The potential impacts of warmer-continent-related lower-layer equatorial westerly wind on tropical cyclone initiation

Global climate models predict that the increasing Amazonian-deforestation rates cause rising tempera- tures (increases of 1.8℃ to 8℃ under different conditions) and Amazonian drying over the 21st century. Observations in the 20th century also show that over the warmer continent and the nearby western South At- lantic Ocean, the lower-layer equatorial westerly wind (LLEWW) strengthens with the initiation of tropical cyclones (TCs). The warmer-continent-related LLEWW can result from the Coriolis-force-induced deflection of the cross-equatorial flow (similar to the well-known heat-island effect on sea breeze) driven by the enhanced land-sea contrast between the warmer urbanized continents and relatively cold oceans. This study focuses on the processes relating the warmer-continent-related LLEWW to the TC initiation and demonstrates that the LLEWW embedded in trade easterlies can directly initiate TCs by creating cyclonic wind shears and forming the intertropical convergence zone. In addition to this direct effect, the LLEWW combined with the rotating Earth can boost additional updraft vapor over the high sea-surface temperature region (factor 1), facilitating a surface-to-midtroposphere moist layer (factor 2) and convective instability (factor 3) followed by diabatic processes. According to previous studies, the diabatic heating in a finite equatorial region also activates TCs (factor 4) on each side of the Equator with weak vertical shear (factor 5). Factors 1-5 are favorable conditions for the initiation of severe TCs. Statistical analyses show that the earliest signal of sustained LLEWW not only leads the earliest signal of sustained tropical depression by >3 days but also explains a higher percentage of total variance.     

A Study on the Dynamic Mechanism of the Formation of Mesoscale Vortex in Col Field

The mesoscale vortex associated with a mesoscale low-level jet(mLLJ) usually causes heavy rainfall in the col field.The col field is defined as a region between two highs and two lows,with the isobaric surface similar to a col.Using a two-dimensional shallow water model,the meso-β scale vortex couplets(MβVCs) induced by eight types of mesoscale wind perturbations in an ideal col field were numerically simulated.With the sizes of ～100 km,the MβVCs induced by northerly perturbation(NP) and southerly perturbation(SP) moved toward the col point.The sizes of MβVCs induced by southwesterly perturbation(SWP),southeasterly perturbation(SEP),northwesterly perturbation(NWP),and northeasterly perturbation(NEP) were relatively small for the perturbations moving toward dilatation axis.The MβVC induced by easterly perturbation(EP) and westerly perturbation(WP) could not develop because they quickly moved away from the col point,before the circulation could form.The size of the circulation was determined by the distance between the vortex and the col point.The closer to the col point the vortex was,the larger the size of vortex.The comparisons of maximum vorticity and vorticity root mean square error(RMSE) of the NP,the SWP,and the WP show that the maximum vorticity and the vorticity RMSE of the NP decreased slower than other perturbations.Therefore,the weak environment of the col field favors the maintenance of vorticity and the formation of vortex.When a mesoscale vortex forms near the col point or moves toward the col point,it may maintain a quasi-stationary state in the stable col field.     

A Climatology of Extratropical Cyclones over East Asia During 1958–2001

A climatology of extratropical cyclones (ECs) over East Asia (20 -75 N, 60 -160 E) is analyzed by applying an improved objective detection and tracking algorithm to the 4-time daily sea level pressure fields from the European Centre for Medium-range Weather Forecasts (ECMWF) reanalysis data. A total of 12914 EC processes for the period of 1958-2001 are identified, with an EC database integrated and EC activities reanalyzed using the objective algorithm. The results reveal that there are three major cyclogenesis regions: West Siberian Plain, Mongolia (to the south of Lake Baikal), and the coastal region of East China; whereas significant cyclolysis regions are observed in Siberia north of 60 N, Northeast China, and Okhotsk Sea-Northwest Pacific. It is found that the EC lifetime is largely 1-7 days while winter ECs have the shortest lifespan. The ECs are the weakest in summer among the four seasons. Strong ECs often appear in West Siberia, Northeast China, and Okhotsk Sea-Northwest Pacific. Statistical analysis based on k-means clustering has identified 6 dominating trajectories in the area south of 55 N and east of 80 E, among which 4 tracks have important impacts on weather/climate in China. ECs occurring in spring (summer) tend to travel the longest (shortest). They move the fastest in winter, and the slowest in summer. In winter, cyclones move fast in Northeast China, some areas of the Yangtze-Huaihe River region, and the south of Japan, with speed greater than 15 m s 1 . Explosively-deepening cyclones are found to occur frequently along the east coast of China, Japan, and Northwest Pacific, but very few storms occur over the inland area. Bombs prefer to occur in winter, spring, and autumn. Their annual number and intensity in 1990 and 1992 in East Asia (EA) are smaller and weaker than their counterparts in North America.     

