共查询到20条相似文献，搜索用时 265 毫秒
1.
The non-uniqueness of solution and compatibility between the coupled boundary conditions in computing velocity potential and streamfunction from horizontal velocity in a limited domain of arbitrary shape are revisited theoretically with rigorous mathematic treatments.Classic integral formulas and their variants are used to formulate solutions for the coupled problems.In the absence of data holes,the total solution is the sum of two integral solutions.One is the internally induced solution produced purely and uniquely by the domain internal divergence and vorticity,and its two components(velocity potential and streamfunction) can be constructed by applying Green’s function for Poisson equation in unbounded domain to the divergence and vorticity inside the domain.The other is the externally induced solution produced purely but non-uniquely by the domain external divergence and vorticity,and the non-uniqueness is caused by the harmonic nature of the solution and the unknown divergence and vorticity distributions outside the domain.By setting either the velocity potential(or streamfunction) component to zero,the other component of the externally induced solution can be expressed by the imaginary(or real) part of the Cauchy integral constructed using the coupled boundary conditions and solvability conditions that exclude the internally induced solution.The streamfunction(or velocity potential) for the externally induced solution can also be expressed by the boundary integral of a double-layer(or singlelayer) density function.In the presence of data holes,the total solution includes a data-hole-induced solution in addition to the above internally and externally induced solutions. 相似文献
2.
Three types of previously used numerical methods are revisited for computing the streamfunction ψ and velocity potential χ from the horizontal velocity v in limited domains. The first type, called the SOR-based method, uses a classical successive over-relaxation (SOR) scheme to compute ψ (or χ) first with an arbitrary boundary condition (BC) and then χ (or ψ) with the BC derived from v . The second type, called the spectral method, uses spectral formulations to construct the inner part of (ψ, χ)—the inversion of (vorticity, divergence) with a homogeneous BC, and then the remaining harmonic part of (ψ, χ) with BCs from v . The third type, called the integral method, uses integral formulas to compute the internally induced (ψ, χ)—the inversion of domain-internal (vorticity, divergence) using the free-space Green?s function without BCs and then the remaining harmonic ψ (or χ) with BCs from v minus the internally-induced part. Although these methods have previously been successfully applied to flows in large-scale and synoptic-scale domains, their accuracy is compromised when applied to complex flows over mesoscale domains, as shown in this paper. To resolve this problem, two hybrid approaches, the integral-SOR method and the integral-spectral method, are developed by combining the first step of the integral method with the second step adopted from the SOR-based and spectral methods, respectively. Upon testing these methods on real-case complex flows, the integral-SOR method is significantly more accurate than the integral-spectral method, noting that the latter is still generally more accurate than the three previously-used methods. The integral-SOR method is recommended for future applications and diagnostic studies of complex flows. 相似文献
3.
流函数和速度势是表示风场的一种变量, 在数值天气预报模式和分析、同化方案中经常使用, 通常可以用风速分量场求解Poisson方程得到。对于有限区域系统, 往往采用差分方法, 但由于存在边界问题, 用计算所得到的流函数和速度势场重建风速场, 在边界附近经常出现明显的偏差。基于差分方法、利用有限区域风速场求解流函数和速度势场的基本方法和特点的分析, 在Arakawa A网格分布的有限区域, 设计了一种用差分方法求解流函数和速度势场的有效方案。在该有效方案中, 通过将有限区域向外扩展二圈, 风速场线性外推, 改进计算边界风速值和边界定解条件的效果; 尽可能使用协调、一致的差分格式, 提高求解精度; 最后利用一种增量订正迭代方法, 迭代2~3次就可以获得令人满意的结果。实例试验的对比、检验显示, 用该方案计算求得的流函数和速度势场重建风速场, 具有非常高的精度。 相似文献
4.
