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1.
In a sharp contrast to tropical cyclone(TC) genesis over the main development region of the western North Pacific(WNP), near-equatorial(0°–5°N) TCs exhibit a distinctive annual cycle, peaking in boreal winter and being inactive in boreal summer. The relative roles of dynamic and thermodynamic background states on near-equatorial TCs formation were investigated based on the observational diagnosis of the genesis potential index(GPI) and high-resolution model simulations. It is found that the background vorticity makes a major contribution to the distinctive annual cycle, while mean temperature and specific humidity fields are not critical. Numerical simulations further indicate that seasonal mean cyclonic vorticity in boreal winter has three effects on TC genesis near the equator. First, the environmental cyclonic vorticity interacts with TC vortex to promote a mid-level outflow, which strengthens boundary layer friction induced ascending motion and thus condensational heating. Second, it produces an equivalent Coriolis effect(via enhanced absolute vorticity), which strengthens positive feedback between primary and secondary circulation. Third, it helps to merge small-scale vortical hot towers(VHTs) into a mesoscale core through vorticity segregation process. However, background vorticity in boreal summer has an opposite effect on TC development near the equator.  相似文献   

2.
In this study,the effect of vertical wind shear(VWS)on the intensification of tropical cyclone(TC)is investigated via the numerical simulations.Results indicate that weak shear tends to facilitate the development of TC while strong shear appears to inhibit the intensification of TC.As the VWS is imposed on the TC,the vortex of the cyclone tends to tilt vertically and significantly in the upper troposphere.Consequently,the upward motion is considerably enhanced in the downshear side of the storm center and correspondingly,the low-to mid-level potential temperature decreases under the effect of adiabatic cooling,which leads to the increase of the low-to mid-level static instability and relative humidity and then facilitates the burst of convection.In the case of weak shear,the vertical tilting of the vortex is weak and the increase of ascent,static instability and relative humidity occur in the area close to the TC center.Therefore,active convection happens in the TC center region and facilitates the enhancement of vorticity in the inner core region and then the intensification of TC.In contrast,due to strong VWS,the increase of the ascent,static instability and relative humidity induced by the vertical tilting mainly appear in the outer region of TC in the case with stronger shear,and the convection in the inner-core area of TC is rather weak and convective activity mainly happens in the outer-region of the TC.Therefore,the development of a warm core is inhibited and then the intensification of TC is delayed.Different from previous numerical results obtained by imposing VWS suddenly to a strong TC,the simulation performed in this work shows that,even when the VWS is as strong as 12 m s-1,the tropical storm can still experience rapid intensification and finally develop into a strong tropical cyclone after a relatively long period of adjustment.It is found that the convection plays an important role in the adjusting period.On one hand,the convection leads to the horizontal convergence of the low-level vorticity flux and therefore leads to the enhancement of the low-level vorticity in the inner-core area of the cyclone.On the other hand,the active ascent accompanying the convection tends to transport the low-level vorticity to the middle levels.The enhanced vorticity in the lower to middle troposphere strengths the interaction between the low-and mid-level cyclonical circulation and the upper-level circulation deviated from the storm center under the effect of VWS.As a result,the vertical tilting of the vortex is considerably decreased,and then the cyclone starts to develop rapidly.  相似文献   

3.
A statistical analysis of the initial vortexes leading to tropical cyclone(TC) formation in the western North Pacific(WNP) is conducted with the ECMWF ERA5 reanalysis data from 1999 to 2018. It is found that TCs in the WNP basically originate from three kinds of vortexes, i.e., a mid-level vortex(MV), a low-level vortex(LV), and a relatively deep vortex with notable vorticity in both the lower and middle troposphere(DV). Among them, LV and DV account for 47.9% and24.2% of tropical cyclogenesis e...  相似文献   

