趋势。     

登陆我国过湖泊热带气旋的统计特征

利用1949～2001年热带气旋年鉴资料,对53年登陆我国并经过内陆湖泊的热带气旋特征进行统计分析.结果表明:登陆过湖泊热带气旋在陆上维持时间长,登陆时强度较强;湖泊地带能延缓登陆TC强度的衰减;热带气旋经过内陆湖泊时大多表现为中心气压维持不变或降低,风速增大;长江中游地区是登陆过湖泊热带气旋消失数最多的区域.     

麦莎台风登陆后能量过程与水汽供应的诊断研究

对0509号台风"麦莎(Matsa)"登陆后长时间维持并轻度加强的过程进行了诊断研究,此过程涉及到多种因素.位涡分析表明,当中纬度西风槽东移,该槽底部分裂出的一个较小的正位涡中心与"麦莎"合并使"麦莎"的涡旋动能增强,而在9日之后"麦莎"与槽主体合并的阶段,槽区主要的正位涡中心与"麦莎"融合.动能收支分析发现,"麦莎"登陆北移过程中,高层的无辐散风穿越等高线将位能转换为动能这一过程较"麦莎"的整体加强为早,而辐散风是低层动能的主要来源.中低层天气尺度系统为积云对流的发展提供动能,而积云对流释放潜热又为高层动能的维持提供了帮助.将"麦莎"与北美"Agnes"飓风比较后发现,"麦莎"加强程度比"Agnes"较弱的原因之一,是高层的无辐散风把台风环流内的动能向环境输出,而"Agnes"飓风则是环境区有大量动能向台风环流区输送.分析水汽来源可知,在"麦莎"登陆期间及其后副热带和热带的两条水汽通道同时或分别为"麦莎"的积云对流提供了足够的水汽供应.     

Variation characteristics of tropical cyclones making landfall in China at different intensities

According to the national standard(2006)on tropical cyclone(TC)intensity,TCs are categorized into six intensity types,namely,tropical depression(TD),tropical storm(TS),severe tropical storm(STS), typhoon(TY),severe typhoon(STY),and super severe typhoon(SSTY).Fifty-eight years(1949–2006)of the datasets from the Yearbook of Typhoons and Yearbook of Tropical Cyclones were used to study the variation characteristics of TCs making landfalls in mainland China,Hainan and Taiwan islands.The main results are as foll...     

STATISTICAL CLASSIFICATION AND CHARACTERISTICS ANALYSIS OF BINARY TROPICAL CYCLONES OVER THE WESTERN NORTH PACIFIC OCEAN SUMMER: CONTRASTIVE ANALYSIS FOR ACTIVE AND INACTIVE YEARS OF TROPICAL CYCLONES

Using the 1949-2007 western North Pacific tropical cyclones (TCs) best-track data archived at the Shanghai Typhoon Institute of China Meteorological Administration for the western North Pacific from 1949 to 2007,both the characteristics of binary and multiple TCs and samples of interactions among TCs and multi-TCs are identified and statistically analyzed.According to the various features of individual TC tracks and interacting tracks,seven distinct types are proposed to describe the binary system of TCs and their interaction samples.The mean trajectories of the west and east component of binary TCs in each type are obtained using a new cluster analysis technique.These types are then analyzed in terms of landfall process,occurrence seasonality,coexistent lifetime,especially the large-scale patterns of atmospheric circulation.Finally,typical steering flows and conceptual models of the binary TCs at different phases are established based on six-hourly flow maps of the binary system and the averages are determined of the mean steering flow of ten representative binary TCs.Then,typical steering flows and conceptual models at the beginning,middle and final phase in each type are established to describe the large-scale circulation patterns of the binary system interaction types.     

Maintenance of Tropical Cyclone Bill （1988） after Landfall

The mechanism for the maintenance of Tropical Cyclone Bill （1988） after landfall is investigated through a numerical simulation. The role of the large-scale environmental flow is examined using a scale separation technique, which isolates the tropical cyclone from the environmental flow. The results show that Bill was embedded in a deep easterly-southeasterly environmental flow to the north-northeast of a large-scale depression and to the southwest of the western Pacific subtropical high. The depression had a quasi-barotropic structure in the mid-lower troposphere and propagated northwestward with a speed similar to the northwestward movement of Bill. The moisture budgets associated with both the large-scale and the tropical cyclone scale motions indicate that persistent low-level easterly-southeasterly flow transported moisture into the inner core of the tropical cyclone. The low-level circulation of the tropical cyclone transported moisture into the eyewall to support eyewall convection, providing sufficient latent heating to counteract energy loss due to surface friction and causing the storm to weaken relatively slowly after landfall. Warming and a westward extension of the upper-level easterly flow led to westward propagation of the environmental flow in the mid-lower troposphere. As a result, Bill was persistently embedded in an environment of deep easterly flow with high humidity, weak vertical wind shear, convergence in the lower troposphere, and divergence in the upper troposphere. These conditions are favorable for both significant intensification prior to landfall and maintenance of the tropical cyclone after landfall.     

全球变暖背景下我国热带气旋灾害趋势分析

利用热带气旋(TC)灾害等资料,对全球变暖背景下我国TC灾害和防台减灾工作的成效等进行了初步分析.在过去25年中,每年平均有6～7个TC登陆我国沿海省份及海南岛,登陆时的平均强度达到29.9 m/s,平均在陆地上维持15.6个小时,导致505人死亡,造成370亿(2008年人民币水平)的直接经济损失,占GDP的0.4％...