双涡藤原效应的数值模拟研究

本文利用正压无辐散模式在无环境基本气流的情况下对双涡的藤原效应及其“合并”后涡旋移动路径的特征进行了数值模拟研究。结果表明，只有当双涡间的距离小于某一临界距离时才会出现藤原效应，这一临界距离决定于涡旋的结构及双涡的相对强度；双涡的“合并”过程实际上是其中一个涡旋减弱消失而另一涡旋维持的过程；在考虑β效应的情形下，两个同等强度的涡旋“合并”后表现为较平直的向西北的β飘移，而两个不同强度的涡旋“合并”后表现为陀螺运动和β飘移的叠加。这些结果与实际大气中双台风相互作用的许多现象极为相似。     

双热带气旋相互作用的机制分析及数值研究(一)——物理机制的分析

本文对正压情况下双热带气旋的相互作用进行了机制分析.通过对两个理想涡旋间非线性涡度平流过程的分析,揭示了双涡气旋性互旋及其中心间距变化的涡度平流机制.分析表明,一个涡旋的切向风场对另一涡旋涡度场的平流相互作用可造成两者气旋性互旋;而一个涡旋的切向风场与另一涡旋涡度梯度间的相互作用所引起的次级环流可造成双涡中心间距的增大或减小(定义为排斥或吸引),由此提出了双涡相互作用的临界距离效应概念.对几类常用的理想热带气旋及合成热带气旋的分析证实,双热带气旋的相互作用存在这种临界距离效应,且临界距离平均在6—7个纬距     

一个β中涡对双台风相互作用影响的物理过程

利用高分辨率f平面正压拟谱模式,分析一个β中尺度涡对双台风相互作用影响的物理过程。结果表明：β中涡的存在可以使原本排斥的两个DeMaria型台风涡旋合并;β中涡改变双台风相互作用终态的物理机制是：初始时段处于某一台风涡旋正影响区内的β中涡,构成了非对称涡度场,进而在该台风涡旋内部产生指向另一个台风涡旋的气流。该气流如果足够强,则会使两个台风的中心距离在短时间内下降到合并临界距离之内,触发双涡合并过程的发生。     

环境流场对双涡相互作用的影响

本文在文献[1]的基础上,进一步应用正压无辐散模式就环境流场对双涡相互作用的影响进行了数值模拟研究。试验结果表明,环境流场对双涡的相互作用确有重要影响,它可以加剧双涡的气旋性互旋和促使其相互趋近.也可以抵销双涡间的互旋作用,并且还可使相距较近的两个涡旋重新分开。上述各种情况取决于双涡的相对位置及其与环境流场的不同配置。      

小尺度系统对涡旋自组织的影响

在涡旋自组织动力学的框架内,利用．厂平面准地转正压模式讨论小尺度涡旋系统对两个中β尺度涡旋的自组织过程的影响。4组数值试验表明：小尺度涡旋的存在,可能会改变双涡相互作用的终态,使原本不合并的两个涡旋组织起来;双涡相互作用的终态对小尺度涡旋的初始位置敏感;存在“Z”型敏感区域,当小尺度涡旋出现在这一区域时,就有可能改变双涡相互作用的终态;小尺度涡旋对双涡相互作用产生影响需具备4个条件,即初始位于敏感区内,有足够的强度,距离适当且生存时间足够长。     