4.
Using the NCEP 1°×1°reanalysis data,several obvious differences of the structural characteristics of developed versus undeveloped mid-level vortexes are studied.First,the central vorticity of the developed mid-level vortex increases towards higher levels while the undeveloped one decreases.The low-level convergence structure maintains well in the developed mid-level vortex whereas the undeveloped one does badly.Second,on the one hand,according to the symmetric analysis,the horizontal wind field and wind vertical section of the developed mid-level vortex are well symmetric while those of the undeveloped one are less symmetric.Meanwhile,weak wind vertical shear help the developed mid-level vortex to establish a warm core in upper-and mid-levels of the troposphere.On the other hand,according to the balance analysis,better balance between wind and pressure is shown in the mid-and lower-levels of the troposphere of the developed mid-level vortex than in those of the undeveloped vortex.Third,positive anomaly of potential vorticity is enhanced and developed in the vertical direction of the developed vortex.However,the undeveloped vortex weakens with a weak positive anomaly.  相似文献   

5.
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.  相似文献   

6.
Based on 6-hourly sensible heat flux and latent heat flux from the NCEP Climate Forecast System Reanalysis (CFSR) and circulation data from the Japanese 25-year Reanalysis (JRA-25), the initial developing process of tropical cyclone Mindulle (1005) in 2010 has been diagnosed to reveal the impact of air-sea interaction over the South China Sea (SCS) on the genesis of its incipient vortex. The results show that the incipient vortex first occurred east of the Luzon Island on 0000 UTC 20 August, suggesting that the topographic forcing of the Luzon Island for easterly winds over the western Pacific might be one of the factors responsible for the formation of the incipient vortex. During the formation stage of the incipient vortex, strong southeasterlies over the SCS caused warm water of the middle and eastern SCS to flow toward the Luzon Island due to Ekman transport resulting from wind stress, leading to an increase of the sea surface temperature and sensible heat flux into the atmosphere. Although the anomalous sensible heating favored surface pressure to reduce, it was not conducive to the increase of local vorticity associated with the vortex above the heating area because, according to the atmospheric thermal adaptation theory, the anticyclonic vorticity would be created in the lower troposphere due to the decreased vertical gradient of the sensible heating. However, the ascending motions occurred over the eastern area of the anomalous sensible heating due to the augmentation of the vorticity advection with increasing height, causing water vapor to condense in the middle and upper troposphere. In turn, cyclonic vorticity was generated in the lower troposphere due to the increased vertical gradient of the condensation latent heating, resulting in the formation and further growth of the incipient vortex. Therefore, the vorticity creation due to the condensation heating played a dominant role during the subsequent enhancing stage of the incipient vortex.  相似文献   

7.
The structure and organization of the extreme-rain-producing deep convection towers and their roles in the formation of a southwest vortex(SWV) event are studied using the intensified surface rainfall observations, weather radar data and numerical simulations from a high-resolution convection-allowing model. The deep convection towers occurred prior to the emergence of SWV and throughout its onset and development stages. They largely resemble the vortical hot tower(VHT) commonly seen in typhoons or hurricanes and are thus considered as a special type of VHT(sVHT). Each sVHT presented a vorticity dipole structure, with the upward motion not superpose the positive vorticity.A positive feedback process in the SWV helped the organization of sVHTs, which in turn strengthened the initial disturbance and development of SWV. The meso-γ-scale large-value areas of positive relative vorticity in the mid-toupper troposphere were largely induced by the diabatic heating and tilting. The strong mid-level convergence was attributed to the mid-level vortex enhancement. The low-level vortex intensification was mainly due to low-level convergence and the stretching of upward flow. The meso-α-scale large-value areas of positive relative vorticity in the low-level could expand up to about 400 hPa, and gradually weakened with time and height due to the decaying low-level convergence and vertical stretching in the matured SWV. As the SWV matured, two secondary circulations were formed,with a weaker mean radial inflow than the outflow and elevated to 300-400 hPa.  相似文献   

