A laboratory simulation of mesoscale flow interaction with the Alps     

E. Ferrero  A. Longhetto  L. Briatore  G. Chabert d&#x;Hieres  H. Didelle  C. Giraud  P. Gleizon 《Dynamics of Atmospheres and Oceans》2002,35(1):308

A series of laboratory experiments, aimed at the simulation of some aspects of Alpine lee cyclogenesis has been carried out in the rotating tank of the Coriolis Laboratory of LEGI-IMG in Grenoble. Dynamic and thermodynamic processes, typical of baroclinic development triggered by the orography, were simulated. The background flow simulating the basic state of the atmosphere consisted of a stream of intermediate density fluid introduced at the interface between two fluid layers. The structure of the intermediate current was established by mixing fluid obtained from the upper layer of fresh water with fluid removed from the heavier salty layer below.The dynamical similarity parameters are the Rossby (Ro), Burger (Bu) and Ekman (Ek) numbers, although this last, owing to its small values, need not be matched between model and prototype, since viscous effects are not important for small time scales. The flow in both the prototype and laboratory simulation is characterized by hydrostatics; this requires (Ro²δ²/Bu)1 (where δ=H/L is the aspect ratio of the obstacle) which is clearly satisfied, in the atmosphere and oceans, and for the laboratory experiment.A range of experiments for various Rossby and Burger numbers were conducted which delimited the region of parameter space for which background flows akin to that found to the northwest of the Alps prior to baroclinic cyclogenesis events, were observed.One such experiment was carried out by placing a model of the Alps at the appropriate place in the flow field. The subsequent motion in the laboratory was observed and dye tracer motions were used to obtain the approximate particle trajectories. The density field was also analyzed to provide the geopotential field of the simulated atmosphere. Using standard transformations from the similarity analysis, the laboratory observations were related to the prototype atmosphere. The flow and the geopotential fields gave results compatible with the particular atmospheric event presented.  相似文献   

Influence of wave groups on SSC patterns over vortex ripples     

Paul V. Villard  Philip D. Osborne  Christopher E. Vincent 《Continental Shelf Research》2000,20(17):1

Estimates of spatial and temporal variations in suspended sand concentrations (SSC) made with a multi-transducer Acoustic Backscatter Sensor (ABS) under a repeated wave group over a mobile rippled bed in the wave research flume at the National Hydraulics Laboratory in Ottawa, Canada, reveal an number of complex and intriguing patterns. Ensemble averages of 8 nearly identical wave groups provided much more robust estimates of SSC and allowed a detailed examination of the wave group effects. The largest SSC near the bed (< 0.10 m) occurs in phase with the largest waves in the group. Above approximately 0.10 m elevation, SSC lags behind the near bed SSC by as much as 2–3 waves; introducing significant curvature (on a semi-log plot) to the SSC profile. The log linear segments of the SSC profile grow and decay systematically on the scale of the wave group. The range in lengths of log-linear profile segments ( 0.03–0.355 m) suggest that the boundary layer thickness also fluctuates throughout the passage of the wave group. Furthermore, there are significant variations in the patterns of SSC, which occur under the largest and smallest waves in the group. Under the largest waves vertical bands of alternating high and low SSC produce an intra-wave modulation in the upper water column ( 0.075–0.30 m). The equivalent horizontal excursion of these bands scales to the ripple length. Under the smaller waves the intra-wave modulation of the SSC disappears and is replaced by temporally homogenous suspension that expands vertically through several individual wave cycles. The former pattern of homogenous suspension appears to be associated with growth of a boundary layer due to the persistent uni-directional horizontal flow during this part of the group together with the persistence of antecedent bed generated turbulence and vorticity which maintains the suspension. The latter pattern of bands of high and low SSC indicates a strong temporal and spatial constraint on the SSC (phase coupling) induced by the presence of the bedforms which may be enhanced by strong reversals in both flow and vorticity under the large waves in the group.  相似文献   

