Comparison of two types of persistent heavy rainfall events during sixteen warm seasons in the Sichuan Basin

本文筛选出四川盆地西部(盆西型)和盆地东部(盆东型)持续性暴雨个例,深入对比两类持续性暴雨的大气环流特征和直接造成持续性暴雨的西南低涡维持的机理.四川盆地的短波槽和西太平洋副热带高压的配置有利于持续性暴雨的维持,盆东型的降水强度较盆西型个例强,高空急流位置偏南,南亚高压的强度更强,高层辐散更强,对流层中层副热带高压偏东偏南.盆西型的水汽输送主要来自南海,而盆东型的水汽输送主要来自南海和孟加拉湾.合成涡度收支的结果表明散度项是两类持续暴雨中西南涡维持的主要原因,但盆西型中,垂直平流的作用更强.     

Vorticity and intermittency within the pre-breaking region of spilling breakers

This paper presents measurements and analysis of fluid velocity within the context of spilling waves. The data have been collected using 2-D Laser Doppler Velocimetry in pre-breaking monochromatic waves generated in a wave tank. The analysis is performed using orthogonal wavelets and, in addition to the classical criterion adopted in applying Taylor's hypothesis, a new algorithm is proposed for the eduction of eddies at different length scales. The contribution of different scale vortices is computed, and phase is resolved. Microvortices (smaller than the breaker height but larger than the dissipative vortices) and mid-size vortices (with length ranging from the breaker height to the wave length) carry out most turbulence energy under wave crest. The phase average vorticity and strain rate is computed at different wave lengths, with the analysis of intermittence. The intermittency factor shows spikes in the wave crest, especially for turbulence in small vortices.     

AN EXPERIMENT STUDY OF FRONTAL CYCLONES OVER THE WESTERN ATLANTIC OCEAN*

By using PSU/NCAR MM5 mesoscale model,a 60-h simulation is performed to reproduce a frontal cyclogenesis over the Western Atlantic Ocean during March 13-15 1992.The model reproduces well the genesis,track and intensity of the cyclone,its associated thermal structure as well as its surface circulation.The major cyclone (M) deepens 45 hPa in the 60-h simulation and 12 hPa in 6 hours from 36 h to 42 h (model time) and 27 hPa in 24 hours from 36 h to 60 h (model time).Cross-section and isentropic analysis tell us that the cyclogenesis is in very close relation with slantwise isentropic surfaces;the cyclone is always superposed on the core of neutral convective stability with nearly vertical isentropic surfaces,which coincides with what the theory of Slantwise Vorticity Development (SVD) says.Beginning with the theory of SVD,the development and propagation of the oceanic frontal cyclone are studied by using high-resolution model output in the context of slantwise isentropic surfaces.The results show that the frontal cyclone deepens rapidly by the interaction with the large-scale environment after occurring over the ocean with weak static stability;and the theory of SVD can well interpret the development and propagation closely related with slantwise isentropic surfaces,The downstream slantwise up-sliding movement along canting isentropic surfaces makes vorticities develop (USVD) under favorable condition (C_D<0,where CD is SVD index),and results in the moving and development of the cyclone.     

Flow fields around tandem and staggered piers on a mobile bed

An experimental investigation on flow fields within the scour holes upstream and downstream of circular piers positioned in tandem and staggered arrangements is reported and compared with isolated piers on mobile beds with uniform sediment. The instantaneous bed elevations and instantaneous three dimensional (3D) velocities were measured using a 5 MHz Ultrasonic Ranging system and 16 MHz micro down looking acoustic Doppler velocimeter, respectively. The velocity and flow depth were measured at different locations under near equilibrium bed scour conditions. The measured 3D velocities were processed for the computation of flow parameters, such as velocity fields, streamline patterns, vorticity fields, and circulation. Furthermore, turbulence intensities, turbulent kinetic energy, Reynolds shear stresses, and bed shear stresses around the piers for all three pier configurations were computed from the detrended velocity signals to identify significant differences in the flow parameters and turbulence in the tandem and staggered pier arrangements as compared to those for an isolated pier. A recirculation zone was found near the bed in front of the rear pier in the tandem case from the streamline patterns. The vortices in the bi-vortex system were observed to be opposite to each other in the gap between the three piers in the staggered case. A strong secondary vortex also was observed apart from the primary vortex at the foot of the pier (θ = 0°) in all the three configurations. The strength of the horseshoe vortex (combination of primary and secondary vortices) was found to be higher at the front piers of the staggered arrangement as compared to those of the tandem piers, followed by the isolated pier. The bed shear stresses were found to be higher for the staggered piers than for the tandem piers in the direction of flow (θ = 0°). However, a 50% reduction in the bed shear stresses was observed behind the tandem piers at θ = 180°. The study reported in this paper provides the foundation for further investigation of countermeasures against local scour around tandem and staggered bridge piers on a mobile bed with non-uniform sediment.     

