Linear and angular momentum conservation in hydraulic jump in diverging channels     

Alessandro Valiani Valerio Caleffi 《Advances in water resources》2011,34(2):227-242

This paper addresses the integral conservation of linear and angular momentum in the steady hydraulic jump in a linearly diverging channel.The flow is considered to be divided into a mainstream that conveys the total liquid discharge, and a roller where no average mass transport occurs. It is assumed that no macroscopic rheological relationship holds, so mass, momentum and angular momentum integral balances are independent relationships. Normal stresses are assumed to be hydrostatic on vertical, cylindrical surfaces. Viscous stresses are assumed to be negligible with respect to turbulent stresses. Assuming that the horizontal velocity distribution in the mainstream is uniform and that the horizontal momentum and angular momentum in the roller are negligible with respect to their mainstream counterparts, an analytical solution is obtained for the free surface profile of the flow. This solution is fundamental for finding the sequent depths and their positions. Consequently, it permits solving for the length of the jump, which is assumed to be equal to the length of the roller. Mainstream and roller thicknesses can also be derived from the present solution. This model may also be theoretically used to derive the average shear stresses exerted by the roller on the mainstream and the power losses per unit weight. This second relationship, which returns the well-known classical expression for total power loss in the jump, demonstrates that the strongly idealized mechanical model proposed here is internally consistent.  相似文献   

基于ERA40资料的边界层厚度气候特征分析     

王学忠  胡邦辉  李昀英 《气象科学》2011,31(3):339-346

为了解边界层厚度的大尺度分布及其气候循环特征,基于逐日ERA 40 再分析边界层厚度资料,采用统计学方法计算边界层厚度分4个时次的季节循环、年较差等气候特征场,对全球边界层厚度分布及其季节演变特征进行了研究。结果表明：全球较厚的边界层分布与大地形、沙漠等下垫面的分布有直接联系;陆地强边界层分布存在“08时”现象,即陆地较厚边界层中心分布在当地时间08时经线附近;在大洋上南北纬30°~40°两条纬度带内边界层厚度的月尺度内变率较大,与大气强斜压性的天气系统活跃区一致;月平均日较差的年循环特征存在着季节性南北进退的现象。研究结果对于全面认识和科学利用ERA 40逐日边界层再分析资料有一定帮助。  相似文献   

Two Northward Jumps of the Summertime Western Pacific Subtropical High and Their Associations with the Tropical SST Anomalies   总被引：1，自引：0，他引：1       下载免费PDF全文

SU Tong-Hu  XUE Feng 《大气和海洋科学快报》2011,4(2):98-102

Based on the pentad mean ridgeline index of the western Pacific subtropical high (WPSH), the authors identified the two northward jumps of the WPSH from 1979 to 2008 and revealed their associations with the tropical SST anomalies. The authors show that the northward jumps, especially the second jump, exhibited remarkable interannual variability. In addition, the authors find that the two northward jumps were mutually independent and were influenced by the SST anomalies in the different regions of the tropical Pacific. The first jump was positively correlated with the SST anomalies in the tropical central Pacific from the preceding winter to June. In contrast, the second jump was positively related to ENSO in the preceding winter, but this correlation tended to weaken with the decay of ENSO and disappeared in July. Instead, a positive correlation was found in the Indian Ocean. We therefore suggest that ENSO plays an indirect role in the second jump through the capacitor effect of the Indian Ocean.  相似文献   

Holocene transgression of the Rhine river mouth area,The Netherlands/Southern North Sea: palaeogeography and sequence stratigraphy     

KIM M. COHEN 《Sedimentology》2011,58(6):1453-1485

This study presents a detailed reconstruction of the palaeogeography of the Rhine valley (western Netherlands) during the Holocene transgression with systems tracts placed in a precise sea‐level context. This approach permits comparison of actual versus conceptual boundaries of the lowstand, transgressive and highstand systems tracts. The inland position of the highstand Rhine river mouth on a wide, low‐gradient continental shelf meant that base‐level changes were the dominant control on sedimentation for a relatively short period of the last glacial cycle. Systems in such inland positions predominantly record changes in the balance between river discharge and sediment load, and preserve excellent records of climatic changes or other catchment‐induced forcing. It is shown here that the transgressive systems tract‐part of the coastal prism formed in three stages: (i) the millennium before 8·45 ka bp , when the area was dominated by fluvial environments with extensive wetlands; (ii) the millennium after 8·45 ka, characterized by strong erosion, increasing tidal amplitudes and bay‐head delta development; and (iii) the period between 7·5 and 6·3 ka bp when the Rhine avulsed multiple times and the maximum flooding surface formed. The diachroneity of the transgressive surface is strongly suppressed because of a pulse of accelerated sea‐level rise at 8·45 ka bp . That event not only had a strong effect on preservation, but has circum‐oceanic stratigraphical relevance as it divides the early and middle Holocene parts of coastal successions worldwide. The palaeogeographical reconstruction offers a unique full spatial–temporal view on the coastal and fluvial dynamics of a major river mouth under brief rapid forced transgression. This reconstruction is of relevance for Holocene and ancient transgressive systems worldwide, and for next‐century natural coasts that are predicted to experience a 1 m sea‐level rise.  相似文献   

全球变化背景下新疆地区气候跃变的可能影响因素分析   总被引：1，自引：0，他引：1  

李稚  李卫红  陈亚宁 《冰川冻土》2011,33(6):1302-1309

基于新疆过去50a气温和降水时间序列长期趋势的研究结果,结合对1986年及附近时段对全球气候产生重大影响的一系列事件（厄尔尼诺、火山爆发、核电站爆炸事故、温室气体增加等）的分析,探讨了1986年新疆气温和降水出现跳跃式变化的原因.具体表现为：当厄尔尼诺现象发生时,常常出现暖冬、早春,在该现象消退过程,往往降水有所增多;...  相似文献   

