中国东南沿海冬季渐近线型锋生辐合线及其强降水的发展机制

基于中国东南沿海冬季强降水的统计分析,采用EOF、REOF、North检验等方法对2011—2016年冬季(12、1、2月)欧洲中期天气预报中心降水资料进行分类,选取位于内陆的第1、第4 REOF模态,对该两模态的降水样本进行合成分析,合成的降水中心与东南沿海福建的多年统计暴雨中心吻合。与强降水相配合,1 000 hPa上有自北向南的渐近线型辐合气流,并伴有锋区,从而形成天气尺度渐近线型锋生辐合线,强降水位于辐合轴线左侧气旋式风切变处。这是一类以前未曾受到关注和讨论的东南沿海地区冬季暴雨系统。利用客观判定方法和建立系统坐标系,以确认并诊断该系统的结构。在冷干少雨、低层盛行偏北风的冬季,此类系统兼有锋区热力抬升与辐合气流动力抬升,在雨区形成旺盛的上升运动;同时,通过辐合线正交风分量将邻近的海面水汽汇集到降水区,与中高层副高边缘偏南气流相向而行,构成较为深厚的交汇式水汽输送层;通过非绝热加热,形成深厚的热力对流不稳定,并通过干区向湿气团下楔入,形成下干上湿的湿动力不稳定,以及假相当位温随高度增加而递减,形成上暖湿、下冷干的对流不稳定层。因此,该系统对冬季强降水的发生发展及落区具有重要影响。通过WRF模式的模拟结果探讨环境热力机制的影响,结果显示,凝结潜热加热可影响辐合线的辐合位置和强度、锋生区的位置及强度,进而影响系统的活跃程度。中层潜热加热抑制平流感热冷却进入暖气团,维持降雨区的热力不稳定和降水强度。渐近线型锋生辐合线有利于东南沿海冬季大范围降水出现暴雨,其中凝结潜热释放具有重要贡献。     

Experimental study on the influence of river flow confluences on the open channel stage–discharge relationship

ABSTRACT

Accurate assessment of stage–discharge relationships in open channel flows is important to the design and management of hydraulic structures and engineering. Flow junctions commonly occur at the confluence of natural rivers or streams. The effect of flow junctions on the stage–discharge relationship at mountain river confluences was found by measuring velocity fields and water levels in experimental models. The results show that the backwater and accumulation–separation at flow junctions affect the flow structures and patterns in the channel; also, flow confluences may induce complex flow characteristics of backwater and flow separation at river junctions, indicating potential submerged flooding disasters within the confluence zone. The impacts of flow junctions on the stage–discharge relationship are investigated for two physical confluence models built from river confluence prototype systems in southwest China. The results show that the presence of tributary river inflows tends to increase the water level of the main river. This is important for flood control, flood-risk evaluation and engineering (e.g. hydropower station construction) in mountain rivers. Finally, a comparative quantitative analysis based on flow motion equations is conducted to evaluate the stage–discharge relationship in both uniform and regular confluence systems. The results indicate that more accurate prediction can be made when taking into account the flow non-uniformity induced by flow separation, backwater and distorted bed in the junction region.     

基于错峰型洪水的伊洛河交汇区水动力特性

采用平面二维水动力数值模拟方法,对伊洛河交汇区河段的错峰洪水特性进行了系统分析,讨论了洪水水位、流态及两支流的相互影响,结果表明:数值模拟成果与实测数据有很好的一致性。研究发现:交汇区流场呈现马鞍形水面形态,并随流量增大向下游推进;错峰洪水过程中强势支流产生的漫滩水流对弱势支流形成壅堵,交汇区水动力要素与流态均与强、弱支流流量比有明显的关联性,交汇点下游的螺旋流结构与转向受当时强势支流控制;错峰洪水使交汇区水位随时间变化呈现三段式二次曲线特征,在两洪峰期间交汇区保持高水位的时间比单峰或同峰长5~7倍;错峰洪水使交汇区洪水位与流量形成特殊的Y型曲线关系,在两个洪峰点附近,水位与汇流比的对应关系出现前、后两个拐点。     

Experimental study on lag time for a small watershed

This paper is focused on the lag time of flow confluence. Lag is described empirically by the precipitation factor in hydrological modelling, but the traditional way to establish the relationship between lag and intensity is not very satisfactory in arid and semi‐arid regions. A total 215 rainfall–runoff experiments were conducted to reveal the effects of net rain duration on lag and intensity. The results show that a correlation between lag and rain intensity exists under certain conditions. There is a critical value in net rain intensity (0·8–1 mm min^?1) in the correlation curve of lag and net rain intensity in the given experimental watershed in the laboratory. Similarly, a critical value of net rain duration also exists. This value is the total confluence time. The features of lag time change significantly if intensity or duration is less or greater than the critical value. The study also explores the joint effects of net rain intensity and duration on lag. The formula established among lag, intensity and duration resulted in a better fit. Therefore, the two‐parameter (intensity and duration) empirical formula for lag is better than the traditional single‐parameter (intensity) method. This two‐parameter correlation formula can also be applied to a temporally and spatially uneven runoff processes. A typical field watershed is selected to test the results of the experiments. Copyright © 2006 John Wiley & Sons, Ltd.     

