基于新型探测资料对西安一次弱降水预报失误的原因分析

利用微波辐射计、激光测风雷达、多普勒雷达、相控阵雷达等新型探测资料、地面加密观测资料、ERA5再分析及多模式数值预报结果对2022年4月24日西安城区一次弱降水预报出现明显失误的原因进行分析,结果表明：(1)全球数值预报模式和中尺度数值预报模式对本次过程西安城区均报有明显降水,主要原因为模式对低层相对湿度预报明显偏大;(2)多种新型探测数据分析认为近地层湿度条件较差及中层的绝对水汽含量低,中层的干暖空气不利于成云致雨,垂直上升运动不强,且由于低层非常干燥,使水滴在下沉过程中蒸发,从而无法形成雨滴下落,这些原因共同造成西安城区无降水,低层相对湿度预报偏大是造成这次西安城区降水预报失误的主要原因之一;(3)造成西安城区近地层湿度条件差的原因是城市干热岛效应和低层干暖平流输送,且降水云团翻越秦岭后其湿空气绝热下沉至城区后出现增温降湿,使得城区形成较为深厚的干层,即使有雨滴在下落过程中也会造成更强的蒸发,这也是城区没有降水的重要原因之一;(4)预报员主观预报订正出现空报主要是源于对边界层水汽、抬升条件等关键降水要素缺乏订正能力,且对大城市的干热岛效应和秦岭山区地形影响研究不足。     

Numerical study on scale effect of form factor

The scale effect of form factor is investigated via a numerical approach in this paper, where the turbulent ship flow is computed by solving the steady and incompressible Reynolds-averaged Navier-Stokes and continuity equations. A wall function approach is employed to bridge the near-wall and outer turbulent flow region. The numerical scheme based on a finite-volume formulation is applied to discretize the coupled governing equation. For the sake of numerical stability, accuracy and economy, an identical grid is employed to compute ship flow at different Reynolds number, where the grid is optimized for the medium Reynolds number of the investigated range. Four surface ships and two sub-bodies with notably different geometrical characteristics are chosen as the investigated cases, where double-model flow without appendages is computed. The calculated total resistance coefficient shows a decreasing tendency against Reynolds number among all studied hulls. Similar to the calculated total resistance coefficient, the calculated friction resistance coefficient decreases with the Reynolds number and varies relatively little for a given Reynolds number among different hulls. The viscous pressure resistance coefficient is less insensitive to the Reynolds number but apparently depends on hull form. Compared with the form factor calculation based on empirical friction lines, the flat-plate friction prediction based on CFD approach clearly gives smaller Re-dependent form factor, which should more realistically reflect the scale effect of form factor. The form factor exhibits a near linear and increasing dependence on Reynolds number. The numerical results show that the dependence of r_P on Reynolds number mainly governs the scale effect of form factor.     

不同形状和材料的鱼礁模型对短蛸诱集效果的初步研究

采用鱼类行为学方法,观测了短蛸(Octopus ocellatus)对同为PVC材质的3种形状的有孔和无孔模型礁以及对同为管状的3种不同材料的单体和叠加模型礁的行为反应,并对各组内模型礁的诱集效果进行了比较.实验结果表明:未投放模型礁条件下短蛸在水槽四角分布率较高,在鱼礁标志区分布率只有0.5%;PVC材质3种形状的模型礁放入后对短蛸的聚集率均在10%以上,其中内部空间最大的正方体模型礁聚集率最大,每种形状的模型礁中有孔模型礁聚集率高于无孔;3种不同材料的管状模型礁放入后诱集效果均较明显,其中陶瓷材质的管状模型礁效果最好,每种材料的模型礁中3单体叠加后的组合礁聚集率明显高于单体,其中叠加后的陶瓷模型礁对短蛸的聚集率达到39.3%;研究发现短蛸的领域行为对鱼礁模型的诱集效果影响较大.     

GNSS地面测试环境中关于影响站址选择因素的考虑

卫星导航产品在投入正式运行之前需要在地面上对其进行测试和校准,因此需要在地面建立模拟天上GPS卫星星座的测试环境,并通过修建伪卫星发射塔来发射类GPS卫星信号。测试环境中关于伪卫星发射塔站址选择问题是一个值得关注的硬件建设环节,它直接影响了系统的运行性能,本文通过对五个影响站址选择的因素进行论述,详细地说明了它们各自的影响特征,给出了相应的结论。     