双热带气旋合并过程涡旋强度与吸引效应相关特征数值模拟

利用中尺度模式HWRF(Hurricane Weather Research and Forecast System)模拟双热带气旋"狮子山"(2010)与"南川"(2010)涡旋合并过程,并通过强度敏感性试验揭示两涡旋强度对合并过程的影响。分析表明:在两者的合并过程中,"狮子山"涡旋强度明显大于"南川";"狮子山"涡旋对"南川"涡旋具有更大的"吸引"效应,两者西侧呈相对强的能量、水汽"连体"通道。HWRF能够较好的模拟出双热带气旋"狮子山"与"南川"的强度、移动路径,尤其是两涡旋的合并过程。进一步分析控制试验双热带气旋水平与垂直结构揭示出两涡旋"互旋"过程中,"弱涡旋"并入"强涡旋"相互影响特征。有关"狮子山"与"南川"强度的敏感试验亦表明,两者各自涡旋强度"合并方向"具有关键影响。在敏感性试验中,改变涡旋强度后两者路径亦存在"互旋"现象,但与控制试验两涡旋"合并方向"相反,即敏感性试验热带气旋"狮子山"涡旋削弱,而"南川"涡旋强度相对增强,导致原涡旋西侧水汽、能量输送连体通道明显削弱,同时由于"南川"涡旋的强度强于"狮子山",两者东侧水汽、能量输送通道亦加强,导致"南川"涡旋对"狮子山"的涡旋存在"吸引"效应。"狮子山"涡旋残留云带一部分合并入"南川",一部分则随西南气流进入台风"圆规"。     

双热带气旋相互作用的机制分析及数值研究 II:数值模拟

在本文第Ⅰ部分关于双热带气旋相互作用物卵机制分析的基础上,利用正压原始方程模式对,平面上无大尺度环境流场情况下双热带气旋的相互作用进行了数值模拟研究,重点考察了双热带气旋水平流场相互作用的次级环流机制,文[1]中所提出的双热带气旋相互作用的临界距离效应概念在模式中得到了验证.同时通过模拟还发现,双热带气旋的联合强度越强、中心间距越小,则互旋越快;对于相互吸引的双热带气旋而言,合并后范围有所扩大,强度有一定的加强,并且较强的两个热带气旋比一强一弱的两个热带气旋维持的时间要长、较难于合并.这些结果与实际双热带气旋相互作用的观测事实极为一致.     

双热带气旋相互作用的机制分析及数值研究 Ⅱ:数值模拟

在本文第Ⅰ部分关于双热带气旋相互作用物卵机制分析的基础上,利用正压原始方程模式对,平面上无大尺度环境流场情况下双热带气旋的相互作用进行了数值模拟研究,重点考察了双热带气旋水平流场相互作用的次级环流机制,文[1]中所提出的双热带气旋相互作用的临界距离效应概念在模式中得到了验证.同时通过模拟还发现,双热带气旋的联合强度越强、中心间距越小,则互旋越快;对于相互吸引的双热带气旋而言,合并后范围有所扩大,强度有一定的加强,并且较强的两个热带气旋比一强一弱的两个热带气旋维持的时间要长、较难于合并.这些结果与实际双热带气旋相互作用的观测事实极为一致.     

非轴对称双涡相互作用的研究

在平流动力学的框架内,用准地转正压涡度方程模式实施了19组试验,研究双涡合并的条件及较大尺度涡旋自组织的问题。结果指出:(1)存在着两个影响双涡合并的因素,即初始双涡中心之间的距离和初始涡旋的非轴对称分布。初始两个对称涡旋合并具有明显的临界距离效应,但初始两个非轴对称涡旋能否合并还受到初始涡旋的非对称结构的复杂影响。(2)存在着两类不同的较大尺度涡旋的自组织过程,形成较大尺度涡旋。第一类,初始两个涡旋相同,均呈轴对称分布。双涡作用经历了缓变、快变,以及涡量羽翼的生成、拉伸和发展的过程,合并后呈对称性流型;终态涡内区涡量的堆积来源于两个初始涡,终态涡外区的螺旋带来源于两个初始涡外缘线涡量羽翼的拉伸。第二类,初始两个涡旋不同,一个为椭圆型,一个为偏心型,均呈非轴对称分布。双涡作用中,椭圆涡一边互旋,一边向计算区域中心靠近,同时涡量范围加大,形成了终态涡的内核区;偏心涡一边互旋,一边被不断拉伸,形成了终态涡的螺旋带区;表现出终态涡内区的涡量堆集来源于椭圆涡,终态涡外区螺旋带主要来源于偏心涡的反复拉伸及断裂的特性。     