Confluence and intersection of interacting conjugate faults: A new concept based on analogue experiments

Examination of the interaction area of conjugate faults yields evidence of a yet unknown structural pattern. Analogue experiments revealed the evolution of this pattern during the interaction process. A fault propagating towards the highly active section of another conjugate fault becomes increasingly deflected towards the obtuse angle side of the conjugate system while approaching until joining it. During this process, the tip of the curved propagating fault retains its shear sense until immediately before joining with the highly active fault—a process called “confluence” in this report. Under constant remote stress, the deflected fault now becomes dominant. A fault branch splits off at the point where deflection commences, propagating straight ahead of the original direction, and may dissect the former master fault. Alternating activity at both of the faults finally produces the known intersection patterns. Further studies of the proposed concept on natural examples are recommended due to the close geometric similarity of the evolutionary structural stages and natural fault patterns.     

试论喀斯特管流水特征

喀斯特水系统是由管流水与隙流水共同组成的水文地质系统,它们之间存在着复杂的耦合与转化关系。偏流作用引起夺水现象,进而导致汇流。管流水分为伏流与暗河,在非灌入式补给情况下,喀斯特水运动的基本模式是:降雨(补给)→隙流水(赋存)→管流水(汇排)。一般情况下,暗河起着汇排隙流水的功能。     

Depth Distribution of the Subtropical Gyre in the North Pacific

Large-scale aspects of the North Pacific subtropical gyre have been investigated using a climatology of temperature and salinity (World Ocean Atlas 1998). In the central and eastern parts of the basin, the axis of the subtropical gyre, defined as the meridional maximum of dynamic height, tends to move poleward from about 25°N near the surface to about 40°N in the upper intermediate layers. In the western part of the basin, the axis is seen at about 30°N, remaining almost unchanged with depth. Striking features associated with this vertical distribution include a northward shift of the bifurcation latitude of the North Equatorial Current at increasing depth and a barotropic nature of the confluence point between the Kuroshio and Oyashio at their respective western boundaries. The former occurs at about 14°N near the surface and extends north of 20°N at depths around 800 m. The latter, situated at about 36.4°N off Japan, does not appear to have a strong signature of depth-dependence. While some of these results are already known from sporadic hydrographic observations, they have not hitherto been represented in a three-dimensional climatology. This revised version was published online in July 2006 with corrections to the Cover Date.     

90°支流入汇区域时均流速分布特征试验研究

通过多组次水槽试验,应用声速多普勒测速仪研究入汇角为90°时,不同汇流比水流条件下支流入汇区域及其附近的水面比降变化和时均流速分布特征。试验表明,入汇口上游的干支流水面坡降取决于干支流相互顶托影响,而入汇口处及其下游合流掺混区水面坡降随干流流量增大而增大,随汇流比增大而有所减小。不同位置垂线时均流速分布主要受主支汇流比和水流流态的双重影响,随着汇流比的增大,入汇口上游断面时均流速逐渐减小,而入汇口下游断面产生了明显的高、低流速带,形成回流分离区。     

天山东段冰雪消融与产汇流水文过程——以乌鲁木齐河源区为例

应用天山北坡乌鲁木齐河源区1 号冰川、空冰斗和总控3 个水文断面2011 年5-9 月每10min 实测水位数据、15 min 的气温、降水数据、1 号冰川消融及空冰斗区积雪观测数据,采用排除和不排除降水对冰雪消融产流影响的研究方法,系统分析了不同冰川覆盖率下水文断面冰雪消融特征及产汇流过程。结果表明：（1） 乌鲁木齐河源区3 个水文断面流量昼夜差异明显,1 号冰川水文断面白天径流大于夜晚径流,空冰斗和总控断面则相反;3 个断面流量亦具有显著日变化过程,且流量峰值大小和到来的时间存在差异。（2） 在排除与不排除降水两种天气影响下,1 号冰川最大流量滞后最高气温分别为1~3 h 和0~1 h;空冰斗断面分别为10~16 h和13 h,总控断面分别为5~11 h 和6~7 h,反映了1 号冰川从消融产流到汇流时间最短,空冰斗积雪消融产流时间最长,总控位于二者之间,同时亦反映伴随降水过程冰雪融水汇流迅速,即从产流到汇流时间有一定的缩短。（3） 影响3 个水文断面流量变化的因素不同,冰川区热量条件是影响1 号冰川水文断面的关键,1 号冰川在过去20 余年间,冰内、冰下排水道变得更为单一,对融水的阻滞和贮存作用弱化,融水汇流过程变得更为迅速。（4） 冰川覆盖山区流域水文断面在冰川消融期的流量过程线变化及最大流量与最高气温时滞变化规律在一定程度上对于认识冰川覆盖率有差异背景下的流域下垫面水系演化、冰雪消融过程及水文断面径流补给具有重要的指示意义。     

Total deformation and its role in heavy precipitation events associated with deformation-dominant flow patterns

In this paper, it is elucidated that the total deformation （TD）, defined as the square root of the sum of squared stretching deformation and squared shearing deformation, is an invariant independent of the coordinate system used. An idealized flow field is then constructed to demonstrate the confluence effect of a non-divergent and irrotational deformation field on moisture transport. To explore the characteristics and role of TD, one heavy rainfall case that occurred in the middle and lower reaches of the Yangtze River （MRYR） over China, associated with a front with shear line, is analyzed using the Weather Research and Forecasting （WRF） model output data. It is found that right before the occurrence of precipitation, the effect of the confluence induced by deformation on moisture transport provides a favorable condition for precipitation. During the precipitation, both location and orientation of the zone of large TD coincide with the confluent shear line. The rainhands are nearly parallel with, and located lightly to the south of the zones of large TD and the confluent shear line. The TD in the lower troposphere increases in value as precipitation persists. When TD approaches its maximal value, the next 6-hour precipitation reaches its peak correspondingly. A tendency equation for TD is derived. The analysis of linear correlation and RMS difference between individual terms in the total deformation equation and the sum of the terms shows that the pressure gradient plays a major role in determining the local change of total deformation.