Glenn R. Flierl 《Dynamics of Atmospheres and Oceans》1977,1(5):443-453
We have solved several linear quasigeostrophic problems in singly and multiply connected domains to illustrate the use and effects of correct boundary conditions which balance circulation changes with forcing along each boundary. The results with these boundary conditions differ markedly from those obtained by setting the streamfunction to zero on the walls either when the fluid is forced by a time-dependent wind stress curl or when the fluid oscillates in a free basin mode in a domain which contains interior islands. 相似文献
5.
A series of idealized model simulations are analyzed to determine the sensitivity of model results to different configurations of the lateral boundary conditions (LBCs) in simulating mesoscale shallow convection over hilly terrain, In the simulations with steady thermal forcing at the model surface, a radiation condition at both boundaries is the best choice under high wind conditions, and the best results are produced when both the normal velocities and the temperature are treated with the radiation scheme in which the phase speed is the same for different variables, When the background wind speed is reasonably small, the LBC configuration with either the radiation or the zero gradient condition at both boundaries tends to make the numerical solution unstable. The choice of a constant condition at the inflow boundary and a radiation outflow boundary condition is appropriate in most cases, In the simulations with diurnal thermal forcing at the model surface, different LBC schemes are combined together to reduce spurious signals induced by the outflow boundary, A specification inflow boundary condition, in which the velocity fields at the inflow boundary are provided using the time-dependent results of a simulation with periodic LBCs over a flat domain, is tested and the results indicate that the specification condition at the inflow boundary makes it possible to use a smaller model domain to obtain reasonable results. The model horizontal domain length should be greater than a critical length, which depends on the domain depth H and the angle between gravity wave phase lines and the vertical, An estimate of minimum domain length is given by [(H-zi)/πU]√N^2L2x-4π^2u^2, where N and U are the background stability and wind speed,respectively, Lx is the typical gravity wavelength scale, and zi is the convective boundary layer (CBL)depth. 相似文献
6.
A regional climate model is employed to simulate the aerosols(dust,sulfate,black carbon,and organic carbon) and their direct effect on the climate over China.The emphasis is on the direct radiative forcing due to the change in mixing state of aerosols.The results show that direct radiative forcing is significantly different between externally and internally mixed aerosols.At the top of the atmosphere(TOA),the radiative forcing of externally mixed aerosols is larger than that of internally mixed ones,especially in the Tarim desert region where the difference is about 0.7 W m 2.At the surface,however,the situation becomes opposite,especially in the Sichuan basin where the difference is about-1.4 W m 2.Nonetheless,either externally or internally mixed aerosols in China can result in a significant cooling effect,except for the warming in South China in winter and the slight warming in North China in February.The cooling effect induced by externally mixed aerosols is weaker than that induced by internally mixed aerosols,and this is more obvious in spring and winter than in summer and autumn.In spring and summer,the inhibiting effect of externally mixed aerosols on precipitation is less than that of internally mixed aerosols,whereas in autumn and winter the difference is not obvious. 相似文献
7.
8.
Since the solution of elliptic partial differential equations continuously depends on the boundary condition, the Euler equation derived from variational method cannot be solved without boundary condition. It is often difficult to provide the exact boundary condition in the practical use of variational method. However, in some application problems such as the remote sensing data assimilation, the values can be easily obtained in the inner region of the domain. In this paper, the boundary condition is tried to be retrieved by using part solutions in the inner area. Firstly, the variational problem of remote sensing data assimilation within a circular area is established. The Klein-Gordon elliptic equation is derived from the Euler method of variational problems with assumed boundary condition. Secondly, a computer-friendly Green function is constructed for the Dirichlet problem of two-dimensional Klein-Gordon equation, with the formal solution according to Green formula. Thirdly, boundary values are retrieved by solving the optimal problem which is constructed according to the best approximation between formal solutions and high-accuracy measurements in the interior of the domain. Finally, the assimilation problem is solved on substituting the retrieved boundary values into the Klein-Gordon equation. It is a type of inverse problem in mathematics. The advantage of this method lies in that it overcomes the inherent instability of the inverse problem of Fredholm integral equation and alleviates the error introduced by artificial boundary condition in data fusion using variational method in the past. 相似文献
9.
