Variations in High-frequency Oscillations of Tropical Cyclones over the Western North Pacific

Variations in the high-frequency oscillations of tropical cyclones(TCs) over the western North Pacific(WNP) are studied in numerical model simulations. Power spectrum analysis of maximum wind speeds at 10 m(MWS_(10)) from an ensemble of15 simulated TCs shows that oscillations are significant for all TCs. The magnitudes of oscillations in MWS_(10) are similar in the WNP and South China Sea(SCS); however, the mean of the averaged significant periods in the SCS(1.93 h) is shorter than that in the open water of the WNP(2.83 h). The shorter period in the SCS is examined through an ensemble of simulations,and a case simulation as well as a sensitivity experiment in which the continent is replaced by ocean for Typhoon Hagupit(2008). The analysis of the convergence efficiency within the boundary layer suggests that the shorter periods in the SCS are possibly due to the stronger terrain effect, which intensifies convergence through greater friction. The enhanced convergence strengthens the disturbance of the gradient and thermal wind balances, and then contributes to the shorter oscillation periods in the SCS.     

西北太平洋热带气旋达到不同强度时所处位置的变化趋势研究

本文利用美国联合台风预警中心（JTWC)、中国气象局上海台风研究所（CMA）及日本气象厅东京台风中心（JMA）3 套热带气旋（TC）数据集,分别选取TC 达到生命史极值强度时、达到台风等级时以及达到热带风暴等级时所在的位置作为研究指标,分析了1980—2013 年5—11 月西北太平洋TC 达到不同强度时所在位置的长期变化趋势。研究得出如下结论：西北太平洋TC 在达到生命史极值强度时所处的位置表现为显著向北移动的趋势,3 个不同数据集向北移动趋势值分别为90、93、113 km/10a。同时TC 在达到台风和热带风暴级别时还存在相对明显的向北和向西移动趋势。本文进一步从环境场出发,分析了垂直风切变、海表温度以及潜在生成指数等影响因子的变化特征,为TC达不同强度时所处位置的长期变化趋势给出可能的物理解释。     

1949—2009年西北太平洋热带气旋气候特征

利用1949—2009年中国气象局热带气旋（TC:Tropical Cyclone）最佳路径数据集,对西北太平洋TC生成源地的时空分布、生成和登陆我国TC的年、月频数分布、强度分布和地理分布等气候特征进行统计分析。研究结果表明：在这61年中生成的TC呈减少趋势,登陆我国的TC数量比较稳定,不过近10年来登陆的较强TC数量却呈现出上升趋势;每年6—10月是TC高发期,强度等级越高的TC生成季节越偏晚,而8月是生成和登陆数量最多的月份;TC平均生命期随强度等级的增加而增大,且逐渐表现出单峰值分布特征;TC频数的地理分布以我国南海和菲律宾以东洋面为中心,向四周呈辐射状减少,近10年来其活动范围有所西伸加强。     

Impact of the Western Pacific Tropical Easterly Jet on Tropical Cyclone Genesis Frequency over the Western North Pacific

Although it is well known that the tropical easterly jet(TEJ)has a significant impact on summer weather and climate over India and Africa,whether the TEJ exerts an important impact on tropical cyclone(TC)activity over the western North Pacific(WNP)remains unknown.In this study,we examined the impact of the TEJ on the interannual variability of TC genesis frequency over the WNP in the TC season(June-September)during 1980-2020.The results show a significant positive correlation between TC genesis frequency over the WNP and the jet intensity in the entrance region of the TEJ over the tropical western Pacific(in brief WP_TEJ),with a correlation coefficient as high as 0.66.The intensified WP_TEJ results in strong ageostrophic northerly winds in the entrance region and thus upper-level divergence to the north of the jet axis over the main TC genesis region in the WNP.This would lead to an increase in upward motion in the troposphere with enhanced low-level convergence,which are the most important factors to the increases in low-level vorticity,mid-level humidity and low-level eddy kinetic energy,and the decreases in sea level pressure and vertical wind shear in the region.All these changes are favorable for TC genesis over the WNP and vice versa.Further analyses indicate that the interannual variability of the WP_TEJ intensity is likely to be linked to the local diabatic heating over the Indian Ocean-western Pacific and the central Pacific El Ni?o-Southern Oscillation.     

