台风生成阶段周期性对流爆发机制及辐射的影响

最新观测统计分析表明台风生成前对流活动存在周期性特征,周期集中在18~26 h,峰值多出现在夜间。本文通过一系列理想的云分辨数值控制试验和辐射敏感性对比试验讨论了台风生成阶段周期性对流爆发的成因以及辐射日变化在此过程中产生的影响。结果表明,在给定无日变化的恒定太阳辐射情况下,台风生成过程中也会出现与观测一致的周期性对流爆发,说明辐射日变化不是该周期性形成的控制因素。对湿静力能扰动的收支分析发现边界层中湿静力能的周期性增减是导致周期性深对流爆发的关键因素。深对流爆发形成的冷池通过平流作用降低了边界层内的湿静力能,需要通过辐射与海表面通量过程使其恢复,才能再次触发对流。辐射日变化对于一些较弱的热带低压涡旋有调制作用,能使其周期性对流爆发的峰值时间相位改变并趋于在夜晚出现;但对于较强的涡旋,辐射的这种调制作用并不显著,涡旋会保持其最初对流爆发的周期和相位。本研究对观测到的台风生成阶段对流峰值出现在白天或夜间的情况都可以作出合理解释,也深化了对台风生成过程对流和辐射的作用和影响科学问题的理解。

THE STRUCTURE OF TROPICAL CYCLONE BY TOVS AND ITS APPLICATION IN NUMERICAL WEATHER PREDICTION

The TOVS data are used to study the structure of a number of tropical cyclones for the year 2000. Differences are found to some extent between what is found and classic conceptual models in that (1) the horizontal structure is asymmetric and variable so that the low-value centers at low levels of the geopotential height field (or the high-value centers at high levels) do not necessarily coincide with the high-value centers of the temperature field; (2) the vertical structure is also variable in the allocation of the anomalies of the geopotential height field between low values at low levels and high values at high levels. It is especially noted that the centers of the anomalies are tilting at both high and low levels or the high level is only at the edge of a high-pressure zone. There is not any significant high-value anomalous center in a corresponding location with the tropical cyclone. The structure of tropical cyclone in the TOVS is also used as reference to modify the structure of typhoon BOGUS in the numerical prediction model system of tropical cyclones. It is found that the modified BOGUS performs better in coordinating with the environment and predicting the track of the tropical cyclone. The demonstration is two-fold the structure of the typhoon BOGUS is such that it means much in the track prediction and the use of the TOVS-based tropical cyclone structure really helps in improving it. It provides the foundation for modification and evolution of typhoon BOGUS.

Track Prediction of Typhoon Neoguri 2008

In this paper, the prediction of the track for No.1 tropical cyclone, Neoguri, is concretely provided by using a special solution to （2-1）-dimensional Euler Equation.

Extratropical Transition and Re-Intensification of Typhoon Toraji(2001):Large-Scale Circulations, Structural Characteristics, and Mechanism Analysis

With the use of data from the National Centers for Environmental Prediction Climate Forecast System Reanalysis,the environment and structure of typhoon Toraji(2001)are investigated during the re-intensification(RI)stage of its extratropical transition(ET),a process in which a tropical cyclone transforms into an extratropical or mid-latitude cyclone.The results provide detailed insight into the ET system and identify the specific features of the system,including wind field,a cold and dry intrusion,and a frontal structure in the RI stage.The irrotational wind provides the values of upper-and lower-level jets within the transitioning tropical cyclone and the cyclone over Shandong Peninsula,accompanied with the reduced radius of maximum surface winds around the cyclone center in the lower troposphere.Simultaneously,dry air intrusion enhances the formation of fronts and leads to strong potential instability in the southwest and northeast quadrants.The distribution of frontogenesis shows that the tilting term associated with vertical motion dominates the positive frontogenesis surrounding the cyclone center,especially in the RI stage.The diagnostics of the kinetic energy budget suggest that the divergent kinetic energy generation whose time evolution corresponds well to that of cyclone center pressure is the primary factor for the development of Toraji in the lower troposphere.The ET of Toraji is a compound pattern that contains a development similar to that of a B-type extratropical cyclone within the maintaining phase and an A-type extratropical cyclone within the strengthening period,which corresponds to the distribution of the E-P fluxes with vertically downward propagation in the maintaining stage and upwards momentum in the strengthening phase.