A non-local parameterisation of shear turbulence is proposed, which includes a dimensionless multiplicative constant as the
sole tunable parameter. Analytical and numerical solutions in the case of plane Couette flow exhibit sheared velocity profiles
with logarithmic behaviour near the boundaries, and the classical logarithmic flow profile is reproduced for a semi-infinite
domain. We also prove that the families of analytical solutions obtained are locally unique: if the velocity is a strictly-increasing
function of the distance from the boundary, a small perturbation of the velocity profile must be of the same functional form
as the basic flow. 相似文献
10.
The air quality model system RAMS (Regional Atmospheric Modeling System)-CMAQ (Models-3 Community Multi-scale Air Quality) coupled with an aerosol optical/radiative module was applied to investigate the impact of different aerosol mixing states (i.e., externally mixed, half externally and half internally mixed, and internally mixed) on radiative forcing in East Asia. The simulation results show that the aerosol optical depth (AOD) generally increased when the aerosol mixing state changed from externally mixed to internally mixed, while the single scattering albedo (SSA) decreased. Therefore, the scattering and absorption properties of aerosols can be significantly affected by the change of aerosol mixing states. Comparison of simulated and observed SSAs at five AERONET (Aerosol Robotic Network) sites suggests that SSA could be better estimated by considering aerosol particles to be internally mixed. Model analysis indicates that the impact of aerosol mixing state upon aerosol direct radiative forcing (DRF) is complex. Generally, the cooling effect of aerosols over East Asia are enhanced in the northern part of East Asia (Northern China, Korean peninsula, and the surrounding area of Japan) and are reduced in the southern part of East Asia (Sichuan Basin and Southeast China) by internal mixing process, and the variation range can reach 5 W m－2. The analysis shows that the internal mixing between inorganic salt and dust is likely the main reason that the cooling effect strengthens. Conversely, the internal mixture of anthropogenic aerosols, including sulfate, nitrate, ammonium, black carbon, and organic carbon, could obviously weaken the cooling effect. 相似文献
11.
GRAPES全球集合预报系统模式扰动随机动能补偿方案初步探究 总被引：10，自引：5，他引：5
为了体现次网格尺度能量升尺度转换过程中存在的不确定性, 文中将随机动能补偿(Stochastic Kinetic Energy Backscatter, SKEB)方案应用于GRAPES(Global/Regional Assimilation and Prediction System)全球集合预报系统(GRAPES-GEPS), 以更好地表征模式误差并且增大集合离散度。使用的SKEB方案基于具有一定时、空相关特征的随机型以及由数值扩散导致的局地动能耗散率来构造随机流函数强迫。并根据流函数与水平风速旋转分量的关系, 将SKEB方案中的流函数强迫转化为适用于GRAPES全球模式的水平风速扰动。结果表明, SKEB方案的使用一方面能够提高GRAPES对大气动能谱的模拟能力; 另一方面能够改善GRAPES-GEPS的集合离散度与集合平均误差的关系, 增加了集合离散度, 并在一定程度上减小了集合平均误差, 尤其是在热带地区这种改进更为显著。而且该方案使得热带地区连续分级概率评分(CRPS评分)显著减小。就降水预报而言, 从Brier评分与相对作用特征面积(AROC, Area under the Relative Operating Characteristics)的结果来看, SKEB方案有助于改善中国地区小雨[0.1 mm, 10 mm)、中雨[10 mm, 25 mm)与大雨[25 mm, 50 mm)量级降水的概率预报技巧, 而对暴雨[50 mm, ∞)量级降水预报技巧影响很小(24 h降水量)。总体上, 模式扰动随机动能补偿方案提高了GRAPES-GEPS的概率预报技巧。 相似文献
12.