8.
A relatively independent and small-scale heavy rainfall event occurred to the south of a slow eastward-moving meso-α-scale vortex. The analysis shows that a meso-β-scale system is heavily responsible for the intense precipitation. An attempt to simulate it met with some failures. In view of its small scale, short lifetime and relatively sparse observations at the initial time, an adjoint model was used to examine the sensitivity of the meso-β-scale vortex simulation with respect to initial conditions. The adjoint sensitivity indicates how small perturbations of initial model variables anywhere in the model domain can influence the central vorticity of the vortex. The largest sensitivity for both the wind and temperature perturbation is located below 700 hPa, especially at the low level. The largest sensitivity for the water vapor perturbation is located below 500 hPa, especially at the middle and low levels. The horizontal adjoint sensitivity for all variables is mainly located toward the upper reaches of the Yangtze River with respect to the simulated meso-β-scale system in Hunan and Jiangxi provinces with strong locality. The sensitivity shows that warm cyclonic perturbations in the upper reaches can have a great effect on the development of the meso-β-scale vortex. Based on adjoint sensitivity, forward sensitivity experiments were conducted to identify factors influencing the development of the meso-β-scale vortex and to explore ways of improving the prediction. A realistic prediction was achieved by using adjoint sensitivity to modify the initial conditions and implanting a warm cyclone at the initial time in the upper reaches of the river with respect to the meso-β-scale vortex,as is commonly done in tropical cyclone prediction.  相似文献   

9.
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.  相似文献   

10.
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.  相似文献   

11.
台风榴莲（2001）生成初期中尺度涡旋合并过程研究   总被引：3，自引：1，他引：2  
由于热带海洋上观测资料的稀缺和热带气旋系统本身发生、发展的复杂性,热带气旋生成机制研究领域至今仍然存在很多未解之谜。已有的观测和模拟研究证明,中尺度涡旋合并过程对于热带气旋的生成可能有触发作用,但尚未见到南海季风槽内热带气旋生成过程中中尺度涡旋合并现象的实例模拟研究。利用新一代中尺度天气研究与预报模式WRF对南海热带气旋榴莲(2001)生成过程中的中尺度涡旋合并过程进行了高分辨率(4 km)数值模拟,并与观测资料进行对比,利用模式输出结果重点分析两个中尺度涡旋合并过程中的主要动力学和热力学特征,并在此基础上进一步分析了合并过程中系统中心附近涡度方程中各项涡度收支的演变情况,最后通过两个敏感性试验与控制试验结果的对比,初步探讨中尺度涡旋合并过程对于热带气旋榴莲生成的作用。结果表明,南海季风槽中的新生中层中尺度涡旋V2,是榴莲生成过程中的主导涡旋,预先存在的东部低层的中尺度涡旋V1对于台风榴莲的生成则起到了辅助作用,两个不同高度的涡旋合并叠加促使涡度的辐合、辐散项率先在低层引起涡度的快速增长,随后垂直输送项在对流层中层对涡度的增长起主要作用。两个涡旋的最终合并,使热带气旋系统正绝对涡度在垂直方向上从低层到中层得以贯通,进而触发榴莲的生成。  相似文献   

12.
In this study, we first show that tropical cyclone (TC) Usagi evolved from a mid-level vortex over the South China Sea (SCS) in August 2001. The initial disturbance of TC Usagi had a maximum potential vorticity (PV) near 500 hPa, and an anticyclonic circulation with a cold core near the surface. The cyclonic circulation and its warm core of the mid-level vortex developed gradually downward toward the surface when environmental easterly and dry air intruded from the upper troposphere; finally, the mid-level vortex evolved into TC Usagi under favorable environment conditions such as weak vertical wind shear, deep moist layer, etc. To investigate the dynamic and thermodynamic processes during TC Usagi genesis, the technique of piecewise PV inversion is employed. The results show that the actions of upper-layer PV and potential temperature anomalies were not important in TC Usagi genesis. Surface-layer thermal anomalies mainly produced negative disturbances of temperature at the vortex center below 800 hPa, which was unfavorable to the genesis of a cyclonic circulation near the surface. Middle-to-lower-layer latent heat played a key role in TC Usagi genesis and downward development of dynamic and thermodynamic processes. The actions of dry air intrusion from the upper troposphere, environmental westerly changing into easterly in the middle and lower troposphere, and baroclinic structure of the vortex were also important. The cyclonic circulation of the mid-level vortex could develop downward quickly from the middle troposphere toward the surface. However, whether the warm core of the vortex developed near the surface depended on the combined actions of surface-layer thermal anomaly and middle-to-lower-layer latent heat. Finally, we present a conceptual model of TC Usagi genesis induced by a mid-level vortex over the SCS.  相似文献   