A bed‐load transport model for rough turbulent open‐channel flows on plane beds     

Athol D. Abrahams  Peng Gao 《地球表面变化过程与地形》2006,31(7):910-928

Data from flume studies are used to develop a model for predicting bed‐load transport rates in rough turbulent two‐dimensional open‐channel flows moving well sorted non‐cohesive sediments over plane mobile beds. The object is not to predict transport rates in natural channel flows but rather to provide a standard against which measured bed‐load transport rates influenced by factors such as bed forms, bed armouring, or limited sediment availability may be compared in order to assess the impact of these factors on bed‐load transport rates. The model is based on a revised version of Bagnold's basic energy equation i_bs_b = e_bω, where i_b is the immersed bed‐load transport rate, ω is flow power per unit area, e_b is the efficiency coefficient, and s_b is the stress coefficient defined as the ratio of the tangential bed shear stress caused by grain collisions and fluid drag to the immersed weight of the bed load. Expressions are developed for s_b and e_b in terms of G, a normalized measure of sediment transport stage, and these expressions are substituted into the revised energy equation to obtain the bed‐load transport equation i_b = ω G ^3·4. This equation applies regardless of the mode of bed‐load transport (i.e. saltation or sheet flow) and reduces to i_b = ω where G approaches 1 in the sheet‐flow regime. That i_b = ω does not mean that all the available power is dissipated in transporting the bed load. Rather, it reflects the fact that i_b is a transport rate that must be multiplied by s_b to become a work rate before it can be compared with ω. It follows that the proportion of ω that is dissipated in the transport of bed load is i_bs_b/ω, which is approximately 0·6 when i_b = ω. It is suggested that this remarkably high transport efficiency is achieved in sheet flow (1) because the ratio of grain‐to‐grain to grain‐to‐bed collisions increases with bed shear stress, and (2) because on average much more momentum is lost in a grain‐to‐bed collision than in a grain‐to‐grain one. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

关于上海地区金山群大地构造属性的思考   总被引：1，自引：2，他引：1  

顾澎涛 《上海国土资源》2006,(4):10-13

根据上海地区金山群的岩石组合、原岩及原岩建造、变质作用、构造形变特征,以及已有的同位素年龄值,认为金山群与浙东南陈蔡群基本一致,是华厦古陆块的基底组分。参照华厦古板块与扬子古板岩多期聚合的观点,推断分布在(?)径——川沙断裂南东侧的金山群块断体,是晋宁期末二古板块碰撞对接、于结合带内形成的构造推叠岩片。  相似文献   

“96·8”华北暴雨数值模拟与稳定性分析   总被引：3，自引：3，他引：3  

边清河  丁治英  董金虎 《气象》2006,32(8):17-22

分析1996年8月发生在华北地区的台风暴雨过程的环流形势，发现：副热带高压与台风低压之间的气压梯度很大，宽广的偏南急流源源不断地向北输送水汽和能量，而太行山一带正处于汇合区，构成十分有利的暴雨天气形势。应用MM5数值预报方法对1996年8月4—5日的降雨天气过程进行数值模拟，并依据天气学原理和位涡理论对此过程的稳定性进行分析认为：（1）MM5数值预报模式较好地模拟出了台风暴雨的物理过程。（2）此次降雨的不稳定层结有南高北低现象，同时有对称不稳定和对流不稳定存在；条件性对称不稳定可使环流加速，对降水有一定的增幅作用。  相似文献   

2005年3月22日华南飑线的综合分析   总被引：8，自引：1，他引：8  

罗建英  廖胜石  梁岱云  马艺  寿绍文 《气象》2006,32(10):70-75

2005年3月22日华南地区发生了一次飑线天气过程。利用常规观测、雷达回波、自动气象站资料及NCEP1°×1°的逐6小时资料,从天气形势、雷达回波、物理量场等多角度综合诊断分析了该飑线过程。结果表明:该过程具有低层增温、增湿,中高层降温、低湿的特征。飑线发生在快速东移的高空槽前上干冷、下暖湿的不稳定区域,华南地区700hPa低空急流的爆发及低层急流核向东传输对不稳定能量的突然释放有很大的触发作用。飑线系统在低空增温、增湿与对流层中层干侵入的相互作用下形成,产生大范围的雷雨大风、冰雹等强对流天气。  相似文献   

