Alignment Trends of Velocity Gradients and Subgrid-Scale Fluxes in the Turbulent Atmospheric Boundary Layer

Field experimental data in the atmospheric surface layer are analyzed using toolsfrom statistical geometry. The data consist of velocity measurements from sonicanemometer arrays. In the context of large eddy simulations (LES), these arrayspermit the spatial filtering needed to separate large from small scales. Time seriesof various quantities relevant to LES are evaluated from the data. Results show thatthe preferred filtered fluid deformation is axisymmetric extension and the preferredsubgrid stress state is axisymmetric contraction. The filtered fluctuating vorticityshows preferred alignments with the mean vorticity, with the streamwise direction,and with the intermediate strain-rate eigenvector. The alignment between eigenvectorsof the subgrid-scale stress and filtered strain rate is used to test eddy viscosity andmixed model formulations. In qualitative agreement with prior laboratory measurements at much lower Reynolds numbers, a bimodal distribution is observed, which can be reduced to good alignment with eddy viscosity closure using the mixed model.     

Prediction of monsoon rainfall with a nested grid mesoscale limited area model

At the India Meteorological Department (IMD), New Delhi, a 12-level limited area model with 100 km horizontal resolution has been in use for weather forecasting. The present study uses this model together with a higher horizontal resolution (50 km) and vertical resolution (16-levels) model to examine the impact of increased resolution to simulate mesoscale features of rainfall during monsoon disturbances. The model was run for 22 days in the month of August 1997 and one week in September 1997 during three monsoon depressions and one cyclonic storm in the Bay of Bengal. The model results are compared with observations. The study shows that the model can capture mesoscale convective organization associated with monsoon depression.     

漫滩水流摩阻因子与涡粘性系数的研究

对复式河道漫滩水流进行了研究,建立了摩阻因子与一维明渠流摩阻因子的关系,揭示出滩地的量纲为一涡粘性系数随着摩阻因子的增加而增加,并将二次流的影响表达成与雷诺切应力相同的形式,从而将其归入表观切应力中,由量纲为一表观涡粘性系数确定,后者可通过量纲为一表观涡粘性系数与摩阻因子之间的一个关系加以计算,该关系类似于量纲为一涡粘性系数与摩阻因子之间的关系。对光滑边界上对称的漫滩水流进行了计算,计算结果与实验资料吻合良好。     

The South China Sea warm-core ring 94S and its influence on the distribution of chemical tracers

 In this paper, the distributions of currents and chemical tracers were studied along two hydrographic sections across ring 94S, a warm-core ring found in the South China Sea. Results suggest that while currents on its offshore side maintained quasigeostrophic balance, such a balance was not reached on its onshore side. Therefore, it is suggested that interactions of ring 94S with the slope may play an important role: it may break down the quasigeostrophic balance and cause a deformation of the current field. The observed distribution of δ¹⁸O supports the hypothesis that water masses inside the ring originate from the Kuroshio. Distributions of chemical tracers reveal a strong vertical disturbance of the isolines at the edge of ring 94S, where it approaches the shelf. This phenomenon may play an important role in the vertical exchange of biochemical elements across the thermocline. Possible reasons, i.e., enhanced vertical mixing and upwelling, are discussed. Received: 16 May 2001 / Accepted: 5 November 2001     

影响北京地区东北方路径带状雷暴天气分析

通过对１９９１年４次东北方路径的带状雷暴天气的大尺度合成分析，指出了这类天气发生的大尺度控制气流及热力环境的主要特征，并分析了在这类天气了具体发展演变过程中，中尺度地面形势场及地形对其影响的事实。     

1998年7月21～22日鄂东沿江连日特大暴雨成因探讨

分析了1998年7月21～22日连续两天发生在鄂东沿江的特大暴雨的环流背景特点和中尺度演变过程,并对其成因作了初步探讨.揭示了一种晚梅雨期特有的能把沿江东移的暴雨系统阻滞在鄂东形成持续和重复大暴雨的环流形势.结合当时的环境气流背景,讨论了大别山与鄂东南山地之间的NW-SE向河谷地形导致暴雨在夜间沿河谷产生和持续的可能机制.其中包括地形的机械和热力作用导致的中尺度环流变化,和在此情况下形成的一种长生命,强对流云系动态结构.     

全球谱模式不同垂直分层对纬向基本气流和行星尺度波动物理量的影响

本文用σ坐标原始方程全球谱模式，对１９７９年１月２３日个例，采用３种不同的垂直分层方案，实行了５天积分，输出了若干模式诊断物理量。发现提高了模式顶高度和增加垂直分辨率的１５层模式，对北半球冬季行星尺度波动的水平感热通量、波数１的振幅强度以及对流层西风急流强度的预报均有明显地改进，与实况符合很好。     

对流层及平流层低层全球动能的季节性急变

利用１９８０－１９８８年ＥＣＭＷＦ的资料，分析对流层５００ｈＰａ，３００ｈＰａ和平流层低层１００ｈＰａ全球范围的纬向平均动能和涡动动能的季节过渡，比较不同层次不同纬度带动能模态的季节性急变，得到如下结论：（１）北半球热带与温带各层次的ＫＺ都有季节性急变，发生在３－４月和１０月附近，而南半球仅在热带有急变。（２）北半球热带地区ＫＺ在６月还有一次季节性急变，出现有１００ｈＰａ和５００ｈＰａ．（３）１０     

A Boussinesq Equation-Based Model for Nearshore Wave Breaking

Based on the wave breaking model by Li and Wang (1999), this work is to apply Dally‘ s analytical solution to the wave-height decay instead of the empirical and semi-empirical hypotheses of wave-height distribution within the wave breaking zone. This enhances the applicability of the model. Computational results of shoaling, location of wave breaking, wave-height decay after wave breaking, set-down and set-up for incident regular waves are shown to have good agreement with experimental and field data.