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11.
On rip-channelled beaches, intense rip currents are driven by waves due to alongshore variations in breaking-induced wave energy dissipation. This study addresses for the first time the potential development of tidal currents superimposed onto the wave-driven circulation. This phenomenon is observed on a rip-channelled meso-macrotidal beach (Biscarrosse, SW France). Field measurements show 20 to 45% stronger mean rip velocities during ebb than during flood. Numerical experiments reveal that this asymmetry is the signature of tidal currents developing over the rip channel morphology. This asymmetry is found to increase roughly linearly with increasing tidal range. These results are significant to beach safety and lifeguarding and stimulate further numerical exercises. 相似文献
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Using an eddy-resolving two layer primitive-equation model forced by symmetric wind stress, we investigate the asymmetry of the recirculation of a double gyre (subpolar gyre and subtropical gyre). In the case where the layer thickness change is large, cyclonic recirculation becomes unstable and splits into meso-scale vortices more easily than anti-cyclonic recirculation in their developing stage. The subpolar gyre is, therefore, filled with more vortices than the subtropical gyre. Moreover, the effect of the upper layer potential vorticity on the lower layer potential vorticity in the subpolar gyre is stronger than that in the subtropical gyre. The characteristic of turbulence in the subpolar gyre is different from that found in the subtropical gyre and, therefore, the asymmetry of the recirculation of the double gyre is maintained by this difference. The asymmetry can not be produced in a quasi-geostrophic model because it ignores the nonlinearity associated with layer thickness change. Moreover, we investigate the effects of layer thickness and lateral viscosity on the asymmetry of the recirculation of the double gyre. In the case of realistic physical parameters, the asymmetry of the recirculation of the double gyre is noticeable from the view of the activities of the eddies. In the case with the shallowest upper layer, the position of separation of the western boundary current moves further southward. 相似文献
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利用哈得来中心海表温度（HadISST）和美国马里兰大学海洋同化（Simple Ocean Data Assimilation,SODA 2.2.4）数据集,分析了1979-2015年赤道太平洋地区海表温度异常的偏度特征;并通过诊断混合层热量收支平衡方程着重探讨了非线性项在不同类型ENSO发展衰亡过程中起的不同作用。研究结果表明,中部型厄尔尼诺和拉尼娜事件的偏度很弱,且符号相反、强度相当,而东部型厄尔尼诺的偏度为很强的正值,即ENSO的强度非对称性主要来自东部型厄尔尼诺的贡献,中部型厄尔尼诺和拉尼娜的海表温度异常呈现准对称特征。在ENSO事件的发展阶段,非线性项对东部型厄尔尼诺和拉尼娜均有很强的正贡献,而对中部型厄尔尼诺的正贡献相对较弱,这会促进东部型厄尔尼诺强度的增强而抑制拉尼娜振幅的发展,从而有利于ENSO强度非对称性的产生;在ENSO事件的衰亡阶段,非线性项对拉尼娜维持正贡献但强度大为减弱,对东部型厄尔尼诺和中部型厄尔尼诺的衰减则分别起促进和抑制作用,这可能是东部型厄尔尼诺衰减速度较快而中部型厄尔尼诺衰减速度较慢的重要原因之一。 相似文献
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We use interferometric synthetic aperture radar (InSAR) observations to investigate the coseismic deformation and slip distribution of the 1997 Mw7.5 Manyi earthquake, a left-lateral strike-slip earthquake occurred on the west portion of the Kunlun fault in the northern Tibet, China. The fault trace is constrained by the combination of interferometric coherence image and azimuth offset image. The total length of the identified fault is about 170 km. We estimate the source parameters using a seven-segment fault model in a homogeneous elastic half-space. We first use a uniform slip model to estimate the slip, width, dip and rake for each segment, resulting in a maximum slip of 5.5 m with a depth