初始涡的结构与尺度对涡旋自组织影响的研究   总被引：5，自引：2，他引：5  

周嘉陵  马镜娴  陈联寿  罗哲贤 《气象学报》2006,64(5):537-551

在涡旋自组织动力学的框架内,实施了9组积分时间为72 h的试验,分析初始涡廓线与初始涡尺度对多涡自组织的作用。试验的初始场上,存在着12个大小不等的β和γ中尺度的涡。若初始涡廓线为高斯型,则这些涡不能自组织形成一个α中尺度的涡;若初始涡廓线为双正弦型、抛物线型或压缩型,则这个α中尺度的涡可以形成。此外,涡廓线不同,三涡流型出现的时间迟早不一,较大尺度α中尺度涡出现的时间也迟早不一。同时,初始涡的半径大小也是影响自组织过程成败的一个重要因素。  相似文献   

华北盛夏暴雨过程的能量特征分析   总被引：1，自引：0，他引：1       下载免费PDF全文

赵桂香  李新生  袁崇民  牛永波  王莉 《山西气象》2006,(1):3-4,7

本文利用实况资料和T213输出资料,对发生在2005年8月16日～17日的华北暴雨过程进行了能量学特征分析,结果表明:暴雨前24h～36h,对流层上下层有暖湿空气同时发展,是暴雨发生的重要动力机制;强的干、湿静力能锋区稳定维持是这次大范围暴雨产生的能量基础,中低层湿静力能差值大于0,且数值很大,说明对流不稳定层结很强,暴雨发生在高能、高不稳定度的条件下;水汽的充足补给作用,是这次暴雨过程能量转换的基础。  相似文献   

Properties and Stability of a Meso-Scale Line-Form Disturbance   总被引：1，自引：0，他引：1  

沈新勇  丁一汇  赵南 《大气科学进展》2006,23(2):282-290

By using the 3D dynamic equations for small- and meso-scale disturbances, an investigation is performed on the heterotropic instability (including symmetric instability and traversal-type instability) of a zonal line-like disturbance moving at any angle with respect to basic flow, arriving at the following results: (1) with linear shear available, the heterotropic instability of the disturbance will occur only when flow shearing happens in the direction of the line-like disturbance movement or in the direction perpendicular to the disturbance movement, with the heterotropic instability showing the instability of the internal inertial gravity wave; (2) in the presence of second-order non-linear shear, the disturbance of the heterotropic instability includes internal inertial gravity and vortex Rossby waves. For the zonal line-form disturbance under study, the vortex Rossby wave has its source in the second-order shear of meridional basic wind speed in the flow and propagates unidirectionally with respect to the meridional basic flow. As a mesoscale heterotropic instable disturbance, the vortex Rossby wave has its origin from the second shear of the flow in the direction perpendicular to the line-form disturbance and is independent of the condition in the direction parallel to the flow; (3) for general zonal line-like disturbances, if the second-order shear happens in the meridional wind speed, i.e., the second shear of the flow in the direction perpendicular to the line-form disturbance, then the heterotropic instability of the disturbance is likely to be the instability of a mixed Rossby–internal inertial gravity wave; (4) the symmetric instability is actually the instability of the internal inertial gravity wave. The second-order shear in the flow represents an instable factor for a symmetric-type disturbance; (5) the instability of a traversal-type disturbance is the instability of the internal inertial gravity wave when the basic flow is constant or only linearly sheared. With a second or nonlinear vertical shear of the basic flow taken into account, the instability of a traversal-type disturbance may be the instability of a mixed vortex Rossby – gravity wave.  相似文献   

2003年江淮梅雨暴雨与湿位涡的关系   总被引：11，自引：3，他引：11  

张端禹  丁治英  张兴强  徐海明 《南京气象学院学报》2006,29(4):526-532

统计了2003年发生在江淮地区的梅雨暴雨，根据暴雨中心对流稳定度及降雨性质将暴雨分为4种类型。分析湿位涡与梅雨暴雨中的惯性不稳定、对称不稳定、对流不稳定的关系发现：第1类暴雨往往与对称不稳定有关，第2类暴雨与对流不稳定有关，且这两类暴雨高层多具有强惯性不稳定，降水相对较大；第3、第4类暴雨是稳定性降雨，高层一般不具备惯性不稳定，雨量一般较小。高层的惯性不稳定对降水的加强作用明显。  相似文献