涡群中双涡相互作用的研究

在涡旋自组织动力学的框架内，数值研究了涡群中双涡相互作用的问题，对比了弱环境流与涡群环境流两类条件下双涡作用的异同。指出：弱环境流条件下双涡合并的过程，在涡群环境条件下不再存在；同时，涡群环境流的引进可使涡强度显著增强。     

The interaction of binary vortices in a barotropic model

Summary The interaction of binary cyclonic vortices is investigated using the nondivergent barotropic model of Chan and Williams (1987) under two situations: a quiescent environment and a linearly-sheared background flow. It is found that the mutual interaction between the vortices results from a combination of two processes: the advection of symmetric vorticity by the asymmetric flow and the advection of asymmetric vorticity by the symmetric flow. The latter contribution is rather significant. Whether the vortices in a binary system attract or repel each other depends on the asymmetric vorticity distribution associated with the two vortices. Such a distribution is governed by the structure (size) of and the separation between the vortices. In the presence of a sheared flow, the contribution from the advection of asymmetric vorticity by the symmetric flow may also become appreciable depending on the structure and magnitude of the shear. Furthermore, the geographical locations of the vortices in relation to the sheared flow are also important in determining the relative movement of the vortices.In the presence of , the movements of the vortices are modified by the northwestward -drift However, the relative motion between the vortices is almost identical to that on an f-plane. In other words, the mutual interaction between the vortices is largely independent of . Alternatively, the two vortices can be considered to be one system which drifts towards the northwest under the influence of while they interact with each other within the system. Physically, this independence arises because the two relative vorticity advection terms have much larger magnitudes than the planetary vorticity advection term. However, the -effect is still important in that it modifies the asymmetric flow associated with each vortex and hence the asymmetric vorticity. Such modifications change the advection patterns compared with the =0 case and hence lead to different vortex movements. The presence of a linear shear causes the binary system to move as if it was a large (for a cyclonic shear) or smaller (for an anticyclonic shear) vortex under the influence of .With 22 Figures     

A review of ground-based,mobile, W-band Doppler-radar observations of tornadoes and dust devils

A ground-based, mobile, W-band Doppler-radar has been used in the U.S. during the last decade to obtain high-spatial resolution maps of the radar reflectivity and wind fields in tornadoes and dust devils. This radar is one of the best tools available for studying the substructure of intense, small-scale vortices in the boundary layer. The most significant findings to date are summarized.In one case, it was found that just prior to tornadogenesis in a supercell, a 100–200 m scale cyclonic vortex formed at the leading edge of a bulge in the rear–flank gust front. This vortex appeared to interact with a larger-scale (500 m to 1 km wide) cyclonic vortex, just as the tornado formed. Other small-scale cyclonic vortices were present along the rear–flank gust, but they did not develop into tornadoes. The mature tornado-vortex was dominated by quasi-stationary wavenumber-two disturbances, while the mean vortex resembled a two-celled, Rankine combined vortex. The diameter of the mean vortex narrowed as it intensified and widened as it weakened, even though the tornado condensation funnel narrowed as the tornado was dissipating. Evidence was also found of short-term, inertial-like oscillations in vortex diameter and intensity. Spiral bands and eyes were ubiquitous. The eye in one well-documented case was broader in the lowest few hundred meters than it was aloft. Multiple vortices and “umbilical” cords of very narrow bands of reflectivity have also been found.Both cyclonic and anticyclonic dust devils have been documented. Some dust devils resemble a relatively narrow, Rankine combined vortex, while others are wider and have a broad, calm eye and a narrow annulus of intense vorticity just within the radius of maximum wind (RMW), and rising motion just inside the RMW and sinking motion well inside the RMW. Multiple-vortex structure, Rossby-like wave motion, and the Fujiwhara effect have also been documented.     