Ocean coast effect on magnetotelluric data: a case study from Kachchh, India

The effects of the ocean coast on magnetotelluric (MT) data have been studied and results are described in this article. MT soundings from various sites along coastal plains of Kachchh were acquired where volcanic rocks overlie Mesozoic sediments having potential resource prospects. The region of our study lies in the proximity of Gulf of Kachchh and Arabian Sea. This article describes the effects the presence of a conductive body on the acquired MT data. With the help of synthetic MT modelling and induction arrows we demonstrate the influence of shallow conductive body on the MT data. The modelling results when compared to the field observations show that much of the high frequencies are unaffected by the coast, however low frequencies of the data do seem to be distorted by onshore–offshore resistivity contrasts. The least affect on high frequencies can be attributed to very shallow water depth as well as to the fact that the onshore–offshore resistivity contrasts are not large enough to influence the high frequency data badly. The results presented here report for the first time the effect of the coast on the analysis on MT data from Kachchh. Due to several academic as well as industrial onshore–offshore MT experiments presently being carried out to explore its hydrocarbon potential, our results have an important bearing on designing and acquisition of future MT surveys in this region.     

A finite differences formulation for the linear and nonlinear dynamics of 2D catenary risers

A finite differences (FD) solution method is proposed for the numerical treatment of the dynamic equilibrium problem of 2D catenary risers. The method is based on the so-called Box approximation, which in the scope of the present contribution is applied to the complete nonlinear model as well as to the reduced linearized formulation. The application of the Box method transforms the original governing systems into convenient sets of algebraic equations, which in turn are solved efficiently by the relaxation method. Extensive numerical calculations are presented that describe the dynamic behaviour of the structure and evaluate the amplification in loading due to the dynamic components. The effect of the geometric nonlinearities is assessed through comparative calculations that concern both mathematical formulations examined in the present, i.e. the complete nonlinear, and the reduced linearized model. Special attention is paid to the heave excitations as they amplify significantly the magnitudes of the loading components.     

Physical and finite element modeling of lateral stability of offshore skirted gravity structures subjected to iceberg impact load

Static stability mainly against sliding of a typical, relatively large skirted gravity structure was investigated using three-dimensional finite element modeling. The numerical model was validated against centrifuge test results. A specific set of dimensions was chosen to model a typical skirted gravity structure in a centrifuge with two types of foundation soils: uniform saturated sand and a clay zone sandwiched between two sand layers. Soil shear strength parameters used in the finite element models were estimated from in-flight cone penetration resistance measurements obtained in the centrifuge. Numerical parametric studies were conducted using the validated finite element model. The parameters included were the depth and strength of the clay zone and the inclination of external load. It is shown that a relatively simple three-dimensional finite element model was effective in providing information that would be needed to design such a critical and expensive offshore structure. Basic Mohr-Coulomb strength parameters and moduli based on cone penetration resistance measurements and published empirical correlations were appropriate in modeling the soils in the finite element simulations.     

A two-dimensional finite volume hydrodynamic model for coastal areas: Model development and validation

A two dimensional implicit finite volume scheme for solving the shallow-water equations is developed. The effects of the Coriolis force, surface wind stress, and waves are included. A non-uniform rectilinear forward staggered grid is used with Cartesian coordinates. The time integration is performed using the Euler implicit technique. The convective flux is treated using the deferred correction method. The viscous terms are discretized using a second order central difference approximation. The SIMPLE (Semi-Implicit Method for Pressure-Linked Equations) algorithm is used for coupling the velocity components and the water elevation gradient for the water level correction. The system of equations is solved sequentially using the Strongly Implicit Procedure (SIP). To simulate wave driven current, a phase averaged wave model is used first to simulate wave transformation and calculate radiation stresses. The performance of the developed model is validated for different sources of external forces and different combinations of boundary conditions. The validation cases include tidal circulation in a harbor and wave induced currents behind a breakwater parallel to the coastline. The model is finally applied to simulate the flow pattern in a closed artificial lagoon and along the coastline near Damietta Port located along the Northern coast of Egypt. Results of the developed model agree well with the published results for the considered cases.     

Development and validation of a finite element morphological model for shallow water basins

Recognising the importance of understanding sediment dynamics to evaluate the status of a coastal lagoon environment, this work has been focused on the investigation of the hydrodynamic and sediment transport processes occurring in such basins. In order to describe the lagoon system, a modelling approach combining hydrodynamics, waves and sediment dynamics has been developed. The framework of the numerical model consists of a finite element hydrodynamic model, a third generation finite element spectral wave model and a sediment transport and morphodynamic model for both cohesive and non-cohesive sediments. The model adopts the finite element technique for spatial integration, which has the advantage to describe more accurately complicated bathymetry and irregular boundaries for shallow water areas. The developed model has been applied to test cases and to a very shallow tidal lagoon, the Venice Lagoon, Italy. Numerical results show good agreement with water level, waves and turbidity measurements collected in several monitoring stations inside the Lagoon of Venice. Such a model represents an indispensable tool in analysing coastal problems and assessing morphological impacts of human interference.