13.
张文龙  王昂生  崔晓鹏 《大气科学》2008,32(5):1197-1209
西太平洋热带气旋（TC）的生成和季风槽及中尺度对流系统（MCS）的活动有密切关系,但以往这方面的实例数值模拟很少。为了进一步探讨由MCS对流强迫产生的对流层中层中尺度涡旋（MCV）在TC生成中的作用,作者利用非静力平衡的中尺度模式PSU/NCAR MM5对台风榴莲（2001）的生成过程进行了高分辨率（6 km）数值模拟和比较验证。结果表明:模式成功地模拟出榴莲的生成地点,其与MCS的相对位置关系与以往的观测研究结果一致;模拟的TC移动路径、强度变化与最优观测报告比较接近,准确反映了TC未来登陆地点,以及中心气压缓慢下降和迅速下降两个阶段;对云系演变的模拟,成功模拟出了TC初生时的涡旋云系和季风槽中MCS云系的分离现象,以及在TC登陆前达到成熟阶段时出现的台风眼和螺旋云带。此外,模式还成功模拟出中层MCV,它的水平尺度约200 km,位于800~400 hPa之间,具有暖心结构等,均与已有观测结果相近。模式初始场中包含有充分的MCS信息,是模拟取得成功的关键因素之一。  相似文献   

14.
现阶段使用的热带气旋潜在生成指数(Genesis Potential Index,GPI)在气候场的空间分布上能很好地拟合热带气旋的生成情况,但在热带气旋的年际变化拟合上效果很差。本研究考虑了相对涡度在热带气旋年际变化拟合上的重要作用,并以此为出发点,尝试改善GPI在西北太平洋地区的拟合效果。基于对1979—2011年美国联合飓风警报中心提供的热带气旋最佳路径数据和NCEP/NCAR再分析资料数据集的研究,将之前GPI中的绝对涡度项替换为修正过的相对涡度项。科氏力项仍然保留;将南海(100°～120°E,5°～25°N)与西北太平洋地区(120°～180°E,5°～40°N)热带气旋生成的差异性也纳入了考量,并在这两个区域分别构建GPI公式,改善了对热带气旋生成的气候分布模拟。除此之外,较之已存的GPI指数,改进后的GPI还很大程度提高了GPI对热带气旋生成年际变化的拟合效果,特别是对弱热带气旋年际变化的拟合效果有了显著提升。  相似文献   

15.
本文基于PSU/NCAR MM5中尺度模式对台风榴莲 (2001) 生成过程成功的数值模拟, 利用模式输出的较高时空分辨率资料, 对台风榴莲生成过程中对流层中层中尺度对流涡旋 (MCV) 的作用进行了诊断分析。结果表明, 中层MCV在台风榴莲生成中的作用有三个重要方面: 第一, 中尺度组织化作用: 伴随中层MCV的垂直次级环流圈, 使得区域内的积云对流热塔趋向于逐步在中心区域集中, 热塔相互之间容易发生相互作用, 通过合并过程有些热塔得到加强, 而有些趋于消亡。同时, 热塔聚集后的群体效应反馈作用又使得中层MCV加强或维持, 进一步促进热塔的合并以及向轴对称化发展; 第二, 存贮效应: 因为中层MCV的生命史比积云对流热塔长, 能够将消亡对流热塔所携带的热量、 水汽、 涡度加以存贮和保留, 使得中层MCV区域向有利于TC生成的方向发展, 最终成为TC环流的 “胚胎”; 第三, 中层MCV与对流层低层的槽 (涡旋) 以及对流热塔之间通过相互作用, 共同实现中低层系统的垂直耦合。  相似文献   

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