The Evolution of a Meso-β-Scale Convective Vortex in the Dabie Mountain Area

The evolution of a mesoscale convective system (MCS) that caused strong precipitation in the northern area of Dabie Mountain during 21-22 June 2008 is analyzed, along with the evolution of the associated meso-β-scale convective vortex (MCV). The mesoscale reanalysis data generated by the Local Analysis and Prediction System (LAPS) at a 3-km horizontal resolution and a 1-h time resolution during the South China Heavy Rainfall Experiment (SCHeREX) were utilized. The results show that two processes played key roles in the enhancement of convective instability. First, the mesoscale low-level jet strengthened and shifted eastward, leading to the convergence of warm-wet airflow and increasing convective instability at middle and low levels. Second, the warm-wet airflow interacted with the cold airflow from the north, causing increased vertical vorticity in the vicinity of steeply sloping moist isentropic surfaces. The combined action of these two processes caused the MCS to shift progressively eastward. Condensation associated with the MCS released latent heat and formed a layer of large diabatic heating in the middle troposphere, increasing the potential vorticity below this layer. This increase in potential vorticity created favorable conditions for the development of a low-level vortex circulation. The vertical motion associated with this low-level vortex further promoted the development of convection, creating a positive feedback between the deep convection and the low-level vortex circulation. This feedback mechanism not only promoted the maturation of the MCS, but also played the primary role in the evolution of the MCV. The MCV formed and developed due to the enhancement of the positive feedback that accompanied the coming together of the center of the vortex and the center of the convection. The positive feedback peaked and the MCV matured when these two centers converged. The positive feedback weakened and the MCV began to decay as the two centers separated and diverged.     

豫东地区“6.3”与“7.17”两次致灾大风雷达资料对比分析

利用加密自动站常规、非常规资料和多普勒雷达资料,分析了2009年6月3日(简称"6.3"过程)和2010年7月17日(简称"7.17"过程)在豫东地区出现的大范围强对流、大风天气过程。结果表明,"6.3"和"7.17"过程虽然都有高空槽和槽后西北气流引导北方冷空气南下,使中高层气温迅速下降,造成大气层结的极不稳定和高低空风的垂直切变,进而高空动量下传,形成强对流的发生、发展、维持和传播。雷达径向速度和风廓线显示,"6.3"过程"弓形"出现了强回波区并位于回波区前沿,其移动速度快,具有径向风速辐合等飑线特征;"7.17"过程则是在中尺度涡旋云系中形成多个局地对流旺盛的单体,最终造成强降雨、雷电和局地强风暴等强对流天气。     

2009年7月7日南京短时暴雨的中尺度特征分析

利用FY-2C卫星红外辐射亮度温度(T_BB)资料、多普勒天气雷达资料、加密自动站资料、NECP再分析资料、常规观测资料对2009年7月7日发生在南京地区的一次短时大暴雨过程的中尺度特征进行分析,结果表明:在有利的天气尺度背景形势下,多个中尺度对流系统在南京地区合并,合并后的中尺度对流系统强度强,移速慢,造成了南京地区的强降水。这次短时暴雨的中尺度特征在云图T_BB资料上表现为对流云团合并后强度和范围显著增强,移速缓慢,T_BB梯度大值区在南京地区停留;在地面风场上体现为南移的中尺度辐合线与南京地区局地生成的中尺度辐合中心合并,使得地面风场辐合显著增强;在雷达回波上表现为,南京地区上空不断有对流单体并入形成大面积高效率降水回波,镶嵌其中的γ尺度对流单体沿着相同方向依次通过南京地区。分析中还发现,低空急流、低空切变线是这次短时暴雨天气过程的重要影响系统,利用多普勒雷达资料可以识别和分析它们的发展、变化特征,为短时暴雨的临近预报提供依据。     

