热带气旋登陆维持和迅速消亡的诊断研究

采用动态合成方法, 对登陆后长久维持热带气旋(LTC)和迅速衰亡热带气旋(STC)的涡度、动能、热量和水汽的收支平衡进行计算和对比分析.结果表明: (1)LTC在陆上长久维持过程中, 其低层正涡度衰减缓慢并保持一定强度.STC登陆后正涡度减弱较快.(2)热带气旋登陆后涡度的收支主要取决于水平散度项、平流项和剩余项.散度项使LTC低层正涡度增加, 高层减少, 平流项和剩余项则使其低层涡度减弱, 高层涡度增加.总体而言, LTC自高层获得正涡度的补充, STC则没有获得环境正涡度.(3)低层, 摩擦耗散使LTC动能减少, 但动能通量辐合可补充部分动能而减缓衰减.中高层, LTC登陆后36～60 h动能收大于支, 动能的增加一部分来自于斜压动能制造, 一部分来自于次网格尺度.STC有类似的动能耗散, 却无动能补充.(4)LTC登陆后保持一定强度, 并从外界获得热量和水汽补充来支持积云对流发展, 而积云对流对LTC的维持具有正反馈作用.STC登陆后没有这一过程.     

物理过程和分辨率对西南涡演变的中尺度模拟影响

本文运用PSU/NCAR中尺度模式第四改进型(MM_4),对“81.7”四川暴雨期西南涡的演变进行了一系列96和72小时数值模拟;其目的在于研究模式物理过程和空间分辨率以及行星边界层(PBL)参数化对西南涡生成和发展模拟的影响。96小时的控制试验(方案Ⅰ)结果表明:水平分辨率160km和总体PBL的10层MM_4对西南涡发展模拟得较好,但对其生成只有初步模拟的能力;各种物理过程主要影响西南涡演变的强度,而对其位置影响较小;去掉潜热释放会显著影响西南涡在后期的发展;没有地面通量将使西南涡的生成和发展强度明显减弱;于试验不能模拟出西南涡在整个暴雨期的发展,且使其移速略快,无摩擦试验指出,地面摩擦对西南涡的生成是不重要的。提高模式空间分辨率和采用高分辨PBL模式,能使西南涡生成和发展位置的模拟有明显改进,但其强度一般偏强于实况。事实表明,进一步完善模式物理过程、改进PBL参数化和提高模式空间分辨率将是十分必要的。     

黄土高原复杂地形受中尺度运动影响的稳定边界层湍流特征

利用兰州大学半干旱气候与环境观测站(Semi-Arid Climate and Environment Observatory of Lanzhou University,简称SACOL)2008年12月观测资料,研究了稳定边界层湍流特征.使用涡动相关资料研究湍流通量时,定义湍流的平均时间τ内的中尺度运动是造成湍流统计量变化范围大的主要原因,稳定情形? τ取几十秒至几分钟.对梯度理查森数大于0.3的强稳定情形的湍流尺度分解(MRD)谱分析表明,感热通量在112.4～449.9 s存在谱隙,尺度大于谱隙的中尺度运动造成了通量观测资料离散性大,甚至有支配性影响.动量通量的谱隙在112.4～224.9 s之间.弱风时,中尺度运动的影响更大,垂直风速标准差以0.1的比率随中尺度风速变化;垂直风速标准差同广义风速表现出很好的相关性,并随着广义风速消失而消失.三维风速标准差与摩擦速度呈很好的线性关系,垂直、水平、横风风速的无量纲标准差分别为1.35、2.54、2.21.对湍流动能的研究发现,在梯度理查森数大于0.3的条件下,仍然存在连续的湍流.以湍动能为依据,分析了湍流的平稳时间长度,其长度随稳定度变化而变化,2008年12月7～11日从133.5 s变化到856.2 s,湍流平稳时间长度反映了中尺度运动的发生频率.     