Relationship of tropical cyclone size change rate with size and intensity over the western North Pacific

In this study,the relationship of tropical cyclone(TC)size change rate(SCR),within 24 hours,with size,intensity,and intensity change rate(ICR)are explored over the western North Pacific.TC size is defined as the azimuthally averaged radius of gale-force wind of 17 m s−1(R17)based on the Multiplatform Tropical Cyclone Surface Winds Analysis data.The majority of SCRs are mainly distributed in the range from−20 to 80 km d−1.The correlation coefficients between SCR and size(SCR-R17),intensity,and ICR(SCR-ICR)are−0.43,−0.12,and 0.25,respectively.The sensitivity of the SCR-R17 and SCR-ICR relationships to size,intensity,and evolution stage are further examined.Results show that the SCR-R17 relationship is more sensitive to variations of size and evolution stage than that of intensity.The relationship of SCR-ICR is largely modulated by the evolution stage.The correlation coefficient of SCR-ICR can increase from 0.25 to 0.40 when only considering the lifetime stages concurrently before and after the lifetime maximum size(LMS)and lifetime maximum intensity.This demonstrates that ICR is a potential factor in predicting SCR during these evolution stages.Besides,the TC size expansion(shrinkage)is more likely to occur for TCs with smaller(larger)size and weaker(stronger)intensity.The complexity of size change during a TC's lifetime can be attributed to the fact that shrinkage or expansion could occur both before and after LMS.

Does the Asian monsoon modulate tropical cyclone activity over the South China Sea?

To investigate whether the Asian monsoon influences tropical cyclone (TC) activity over the South China Sea (SCS), TCs (including tropical storms and typhoons) over the SCS are analyzed using the Joint Typhoon Warning Center dataset from 1945 to 2009. Results show an increasing trend in the frequencies of TC-all (all TCs over the SCS) and TY-all (all typhoons over the SCS), due mainly to an increase in the number of TCs moving into the SCS after development elsewhere. Little change is seen in the number of TCs that form in the SCS. The results of wavelet analysis indicate that the frequency of typhoons (TY) shows a similar oscillation as that of TCs, i.e., a dominant periodicity of 8-16 years around the 1970s for all TC activity, except for TC-mov (TCs that moved into the SCS from the western North Pacific). To examine the relationship between typhoon activity and the summer monsoon, a correlation analysis was performed that considered typhoons, TCs, and five monsoon indexes. The analysis reveals statistically significant negative correlation between the strength of the Southwest Asian summer monsoon and typhoon activity over the SCS, which likely reflects the effect of the monsoon on TC formation in the western North Pacific (WNP) and subsequent movement into the SCS. There is a statistically significant negative correlation between TY-loc (typhoons that developed from TCs formed over the SCS) and the South China Sea summer monsoon and Southeast Asian summer monsoon.

THE THEORY OF MOIST POTENTIAL VORTICITY AND ITS APPLICATION IN THE DIAGNOSIS OF TYPHOON RAINFALL AND INTENSITY

This paper tests the impacts of cloud-induced mass forcing on the moist potential vorticity （MPV） anomaly associated with torrential rains caused by Typhoon No.9914 （Dan） by using fine model simulation data outputted by the Fifth-Generation NCAR / Penn State Mesoscale Model （MMS）. The diagnostic results show that the positive MPV anomaly region, which is obtained by integrating the MPV from 600 hPa to 300 hPa in the vertical, roughly coincides with the precipitation at their synchronous stages either in position or in the distribution pattem, and the maximum positive MPV area of Dan is located mainly between 600 hPa and 300 hPa, which is much higher than torrential rain cases. Further analyses also showed that the value of positive MPV anomaly increased or decreased with the development of Dan, and the positive MPV anomaly may also be served as a tracer to indicate the evolution of tropical cyclone intensity.

Upper ocean response to tropical cyclone wind forcing: A case study of typhoon Rammasun(2008)

The characteristics of the upper ocean response to tropical cyclone wind （TCW） forcing in the northwestern Pacific were in- vestigated using satellite and Argo data, as well as an ocean general circulation model. In particular, a case study was carried out on typhoon Rammasun, which passed through our study area during May 6-13, 2008. It is found that the local response fight under the TCW forcing is characterized by a quick deepening of the surface mixed layer, a strong latent heat loss to the atmosphere, and an intense upwelling near the center of typhoon, leading to a cooling of the oceanic surface layer that persists as a cold wake along the typhoon track. More interestingly, the upper ocean response exhibits a four-layer thermal structure, including a cooling layer near the surface and a warming layer right below, accompanied by another pair of cooling/warming layers in the thermocline. The formation of the surface cooling/warming layers can be readily explained by the strong vertical mixing induced by TCW forcing, while the thermal response in the thermocline is probably a result of the cyclone-driven upwelling and the associated advective processes.