2012年6月大气环流和天气分析

2012年6月大气环流特征为:北半球极涡呈单极型,中心位于北地群岛附近,较常年略偏强,中高纬环流多小波动;西太平洋副热带高压接近常年同期,南支槽位于90°E附近,槽前西南气流明显。2012年6月,全国平均气温为20.2℃,较常年同期(19.8℃)偏高0.4℃,全国大部地区气温接近常年同期或略偏高。全国平均降水量为108.5 mm,较常年同期(97.6 mm)偏多11.2%。月内,我国主要天气气候事件有:南方降水过程频繁,部分地区洪涝灾害较重;东北大部降水偏多,局部暴雨成灾;25个省(市、区)遭受风雹灾害,其中新疆、山东、吉林、甘肃、宁夏、陕西等省(区)局部受灾较重;西南气象干旱解除,黄淮、江淮气象干旱持续发展;强热带风暴泰利、杜苏芮先后影响我国。     

2012年9月大气环流和天气分析

2012年9月环流特征如下：北半球高纬度地区极涡呈单极型分布,中心略偏于西半球,北半球高纬度环流呈5波型分布,槽区分别位于亚洲西部、亚洲东部、太平洋中部、北美洲东部和欧洲西部。月内,西太平洋副热带高压西脊点位置变化较大,强度比常年同期偏强。9月,全国平均气温为16．6℃,与常年同期持平,全国平均降水量73．8mm,较常年同期（65．2mm）偏多13．2％。月内共出现6次降水过程,西北太平洋有3个热带气旋活动,没有热带气旋在我国登陆。北方多地出现低温冷冻灾害,16个省（区）遭受风雹灾害。     

用支持向量机方法做登陆热带气旋站点大风预报

将支持向量机（SVM）回归方法应用于在登陆热带气旋影响下,每天00、06、12、18 UTC 4时次2分钟平均的站点风速预报。从2002-2007年热带气旋本身强度、站点地形情况和站点附近高低空环境场要素,设计相关因子,建立了4种预报模式,其中模式4的风速拟合误差的标准差为1.591 m·s~（-1）。用2008年8个登录热带气旋做独立样本检验,预报风速与实际风速的平均绝对值误差为1.750 m·s~（-1）,标准差为2.367 m·s~（-1）。结果表明,在适当的样本截取和预报因子选取后,SVM方法建模的风速预报48小时内效果较好。     

越赤道气流准双周振荡对西北太平洋台风路径的调制作用

利用美国联合台风预警中心（Joint Typhoon Warning Center,JTWC）热带气旋（tropicalcy—clone,TC）数据、NCEP／NCAR再分析风场资料,研究了越赤道气流准双周振荡对西北太平洋台风路径的调制作用。将西北太平洋台风路径划分为：西行路径、西北行路径、转向登陆中国路径、转向中日之间路径、转向登陆日本路径、转向日本以东路径和140°E以东路径。利用超前滞后回归方法,合成分析了6—10月不同路径台风对应的越赤道气流准双周振荡的低频环流演变过程。结果表明,925hPa越赤道气流及与其相联系的经向风存在明显10～20d准双周振荡现象,且对西北太平洋台风路径预报具有一定的指示作用。在西太平洋赤道地区,低频越赤道气流强度、演变特征影响着西北太平洋低频气旋的位置和移动方向,调节风场强辐合带与季风槽的位置与强度,继而对台风生成位置、移动路径产生重要的影响。初步认为,强向北低频越赤道气流分量有利于北侧低频气旋加强和向北传播,继而使得强辐合带、季风槽位置偏北,台风易于在此区域生成且沿着强辐合带位置移动。而弱向北低频分量或向南低频分量则不利于台风转向移动。