北欧海的锋面分布特征及其季节变化   总被引：2，自引：0，他引：2  

何琰  赵进平 《地球科学进展》2011,26(10):1079-1091

利用多年月平均格点数据分析了北欧海主要锋面的分布特征和季节变化规律,并讨论了月平均数据分析锋面适合使用的方法。月平均数据显示的锋面出现间断或多重的现象是锋面侧向摆动造成的,这是月平均数据的一大特点。北欧海各锋面主要水文和季节变化特征差异很大。东格陵兰极地锋在夏季锋面强度大,锋面较连续完整,而冬季强度小,锋面结构零散。9月由于东格陵兰寒流势力最强,可观察到温度梯度较大且连续的东格陵兰锋。北极锋的季节变化在水平方向呈＂哑铃型＂分布,中段摆动较南北两端小。由于挪威海流在冬季出现的最大流量引起挪威海流的流幅在该处加宽,莫恩海脊锋冬季向西北移动,对前人文章中基本上没有季节性移动的说法进行了修正和补充。冰岛—法罗群岛锋随深度增加向南移动,锋面强度增强,这是溢流造成的。  相似文献   

武汉天河机场低能见度现象的气候特征分析   总被引：2，自引：0，他引：2  

胡迪 《山西气象》2006,19(3):18-20

用5年滑动平均方法并用最小二乘法建立方程式,计算太阳辐射气候倾向率及日照气候倾向反映太阳辐射的45a变化趋势,用小波分析方法分析太阳总辐射的周期变化规律。结果表明,太原地区的年日照时数减少,年总辐射量呈显著减少趋势,有11a的周期变化,在1981年左右有突变。初步统计能见度低于10km次数在逐年增多。  相似文献   

副热带高压季节性北跳的诊断研究   总被引：6，自引：3，他引：6  

朱玉先  喻世华 《热带气象学报》1997,13(3):246-257

用ＣＥＯＦ、功率谱，带通滤波等方法的１９８８年，１９９１年５￣７月逐日２．５°×２．５°５００ｈＰａ位势高度场资料对北半球副絷带高压季节性北跳进行了诊断研究，得出：副热带高压季节性北跳及西传是全球现象；副高发生季节性跳动的源主要位于阿拉海地区和中太平洋地区，汇主要位于东太平洋地区；副高的季节性跳动主要是扰动场的作用，低频振荡在副高的季节性跳动及西传中起举足轻重的作用，各年副高季节性跳动的差异是扰源  相似文献   

4—6月MJO北传与东亚季风爆发的关系   总被引：2，自引：0，他引：2  

马婷婷  吴志伟  江志红  解文璇 《热带气象学报》2014,30(5):921-931

大气季节内振荡(MJO)对东亚季风的活跃、推进和中断具有显著影响,然而其在东亚季风爆发中的作用一直没有得到深入探讨。利用经验正交函数(EOF)分解和超前滞后回归分析方法,探讨了1979—2011年4—6月MJO的北传与东亚季风爆发的关系。研究表明:4—6月MJO有两次明显的北传过程,分别发生在5月中旬和6月中旬。逐候向外长波辐射资料EOF分析的前两个模态对应了MJO的不同位相,可以表征MJO的向东向北传播。超前滞后回归分析表明,伴随MJO的北传,降水带北移、印度洋到我国南海地区西南风建立、西太平洋副热带高压撤出南海及南亚高压北抬,这些环流异常形势有利于东亚季风的爆发。进一步对PC1所确定的季风爆发候与其它5个季风爆发指数进行对比发现,在大多数年份中,PC1所确立的季风爆发候与其它指数相差2候以内,其中,与其它5个指数所表征的季风爆发时间严格一致的年份分别有15、10、11、10、12年,表明MJO对东亚季风的爆发具有普遍作用。  相似文献   

华南前汛期开始和结束日期的划分   总被引：16，自引：0，他引：16       下载免费PDF全文

强学民  杨修群 《地球物理学报》2008,51(5):1333-1345

本文利用48年（1957～2004年）中国站点逐日降水资料和同期NCEP/ NCAR逐日再分析资料,研究了华南前汛期的开始和结束时间的划分问题.首先,选择了研究华南前汛期问题的区域和代表站点,然后对降水量、水汽（可降水量,水汽通量,水汽通量散度）、垂直速度和假相当位温等物理量的演变特征进行分析,发现：前汛期起、止前后上述要素均有阶段性的突变.其中4月第1候（19候）是华南前汛期的开始,可降水量、水汽通量和假相当位温等增加显著,对流开始活跃,水汽通量散度也由辐散变为辐合,降水量明显增加.但4月份总体雨量不强,主要为锋面降水.5月份随夏季风爆发,水汽继续增加,对流活跃,进入季风降水阶段;夏季风降水盛期时段主要集中在6月份.6月第4候（34候）前汛期结束,各降水指标骤减.然后根据降水和环流指标,提出了华南前汛期开始和结束日期的划分标准,定义了逐年的开始和结束日期.最后对华南前汛期开始期之前、之后以及结束期之前、之后的大气环流背景做了对比分析.指出,前汛期开始前,环流形势有利于华南地区增暖增湿;开始期以后则有利于冷空气南下,造成连续降水,使华南进入前汛期.而前汛期的结束,则是由于东亚大气环流的季节调整,尤其是西太平洋副热带高压的第一次北跳所造成的.  相似文献