of 11 km on the fourth segment. The average dip of the uniform slip model is about 93° northward and the average rake is about −2°. We then use a distributed slip model to estimate the pure strike-slip and oblique slip distribution, respectively. In the distributed slip model, the fault plane is discretized into 225 patches, each of them 4 km × 4 km. We fix the optimal geometric parameters and solve for the slip distribution using a bounded variable least-squares (BVLS) method. We find a geodetic moment of 1.91 × 1020 Nm (Mw7.5), of which almost 68% released in the uppermost 8 km and 82% in the uppermost 12 km. For all the models used in this study, the synthetic profiles along strike show asymmetric displacements on the opposite sides of the fault, which are in agreement with the observations. This suggests that a linear elastic model with variable and non-vertical dips is also reasonable for the mechanism of the Manyi earthquake. 相似文献
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利用中国西藏高原25个台站1971~2000年月平均最高气温 (Tmax)、最低气温 (Tmin) 及气温日较差 (ΔT)，分析了西藏高原最高、最低气温的非对称变化。结果发现，西藏高原普遍存在非对称变化现象，以Tmax、Tmin显著上升，但Tmin上升幅度大于Tmax为主要类型。Tmax上升主要表现在夏季，Tmin增暖以秋季最为明显，ΔT显著变小主要是由于Tmin明显增暖引起的。Tmin的上升幅度随海拔高度的上升而增加，Tmax在3000~4000 m地区增温最大，而ΔT在4000 m以上呈显著的减小趋势。 相似文献
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红外云顶亮温在西北太平洋热带气旋强度预报中的应用 总被引：3，自引：0，他引：3
应用GMS-5气象卫星红外云顶亮温(TBB)资料,分析西北太平洋的热带气旋(TC)TBB、TBB的对称和非对称分量与滞后0—48h TC强度的相关关系。发现,TC眼墙附近东南侧的TBB、距TC中心半径0.8°—1.7°范围内TBB对称分量和1—10波振幅之和与0—48h的TC强度有很好的负相关关系,与滞后24h的TC强度相关极值分别达到-0.52,-0.59和-0.625。考虑气候持续因子、天气因子及TBB因子,针对1996—2002年西北太平洋远海区域(0°—50°N,120°—155°E)热带风暴(TS)等级以上样本,建立12,24h和48h强度预报方程并进行独立样本检验。结果表明,1.0°—1.5°环域平均的TBB对12h强度预报的方差贡献位居第4,TC东南侧TBB的平均值和1.1°—1.5°范围TBB极大与极小值之差对24h强度预报的方差贡献分列第3和第5位。考虑TBB因子的回归方程对TS和强热带风暴(STS)的强度预报能力有较大提高,对12h内强度减弱15m/s以上TC的12h预报、强度稳定TC的24h预报和强度48h增强10m/s以上TC的48h预报均有所改善。 相似文献
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A. D. Knighton 《地球表面变化过程与地形》1981,6(6):581-588
Most river cross-sections display some degree of asymmetry for which there is no adequate quantitative definition. Indices of asymmetry are derived using two principles based on areal differences and maximum depth displacement relative to the channel centreline. The indices are assessed firstly by comparing values for a set of constructed channel shapes which are ranked in order of increasing visual asymmetry, and secondly by analysing a series of 50 cross-sections from a small mountain stream. Two indices, A* and A2, are probably the most valuable, and could be used to test and generate hypotheses regarding the related changes to cross-sectional and planimetric form which occur during the transition from a straight to a meandering channel, particularly if parallel measures of flow asymmetry are defined. Part II of this paper, concerned with ‘Mode of Development and Local Variation’ will be published in a subsequent issue of this journal. 相似文献
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夏季南海对流活动对北半球中低纬大气环流的影响及其可能途径 总被引：21，自引：10，他引：21
分析了１９９２年８月３１日到９月２日９２１６号台风（Ｐｏｌｌｙ）登陆后的结构和暴雨。分析发现，９２１６号台风具有显著的不对称结构。首先，地面台风环流的涡度和散度场不对称。地面正涡度中心位于台风的东侧和北侧。强辐合区从台风中心向北延伸近千公里。其次，８５０ｈＰａ台风流场和热力场不对称。台风东侧的环流强，西侧的环流弱。台风东侧暖，西侧冷。流场和热力场的不对称导致台风外围东北侧存在很强的暖平流。由于９２ 相似文献
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利用1961—2010年多种海、气观测资料,分析了东部型(EP)和中部型(CP)ENSO事件海温异常的非对称性及可能原因,并讨论了两类ENSO事件的非对称性对大气环流的可能影响。结果表明,海温异常非对称性包括空间分布非对称、强度非对称以及持续性非对称。从动力学角度来看,虽然不同事件发展衰减阶段主导的动力作用不尽相同,但就两类事件强度非对称性而言,海洋垂直对流项起关键性的作用。此外,研究发现,无论是通过海温强迫直接影响的赤道地区或是通过遥相关影响的赤道外地区,大气非对称响应都表现出与海温异常非对称较强的一致性,其中东部型ENSO的非对称性分布与事件暖位相分布相似,而中部型ENSO的非对称性分布与事件冷位相分布相似,且东部型ENSO非对称性差异大于中部型ENSO。 相似文献