A Statistical Analysis on the Effect of Vertical Wind Shear on Tropical Cyclone Development

Using tropical cyclone （TC） best track and intensity of the western North Pacific data from the Joint Typhoon Warning Center （JTWC） of the United States and the NCEP/NCAR reanalysis data for the period of 1992-2002, the effects of vertical wind shear on TC intensity are examined. The samples were limited to the westward or northwestward moving TCs between 5°N and 20°N in order to minimize thermodynamic effects. It is found that the effect of vertical wind shear between 200 and 500 hPa on TC intensity change is larger than that of the shear between 500 and 850 hPa, while similar to that of the shear between 200 and 850 hPa. Vertical wind shear may have a threshold value, which tends to decrease as TC intensifies. As the intensifying rate of TC weakens, the average shear increases. The large shear has the obvious trend of inhibiting TC development. The average shear of TC which can develop into typhoon （tropical depression or tropical storm） is below 7 m s^-1 （above 8 m s^-1）.

EFFECTS OF VERTICAL WIND SHEAR ON TROPICAL CYCLONE INTENSITY CHANGE

The effects of vertical wind shear on tropical cyclone(TC) intensity change are examined based on the TC data from the China Meteorological Administration and the NCEP reanalysis daily data from 2001 to 2006.First,the influence of wind shear between different vertical levels and averages in different horizontal areas are compared.The results indicate that the effect of wind shear between 200 and 850 hPa averaged within a 200-800 km annulus on TC intensity change is larger than any other calculated vertical wind shear.High-latitude and intense TCs tend to be less sensitive to the effects of VWS than low-latitude and weak TCs.TCs experience time lags between the imposition of the shear and the weakening in TC intensity.A vertical shear of 8-9 m/s(9-10 m/s) would weaken TC intensity within 60 h(48 h).A vertical shear greater than 10 m/s would weaken TC intensity within 6 h.Finally,a statistical TC intensity prediction scheme is developed by using partial least squares regression,which produces skillful intensity forecasts when potential predictors include factors related to the vertical wind shear.Analysis of the standardized regression coefficients further confirms the obtained statistical results.

A Lagrangian Trajectory Analysis of Azimuthally Asymmetric Equivalent Potential Temperature in the Outer Core of Sheared Tropical Cyclones

In this study,the characteristics of azimuthally asymmetric equivalent potential temperature(θ_(e))distributions in the outer core of tropical cyclones(TCs)encountering weak and strong vertical wind shear are examined using a Lagrangian trajectory method.Evaporatively forced downdrafts in the outer rainbands can transport low-entropy air downward,resulting in the lowestθ_(e)in the downshear-left boundary layer.Quantitative estimations ofθ_(e)recovery indicate that air parcels,especially those originating from the downshear-left outer core,can gradually revive from a low entropy state through surface enthalpy fluxes as the parcels move cyclonically.As a result,the maximumθ_(e)is observed in the downshear-right quadrant of a highly sheared TC.The trajectory analyses also indicate that parcels that move upward in the outer rainbands and those that travel through the inner core due to shear make a dominant contribution to the midlevel enhancement ofθ_(e)in the downshear-left outer core.In particular,the former plays a leading role in suchθ_(e)enhancements,while the latter plays a secondary role.As a result,moist potential stability occurs in the middle-to-lower troposphere in the downshear-left outer core.

INTERDECADAL VARIATION IN THE ACTIVITY OF TROPICAL CYCLONES AFFECTING KOREA

By performing a statistical change-point analysis of activities of the tropical cyclones(TCs)that have affected Korea(K-TCs),it was found that there was a signifi cant change between 1983 and 1984.During the period of 1984-2004(P2),more TCs migrated toward the west,recurved in the southwest,and affected Korea,compared to the period of 1965-1983(P1).These changes for P2 were related to the southwestward expansion of the subtropical western Pacifi c high(SWPH)and,simultaneously,elongation of its elliptical shape toward Korea.Because of these changes,the central pressure and lifetime of K-TC during P2 were deeper and longer,respectively,than fi gures for P1.This stronger K-TC intensity for P2 was related to the more southwestward genesis due to the southwestward expansion of the SWPH.The weaker vertical wind shear environment during P2 was more favorable for K-TC to maintain a strong intensity in the mid-latitudes of East Asia.

