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。

RESEARCH PROGRESS ON THE STRUCTURE AND INTENSITY CHANGE FOR THE LANDFALLING TROPICAL CYCLONES

Landfalling tropical cyclones(LTCs)include those TCs approaching the land and moving across the coast.Structure and intensity change for LTCs include change of the eye wall,spiral rain band,mesoscale vortices,low-layer shear lines and tornadoes in the envelope region of TC,pre-TC squall lines,remote rain bands,core region intensity and extratropical transition(ET)processes,etc.Structure and intensity change of TC are mainly affected by three aspects,namely,environmental effects,inner core dynamics and underlying surface forcing.Structure and intensity change of coastal TCs will be especially affected by seaboard topography,oceanic stratification above the continental shelf and cold dry continental airflow,etc.Rapid changes of TC intensity,including rapid intensification and sudden weakening and dissipation,are the small probability events which are in lack of effective forecasting techniques up to now.Diagnostic analysis and mechanism study will help improve the understanding and prediction of the rapid change phenomena in TCs.

Revisiting the response of western North Pacific tropical cyclone intensity change to vertical wind shear in different directions

The effect of vertical wind shear(VWS)directions on the change in western North Pacific tropical cyclone(TC)intensity is revisited in this study.Results show that the differences in the correlations between VWS in different orientations and the change in TC nondimensional intensity highly diminish,although slight differences are still present.The subtle differences in the correlations are likely associated with different synoptic-scale patterns at upper and lower levels.This result suggests that,in addition to thermodynamic effects,dynamic roles of the synoptic-scale patterns associated with the VWS should also be taken into account when the authors examine how VWS in different directions affects TC intensity change.

VARIATION OF THE MONSOON TROUGH INTENSITY IN THE SOUTH CHINA SEA AND WESTERN NORTH PACIFIC OCEAN AND ITS RELATIONSHIP TO TROPICAL CYCLONE ACTIVITIES

Using the daily average of the NCEP/DOE AMIP-II reanalysis data from 1979 to 2005 and the characteristics of monsoon troughs in the western North Pacific,we established an intensity index and a location index to describe the activity of the monsoon troughs in three different regions and their impacts on tropical cyclones generated therein(MTTCs).The behavior of the monsoon troughs was analyzed.The following conclusions are obtained:(1)The established monsoon trough intensity index has a positive correlation to the location index,indicating that stronger monsoon trough intensity corresponds to more northward location.(2)Monsoon trough intensity exhibits significant interannual variation,with obvious periods of 4–5 years prior to 1994 and 2–3 years afterwards.(3)The affecting factors on monsoon trough intensity are different with areas.The preceding SST anomaly results in anomalous atmospheric circulation, leading to the anomaly of monsoon trough intensity in different areas.(4)The frequency of cyclogenesis and location anomalies of the MTTC are closely related to the intensity and location of the monsoon trough. Most of the anomalously less MTTC years coincide with the years with a weak general monsoon trough and weak regional monsoon troughs.The anomalously more MTTC years are associated with both a strong general monsoon trough and a weak general monsoon trough combined with a strong one over the South China Sea,though with a larger probability for the latter.(5)The interseasonal variation of the intensity of monsoon troughs provides favorable conditions for TC generation and development.The monsoon trough is in the active periods of both quasi-biweekly 10 to 20 day and 30 to 60 day oscillations,which is favorable for MTTC occurrence.