Sensitivity studies on vortex development over a polynya

Summary The development of a cyclonic vortex over a polynya is investigated with the primitive equation mesoscale model METRAS. The impact of different atmospheric processes on vortex development is determined by calculating the terms of the vorticity tendency equation. Sensitivity studies are performed for different large-scale situations (geostrophic winds 1 ms⁻¹, 3 ms⁻¹, 20 ms⁻¹, initial ice-water temperature difference of 35 K or 17.5 K) and for different polynya sizes and shapes. In general, the vortex develops within a few hours. It is intensified by buoyancy, mainly resulting from latent heat release. Advective and diffusive processes hinder the vortex development. The intensification depends on the actual situation and is faster over small polynyas and heterogeneous ice cover. These situations result in intensification periods of only 12 to 18 hours for the vortex, but create very strong vortices. Halved horizontal temperature gradients also about halve the vortex intensity. The lifetime and intensification of a vortex increases with the time the air mass spends over the water. Thus, weak winds show a slower development of the vortex but the vortex intensifies for more than 24 hours. Over big polynyas several vortices develop, a long polynya results in a longer and narrower vortex which intensifies over a longer period.     

夏季青藏高原低涡的切向流场及波动特征分析

从大气动力学原理出发,将高原低涡视为受热源强迫的边界层内涡旋,建立了柱坐标下满足梯度风平衡的低涡控制方程组,分析高原低涡切向流场的基本特征.在此基础上,通过求解线性化涡旋模式,得出高原低涡中各类波动的频散关系及其特征,同时定性讨论了热力作用对混合波动的影响以及混合波动与高原低涡流场特征的联系.使用中尺度数值模式WRF分...     

On the bogusing of tropical cyclones in numerical models: The influence of vertical structure

Summary In this study, idealised conditions are used to study the influence of vertical structure of the bogus vortex on its motion in numerical models by comparing the resultant forecast tracks. Two vortices were used: one has a cyclonic circulation throughout the troposphere and the other has an upper tropospheric anticyclone. Both vortices have the same structure in the middle and lower troposphere. The two vortices were inserted into four different environmental flows on a beta-plane: (a) a resting atmosphere; (b) a uniform flow; (c) a horozontal shear flow and (d) a vertical shear flow. The results show that the forecast tracks are very sensitive to the vertical structure of the bogus vortex, especially when the environmental flow is very weak, or is westerly and has a cyclonic horizontal shear. However, this sensitivity is reduced in moderate vertical shear. This motion sensitivity is found to arise from the vertical coupling mechanism by which the upper-and lower-level circulations interact with each other when a horizontal displacement occurs between them.The vertical structure of the bogus vortex can also affect the intensity of the model cyclone, depending on the configuration of the environmental flow. In general, the bogus vortex without an upper-level anticyclone will intensify quicker and will develop more intense than the one with an upper-level anticyclone. The vertical coupling mechanism can result in different asymmetric rainfall pattern in cyclone core region depending on the vertical structure of the bogus vortex. The asymmetric divergent flow associated with these convective asymmetries may in turn further influence the vortex motion. It is suggested that care needs to be taken in determining the vertical structure of the bogus vortex in numerical models.With 14 Figures     

东移低涡动力学的初步研究

用一个正压原始方程模式实施了六组试验，研究了东移低涡的动力学。结果表明：无论是切变基流与低涡的相互作用，还是涡块与低涡的相互作用，都可引起低涡强度在短暂时段内增强，但整个积分时段内低涡强度的演变仍呈下降趋势。切变基流、低涡和多个涡块的相互作用，可以改变下降的趋势。正相对涡度切变基流中低涡和涡块的合并，是东移低涡强度得以维持和发展的一个直接的原因。     

A numerical study of multiple vortex self-organization as forced by mesoscale topography