Impact of the Subtropical High on the Extratropical Transition of Tropical Cyclones over the Western North Pacific

Recent publications have investigated the interactions between the extratropical transitions （ETs） of tropical cyclones （TCs） and midlatitude circulations; however, studies of ET events have rarely considered the relationship between the storm and the nearby subtropical high. The TC best-track data provided by the Regional Specialized Meteorological Center-Tokyo Typhoon Center of the Japan Meteorology Agency are used in conjunction with the NCEP/NCAR reanalysis data to discuss the potential effects of the subtropical high on ETs over the western North Pacific basin. When the western Pacific subtropical high （WPSH） is weakened and withdrawn toward the east, more TCs follow recurving paths and the midlatitude trough activity is intensified. These changes lead to enhanced ET activity. By contrast, when the WPSH strengthens and extends westward, the number of TCs that follow direct westward paths increases and the midlatitude trough is relatively inactive. These conditions lead to reduced occurrences of ET cases. Abnormal activity of the WPSH should be considered as an important factor in determining ET activity.     

西北太平洋大气准双周振荡对热带气旋活动的影响

利用JRA逐日风场资料、 NOAA/NCEP的逐日OLR场资料以及美国联合台风预报中心的热带气旋 (TC) 数据, 通过对西北太平洋 (WNP) 上空10～20天大气准双周振荡 (QBWO) 不同位相的划分, 深入分析了QBWO对WNP区域生成TC的调制作用。研究结果表明: 在西北太平洋准双周尺度上, 对流与纬向风表现出沿热带地区向西偏北传播的特性。不同位相合成的季风槽位置和强度也发生相应的改变, 由此可见, QBWO是WNP上空季风槽季内变化的重要影响因子。当处于位相1、 4时, WNP生成TC的概率较低, 且登陆我国TC的数量也较少; 当处于位相2、 3时, WNP发生TC的概率较高, 特别是处于位相3时, 不仅TC发生概率最高, 而且登陆我国的TC数量也最多。沿热带地区西传的天气尺度波动 (周期10天以下) 在WNP通过季风槽的纬向风辐合作用, 易于转变为波数较大、 波长较短的热带低压 (TD) 型扰动, 这种扰动在季风槽区通过能量的转换有利于发展成为TC。     

Western North Pacific Tropical Cyclone Database Created by the China Meteorological Administration

This paper describes the access to, and the content, characteristics, and potential applications of the tropical cyclone(TC) database that is maintained and actively developed by the China Meteorological Administration, with the aim of facilitating its use in scientific research and operational services. This database records data relating to all TCs that have passed through the western North Pacific(WNP) and South China Sea(SCS) since 1949. TC data collection has expanded over recent decades via continuous TC monitoring using remote sensing and specialized field detection techniques,allowing collation of a multi-source TC database for the WNP and SCS that covers a long period, with wide coverage and many observational elements. This database now comprises a wide variety of information related to TCs, such as historical or real-time locations(i.e., best track and landfall), intensity, dynamic and thermal structures, wind strengths, precipitation amounts, and frequency. This database will support ongoing research into the processes and patterns associated with TC climatic activity and TC forecasting.     

Three Types of Tropical Waves Related to Tropical Cyclogenesis over the Western North Pacific

The present study applies a space-time filter to identify three dominant types of tropical waves： Madden-Julian oscillations （MJOs）, equatorial Rossby （ER） waves, and tropical depression （TD）-type disturbances. The impacts of these waves on tropical cyclones （TCs） were investigated based on 131 observations during the period 2000-07. The results suggest that 72% of TC geneses were related to the joint impacts of more than one type of wave. The composites for cases in different categories reveal that TCs related to the concurrence of the three types of waves have strong and large initial vortices at the time of TC genesis. In the absence of the MJO, ER- and TD-related TC genesis, embedded in easterly flow, exhibits a relatively fast initiation process and gives rise to a relatively small scale vortex. In contrast, without the ER wave contribution, TCs associated with ER and TD waves did not require strong convection at the time of genesis because an initial vortex can rapidly develop in the MJO active phase through persistent energy transfer. The MJO-related TC geneses were scattered in geographic distribution, as opposed to the clustered and eastward shift observed for genesis cases without contributions from MJOs.     