环境风速垂直切变对西北太平洋热带气旋强度变化的影响

利用2000---2006年中国气象局《热带气旋年鉴》和NCEP再分析日资料，对环境风垂直切变对西北太平洋热带气旋（TC）强度变化的影响进行统计分析。首先比较了不同高度层之间、不同水平区域平均的全风速垂直切变和纬向风速垂直切变对TC强度变化的影响，结果表明，全风速切变对TC强度变化的抑制作用显著大于纬向风速垂直切变；以200～800km的圆环区域平均计算的风速垂直切变与TC强度变化的负相关最显著；中高层的风速垂直切变与TC强度变化的相关优于中低层。其次，全风速切变大于8m／s后抑制TC增强，且这种抑制作用存在6．60h的滞后。全风速垂直切变大时，滞后时间较短：当全风速切变为8～9m／s（9—10m／s）时，TC强度在未来60（48）h开始减弱；当全风速切变大于10m／s时，TC在6h内开始减弱。最后，利用偏最小二乘回归建立TC强度变化的预报模型PLS—STIPSV。结果表明，加入风速垂直切变因子后对TC强度预报有所改进，并通过分析标准化回归系数进一步证实了上述的统计结果。

西北太平洋热带气旋强度与环境气流切变关系的气候分析

采用NCEP/NCAR再分析资料和JTWC(美国关岛联合台风警报中心)资料,对1974~2004年5~10月西北太平洋热带气旋(TC)强度和环境风垂直切变进行了趋势特征、振荡周期和空间结构分析。结果表明:西北太平洋热带风暴强度以上TC的最大风速和环境风垂直切变在时间上有相反的变化趋势,弱的环境风垂直切变有利于TC强度的增大;前12 h的环境风垂直切变对TC强度的发展影响最大。环境风垂直切变在西北太平洋TC最强的年份表现为环境风切变值小,TC发生密集;最弱的年份表现为环境风切变值大,TC发生稀疏。

台风“凤凰”初始扰动生成过程及其结构演变特征分析

热带气旋的生成是一个由初始扰动转变成为暖心结构低压的过程,但已有的经典理论对初始流场的配置及初始扰动的触发都没有详细的描述。利用FY-2C的TBB资料和日本再分析资料,以2008年8号台风"凤凰"为例,跟踪其流场和涡度场演变,追溯该热带气旋从初始扰动生成到转为有暖心结构的热带低压的过程。分析结果显示,初始扰动在水平风垂直切变较强的鞍形场中触发。随着高层环流调整,水平风垂直切变迅速减弱,高空冷涡外围暖区云系为初始扰动的发展提供了有利动力和热力条件。在其发展为热带低压的过程中,结构上出现正涡度从对流层中层向低层加强并有中心下降的特点,低层涡度的快速增加主要受气压梯度增强导致的辐合增强的影响。由个例分析结果可以看到,高空冷涡和副热带高压对热带初始扰动的生成以及发展和结构演变都起到重要的作用。

FORMATION AND STRUCTURAL EVOLUTION OF INITIAL DISTURBANCE OF TYPHOON FUNG-WONG (2008)

The formation of a tropical cyclone is the result of a process in which an initial disturbance evolves into a warm-core low-pressure system;however,the origin of the initial disturbance and the features of the initial fields are overlooked in most existing theories.In this study,based on FY-2C brightness temperature data and the Japan reanalysis dataset,the origin and evolution of the tropical disturbance that became Typhoon Fung-Wong(2008) were examined.The results demonstrated that the initial disturbance emerged within a saddle-type field with large vertical tropospheric wind shear.The vertical wind shear decreased with the adjustment of the upper circulation;moreover,accompanied by convection over the warm section around the upper cold vortex,it provided favorable thermal and dynamic conditions for the development of a tropical vortex.During its development,the zone of associated positive relative vorticity strengthened and descended from the mid-troposphere to lower levels.This rapid strengthening of lower-level vorticity was due to increasing convergence related to the intensification of the pressure gradient southwest of the subtropical high.This indicated that the upper cold vortex and West Pacific subtropical high played very important roles in this case.