Summary There exists a common observational phenomenon over the offshore areas of the northwest Pacific, that is, when several mesoscale vortices evolve suddenly into a larger scale typhoon-like vortex within one day, often with serious consequences. In this paper a series of numerical experiments has been designed and performed to emulate this evolution. The model is based on the Charney-Hasegawa-Mima equation, where there are around 40 initial meso-β vortices with parabolic profiles whose central positions, dimensions and intensities are all set stochastically. The self-organization process of these stochastically-distributed multiple meso-β vortices can be divided into two phases. During the first phase, a larger scale vortex similar to a typhoon-like vortex forms near the computational center through the gradual stretching and merging of adjacent meso-β vortices while there are more than 10 isolated vortices surrounding this typhoon-vortex. During the second phase, the isolated vortices are stretched and drawn into the typhoon-vortex circulation and become its spiral arms which are gradually incorporated into the inner area of the typhoon. This is then repeated as new isolated vortices are stretched and become new spiral arms until all the isolated vortices are drawn into the typhoon-vortex. The center of the self-organized typhoon-vortex rotates counterclockwise around the computational center when no topography is involved and is thus a transient vortex. When topography is present the vortex remain in the NE quadrant of the model domain, locked by the topography, and this quasi-steady vortex is thus capable of causing local disasters. Correspondence: Chongjian Liu, Chinese Academy of Meteorological Sciences (CAMS), State Key Labaratory of Severe Weather, 46 Zhongguancum South Avenue, 100081 Beijing, P.R. China     

Summer circulation patterns related to the upper tropospheric vortices over the Tropical South Atlantic

Summary Daily 200-hPa relative vorticity data have been used to study the dominant patterns related to the cyclonic vortices over the South Atlantic Ocean in the vicinities of northeast Brazil, during the 1980–1989 period. Reference modes were obtained through empirical orthogonal function (EOF) analysis of the 200-hPa filtered vorticity anomalies over northeast Brazil, considering all the southern hemisphere (SH) summers within the study period. The amplitude time series of the first reference mode, separately for each SH summer, was correlated with the corresponding filtered vorticity anomalies in a larger area extending from 20°N to 40°S and between 120°W and 20°W. The correlation patterns feature a wave-like structure along eastern South America, with three main centers: the first one, over the South Atlantic off the northeast Brazil coast, is associated with the cyclonic vortices; the second one, over eastern Brazil, represents the corresponding anomalously amplified ridges; and the third one, over southern Brazil/Uruguay, is related to the equatorward incursions of midlatitude upper level troughs. This wave-like pattern is consistent with the vortex formation mechanism suggested in previous works. Another wave-like pattern southwest-northeast oriented is evident over the tropical southeastern Pacific, for some years. The internannual variability of these patterns is discussed in this paper.With 9 Figures     

Low-Order Point Vortex Models of Atmospheric Blocking

Summary Conceptual models of blocking structures are constructed by reducing the two-dimensional atmospheric vorticity field to a few point vortices. The flow is assumed to be barotropic and divergence-free, and a blocking event is represented by a point vortex dipole. The focus is here on the motion of the blocking dipole under the influence of the zonal mean flow. This is modelled in three different ways: A dipole embedded in a latitude-dependent zonal mean flow exhibits neutrally stable oscillations; their period is estimated analytically. A cyclonic point vortex approaching from upstream can either pass the dipole or break it up, so that an Ω-shaped pattern of three vortices emerges. The stationarity of a blocking between two troughs is modelled by four point vortices. These low-order point vortex models are compared with the dynamics of real blockings in case studies. Despite their high degree of simplification, those models reproduce the kinematics of blocking events properly. This results from the discretization of the flow to its actual physical states, the vortices, in contrast to the common, purely mathematical discretization to grid points. Thus, point vortex dynamics are proposed to be a powerful completion of continuous fluid dynamics in explaining blocking events. Received August 30, 1999 Revised December 22, 1999