基态位涡径向分布对浅水涡旋系统演变的影响分析Part Ⅱ:扰动发展和结构变化

本文在基态位涡（Potential Vorticity,PV）径向分布和基态涡旋强度对热带气旋（Tropical Cyclone,TC）类涡旋系统稳定性特征影响的研究基础上,结合理想试验和数值模拟诊断分析基态PV径向分布对扰动增长和系统结构变化的影响.基于线性正压浅水模型,设计三种典型基态PV中空结构下基态涡旋强度对系统稳定性影响的敏感性试验.结果表明：基态涡旋的强度主要影响稳定性的强弱,强度越强,不稳定增长率越大,而基态PV径向分布对系统最不稳定波动性质起着决定作用.分析不同波数下扰动的发展及不同波数间扰动的相互作用可知,对于宽且实的PV环,系统稳定性主要取决于低波数不稳定,且最不稳定波数扰动的发展具有明显的优势地位;对于窄且空的PV环,系统稳定性主要取决于高波数不稳定,且多个高波数下增长最快模态的不稳定增长率值十分接近.利用模态线性叠加法讨论扰动增长对系统结构变化的影响表明：最不稳定波数的扰动发展对系统结构变化有关键影响,而多个波数的扰动不稳定增长相当时,不同波数的扰动发生相互作用从而影响系统结构变化.最后,利用实际个例模拟资料分析基态PV径向分布及其变化对TC结构和强度的影响表明：TC内核区出现的多边形眼墙结构与当前时刻基态PV径向分布所决定的最不稳定波数有很好的对应关系,同时基态PV径向分布变化所反映出的系统动力稳定性强弱与TC强度发展阶段具有很好的相关性.     

Vorticity budget investigation of a simulated long-lived mesoscale vortex in South China

A vorticity budget investigation is performed using the output data from a numerical simulation of a typical MCV (mesoscale convectively generated votex) case in South China. Results suggest that the divergence caused by convection in the low troposphere is the main producer of positive vorticity, while vertical vorticity transferred by the tilting term from the horizontal vorticity compensates the upward output of cyclonic vorticity. Scale analyses of the vorticity equation suggest that the advection of planetary vorticity can be neglected owing to the low latitude, which is different from the larger scale systems in high latitude areas. In addition, the distribution of relative vorticity tendency on pressure level is not uniform. A vortex will move along the vector from the negative to the positive vorticity tendency region. The mechanism of the phenomenon-that nearly all of the convectively ascending region is located southward/southeastward of the vortex center-is also discussed. Convergence with regard to latent heat release would be in favor of the spin-up of meso-vortex, however, the horizontal vorticity caused by windshear is tilted by vertical motion due to convection. Consequently, the negative and positive vorticity tendencies are located symmetrically about the convective center, which suggests that the vortex southward movement is dynamically driven by convection.     