Insight into the Role of Lower-Layer Vertical Wind Shear in Tropical Cyclone Intensification over the Western North Pacific

Vertical wind shear fundamentally influences changes in tropical cyclone (TC) intensity. The effects of vertical wind shear on tropical cyclogenesis and evolution in the western North Pacific basin are not well understood. We present a new statistical study of all named TCs in this region during the period 2000-2006 using a second-generation partial least squares (PLS) regression technique. The results show that the lower-layer (between 850 hPa and 10 m above the sea surface) wind shear is more important than the commonly analyzed deep-layer shear (between 200 and 850 hPa) for changes in TC intensity during the TC intensification period. This relationship is particularly strong for westerly low-level shear. Downdrafts induced by the lower-layer shear bring low θ e air into the boundary layer from above, significantly reducing values of θ e in the TC inflow layer and weakening the TC. Large values of deep-layer shear over the ocean to the east of the Philippine Islands inhibit TC formation, while large values of lower-layer shear over the central and western North Pacific inhibit TC intensification. The critical value of deep-layer shear for TC formation is approximately 10 ms-1 , and the critical value of lower-layer shear for TC intensification is approximately ±1.5 ms-1 .     

2015年西北太平洋热带气旋预报精度评定

本文以中国气象局上海台风研究所整编的热带气旋(Tropical Cyclme,TC)最佳路径集为依据,对2015年西北太平洋TC定位精度及路径、登陆点、强度预报精度进行了评定,结果表明：2015年中央气象台TC平均定位误差为14.1 km,优于2014年定位水平;中央气象台24、48、72、96和120 h路径预报误差分别为66.2、119.5、176.3、244.3和328.5 km;国内外共6个全球模式在上述预报时效的总平均路径误差分别为86.5、146.5、215.8、321.6和475.8 km;4个区域模式24、48和72 h的总平均路径误差分别为84.1、147.1和230.8 km。2015年的主观方法、全球模式和区域模式的路径预报性能均较2014年有了较大进步,但是强度预报性能仍未得到改善。目前,统计预报方法的强度预报整体性能仍然领先于数值模式。     

西北太平洋热带气旋强度突变的分布特征

用42年（1949～1990）资料在西北太平洋区域统计各小区（5°×5°经纬度）内热带气旋强度突变（12 h内热带气旋近中心最大风速变化#A≥|10| m/s）的频数和频率，给出其分布特征。 其中频率迅速变化地带除与地形有关外， 还与黑潮、常年的副热带西风急流和副热带高压的活动位置有密切关系。     

卑尔根气候模式中亚洲—太平洋涛动和影响西北太平洋热带气旋频数的环流背景的关系

利用挪威卑尔根的全球大气-海洋-海冰耦合模式的300年气候态数值积分结果,结合观测资料,分析了夏季亚洲-太平洋涛动(APO)的年际变化对与西北太平洋热带气旋生成频数相关联的大尺度环流背景的影响。模式结果表明,当夏季APO异常偏强(弱)时,西太平洋副热带高压位置偏东(西)偏北(南),南亚高压位置偏北(南),西北太平洋低层大气异常辐合(辐散),高层大气异常辐散(辐合),对流活动加强(减弱)。这种环流背景条件有(不)利于西北太平洋热带气旋的发生发展,西北太平洋热带气旋频数因而偏多(少)。     

2009年西北太平洋热带气旋活动的若干特点分析

2009年西北太平洋及南海海域共生成23个风暴以上等级的热带气旋(TC),比常年偏少,但登陆我国的频数(10个)多于常年,台风以上等级TC的活动强于常年.TC源地较常年偏西,并且季节性纬向迁移特征明显,而经向位移并无显著异常,在南海海域生成的TC比例(26.1%)远高于气候均值;路径以西行和西北行为主.登陆华南TC较多...     