印度洋海温异常对西北太平洋台风的影响及机制研究

基于近40 a NCEP/NCAR再分析月平均高度场、风场、涡度场、垂直速度场以及NOAA重构的海面温度(sea surface temperature,SST)资料和美国联合台风预警中心(Joint Typhoon Warning Center,JTWC)热带气旋最佳路径资料,利用合成分析方法,研究了前期春季及同期夏季印度洋海面温度同夏季西北太平洋台风活动的关系。结果表明:1)前期春季印度洋海温异常(sea surface temperature anomaly,SSTA)尤其是关键区位于赤道偏北印度洋和西南印度洋地区对西北太平洋台风活动具有显著的影响,春季印度洋海温异常偏暖年,后期夏季,110°～180°E的经向垂直环流表现为异常下沉气流,对应风场的低层低频风辐散、高层辐合的形势,这种环流形势使得低层水汽无法向上输送,对流层中层水汽异常偏少,纬向风垂直切变偏大,从而夏季西北太平洋台风频数偏少、强度偏弱,而异常偏冷年份则正好相反。2)春季印度洋异常暖年,西北太平洋副热带高压加强、西伸;而春季印度洋异常冷年,后期夏季西北太平洋副热带高压减弱、东退,这可能是引起夏季西北太平洋台风变化的另一原因。     

一次黄海爆发性气旋的观测分析和数值模拟研究

本文利用再分析资料和WRFV3.9模式（Weather Research and Forecasting Model）对2020年7月22－24日发生在黄海海域的一次爆发性气旋进行了研究,并对其演变过程和发展机制进行了详细分析。该气旋22日12UTC在山东南部生成,入海后开始爆发性发展,最大加深率达到1.2 Bergeron,23日在黄海中部气压降至最低990 hPa左右,24日在韩国登陆。高空强辐散、低层的暖舌结构、水汽输送和下垫面热通量的变化增强了大气斜压性,使其迅速发展。使用WRF模式对气旋进行模拟,涡度的诊断分析表明,大气低层强斜压性主要通过涡度方程的散度项对气旋的发展起作用,对流项在涡度发展旺盛的时刻也有一定影响。海温的敏感性试验表明,海温变化对气旋移动路径和中心气压影响明显。     

A MASCOTTE-based characterization of MCSs over Spain, 2000–2002

A 3-year Mesoscale Convective Systems (MCSs) database, which extends from 2000 to 2002, has been built for the Iberian Peninsula and the Balearic Islands using the objective method MASCOTTE (MAximum Spatial COrrelation Technique). It was originally developed to track the evolution of convective systems over the Amazon region; after modifications, it is able to track MCS evolution even with an hour and a half of missed images and provides essential information of both dynamical and morphological characteristics of MCSs. MASCOTTE is tested against a visual and subjective method, and is found to offer advantages such as automation and a simple and efficient operation that make it very useful for building large MCS databases.Thirty-five MCSs were found between June and October, most of them originated along the Mediterranean coast and near the Pyrenees, showing an increasing occurrence from June to September, when the maximum is found. The regions most influenced by MCS occurrences are Balearic Islands, Valencia, Catalonia, Murcia and the Basque Country. The MCSs tend to be small, short-lived and linear, usually moving eastward or northeastward with low velocities.The MCSs-associated precipitation presents high variability, ranging from 80 to 0 mm h⁻¹ as maximum hourly records. Two different convective regions are identified based on MCS behavior in extreme precipitation events: Northern Spain (the Basque Country) with abundant and continuous precipitation regime but little MCS influence, and the Mediterranean coast, where precipitation is sporadic but much more intense.     

The Key Supply Source of Long-Distance Moisture Transport for the Extreme Rainfall Event on July 21, 2012 in Beijing

In this study, the Weather Research and Forecasting (WRF) model and meteorological observation data were used to research the long-distance moisture transport supply source of the extreme rainfall event that occurred on July 21, 2012 in Beijing. Recording a maximum rainfall amount of 460 mm in 24 h, this rainstorm event had two dominant moisture transport channels. In the early stage of the rainstorm, the first channel comprised southwesterly monsoonal moisture from the Bay of Bengal (BOB) that was directly transported to north China along the eastern edge of Tibetan Plateau (TP) by orographic uplift. During the rainstorm, the southwesterly moisture transport was weakened by the transfer of Typhoon Vicente. Moreover, the southeasterly moisture transport between the typhoon and western Pacific subtropical high (WPSH) became another dominant moisture transport channel. The moisture in the lower troposphere was mainly associated with the southeasterly moisture transport from the South China Sea and the East China Sea, and the moisture in the middle troposphere was mainly transported from the BOB and Indian Ocean. The control experiment well reproduced the distribution and intensity of rainfall and moisture transport. By comparing the control and three sensitivity experiments, we found that the moisture transported from Typhoon Vicente and a tropical cyclone in the BOB both significantly affected this extreme rainfall event. After Typhoon Vicente was removed in a sensitivity experiment, the maximum 24-h accumulated rainfall in north China was reduced by approximately 50% compared with that of the control experiment, while the rainfall after removing the tropical cyclone was reduced by 30%. When both the typhoon and tropical cyclone were removed, the southwesterly moisture transport was enhanced. Moreover, the sensitivity experiment of removing Typhoon Vicente also weakened the tropical cyclone in the BOB. Thus, the moisture pump driven by Typhoon Vicente played an important role in maintaining and strengthening the tropical cyclone in the BOB through its westerly airflow. Typhoon Vicente was not only the moisture transfer source for the southwesterly monsoonal moisture but also affected the tropical cyclone in the BOB, which was a key supply source of long-distance moisture transport for the extreme rainfall event on July 21, 2012 in Beijing.     