Dynamic model of mesoscale eddies. Eddy parameterization for coarse resolution ocean circulation models

In the framework of the eddy dynamic model developed in two previous papers (Dubovikov, M.S., Dynamical model of mesoscale eddies, Geophys. Astophys. Fluid Dyn., 2003, 97, 311–358; Canuto, V.M. and Dubovikov, M.S., Modeling mesoscale eddies, Ocean Modelling, 2004, 8, 1–30 referred as I–II), we compute the contribution of unresolved mesoscale eddies to the large-scale dynamic equations of the ocean. In isopycnal coordinates, in addition to the bolus velocity discussed in I–II, the mesoscale contribution to the large scale momentum equation is derived. Its form is quite different from the traditional down-gradient parameterization. The model solutions in isopycnal coordinates are transformed to level coordinates to parameterize the eddy contributions to the corresponding large scale density and momentum equations. In the former, the contributions due to the eddy induced velocity and to the residual density flux across mean isopycnals (so called Σ-term) are derived, both contributions being shown to be of the same order. As for the large scale momentum equation, as well as in isopycnal coordinates, the eddy contribution has a form which is quite different from the down-gradient expression.     

Qualitative and quantitative analysis of vorticity,strain and area change in general non-isochoric 2D deformation

A scale free representation of a general non-isochoric 2D deformation is presented which is amenable to mathematical analysis. By describing deformation in 2D in terms of polar coordinates the stretching and rotational histories of linear elements separate and are easily analysed both qualitatively and quantitatively. An analysis of finite strain combined with dynamical considerations allows the derivation of equations which may be used to estimate finite strain, area change and kinematic vorticity number. Numerical investigation of method developed here was carried out and it was found to perform well unless large area changes occur in combination with large components of simple shear. A re-analysis of natural data indicates the method is consistent.     

Deformation temperatures and flow vorticities near the base of the Greater Himalayan Series,Sutlej Valley and Shimla Klippe,NW India

We report new deformation temperature and flow vorticity data from the base of the Greater Himalayan Series (GHS) exposed in the Sutlej Valley and Shimla Klippe of NW India. We focus on three groups of transects across the hanging wall of the Main Central Thrust (MCT). In order of relative foreland – hinterland positions, they are the Shimla Klippe, Western and Eastern Sutlej transects. Deformation temperatures indicated by quartz c-axis fabric opening-angles increase both from foreland to hinterland at a given structural distance above the MCT and up structural section from the MCT within individual transects. Deformation temperatures in the immediate hanging wall to the MCT are estimated at ∼510–535, 535–550 and 610 °C on the Shimla, Western Sutlej and Eastern Sutlej transects, respectively. The steepest inferred field gradients in deformation temperatures are recorded adjacent to the MCT and progressively decrease up structural section following a power law relationship. Comparison with temperature estimates based on multi-mineral phase equilibria data suggests that penetrative shearing occurred at close to peak metamorphic conditions. Vorticity analyses indicate that shearing along the base of the GHS occurred under sub-simple shear conditions (Wm values of 0.9–1.0) with a minor component of pure shear.     

华南沿海2011年7月15—18日持续暴雨过程中的季风槽与中尺度对流系统相互作用

应用NCEP FNL再分析资料及位涡分离反演等方法,对华南沿海2011年7月15—18日持续暴雨过程中季风槽与中尺度对流系统的相互作用进行了研究,主要针对暴雨发生期间季风槽气旋性涡度向上发展的机理及其对季风槽维持发展和中尺度对流系统活动的影响进行分析。结果发现,季风槽的中尺度对流系统发展于弱斜压性环境中,大多在槽东西两端涡度中心区发展最强。南侧盛行的西南低空急流为对流反复发生提供了对流发展的“可维持性”条件,是对流得以组织发展成为中尺度对流系统的重要原因。涡度收支诊断表明,季风槽气旋性涡度生成主要由中尺度对流系统低层辐合引起。位涡分离反演结果证实,季风槽气旋性环流增强主要由与中尺度对流系统潜热加热相关的扰动位涡造成,并随着中尺度对流系统加热峰值高度升高而向上发展,是大尺度环流对中尺度对流系统潜热加热动力响应的结果。在季风槽东西两端,由于中尺度对流系统发展强烈且持续,具有更高的加热效率,引起的气旋性涡度向上发展最为明显。其结果可引起中尺度对流系统西南一侧向北非地转风发展,并在地转偏向力作用下增强西风,维持低空急流的发展,为对流反复发生提供条件。这些都说明季风槽大尺度环流与中尺度对流系统相互作用在中尺度对流系统和持续暴雨形成过程中有重要作用。