A Western North Pacific Tropical Cyclone Intensity Prediction Scheme

A western North Pacific tropical cyclone (TC) intensity prediction scheme (WIPS) is developed based on TC samples from 1996 to 2002 using the stepwise regression technique, with the western North Pacific divided into three sub-regions: the region near the coast of East China (ECR), the South China Sea region (SCR), and the far oceanic region (FOR). Only the TCs with maximum sustained surface wind speed greater than 17.2 m s-1 are used in the scheme. Potential predictors include the climatology and persistence factors, synoptic environmental conditions, potential intensity of a TC and proximity of a TC to land. Variances explained by the selected predictors suggest that the potential intensity of a TC and the proximity of a TC to land are significant in almost all the forecast equations. Other important predictors include vertical wind shear in ECR, 500-hPa geopotential height anomaly at the TC center, zonal component of TC translation speed in SCR, intensity change of TC 12 or 24 h prior to initial time, and the longitude of TC center in FOR. Independent tests are carried out for TCs in 4 yr (2004-2007), with mean absolute errors of the maximum surface wind being 3.0, 5.0, 6.5, 7.3, 7.6, and 7.9 m s-1 for 12- to 72-h predictions at 12-h intervals, respectively. Positive skills are obtained at all leading time levels as compared to the climatology and persistence prediction scheme, and the large skill scores (near or over 20%) after 36 h imply that WIPS performs especially better at longer leading times. Furthermore, it is found that the amendment in TC track prediction and real-time model analysis can significantly improve the performance of WIPS in the SCR and ECR. Future improvements will focus on applying the scheme for weakening TCs and those near the coastal regions.     

Diurnal Variation of Tropical Cyclone Rainfall in the Western North Pacific in 2008-2010

Diurnal variation of tropical cyclone (TC) rainfall in the western North Pacific (WNP) is investigated using the high-resolution Climate Prediction Center's morphing technique (CMORPH) products obtained from the National Oceanic and Atmospheric Administration (NOAA). From January 2008 to October 2010, 72 TCs and 389 TC rainfall days were reported by the Joint Typhoon Warning Center's (JTWC) best-track record. The TC rain rate was partitioned using the Objective Synoptic Analysis Technique (OSAT) and interpolated into Local Standard Time (LST). Harmonic analysis was applied to analyze the diurnal variation of the precipitation. Obvious diurnal cycles were seen in approximately 70% of the TC rainfall days. The harmonic amplitude and phase of the mean TC rainfall rate vary with TC intensity, life stage, season, and spatial distribution. On the basis of intensity, tropical depressions (TDs) exhibit the highest precipitation variation amplitude (PVA), at approximately 30%, while super typhoons (STs) contain the lowest PVA, at less than 22%. On the basis of lifetime stage, the PVA in the decaying stage (more than 37%) is stronger than that in the developing (less than 20%) and sustaining (28%) stages. On the basis of location, the PVA of more than 35% (less than 18%) is the highest (lowest) over the high-latitude oceanic areas (the eastern ocean of the Philippine Islands). In addition, a sub-diurnal cycle of TC rainfall occurs over the high-latitude oceans. On the basis of season, the diurnal variation is more pronounced during summer and winter, at approximately 30% and 32%, respectively, and is weaker in spring and autumn, at approximately 22% and 24%, respectively.     

2013年西北太平洋热带气旋预报精度评定

本文以中国气象局上海台风研究所整编的最佳路径集为依据,对2013年西北太平洋热带气旋（TC）定位、路径和强度预报精度进行了评定,并对部分全球和区域模式在路径和强度预报中存在的系统性偏差进行了分析,结果表明：2013年定位总平均误差为21.7 km,比往年略偏小。国内各省(自治区)主观预报方法路径预报平均误差为80.2 km（24 h）、143.3 km（48 h）和221.7 km（72 h）,与2012年同比分别降低了13.9%、13.4%和20.9%,中央气象台24 h路径预报误差首次低于90 km。全球和区域模式的路径预报性能稳步提升,并表现出一定的系统性偏差。统计预报方法的强度预报整体性能仍然领先于数值模式,而在数值模式中,区域模式的强度预报性能则略优于全球模式。部分全球和区域模式在强度预报中也存在着系统性偏差。各主观方法对台风温比亚(1306)、台风尤特(1311)、台风潭美(1312)和台风菲特(1323)的24 h登陆点预报效果较好,而对台风西马仑(1308)的24 h登陆点预报不是十分理想。     