The importance of specifying realistic lateral boundary conditions in the regional modeling of mineral aerosols has not been examined previously. This study examines the impact of assigning values for mineral aerosol (dust) concentrations at the lateral boundaries of Regional Climate Model version 3 (RegCM3) and its aerosol model over Southwest Asia. Currently, the dust emission module of RegCM3 operates over the interior of the domain, allowing dust to be transported to the boundaries, but neglecting any dust emitted at these points or from outside the domain. To account for possible dust occurring at, or entering from the boundaries, mixing ratios of dust concentrations from a larger domain RegCM3 simulation are specified at the boundaries of a smaller domain over Southwest Asia. The lateral boundary conditions are monthly averaged concentration values (μg of dust per kg of dry air) resolved in the vertical for all four dust bin sizes within RegCM3’s aerosol model. RegCM3 simulations with the aerosol/dust model including lateral boundary conditions for dust are performed for a five year period and compared to model simulations without prescribed dust concentrations at the boundaries. Results indicate that specifying boundary conditions has a significant impact on dust loading across the entire domain over Southwest Asia. More specifically, a nearly 30% increase in aerosol optical depth occurs during the summer months from specifying realistic dust boundary conditions, bringing model results closer to observations such as MISR. In addition, smaller dust particles at the boundaries have a more important impact than large particles in affecting the dust loading within the interior of this domain. Moreover, increases in aerosol optical depth and dust concentrations within the interior domain are not entirely caused by inflow from the boundaries; results indicate that an increase in the gradient of concentration at the boundaries causes an increase of diffusion from the boundaries. Lastly, experiments performed using a climatology of dust concentrations yield similar results to those using actual monthly values. Therefore, using a climatology of dust mixing ratios is sufficient in implementing lateral boundary conditions for mineral aerosols. In short, this work concludes that realistic specification of lateral boundary conditions for mineral aerosols can be important in modeling the dust loading over arid regional climates such as Southwest Asia. 相似文献
13.
Bridging the Transition from Mesoscale to Microscale Turbulence in Numerical Weather Prediction Models 总被引：1，自引：1，他引：0
Domingo Muñoz-Esparza Branko Kosović Jeff Mirocha Jeroen van Beeck 《Boundary-Layer Meteorology》2014,153(3):409-440
With a focus towards developing multiscale capabilities in numerical weather prediction models, the specific problem of the transition from the mesoscale to the microscale is investigated. For that purpose, idealized one-way nested mesoscale to large-eddy simulation (LES) experiments were carried out using the Weather Research and Forecasting model framework. It is demonstrated that switching from one-dimensional turbulent diffusion in the mesoscale model to three-dimensional LES mixing does not necessarily result in an instantaneous development of turbulence in the LES domain. On the contrary, very large fetches are needed for the natural transition to turbulence to occur. The computational burden imposed by these long fetches necessitates the development of methods to accelerate the generation of turbulence on a nested LES domain forced by a smooth mesoscale inflow. To that end, four new methods based upon finite amplitude perturbations of the potential temperature field along the LES inflow boundaries are developed, and investigated under convective conditions. Each method accelerated the development of turbulence within the LES domain, with two of the methods resulting in a rapid generation of production and inertial range energy content associated to microscales that is consistent with non-nested simulations using periodic boundary conditions. The cell perturbation approach, the simplest and most efficient of the best performing methods, was investigated further under neutral and stable conditions. Successful results were obtained in all the regimes, where satisfactory agreement of mean velocity, variances and turbulent fluxes, as well as velocity and temperature spectra, was achieved with reference non-nested simulations. In contrast, the non-perturbed LES solution exhibited important energy deficits associated to a delayed establishment of fully-developed turbulence. The cell perturbation method has negligible computational cost, significantly accelerates the generation of realistic turbulence, and requires minimal parameter tuning, with the necessary information relatable to mean inflow conditions provided by the mesoscale solution. 相似文献
14.