环境场对东海登陆热带气旋陆地路径的影响

采用合成分析的方法,对比分析了在华东南部沿海登陆,维持较短陆地路径和维持较长陆地路径的两类热带气旋的环境场变化,结果表明：环境场对登陆热带气旋的陆地路径的维持有重要影响。当西北太平洋副热带高压迅速加强西伸;中低层低压环流迅速减弱;中低层环境热力条件差,海洋水汽输送中断;高层水平流场呈纬向型,青藏高原反气旋环流相对较弱而西北太平洋副热带高压反气旋环流强大,中纬带西风急流强大时,环境场的配置使热带气旋周围的高层水平辐散弱并衰减,低层水平辐合弱并衰减,热带气旋的对流被抑制,仅能维持较短路径很快就减弱消亡。反之,当西北太平洋副热带高压减弱东退,西风槽南压;低层低压环流长时间维持;中低层环境热力条件好,海洋水汽输送通畅;高层水平流场呈经向型,青藏高原反气旋环流相对强大而西北太平洋副热带高压反气旋环流减弱东退,中纬带西风急流较弱,低空东南风急流强大时,环境场的配置使热带气旋周围高层水平辐散强并维持,低层水平辐合强并维持,热带气旋的对流通畅,能维持较长生命史和较长的路径。     

登陆热带气旋在鄱阳湖区的活动特征及原因分析

鄱阳湖流域是受登陆热带气旋（简称TC）影响最为频繁的内陆地区之一。利用中国气象局热带气旋年鉴资料、地面观测资料、中国气象局一日两次的常规探空资料以及NCEP/NCAR一日四次的2.5°×2.5°再分析资料,研究了过鄱阳湖TC的活动特征及其影响。首先统计1949~2012年进入鄱阳湖区域61个TC的活动特征发现,平均而言进入鄱阳湖地区的TC具有移速减慢,强度衰减变缓及降水增幅特征。大尺度环流场分析表明,登陆进入鄱阳湖区的TC处于太平洋副热带高压和东亚大陆高压之间的鞍形场中,引导气流减弱,水平风垂直切变减小,有利于其移速减慢、衰减减缓以及降水累计。几个典型TC个例的观测分析发现,TC降水降低了鄱阳湖区浅层土壤温度和低层大气温度及其日较差,但增加了土壤和空气湿度。鄱阳湖区域下垫面较高的地面热量有利于TC维持。而湖区对流有效位能在TC进入后明显降低,说明TC触发了该区域对流有效位能释放,有利于其降水增幅。     

2011年全球重大天气气候事件及其成因

2011年,全球气温偏高,为有观测记录以来的第十个暖年,是近60年来出现拉尼娜事件的年份中全球气温最高的一年。2011年,全球热带气旋活动较常年偏少。4月,一次拉尼娜事件结束,9月又一次拉尼娜事件生成。年初,低温、寒流席卷亚洲大部,暴风雪频繁袭击北美地区。西欧和中国东部出现严重春旱。夏季,非洲东部经历了20世纪80年代以来最严重的干旱,而东南亚、巴基斯坦和中南美洲洪涝灾害严重。全球极端偏暖事件主要出现在欧洲西部和西北部、南亚南部、东亚中西部、北美东南部等地;极端偏冷事件主要出现在东亚、澳大利亚、非洲南部和美国东北部和西部等地。而南美中东部、东南亚及中国东南部、日本、澳大利亚北部、非洲西部等地出现了极端强降水事件。研究发现,2010／2011年拉尼娜事件和台风活动是导致东南亚洪涝出现的重要原因,而巴基斯坦洪涝主要与印度洋正位相偶极型海温分布有关。