A New Prediction Model for Tropical Storm Frequency over the Western North Pacific Using Observed Winter-Spring Precipitation and Geopotential Height at 500 hPa

A new seasonal prediction model for annual tropical storm numbers(ATSNs)over the western North Pacific was developed using the preceding January-February(JF)and April-May(AM)grid-point data at a resolution of 2.5°×2.5°.The JF and AM mean precipitation and the AM mean 500-hPa geopotential height in the Northern Hemisphere,together with the JF mean 500-hPa geopotential height in the Southern Hemisphere,were employed to compose the ATSN forecast model via the stepwise multiple linear regression technique.All JF and AM mean data were confined to the Eastern Hemisphere.We established two empirical prediction models for ATSN using the ERA40 reanalysis and NCEP reanalysis datasets,respectively,together with the observed precipitation.The performance of the models was verified by cross-validation.Anomaly correlation coefficients(ACC)at 0.78 and 0.74 were obtained via comparison of the retrospective predictions of the two models and the observed ATSNs from 1979 to 2002.The multi-year mean absolute prediction errors were 3.0 and 3.2 for the two models respectively,or roughly 10% of the average ATSN.In practice,the final prediction was made by averaging the ATSN predictions of the two models.This resulted in a higher score,with ACC being further increased to 0.88,and the mean absolute error reduced to 1.92,or 6.13% of the average ATSN.     

西北太平洋低频振荡对热带气旋生成的动力作用及其物理机制

利用全球再分析资料以及美国联合台风预报中心的热带气旋(TC)数据, 从动力和能量转换方面深入分析了西北太平洋上空30～60天大气低频振荡(MJO)对西北太平洋区域TC生成的调制作用。研究结果表明, 当西北太平洋西侧为MJO的西风位相所控制时, MJO通过纬向风的辐合作用使得在辐合区传播的波动发生波数增加, 波长减短的结构改变, 从而触发较大尺度波动向天气尺度波动的演变; 西风位相期间纬向风的纬向辐合与经向切变可以使得低频波动动能向高频波动的转换得到加强, 从而使得在此区域TC生成的数量明显偏多。相反, 当西北太平洋西侧为MJO的东风位相时, TC生成的数量得到抑制。此外, 随着西风位相中西风的加强(东风位相中东风的加强), TC的生成概率将得到增加 (减少)。但是, 在西北太平洋东侧海域, MJO对TC活动的调制作用要减弱许多。对MJO活动年际变化的研究表明, 在西太暖池处于暖状态年时, 西北太平洋西侧的MJO活动频繁, 西风位相活跃, 从而有利于此区域TC的生成, 而冷年的情况正好相反。     

西北太平洋热带气旋闪电时空尺度和光辐射能

利用1998—2014年热带测雨(Tropical Rainfall Measuring Mission, TRMM)卫星上携带的闪电成像仪(lightning imaging sensor, LIS)数据, 建立西北太平洋热带气旋闪电数据集, 并研究该区域热带气旋闪电属性特征。结果表明: 热带气旋闪电各属性值均呈对数正态分布特征, 热带气旋极大值闪电更倾向于发生在海洋和热带低压强度等级。不同强度等级(热带低压、热带风暴、台风)的热带气旋闪电持续时间无明显差异, 但热带风暴强度等级的闪电空间尺度和光辐射能平均值均低于热带低压和台风。对于热带气旋的不同区域(内核、内雨带、外雨带), 内核闪电的持续时间最大、光辐射能最强, 且持续时间和光辐射能随着闪电与热带气旋中心距离的增加而逐渐减小, 外雨带达到最小值。在海陆差异方面, 热带气旋闪电在海洋上的空间尺度和光辐射能比陆地大, 而闪电持续时间无明显差异。与非热带气旋闪电相比, 热带气旋闪电的延展距离、通道面积和光辐射能均更小, 但闪电平均持续时间更长。