出流边界对京津冀地区强对流局地新生及快速增强的动力效应 总被引：1，自引：0，他引：1
针对2014年7月16日发生在京津冀地区包含三次风暴过程的强对流"事件",通过雷达、探空和自动站等观测资料分析,以及基于雷达资料快速刷新四维变分同化（RR4DVar）和三维数值云模式的高分辨率模拟,研究了在京津冀复杂地形条件下导致对流风暴局地新生及快速增强的对流尺度热力和动力机制,重点分析了出流边界在对流风暴局地新生及快速增强过程中的动力效应。探空观测和模拟结果均显示,16日当天从上午到傍晚,京津冀地区存在有利于对流风暴发生、发展的中尺度环境条件,包括明显的热力不稳定、强的偏南低空急流和低层垂直风切变等。在本次强对流"事件"中,首先是东移的近地面切变线在中午12:00（北京时,下同）左右触发了天津地区多单体对流风暴的局地新生和快速加强,并产生了明显的向西北移动的出流边界。随后,在京津冀西北部山区形成的一个产生向南出流的风暴单体于下午18:00左右抵达北京西北部山边,由于地形强迫,沿山坡加速下滑的风暴出流与沿山坡上行的低层偏南暖湿气流相互作用,增强了山坡附近的低层辐合和垂直上升,同时在向南和向西北移动的出流边界"碰撞"形成的动力不稳定配合下,使得风暴单体在下山过程中迅速发展为强超级单体风暴。两条出流边界在风暴附近的"碰撞"及其和低层偏南暖湿气流的相互作用,具有复杂地形条件下导致风暴新生和加强的"三重点"关键区特征。在22:00左右,由超级单体风暴形成的出流边界抵达京津冀南部平原地区,与偏南低空急流和低层偏东风湿空气产生的辐合区相互作用,形成新的类似于"三重点"的关键区,导致在辐合区内沿出流边界出现暖湿空气的强烈上升。在出流边界的动力不稳定触发下,沿出流边界附近不断有对流单体新生和增强,最终在23:00左右形成了一条近似东西走向的线状多单体风暴系统。 相似文献
15.
In this paper, the atmosphere is divided into two regions which are governed by different equations. Furthermore, multiple-scale
method is used to obtain the Benjamin-Ono equation satisfied by the nonlinear Rossby wave in weak shear zonal flow. The equation
has algebraic solitary wave solution, and the stationary streamfunction fields in-ithe atmosphere are calculated by using
numerical method, and the results demonstrate that the stationary solution is antisymmetric dipoles with the anticyclone north
of the cyclone, and the structure is similar to the equivalent modon obtained by Mcwilliams (1980). Because the modon obtained
here is an algebraic solitary wave, the modon may be called “algebraic modon”, and the stationary algebraic modon is consistent
with observations of blocking pattern in the atmosphere. 相似文献
16.
A coupled ocean-sea ice-atmosphere model is used to study interdecadal variability (∼40 years) of sea ice depth and concentration
in the Greenland-Iceland-Norwegian Sea. This oceanic region is represented by a meridionally aligned channel on a β-plane
with open zonal boundaries at 60 °N and 80 °N. The model consists of a one and a half layer reduced gravity ocean model, a
thermodynamic/dynamic sea ice model and an energy balance model of the atmosphere. The coupled model is driven by prescribed
surface wind stress, fluxes of heat, salt and ice at inflow points on the northern and southern open zonal boundaries and
annual distribution of solar radiation. It is shown that the coupled model supports unforced modes of interdecadal oscillation
resulting from a form of hydraulic control which regulates the total fluid volume in the oceanic active layer. The mechanism
for the oscillations relies on the presence of three key features: (1) a region of intense oceanic entrainment located in
the eastern part of the domain, (2) a vigorous southward flowing western boundary current, representing the East Greenland
Current (EGC), which supports most of the meridional transport across the domain, and (3) a marked buoyancy contrast between
the relatively salty domain interior and the much fresher western boundary region. During an oscillation excess water is pumped
into the domain via entrainment, thereby creating an active layer depth anomaly, which then propagates westward via long baroclinic
Rossby waves until it reaches the EGC where it is subsequently drained out of the domain across the southern open zonal boundary.
As the depth anomaly traverses the basin, an anomalous geostrophic circulation is established in which cold fresh Arctic water
enters the domain interior, and this eventually promotes enhanced thermodynamic sea ice growth. Consequently, the interdecadal
oscillations of the coupled model are characterised by pulse periods, typically spanning 20 years, during which there is an
abnormally large winter sea ice cover, separated by interpulse periods, lasting another 20 years, during which the winter
sea ice extent is nearly uniform and significantly smaller than in a pulse maximum. The duration of the interpulse periods
is dictated by the time it takes for the Rossby waves to traverse the basin. In addition to the interdecadal oscillation solution,
the coupled ocean-sea ice-atmosphere model is found to also have a stable cyclostationary state, with no interannual variability.
Stochastic forcing, in the form of randomly specified interannual anomalies of salinity (of maximum amplitude 0.1 ppt) or
ice inflow (of maximum amplitude 0.1 Sv) at the northern open zonal boundary, in both cases is capable of driving the model
from the cyclostationary state solution to the interdecadal variability one.
Received: 16 August 1996 / Accepted: 27 July 1998 相似文献
17.
The ability of a nested model to accurately simulate the subarctic climate is studied here. Two issues have been investigated: Model??s internal variability (IV) and the impact of domain size (DS). For this purpose we combine the ??perfect model?? approach, Big-Brother Experiment (BBE) (Denis et al. in Clim Dyn 18:627?C646, 2002) with the ensemble of simulations. The advantage of this framework is the possibility to study small-scale climate features that constitute the main added value of RCM. The effects of the DS on result were studied by employing two Little-Brother (LB) domain sizes. IV has been evaluated by introducing small differences in initial conditions in an ensemble of 20 simulations over each LB. Results confirm previous findings that the IV is more important over the larger domain of integration. The temporal evolution over two domain sizes is rather different and depends strongly on the synoptic situation. Small-scales solution over the larger domain diverges freely from the boundary forcing in some periods. Over the smaller domain, the amplitude of small-scale transient eddies is systematically underestimated, especially at higher altitude characterized by the strongest winds along the storm tracks. Over the larger domain, the amplitude of small-scale transient eddies is better represented. However, the weaker control by the lateral boundaries over the larger domain results in solutions with large internal variability. As a result, the ensemble average strongly underestimates the transient-eddy variance due to partial destructive interference of individual ensemble member solutions. 相似文献
18.
自适应网格在大气海洋问题中的初步应用 总被引：15，自引：4，他引：15
自适应网格法是80年代兴起的通过求解椭圆型方程的边值问题来数值生成网格的一种新方法。它是在任意形状的区域上求偏微分方程的数值解的一种非常有效的工具。该方法抛弃了等距均匀的差分网格，代之以能够自动地适应所研究问题中解的特征的疏密程度不均的曲线网格。如在边界上计算网格与实际边界相重合，在区域内部可任意调节网格点的疏密程度等。本文扼要地介绍了自适应网格的原理及其构造方法。并将其应用于生成南海区域的计算网格以及数值预报台风路径的自适应网格。 相似文献
19.
"96.1"高原暴雪过程三维条件性对称不稳定的数值研究 总被引：37，自引：22，他引：15
利用“96．1”青藏高原东北部暴雪过程的较成功的中尺度模拟的输出资料,用非纬向非平行基流中的对称不稳定模式,对“96．1”暴雪发生发展过程的动力学机制进行了三维基流中二维非线性对称不稳定数值模拟试验。结果表明,ψ场和ω场的三维配置与切变线的演变过程相当一致,说明用三维基流中二维非线性对称不稳定可能能较好地描述SI在切变线东伸南移过程中所起的作用。 相似文献
20.
Recent Developments in Computing Streamfunction and Velocity Potential in a Limited Domain 
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下载免费PDF全文 《大气和海洋科学快报》2015,(6)
After first reviewing historical and current difficulties in solving streamfunction and velocity potential in a limited domain,and describing recent developments in obtaining accurate solutions in a limited domain with arbitrary shape,a newly proposed approach is introduced and its application to a torrential rain event is reported.The results show that the newly developed method has advantages in capturing mesoscale information,compared with horizontal winds. 相似文献